Three-dimensional integration of power electronic converters on printed circuit board

power electronic converters on

printed circuit board

PROEFSCHRIFT

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 9 januari 2007 om 10:00 uur door

Erik Cornelis Wytze DE JONG

Prof.dr. J.A. Ferreira

Samenstelling promotiecommissie: Rector Magnificus, voorzitter

Prof.dr. J.A. Ferreira, Technische Universiteit Delft, promotor Prof.dr.ir. P. Kruit, Technische Universiteit Delft

Prof.dr. J.J. Smit, Technische Universiteit Delft

Prof.dr.ir. A.J.A. Vandenput, Technische Universiteit Eindhoven Prof.dr.ir. F.B.J. Leferink, Universiteit Twente

Prof.ir. M. Antal, Technische Universiteit Eindhoven (emiritus) Dr.ir. P. Bauer, Technische Universiteit Delft

Dr.ir. P. Bauer heeft als begeleider in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen.

This research was supported financially by SenterNovem in the framework of the “Innovatiegerichte OnderzoeksProgramma ElektroMagnetische VermogensTechniek” (IOP-EMVT).

ISBN 978-90-6464-048-3

Printed by Grafisch bedrijf Ponsen & Looijen BV, Wageningen, The Netherlands Copyright © 2007 by E.C.W. de Jong

This thesis, approximately four years in the making, is a culmination of creative and scientific impetus, brought about by the insatiable curiosity to explore the seemingly endless possibilities that power electronics has to offer. It is, however, a thesis which would not have seen the light of day had it not been for the support provided by a network of skilled, dedicated and caring individuals. It is to these individuals, too many to mention all by name, to which I would like to convey my sincerest gratitude and appreciation here.

A special word of gratitude is hereby extended to:

my promotor: Professor Braham Ferreira, whose insightful guidance proved to be invaluable during my entire Ph.D experience, and whose brilliant ideas (that, in-terestingly enough, usually originated while under the shower, on the bicycle, or merely waiting next to the “Kerkpolder” swimming pool) could almost always be transformed into a worthwhile scientific publication (or two);

my supervisor: Dr. Pavol Bauer, whose door always stood open to answer all my questions or to give advise on matters not necessarily related to the work pertaining to this thesis, and whose assistance in organising and executing the many company visits, as well as in preparing the numerous publications (including this thesis) made an enormous difference;

the IOP-EMVT coaching committee, whose bi-annual meetings not only contributed to keeping the project on its intended course by providing valuable comments and suggestions, but also added value to the research by constantly reminding me of th ´ose requirements which the cut-throat power electronics industry so desperately desires.

Furthermore, to Mr. Jos Bentvelsen, from Thales Special Products in Zoetermeer, Ev-ert Raaijen from Exendis in Ede, Marcel Hendrix from Philips Lighting in Eindhoven and Dr. Frank van Horck from Philips Power Solutions also in Eindhoven for their hospitality during my visits to their companies in aid of gathering vital data for the thermal management survey presented in the thesis.

Vandenput, Prof. Frank Leferink and Prof. Mih´aly Antal for investing a significant amount of their valuable time to study the draft thesis and give valuable comments and suggestions;

my language editor: Prof. Craig MacKenzie of the University of Johannesburg in South Africa, whose lightning fast, yet meticulous language check sorted out all the gram-matical and typographical errors that mysteriously found their way into early ver-sions of the draft thesis;

my colleagues and friends at the EPP group during the past few years, who have made my entire Ph.D experience, in all its facets, an enjoyable and memorable one. In particular my predecessors, and dear friends, whose own defences I have witnessed with great awe and admiration. These include both Doctors Mark & Jelena Gerber and Doctor Martin Pavlovsk ´y, who have not only provoked interesting discussions with regards to power electronics but also extended their hand of friendship and proved beyond any doubt that they can be counted on in times of need, and fortu-nately also in times of felicity;

my family, whose unwavering support and motivation gave me the strength to perse-vere through all the trying times and to appreciate thoroughly all the good moments in between. In particular my uncle Jacob and aunt Ineke van den Beukel who were kind enough to open their home for me and make me feel at home during my first visit, as well as during the first year of my stay in the Netherlands;

my parents, my father Henk and my mother Herna, for giving me the freedom to set my own course in life even if it means putting vast distances between us. Their strength and encouragement is, and always will be, a source of inspiration to me. my only sister: Lieutenant-colonel, Dr. Angela, who, by always setting the (academic) bar high, encourages and motivates me to give my best in everything I do and to ac-cept nothing less.

my extended family: Geert, Jos´e and Adam, for their meticulous attention to detail in the not so trivial task of translating the propositions and summary into Dutch, as well as Gerhard, Tonny, Gert & Jos Loorbach, whose bustling company and hospi-tality always invigorates me;

my better half : Marieke Loorbach, whose undying love and support is something I would like to build on for many more years to come.

I hope you will enjoy reading this thesis and will find it to be just as interesting and invigorating as it was for me to write it!

Foreword vii

List of symbols xiii

1 Introduction 1

1.1 PCB-assembled power converters . . . 2

1.1.1 What constitutes a PCB-assembled power converter? . . . 2

1.2 Problem definition . . . 4

1.2.1 Thesis objectives . . . 5

1.3 Thesis layout . . . 6

2 Overview of printed circuit board technology 7 2.1 State of the art . . . 8

2.1.1 Interconnection technology . . . 10

2.1.2 Substrate technology . . . 12

2.1.3 Substrates enabling functional integration . . . 14

2.2 Printed circuit technology comparison . . . 18

2.2.1 Rigid PCB . . . 18

2.2.2 Flex PCB . . . 19

2.2.3 Rigid-flex PCB . . . 22

2.3 Printed circuit board in power converters . . . 24

2.3.1 Technology challenges . . . 25

2.4 Summary . . . 26

3 Overview of thermal management in PCB converters 27 3.1 Introduction . . . 27

3.3 PCBs - Beating the heat . . . 30

3.3.1 Heat spreading . . . 32

3.3.2 Heat sinking . . . 38

3.3.3 Layout & geometry . . . 40

3.3.4 Substrate technologies . . . 42

3.4 Survey: Thermal management in industry . . . 43

3.5 Summary . . . 44

4 Designing for improved thermal management in compact PCB converters 45 4.1 Thermal management performance indicators . . . 45

4.1.1 Thermal management loss density . . . 45

4.1.2 Thermal design rating . . . 48

4.1.3 Example — Determining thermal management effectiveness . . 53

4.1.4 Results of the thermal management analysis . . . 54

4.1.5 Conclusion of example . . . 59

4.2 Designing with the performance indicators . . . 60

4.2.1 Design technique . . . 60

4.2.2 Case study: Finite difference method approach . . . 66

4.3 Conclusion . . . 69

5 Geometrical packaging 71 5.1 Packaging of PCB-assembled converters . . . 72

5.1.1 System level . . . 72

5.1.2 Component level . . . 73

5.2 Geometrical packaging in 3D . . . 76

5.3 3D geometrical packaging design approach . . . 80

5.4 Implementing geometrical packaging(Case study) . . . 83

5.4.1 Electrical connectivity optimisation . . . 84

5.4.2 3D Shape optimisation . . . 84

5.4.3 Thermal pathway optimisation . . . 90

5.4.4 Verification . . . 94

5.5 Conclusion . . . 99

6 Integrated 3D PCB converter design 101 6.1 Electrical design . . . 102

6.1.1 Suitable topologies . . . 102

6.1.2 Electrical analysis . . . 104

6.1.3 Electromagnetic design including integration . . . 105

6.2 Spatial design . . . 109

6.2.1 PCB as enabling technology . . . 110

6.2.2 Winding construction . . . 111

6.3 Influence on geometrical packaging . . . 119

6.6 Case study . . . 121

6.7 Conclusion . . . 143

7 Conclusions and recommendations 145 7.1 Conclusions . . . 146

7.1.1 Quantitative means towards thermal management design . . . 146

7.1.2 Enhanced functionality of the PCB . . . 146

7.1.3 Packaging and integration of components . . . 147

7.1.4 Shape and size optimisation of components . . . 148

7.1.5 3D component placement . . . 148

7.1.6 Thesis contributions . . . 149

7.2 Recommendations for further research . . . 150

Bibliography 153 Appendices 163 A Survey on thermal management strategies used in industry 163 B Parameter criteria: thermal management performance indicators 173 C Resonant converter topology study 183 D Analysis and design of 3D PCB technology demonstrator 225 D.1 Derivation of steady-state equations . . . 226

D.2 Mathematical model of topology . . . 234

D.3 Loss analysis . . . 235

Summary 242

Samenvatting 245

Curriculum Vitae 247

Latin letters Z _{Integer value} < Reluctance [1/H]=[A/Wb] A Area [m2_] C Capacitance [F] d Thickness or depth [m] f Frequency [Hz]

H Magnetic field intensity [A/m]

hc Coefficient of convective heat transfer [W/m2]

I Current [A]

Ki Functional elements integration level [-]

Kp Packaging elements integration level [-]

L Inductance [H]

l Length [m]

N Amount of windings or elements [-]

n Amount of panels [-]

P Power [W]

q Heat flow rate [W]

R Resistance [Ω]

r Radius [m]

T Temperature [K]

t Time [s]

TDR Thermal design rating [%]

TMLD Thermal Management Loss Density [W/m3_]

V Voltage [V] V Volume [m3] Greek letters α Optimal band [%] ∆ _{Difference [-]} δ Skin depth [m] e Permittivity [F/m] η Electrical efficiency [%] η Power density [W/l]

ηv Volumetric packaging effectiveness [%]

γ Angular length [rad]

λ Thermal conductivity [W/mK]

µ Permeability [H/m]

ω0 Resonant angular frequency [rad/s]

ωs Switching angular frequency [rad/s]

φ Diameter [m]

τ Material usage factor [%]

in Input j Junction L Leakage max Maximum min Minimum p Primary PD Power Density PE Packaging Element PEv Virtual Packaging Element

r Reliability (Chapter 5), Resonant (Chapter 6), Relative (Chapter 2) s Switching, Secondary

TM Thermal Management

w Winding

ws Weighted sum

Acronyms

ALIVH Any Layer Inner Via Hole

B2it Buried Bump Interconnect Technology Bit Bump Interconnect Technology CTE Coefficient of Thermal Expansion CVDD Chemical Vapor Deposition Diamond DBC Direct Bonded Copper

DIP Dual Inline Package DVD Digital Versatile Disc

EMI Electro-Magnetic Interference

emPIC Embedded Passives Integrated Circuit FB Full Bridge

FDM Finite Difference Method FPC Ferrite Polymer Composite FPC Flexible Printed Circuit

HB Half Bridge

IC Integrated Circuit

IMS Insulated Metal Substrate

IR Infrared

IVH Inner Via Hole

LCD Liquid Crystal Display

LCT Inductor-Capacitor-Transformer structure MLB Multi Layer Board

MOV Metal Oxide Varistor

MTBF Mean Time Between Failures

PA Phase Arm

PC Personal Computer PCB Printed Circuit Board PDA Personal Digital Assistant PFC Power Factor Correction PRC Parallel Resonant Converter PSRC Partial Series Resonant Converter PWM Pulse Width Modulation

SMD Surface Mount Device SO Small Outline

SRC Series Resonant Converter SVH Surface Via Hole

TFA Thick Film Alumina THD Total Harmonic Distortion UPS Uninterruptible Power Supply ZCS Zero Current Switching ZVS Zero Voltage Switching

CHAPTER

Introduction

In this technologically driven age, many a traveller has already experienced the bur-den of carrying as many power adaptors as the technical ‘gadgets’ they are intended to power — and these in a diverse range of forms. Increasingly, these power adaptors take up more space than the ‘gadget’ itself. The power adaptors for mobile phones, ‘pocket-size’ digital photo cameras and personal digital assistants (PDAs) are good examples of this phenomenon. A miniaturisation of such power converters, ideally at no extra cost to the consumer, would undoubtedly be welcomed by frequent trav-ellers and home dwtrav-ellers alike.

Portable products, however, are not the only applications that could benefit from miniaturisation. Power converters for domestic consumer products such as flat-panel monitors, personal computer (PC) peripherals and even energy-saving light bulbs, would also benefit from volume reduction to such an extent that embedding them in the product could become feasible for higher power applications. Similarly, power converters on which telecommunication infrastructure [MC97] and data cen-ters [PHH+05] rely can benefit from increased power densities to serve more ‘loads’ (telephone exchanges, data servers, etc.) per converter, as opposed to a volume re-duction. This illustrates that more power per volume is desired for many applica-tions.

This thesis therefore investigates means to reliably and cost-effectively increase the power density of such ac-dc power converters in general in order to promote the miniaturisation and increased volumetric performance capability of such ac-dc converters.

More specifically, this research will focus on PCB-assembled power converters for appli-cations which could benefit directly from increased power density. The framework for this research effort is given next.

1.1

PCB-assembled power converters

Printed circuit board (PCB) assembled power converter is a broad definition because of the diversity in electrical design, materials and thermal management implementa-tions. To define the converters under investigation in this thesis, a definition of what constitutes a PCB-assembled power converter is given, followed by the history and future evolution thereof with respect to the advances that will be investigated here.

1.1.1

What constitutes a PCB-assembled power converter?

Power converters consist of many discrete components, mechanically held together by the circuit carrier, in this case a printed circuit board. In the case of PCB-assembled power converters the circuit carrier also interconnects these components and adheres to the specification shown in Figure 1.1, hence the definition.

→ Weight restricted → Volume restricted → Natural convection cooling → Fully enclosed in housing

@ @ @ I 6 ¾ Boundary conditions: → Cost → Performance → Reliability → Power density → Manufacturability Enhancement objectives: → Output power ≤ 500W

→ Galvanic isolation present → Ac-dc conversion

→ Universal input, “off-the-line” Specification: ¢ ¢¢ ¢¢ ¢¢¸ ¢¢¢ ¢¢ ¢¢ ¢¢¢¸ -→ Mechanical support → Electrical interconnection Circuit carrier: PCB technology

@ @@R

Figure 1.1: PCB-assembled power converter definition and boundary conditions, as applicable to the investigation performed in this thesis

The boundary conditions applied to the PCB-assembled power converters under in-vestigation here are also shown in Figure 1.1 and highlighted next.

Volume restriction usually applies without exception, as the converter is typically a sub-assembly to a larger system whose application determines the volumetric distribution. The internal power supplies for high-fidelity equipment and the energy-saving light bulb serve as examples.

Weight plays an important role in portable consumer applications. Battery chargers intended for mobile devices such as PDAs, mobile phones or notebook com-puters serve as examples.

PCB-assembled power converter evolution

The evolution of PCB-assembled power converter technology from state-of-the-art to the advanced prototypes investigated here is illustrated in Figure 1.2.

-6

Low Level of component packaging density & technological investment High

L o w P o w er d en si ty (a c-d c co n fi g ur at io n ) & lo ss d en si ty Hi g h I

Technology base : SS PCB, Disc., Lead. Power density : 100W/l

Role of PCB : EI,MS

TM method : Forced convection Spec : 200W;220Vac→5,12Vdc II [Jit00] ML PCB, SMD, Planar <_{150W/l (ac-dc)} 10kW/l (dc-dc) EI, MS, EMR Nat. conv., HS 1kW; 350Vdc→170-380Vdc III [WF02] emPIC, SMD <_{200W/l (with ac filter)} 1.7kW/l(no ac filter) EI, MS, EMR Nat. conv., HS 60W,220Vac→200Vdc [this thesis] flex PCB, Disc., Lead. 300W/l

EI, MS, TM, 3D Cond., Nat. conv. 20W,220Vac→5,12Vdc IV V [this thesis] rigid-flex PCB,SMD 250W/l EI, MS, TM, 3D, EMR Cond., Nat. conv. 20W,220Vac→12Vdc

SS : Single sided

ML : Multilayer

PCB : Printed circuit board

SMD : Surface mount

Disc. : Discrete components

Lead. : Leaded components

EI : Electrical interconnection

MS : Mechanical support

TM : Thermal managament

3D : 3D layout

EMR : Electromagnetic integration

Nat. conv. : Natural convection

Cond. : Conduction

HS : Heat sink

(1990) (2000) (2002) (2004) (2006)

³³1

Figure 1.2: Time-line of PCB-assembled power converter evolution

It can be seen that PCB only performed electrical interconnection and mechanical support prior to the 1990s (converter I: PC power supply). As technological invest-ment increased, the level of component packaging density increased accordingly, allowing for higher power densities to be achieved. This simultaneously increased the loss density in the converter, shifting attention towards thermal management. Migration to planar technology (converter II) dominated the first period (1990-2000). The height became lower as planar cores replaced wire-wound bobbin magnetic components. Here the component surface profile in the converter has taken on more compatible geometries to be cooled by conventional cooling technologies.

This evolution is perfectly suited for flat-panel monitors and low-profile home en-tertainment applications such as slimline digital versatile disc (DVD) players. Integration methods, including three-dimensional (3D) spatial layout and enhanced thermal management, are currently on the rise. Two technology demonstrators (con-verters IV&V) are the result of applying such methods, as investigated in this thesis, and offer a glimpse of the future.

1.2

Problem definition

The current construction technology for PCB-assembled power converters is based on the assembly of pre-manufactured discrete components. The fundamental limits of this construction method are steadily being reached as power converters tend to exploit higher processing speeds to gain advantages in both magnetic component size and overall power density. Therefore new approaches need to be investigated to push the limits further.

Establishing meaningful progress requires revisiting the complete design and man-ufacturing process currently in place for PCB-assembled power converters. This is because achieving high power density, reliability and low cost simultaneously does not allow for excess material or processes. A systematic approach to this major over-haul underlines the main target of this thesis.

Figure 1.3 illustrates the fundamental design aspects which every power electronic converter design should address systematically [VW00], together with the interre-lated improvements that will be used in this thesis.

Key aspects that have been identified include thermal management, packaging, man-ufacturing, passive component realisation and the power-switching strategy. System layout and interconnect medium (limited to PCB only), related to packaging, will be shown to play a predominant role in this.

Thermal management: the challenge of quantifying the effectiveness of thermal man-agement in power converters, with respect to the invested material and operating temperatures of the vital components, is addressed in a systematic way. Figures of merit, enabling critical comparison of thermal designs and which form an integral part of the overall design cycle, are delivered.

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Figure 1.3: Design improvements being addressed in this thesis, shown as links be-tween the respective, fundamental design aspects (discs)

process becomes increasingly more complex as the various technologies, with vastly different properties, are forced closer together during these integration efforts, re-quiring an extremely careful balance of electromagnetic, physical, thermal and spa-tial aspects. This thesis investigates single-technology platform integration, in order to reduce the cost and complexity of the integral manufacturing process.

1.2.1

Thesis objectives

With the foregoing problem description in mind, the main goal of the thesis is to: develop a means to systematically design, in a reliable and cost-effective man-ner, PCB-assembled power converters which perform “off-the-line” ac-dc power conversion and are optimised for power density without compromising manu-facturability.

To reach this goal the following objectives are required:

A. Create a means to quantitatively compare and design for thermal management in power converters in order to ensure their prolonged reliability.

B. Add more functionality to the printed circuit by incorporating capacitive, in-ductive, thermal, resistive and mechanical functions into it; to ultimately re-duce manufacturing complexity and cost as well as increase power density. C. Improve the physical components by modifying their packaging, as well as by

D. Adjust the shape and size of component packaging so that the assembly method lends itself better to automated manufacturing and miniaturisation.

E. Address the three-dimensional placement of bulky, low-frequency components associated with ac-dc converter operation in order to increase power density.

1.3

Thesis layout

The layout of the thesis is illustrated in Figure 1.4.

PCB technology Chapter 2 Technology overview Thermal management Chapter 3 Technology overview Thermal management Chapter 4 Design guideline Packaging Chapter 5 Design approach Integrated design Chapter 6 Design guideline Conclusions Chapter 7

State of the art & Technology trends

Areas for improvement

? ¾ ? ? » Technological demonstrators ½ ? ¼

CHAPTER

Overview of printed circuit board technology

Printed circuit board technology, with a worldwide market share of 34.3 billion1_US

dollars in the year 2000 [Cus00], can be seen as an attainable and well-used intercon-nect medium in many engineering disciplines.

The Netherlands showed an 7.8% annual growth in this market, the second-largest increase in Europe next to Italy with 8.8% in that same year [Cus00]. This clearly shows an increasing amount of activity in this sector, and it will continue to grow with the many new electrical power conversion applications that are emerging. The global distribution of the PCB market clearly favours the Far East with approxi-mately 72% market share in 2003 [Lon05], as shown in Figure 2.1.

Europe produced a respectable 10.43% of the world market share of printed circuit boards in that same year, which indicates a strong demand for the commodity in Europe.

In this chapter the state of the art of this billion euro commodity is investigated. The advances in PCB materials and associated technologies are presented concisely with emphasis on what advantages they have brought to applications which are not necessarily within the power electronic field. The remainder of the thesis will then be dedicated to applying these identified advantages towards enhancing power electronic converters.

1_{billion is defined here as 10}9_{(used in the USA) and is equivalent to the European definition of milliard}

K LMNOP QR S TUV W XY QT S TZV [\] ^_ Q ` S aa V Y b ] _ c _d ]e] d aa S UZV f ]ggh P K _b i jYe M ] d_ R S TV [ _^_ g _ Q S TaV R S kZ V X NL i \ Y lPM ] d_

Figure 2.1: World PCB output during the year 2003, divided by region [Lon05]

2.1

State of the art

Figure 2.2 shows the layer buildup of a current, state of the art, printed circuit board, whose characteristic features will be highlighted throughout the remainder of this chapter.

Figure 2.2 shows multiple conductor layers containing the circuit artwork, inter-spersed by substrate and bonding (prepreg) layers. Epoxy glass laminate material (FR4) is typically implemented as substrate layer and is primarily responsible for providing electrical isolation between the conductor layers. Advances in substrate materials, however, are slowly enabling limited amounts of capacitance, inductance or resistance to be realised with these substrate layers (discussed in more detail later in this section). Furthermore, the outermost layers carry the components.

The circuit artwork is imprinted on the conductor layers by one of three common production methods:

Silk screen printing using etch-resistant inks to protect the copper foil. Subsequent etching removes the unwanted copper. Alternatively, the ink may be conduc-tive, printed on a blank (non-conductive) board. The latter technique is also used in the manufacture of hybrid circuits.

Bottom conductor layer → FR4 → Resistive → Inductive → Capacitive Substrate layer (multi-layer) Prepreg layer

Inner conductor layer

→ Polyimide (Flexible) → FR4 (Rigid) Substrate layer (core) Inner conductor layer Prepreg layer

→ FR4 → Resistive → Inductive → Capacitive Substrate layer (multi-layer) Top conductor layer

Through-hole via ¾ Blind via -Buried via © © © © © ¼

Figure 2.2: Printed circuit board buildup (exploded view)

The vertical conductors, shown as tubes in Figure 2.2, provide electrical intercon-nection between the horizontal conductor layers. Three possible variations are por-trayed here. These and more variations are discussed next.

2.1.1

Interconnection technology

Electrical connections between layers of the PCB have traditionally been accom-plished by drilled and plated through-holes, labelled vias. With the growing number of layers in a PCB structure the number and type of vias have grown considerably. An overview on the existing types of vias used today is presented here.

through-hole via The plated through-hole via is, as the name suggests, an inter-layer connection established by electroplating a mechanically drilled hole span-ning all layers of a multi-layer PCB. Interconnection between respective layers can be established, or avoided, by connecting to, or staying clear from, the electroplated via structure, as illustrated in Figures 2.2 and 2.3(a). Through-hole vias serve two purposes: to provide inter-layer connectivity and to allow mounting of through-hole components. The growth of surface-mount compo-nent technology since the middle 1980s has since relegated the via to the single function of layer-to-layer interconnection [KK00]. The through-hole via ex-tends to both outer surfaces of the PCB, occupying valuable component mount-ing and electrical routmount-ing space. It therefore limits the packagmount-ing density of the overall PCB circuit to an extent.

blind via The blind via, or plated inner via hole (IVH) [Ish98], is similar to the through-hole via as regards manufacturing but is only applied to inner lay-ers of a multi-layer PCB assembly. Additional laylay-ers eventually enclose the via completely, leaving the blind outer surface areas free to be used for com-ponent placement and electrical routing. Figures 2.2 and 2.3(b) illustrate the construction.

buried via The buried via, or surface via hole (SVH) [Ish98], is an inter-layer connec-tion formed in the surface layer of a multi-layer PCB, by the electroplating of either mechanically drilled (Fig. 2.3(c)) or etched (Fig. 2.3(d)) holes. It extends from one outer surface to an appropriate inner layer, but does not protrude through to the other outer surface, as shown in Figure 2.2. This via is easier to manufacture than its blind via equivalent, as the via can be created directly from the multi-layer stack, as one outer surface is always available for process-ing (reduced intermediate drillprocess-ing and etchprocess-ing steps). The outer surface area has only been reduced on one of the two outsides and therefore an incremental increase in packaging density can be expected, but at a reduced complexity of manufacturing.

via (drilled) ¡ ¡ ª (a) Through-hole via (drilled) ¢ ¢ ¢® (b) Blind via (drilled) ¡ ¡ ª (c) Buried-drilled via (etched) @@R (d) Buried-etched

component via (filled)

XXXXz ¢ ¢ ¢ ¢ ¢® XXX ¢ ¢ ¢ (e) Thermal via (etched/laser ) @@R (f) Micro bump £ £ ££° (g) B2_it vias (paste) @@R A A AU ¡ ¡ ¡ ¡ ¡ ª (h) ALIVH

Figure 2.3: PCB interconnection technology - vias (cross-section through via illus-trated)

electro-plating process or alternative method of via hole metallisation [BM00, WN98]. micro via The micro via is a miniature equivalent of the surface via hole, with hole diameters smaller than 150µm. Micro vias are too small to be manufactured by mechanically drilled holes, they require chemical etching (photo-defined vias), laser ablation, or plasma etching technologies to generate the miniature holes [Hol98,LC99,KK00], as illustrated in Figure 2.3(f). Typical applications of micro via multi-layers appear in the new Sony Handycam PC7 and microcamcorders like the JVC GRDV1 and GRDVM1. These assemblies contain up to 77 leads per cm2[Hol98].

(buried) bump interconnect technology Unlike the mechanically drilled or chemi-cally etched via interconnect technology, bump (Bit) or buried bump intercon-nect technology (B2it) uses miniature copper cones (bumps) to pierce through the prepreg cover layers and establish electrical contact with adjacent layers [GOF00]. No holes are required and the via is completely filled with metal, so it can directly perform the function of a thermal via as well. Figure 2.3(g) illustrates the concept. The manufacturing is more complex than any other via method and involves printing several conductive pastes on the copper foil (inner layer) and leaving them to dry to create the conductive bumps. These bumps are controlled to be cone shaped so that they can easily pierce into the dielectric layer (prepreg). The interconnection between the copper foil and the silver bumps is made using a combination of physical phenomena such as the anchor effect, rivet effect and diffusion between metals [GOF00].

any layer inner via hole (ALIVH) The “any layer inner via hole”, is a via technol-ogy which enables any layer-to-layer interconnection by processing the indi-vidual layer-to-layer interconnections in a modular way [Ish98]. Each layer pair is processed separately and then these layer pairs serve as building blocks to form the PCB layer stack, once stacked. This modular approach allows for higher packaging density as via layout has become virtually unrestricted. This method differs from the above via technologies in the sense that it builds up the necessary vias per layer only where they are required (no bridging of unnec-essary layers), which is very difficult to achieve by electroplating a complete drilled or etched via hole. Figure 2.3(h) shows the flexible ALIVH via construc-tion.

2.1.2

Substrate technology

The substrate material has evolved as enhanced materials became available, address-ing the straddress-ingent and more demandaddress-ing application requirements. Therefore, a broad range of substrate materials can be found in modern printed circuit board designs. The fundamental substrates are listed here, whilst the advanced substrates, enabling more functionality of the PCB, are listed separately in Section 2.1.3.

impregnated paper substrates Paper impregnated with phenolic resin, sometimes branded “Pertinax”, is commonly used in low-end consumer-grade PCBs be-cause the material is inexpensive, easy to machine by drilling, shearing and cold punching, and it causes less tool wear than glass-fibre-reinforced sub-strates. Flame retardants are typically added, and this gives the material its name, FR-2. This rigid substrate is strong enough to mechanically support small- to medium-sized power circuits and with single-sided lamination it ful-fils the sole function of providing electrical connectivity.

glass-fibre-reinforced substrates The most widely used rigid PCB substrate is FR-4, a glass-fibre-reinforced epoxy resin with a brominated flame retardant in the epoxy material. It can be drilled, punched and sheared, but due to its abrasive glass content requires tools made of tungsten carbide for high-volume produc-tion. Due to the fibreglass reinforcement, it exhibits about five times higher flexural strength and resistance to cracking than phenolic impregnated paper types, albeit at higher cost. Multi-layer PCBs are mostly constructed from FR-4 material, and due to the flexural strength of the material the thin multi-layer sheets can be readily manufactured.

polyester film substrate Applications with limited space often require circuit boards which are designed to be completely or partially flexible. The disposable or low-cost applications among these often use less expensive polyester mate-rial as a PCB substrate. It can be employed when no through-hole technol-ogy is required and when high electronic reliability and challenging environ-mental conditions of temperature and humidity are not considerations [Dor00]. Polyester flex circuits are used in telephones as well as in industrial controls, point-of-sale (POS) terminals, medical equipment, membrane switches, dash-board circuitry in automobiles and scientific instruments.

polyimide film substrate Applications with limited space, requiring flexible printed circuits as well as through-hole or multi-layer technology with high reliability, require a more expensive polyimide-based substrate material2_{. This substrate}

does however have many of the characteristics found in its rigid FR-4 based counterpart [Dor00]. Chemically, polyimide is a high-quality plastic film which is the result of a polycondensation reaction between pyromellitic dianhydride and 4,4’diaminodiphenyl ether. Printed circuits inside cameras and hearing aids are almost always made of polyimide flex circuits so they can be folded up to fit into the limited available space. Such flexible printed circuits can also

be used as flexible interconnection between circuit boards or devices. Exam-ples of this application include the cable connected to a hard disc head and the cable connected to the carriage in an inkjet printer.

silicon hybrid substrate The integration of very thin silicon layers (< 50µm total thickness) into the PCB as laminate material provides a step closer to electrical circuits, components and interconnection medium being monolithically inte-grated. On this scale micro vias are then required as layer-to-layer intercon-nection technology. Silicon also has the advantage of being one of the few well-researched materials to date [Det88]. A silicon hybrid is essentially a miniaturised PCB built on a silicon wafer, and like any other PCB it is made up of layers of metal tracks separated by a suitable dielectric. However, un-like a conventional PCB, a silicon hybrid is manufactured using IC fabrication techniques [JWN+02].

Apart from the fundamental substrates listed above, substrates are emerging which provide added functionality to the electrical circuit in terms of electromagnetic as well as thermal functionality. These are discussed in more detail next.

2.1.3

Substrates enabling functional integration

Apart from providing the mechanical support and insulation to the PCB assembly, modern substrate materials are capable of providing enhanced electrical functional-ity to the circuit they carry. To this extent dielectrically enhanced, ferrite enhanced or resistive substrates are conceivable that exhibit the same characteristics that their discrete component counterparts would, but are now intimately connected inside the circuit carrier, allowing for advantages in power density, interconnection relia-bility as well as low inductance interconnections between electromagnetically sensi-tive components. These three types of functional substrates deserve further attention and are therefore discussed next.

Capacitive

The parallel conductor layers, separated by the substrate of a PCB with two or more layers, is inherently a capacitive structure, the capacitance of which depends on the dielectric constant of the substrate material(εr), the surface area of the overlapping

conductor layers(A) and the thickness of the substrate layer(d): C = ε0εrA

d (2.1)

These embedded capacitors were initially implemented to decouple crucial compo-nents, due to the exceptionally close proximity that can be achieved. The small value of capacitance did not allow for more serious integration, let alone for the realisa-tion of power capacitors. The advent of enhanced dielectricum substrates, however, changed all of this, and enabled higher values of capacitance and a wider range of applicability. Capacitive substrate (16µm) ©©©© © * HH HHHj

(a) Cross-section of embedded capacitive substrate layer in PCB assembly [Pei01b]

(b) Multi-layer winding with enhanced inter-winding capacitance (LCT structure) built on capacitive dielectric substrate [RdJC+06]

Figure 2.4: Implementation of capacitive substrate material

Figure 2.4 shows cross-sections of two different printed circuit boards, both imple-menting integrated C-Ply capacitive laminate.

Inductive

The most basic form of inductive substrate is the planar core. Although not strictly a substrate, it resembles the thin planar magnetic material layer required to ensure low-profile magnetics. Planar cores are sintered ferrite made separately from the PCB and this therefore avoids the incompatibility of the sintering process with the PCB manufacturing process. Low-loss, high-frequency magnetic components can be constructed in this manner. Embedding the whole of such a core inside the circuit carrier, as shown in Figure 2.5(a), has been investigated by Popovi´c [Pop05], with remarkable improvements in power density.

Figure 2.5 shows various implementations of magnetic substrate materials.

mag-netic material itself is still in its infancy, with attempts made with electroplated permalloy (NiFe) [WF02, LDO+03b, SS96], and ferrite polymer composite (FPC) ma-terial [WF02]. Both electroplated permalloy and FPC mama-terial exhibit high losses when implemented as magnetic PCB laminate due to eddy current and magnetising losses respectively. All conductive and magnetic layers are manufactured in an in-tegral production process unlike the planar transformer technology. Concepts orig-inating from embedding inductive laminate material, such as embedded passives integrated circuits (emPIC) [WF02], are shown implementing “MagLam” material3 in Figure 2.5(b).

Soft magnetic material deposition, as used on hard drive platters, is another means of creating a magnetic substrate embedded in the circuit carrier. Ludwig et al. [LDO+03a, LDO+03b] have proven this principle for high-frequency, low-power converters. This concept is illustrated in Figure 2.5(c)

PCB embedded inductor

¡ ¡ ª

Ferrite polymer composite material

@ @ @@R Copper windings HHj

(a) Embedded planar core [Pop05]

¡¡¡ µ Copper windingsHH j MagLam ¡¡µ

(b) EmPIC technology using MagLam from Isola [Neu04, WAF05] Copper windings -Magnetic plates @@R Magnetic through-holes ¡ ¡ ª

(c) NiFe layer ferrite deposition [LDO+03a]

Figure 2.5: Cross-sections of various inductive substrate implementations

Resistive

Like capacitive and inductive laminates, low-power, resistive functionality can be added to the printed circuit by introducing suitable resistive material instead of dis-crete, wire-wound resistors.

Embedding resistive functionality into a substrate does not necessarily mean adding an extra substrate layer to the PCB stack. Creating resistors in conductor traces is

made possible by a thin film Electrodeposited-On-Copper NiP metal alloy (resistor-conductor-material) that is laminated to a dielectric material and subtractively pro-cessed to produce planar resistors4. Because of its thin film nature (approximately 0.1- 1.0 µm thick), it can be buried within layers without increasing the thickness of the board or occupying any surface space like discrete resistors [OT04].

The concept is illustrated in Figure 2.6 on traces leading from a high pin count, ball grid array component landing.

Figure 2.6: Embedded resistors increase the potential for packaging density [OT04] Figure 2.6 illustrates how the in-trace resistor hardly uses any more space than a normal low-resistance conductor trace would. Conforming to these practices allows for high-density designs in the converter segments characterised by resistive com-ponents of low power and high quantity, typically concerning the control and pro-tection circuitry of a power converter.

2.2

Printed circuit technology comparison

The different substrate technologies, discussed above, manifest themselves in the PCB-assembled power-converter market in different technologies. These different PCB technologies in the world market as per 2003 are shown in Figure 2.75.

mno pq rs tuu qpu n v wxy z {|}{u n ~ p y € ‚ƒ„ … † ƒ ‡ˆ‰ Š ‹ Œ ‹  Ž  ‘ ’ “ ” •–— qpu n v x˜y m™z wxy šz › œ o u ~ qt ~ p wwy žš wwy •–— v{v Ÿ qpu n v xy

Figure 2.7: 2003 World PCB output divided by PCB technology base [Lon05] The dominating technology implementation (39%) is the multi-layer board using, typically, FR4 resin (MLB-resin). This cost-effective and versatile technology is used in the mass production market segment for a multitude of applications.

Up to now, power electronics exploited only a single PCB technology, namely MLB resin. The advantages brought about by flexible and rigid-flexible technology in other fields include flexibility, increased reliability and increased power density. Power electronics has yet to exploit these possibilities.

For power-density enhancement high-density interconnects6 are more commonly used and have a significant market share of 11%.

Rigid, flexible and rigid-flex PCB technology are viable for implementation in power electronics and are therefore highlighted next.

2.2.1

Rigid PCB

Single or multi-layer rigid PCB is the workhorse for performing electrical intercon-nection in industrial applications, as indicated by the enormous share of the world PCB manufacturing market shown in Figure 2.7.

5_{MLB:Multi-layer Board; HDI: High Density Interconnects; IC: Integrated Circuit; FPC: Flexible}

Printed Circuit

6_{High density interconnects (HDIs) are defined as substrates or boards with a higher wiring density}

For low-power converters the rigidness of the PCB is enough to provide mechan-ical support for all the components mounted on it. A few examples are shown in Figure 2.8.

(a) Paper based PCB as light ballast

(b) FR4 based PCB as supply (c) Multi-layer PCB as PC motherboard

Figure 2.8: Typical applications for rigid PCB

HDI is a subsidiary of PCB with higher wiring density, used to interconnect inte-grated circuits with a very high pin count. Multi-layer technology then provides means to connect the many component pins with reasonably wide tracks, by extract-ing neighbourextract-ing pins on different PCB layers. An example of this is the personal computer (PC) motherboard, shown in Figure 2.8(c).

Power electronics rarely needs this dense interconnection, as its tracks need to be wide and thick to carry substantial current. This does not mean that there is no use for HDI in power electronics. In high turn ratio “off-the-line” ac-dc converters the transformer winding ratio is usually large (≈ 26 : 1), and for low-power applica-tions the primary windings are therefore plentiful and thin. HDI in multi-layer form brings power-density advantages for these types of windings.

2.2.2

Flexible printed circuits (FPC)

Flexible printed circuits are, as the name suggests, a thin, flexible variant of the rigid PCB, especially suited for dynamic interconnection applications and interconnecting components within complex geometries.

The double-sided flexible circuit construction consists of prelaminated double-sided copper-clad material with top and bottom coverlays for accessing copper from both sides via plated through-holes and surface-mount technology.

In multi-layer flexible circuit construction, multiple layers of single- and double-sided material are laminated together. There can be from 3 to 16 layers. Obviously, in multi-layer applications, the more layers that are laminated together, the less abil-ity to bend there will be in the flexible areas.

The bending radius of the flex determines how tightly the flexible printed circuit can be implemented in an application. If the flex is to be used for dynamic interconnec-tion a larger bending radius is required to keep the printed circuit from delaminating and interconnection failure occurring. Guidelines for determining the appropriate bending radius are given in a Military handbook [U.S93] and summarised as fol-lows:

Table 2.1: Bending radius guidelines [U.S93]

Parameter Description value unit

rbend min Single Metal Layer 3-6x circuit thickness (df lex)

Double-sided Flex 6-10x circuit thickness (df lex)

Multilayer Flex 10->15x circuit thickness (df lex)

Dynamic applications 20-40x circuit thickness (df lex)

Examples of possible applications for flexible printed circuits are shown in Figure 2.9. Flexible printed circuits are widely used in the micro-electronic field. Digital cam-eras implement them to realise the complex, high-density circuitry within the prede-fined classical camera shape. This requires flexible connections to reach strategically placed components that are hidden from the user inside the standard camera hous-ing, as shown in Figure 2.9(a).

(a) Optical camera [Fre04, Wik05a]

(b) Hard disc drive [Fre04] (c) Hearing aid [Wik05b] Keybutton and unitized frame assembly

Top membrane layer

Conductive pattern

Spacer layer coated on both sides with adhesive

Bottom membrane layer with conductive pattern on top and selectively coated on the bottom

Baseplate

(d) Membrane keyboard [Gri99]

Figure 2.9: Typical applications for flexible printed circuits

The flexible printed circuit board is typically more expensive than its rigid equiva-lent, due to stringent requirements on material and the manufacturing complexity of processing very thin substrates.

The flexible printed circuit is already being used to construct windings using inge-nious winding patterns [NAY93, ZPCU02, NAY+90, HCL99, CBJ+99, BMR+98]. An effort has been made by Lostetter et al. [LBE+00] to use flexible printed circuits to perform volumetric optimisation of power converters. Power electronics have yet to exploit this level of 3D packaging technology further to increase power density and reliability in converters. Chapter 5 proposes means of exploiting flexible printed circuits more in power electronics to achieve exactly this.

2.2.3

Rigid-flex PCB

Rigid-flex PCB is a combination of both the rigid and flexible PCB technologies dis-cussed above. A rigid-flex construction consists of single or multiple layers of flex with single- and double-sided FR-4 or polyimide laminated to the outer layers of flex. Layers can range in number from 1 to 16 in the flex area and up to 20 in the rigid areas. Figure 2.10 shows some examples of rigid-flex PCB technology being applied to various fields.

Rigid-flex technology combines the advantages of both the rigid and flexible tech-nologies in exactly the locations where it is required. It enables the designer to place components in more strategic places within a system and still be able to electrically interconnect them, while retaining the mechanical support in certain parts that might be required to hold the assembly together.

The visual user interface to most electronic equipment today, namely the liquid crys-tal display (LCD), utilises rigid-flex PCB to connect the many communication lines from the display unit to the controller in a space saving, planar manner while the controller itself is still realised on rigid PCB, as shown in Figure 2.10(a) in a folded (top) and unfolded state (bottom).

flexible PCB Controller Controller flexible PCB LCD ³³1

(a) LCD and controller interconnection [FPG+_04]

(b) Via-less compact circuit interconnection using rigid-flex [FPG+_04]

(c) Inter board connector [FPG+_04]

©©©©

©©©*

(d) Pacemaker revealing compact, reliable rigid-flex circuitry [aO05]

Figure 2.10: Typical applications relying on rigid-flex PCB

to expand, resulting in less mechanical stress due to thermal expansion or incompat-ible CTEs.

The medical industry uses rigid-flex PCBs for compact, delicate and reliable equip-ment such as pacemakers for the human heart. Figure 2.10(d) shows how the entire control and communication circuit is implemented on two rigid PCB panels con-nected by means of a flexible printed circuit. Here the least number of vias are re-quired and thermal expansion between boards is absorbed by the flexibility of the FPC interconnection, substantially improving the reliability.

Chapter 6 proposes means of implementing rigid-flex PCB technology to perform multiple functions above electrical interconnection, such as electromagnetic integra-tion of passives and thermal management.

2.3

Printed circuit board in power converters

The printed circuit board can in many ways be seen as the heart of a PCB-assembled power converter. It not only governs to a large extent the volume, performance and reliability of the converter but it also governs the cost substantially.

Considering the material cost breakdown of a typical PCB-assembled power con-verter, shown in Figure 2.11, one can see that the printed circuit board7represents 8% of the total material cost, making it the third most expensive individual part in that system, next to the transformer (14%) and the control IC (13%). In more advanced power converters implementing multi-layer, flexible or rigid-flex PCB technology, this contribution will be even more.

Passives 41% Semiconductor dies 24% Remaining packaging parts 27% PCB 8%

Figure 2.11: Material cost breakdown in a typical PCB-assembled power converter, adapted from [Pop05]

To benefit from this substantial investment in a cost-driven market, it is essential to exploit the PCB and all its possible functionality, albeit inherent or artificially en-hanced, to the utmost. Measures to do so include the minimisation of the PCB sur-face area; the sharing of materials between components, such as between the PCB and the transformer, as well as between a large variety of the passives and the PCB; and including the PCB in the thermal management strategy.

The challenge here is to have the cost reduction of the material sharing outweigh the cost increase of the more advanced PCB. A systematic approach to achieving this will be discussed in more detail in this thesis.

This is not the only challenge facing the printed circuit board in PCB-assembled power converters, however. A few more daunting challenges are presented next.

2.3.1

Technology challenges facing the printed circuit in

PCB-as-sembled power converters

The PCB technology described throughout this chapter, has expanded quite rapidly, providing superior electrical interconnection possibilities for compact, reliable and complex shape applications. Power electronics has not followed the same rate of en-hancement and is still mainly implementing the conservative rigid PCB technology, partly due to larger voltage and current handling capabilities but also due to the higher cost brought about by enhanced PCB materials and technologies. It seems that industry has not fully realised the added value that PCB technology still holds and often chooses alternative circuit carrier technologies, such as IMS or DBC, to ad-dress the interconnection challenges encountered.

The question that now remains is: what needs to happen to PCB technology to allow it to continue to be used, or even be used more extensively in power electronics applications? A few challenges and this thesis’s response to them are listed below. PCB technology needs to:

• enable exploitation of the third dimension, especially for high power-density applications

– Develop 3D spatial layout solutions using flexible, as well as rigid-flex printed circuit boards. Using flexible printed circuits to interconnect cir-cuitry within complex geometries will make the resulting converters more compact. Possibility almost exclusively dedicated to PCB technology. • be capable of handling the increased thermal load of the increasingly more

power-dense converters being demanded, and meet or even exceed the ther-mal capability of alternative circuit carrier technologies.

in PCB-assembled power converters. The PCB materials have themselves evolved significantly — far enough to allow higher power processing from them, due to thicker copper layers, better heat-spreading capabilities and measures such as thermal vias to influence the thermal management of such power converters.

• be suitable for functional integration to enable enhanced packaging solutions being developed today

– Investigate possibilities for electromagnetic integration in, and with, PCB technology in combination with spatial enhancement methods to increase the level of integration and packaging.

• remain competitive in manufacturing and assembly complexity and associated cost

– Investigate integral manufacturing solutions, and impact of above chal-lenge solutions to the PCB manufacturing industry

• meet, or exceed, the reliability requirements set by industrial standards for use in power converters

– Investigate and design for reliability by using an integral manufacturing approach, reduced number of interconnections and thermo-mechanical stress relief by means of flex hinges. Using rigid-flex interconnections between boards will reduce the amount of vias and solder joints and im-prove the reliability of the system. It will also provide thermal stress relief between PCB boards located in close proximity and operating under ap-preciable heat changes.

• provide advantages for low-power applications, typically characterised by a large amount of “overhead” volume in relation to the power being processed

– Combine above challenge solutions in technology demonstrator designs aimed at low-power, high-overhead PCB-assembled power converters.

2.4

Summary

CHAPTER

Overview of thermal management in PCB converters

“The goal of thermal management is to allow components to operate at or below their maximum operating temperature, while enhancing electronic designs that achieve all the physical, electrical, and thermal goals of the end product”. -Mark Robins1

3.1

Introduction

Electronic packaging trends continue to increase power density while deploying smaller, more delicate components [BV98]. A logical consequence of this trend is then that the power consumption of power electronic components approaches the power dissipation limits of the implemented component packages [Cor98]. There-fore, as PCBs shrink and components grow in power, more emphasis should be placed on thermal management in electronic design.

The thermal management of power-dense structures requires either higher electrical efficiency, larger cooling surfaces (heat sinks), forced convection, components with good reliability at high temperatures or very good internal thermal management to sustain performance and reliability.

Thermal management also dictates the reliability of a system to a large extent, as can be seen in the breakdown of premature failure causes of electronic equipment2, shown in Figure 3.1 [ZJL97].

1_{from article in Electronic Buyers News, Sept. 17, 2000, by Mark Robins, Associate Editor} 2_{as determined by the US Air Force Avionics Integrity Program (AVIP)}

  ¡ ¢ £¤£ ¥ ¦ §¨© ª£« ¬­ ¥ £ ®¯ °°© ±² ³ ´ µ ¶ · ¸¢¹¸¬­ ¥ ¡¬¸ º»©

Figure 3.1: Breakdown of premature failure causes of electronic equipment [ZJL97]

More specifically, the reliability of a system can be pinpointed to the junction tem-perature of the silicon devices in the system. This is not at all surprising as this junc-tion temperature is the highest in the system due to its deeply embedded physical position in the thermal layout of the system. Figure 3.2 shows the qualitative tem-perature distribution inside a typical system3as investigated in [AB00], from which the reason for the high junction temperature should be evident.

External environment Enclosure PCB Component Enclosure edge PCB edge Component edge Junction Ambient Rise in Temperature (ambient to junction)

Figure 3.2: Qualitative temperature distribution for typical enclosed power supplies

Thermal management is therefore a compulsory step in the design criteria when striving for high power-dense, reliable electronic equipment. The process of thermal management, as it applies to PCB technology, will be addressed next.

3.2

The thermal management process

In trying to understand the thermal management process, its limitations and com-plexity, it is essential that the fundamentals of thermal heat transfer are understood first, before attempting to apply them to the improvement of electronic systems. A revision of the fundamentals of heat transport can be found in [Rem01, Bej93, MGP97].

3.2.1

Heat transfer in PCBs

When the PCB plays an active role in transporting heat from within the system to the ambient, four thermal path legs can be identified. These legs are listed below and illustrated in Figure 3.3 [Rob00].

1. Heat transfer within the component package 2. Heat transfer from the package to a heat spreader 3. Heat spreading throughout material

4. Heat transfer from the heat spreader to the ambient environment

Heat source Component Dissipator (PCB) within component to dissipator Spreading to ambient

Figure 3.3: Thermal path legs through PCBs

Heat transfer within the component package (item 1) falls into the category of pack-age engineering and will be dealt with in Chapter 5. Items 2-4 are influenced, either directly or indirectly, by the characteristics of the PCB itself. The isolation of these crucial characteristics and thereafter the optimisation of them will be the focus of the rest of this section.

Figure 3.4 shows the distribution between the three heat-transfer mechanisms for three component package possibilities that are frequently used in PCB-assembled power supplies, as investigated in [AB00].

Hybrid SO DIP Heat transfer (glass-epoxy PCB) 100% Radiation Conduction Convection

Figure 3.4: The contribution of each heat transport mechanism to component cool-ing for Dual-Inline (DIP), small-outline (SO) and Hybrid packages

Conduction can be seen to be the most efficient mode of heat transfer and about 80% of the heat generated in a component is removed from a power component (to the PCB) in this manner [MGG+98].

High power-dense supplies inevitably implement surface-mount (SO-package) com-ponents, therefore (as Figure 3.4 shows) conduction transports the bulk of the heat in compact PCB-based systems. The focus now narrows to cover ways to improve con-duction using the PCB.

3.3

PCBs - Beating the heat

This section is dedicated to outlining the different possibilities in which the thermal management of PCBs can be improved. The respective technologies are discussed in broad terms to form an understanding of the possibilities that exist and how they can be exploited in achieving higher power densities, for example.

Heat spreading

Heat sinking

Thicker conducters Interfacing materials

Thermal pads & vias

Thermal layers

Clamped layers

Shape of PCB

Layout & geometry

Layout of components

Traces

Thermal management of PCBs

Possible improvements

Substrate technologies _IMS

-Insulated Metal

Substrate DBC

-Direct Bond Copper

TFA

-Thin Film Alumina

It has been sub-divided into the following four main categories.

Heat spreading involves the uniform distribution of heat from the localised source across the available structure surface. Factors such as interface material be-tween the localised source (component package) and the heat spreader (con-ductor plane), the heat spreader layer thickness, thermal pads and thermal vias in the heat spreader play a predominant role in this category.

Heat sinking involves the removal of heat from the heat spreader to the ambient by creating a low thermal-resistive path and large surface area for conduction and convection to the ambient.

Layout & geometry involves the arrangement of components in the available sys-tem volume, the optimal geometry of the circuit carrier’s structure and the interconnections. The layout of components on the circuit carrier plane also strongly influences the heat-spreading capability of that circuit carrier plane. Substrate technologies involve the different realisations of the circuit carrier by

us-ing different substrate materials and construction techniques. Circuit carriers other than PCB are considered here for comparison and completeness.

These factors and their contribution towards good thermal management of PCBs will be discussed individually.

3.3.1

Heat spreading

Heat spreading is required to alleviate thermal hot-spot formation inside critical components, usually isolated by an air layer. Air exhibits a low thermal conduc-tivity (λ = 0.03W/mK) and therefore negligible heat spreading takes place in air. The excessive heat produced by any component should therefore be spread across a surface with a relative high thermal conductivity before it has to cross an air layer, or any other high thermal resistance layer. Heat spreading increases the area over which the heat crosses the air layer, effectively reducing the thermal resistance of the heat path.

In high power-dense applications the heat-spreading function is performed by the already present PCB due to the limited space for extra components and close prox-imity of neighbouring components. The effectiveness of heat spreading in the PCB depends on the properties of the carrier material as well as the presence and position of copper layers.

Thickness of layers

For a one-dimensional system, the conductive heat transfer is governed by: q = −λAc∆T

l (3.1)

where

q = Heat flow rate [W] l = Length of heat transfer [m]

λ = Thermal conductivity [W/m K] Ac = Cross-sectional area for heat transfer [m2]

∆T = Temperature differential [K]

The two intuitive possibilities regarding layer thickness are then (q = constant): 1. Perpendicular heat transport from one layer to another (Figure 3.6(a)), which

is the case for a thermal insulator such as the substrate, plastic enclosure or air. For this case, according to (3.1), holds

∆T ∼ d ∆T ∼ _λ1 ∆T ∼ _A1

1

2. Heat spreading inside the layer (Figure 3.6(b)), which is the case for thermal conductors such as copper or aluminium. For this case, applying (3.1), holds

∆T ∼ 1_d ∆T ∼ l ∆T ∼ 1

λ ∆T ∼

1 A2

where d = Layer thickness [m] l = Length of conductor [m]

A₁

A2

Q

d ∆Τ

λ

(a) Heat flow through layers of the PCB

A₁ A₂ Q d λ ∆Τ

(b) Heat flow inside layers of the PCB

Figure 3.6: Heat flow in PCB layers

The effectiveness of conduction when using PCB, as illustrated in Figure 3.6, can be seen from the concise horizontal and vertical heat transport analysis summarised in Figure 3.7. Here the effect of conduction is displayed by calculating the tempera-ture drop (∆TV) across the top and bottom of a 1cm2PCB test segment, for vertical

heat transport (Figure 3.7(a)), as well as from edge to edge (∆TH) for horizontal heat

? 6 ∆TV FR4(λ =Cu(λ =394W/mK) 0.36W/_mK) Cu(λ = 394W/mK) vias»»: @@R 0 5 10 15 20 25 30 35 40 45 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Temperature difference (Vertical)

∆

T [

°C]

Thickness of substrate layer (FR4) [mm]

q=1W/cm2 Ac=1cm 2 φvia=0.5mm Cu=35µm Single Sided PCB Double Sided PCB Double sided 1 via Double sided 10 vias

(a) Perpendicular heat transport performance analysis (∆TV), based on a 1cm2 PCB segment (with and

without vias) loaded with an uniform 1W/cm2_{heat source (MOSFET), for various PCB implementations}

PP_PPq PP - _∆TH Cu(λ = 394W/m_K) FR4(λ = 0.36W/_mK) Cu(λ = 394W/m_K) 0 50 100 150 200 250 300 10 25 40 55 70 85 100 115 130

Temperature difference (Horizontal)

∆

T [

°C]

Thickness of copper layer [µm] Single sided PCB (Cu=35µm/layer)

Single sided PCB (Cu=70µm/layer)/Double sided PCB (Cu=35µm/layer)

Double sided PCB (Cu=70µm/layer)

q=1W/cm2

Substrate layer = 0.1mm Substrate layer = 0.8mm Substrate layer = 1.6mm

(b) Heat spreading performance analysis (∆TH), based on a 1cm2PCB segment (without vias) loaded with

an uniform 1W/cm2_{heat source, for three substrate thicknesses}

Figure 3.7: Effectiveness of conduction in PCBs

For perpendicular heat transport one can see from Figure 3.7(a) that the major ther-mal obstacle is the material layer with the lowest therther-mal conductivity. The thickness of the intermediate substrate therefore dictates the temperature drop across the top and bottom of the PCB segment. A temperature difference of > 40◦C is shown for the via-less PCB segment for a thickness of substrate (FR4) variation of 1.5mm. A negli-gible temperature drop (∆T) difference can be seen between single-sided (1 copper layer) and double-sided (2 copper layers) PCBs. The addition of vias4reduces this

∆T considerably by creating low thermal-resistance paths for effective perpendicu-lar heat transport, as can also be seen in Figure 3.7(a) for 1 and 10 vias respectively. For heat spreading across the PCB it can be seen from Figure 3.7(b) that the number and thickness of the copper layers greatly influence the resulting temperature drop across the PCB, again due to the high thermal conductivity of copper. It is however not a linear relationship, and this results in a smaller advantage as the thickness of copper is increased beyond that of a double-sided PCB with each layer consisting of a 70µm thick layer of copper. From Figure 3.7(b) one can also deduce that the cost-effective, single-sided PCB, with only 35µm thick copper, has less thermal con-duction potential to assist in any thermal management strategy. Furthermore, the figure shows the negligible effect of the substrate thickness in heat spreading. All heat spreading can therefore be seen to take place in the material with the highest thermal conductivity.

It is therefore advantageous, from a thermal point of view, to have a thin substrate that exhibits a low thermal resistance to the perpendicular heat flow, and thick conductor layers for optimal heat spreading. This is not always possible because the substrate usually contributes to the mechanical strength of the structure, and therefore cannot be exceptionally thin.

Furthermore, the maximum heat that a PCB can successfully conduct (q) can be de-termined from a reverse approach to the above analysis and depends on the allowed temperature drop across the thermal heat path through the PCB. The thermal resis-tance of this heat path is dependent on the number and thickness of both the high thermal conductive (copper), and low thermal conductive (substrate) layers, as well as the number of thermal conduction enhancements (vias).

Interface materials

Interface materials between the component and the PCB are necessary for electrical isolation. However, adequate provision must be taken during the placement of com-ponents that no unnecessary air is enclosed between the component and the heat spreader (PCB) that could increase the thermal resistance of the heat path. Figure 3.8 illustrates the use of a typical interface material, in this case some grease compound.

interface material

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