System-in-Package
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen
op dinsdag 29 Januari 2013 om 12.30 uur
door
Nuria Berenice PALACIOS AGUILERA
Master of Science in Electronic Engineering
Instituto Tecnológico y de Estudios Superiores de Monterrey
Campus Monterrey
Copromotor: Dr.ir. A. Bossche
Samenstelling promotiecommissie:
Reservelid:
De Stichting Technische Wetenschappen (STW) heeft als begeleider in belang- rijke mate aan de totstandkoming van het proefschrift bijgedragen.
ISBN:
Copyright (C) 2013 by N. B. Palacios Aguilera
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the publisher.
Printed in the Netherlands. Rector Magnificus, Prof.dr. P. J. French, Dr.ir. A. Bossche, Prof.dr.ir. P. M. Sarro, Prof.dr.ir. R. Akkerman, Prof.dr. G. Q. Zhang, Prof.dr.ir. J. Westerweel, Prof.Dr.-Ing. J. N. Burghartz, voorzitter
Technische Universiteit Delft, promotor Technische Universiteit Delft, copromotor Technische Universiteit Delft
Universiteit Twente
Technische Universiteit Delft Technische Universiteit Delft Institut Für Mikroelektronik Stuttgart, Universität Stuttgart
1.3 Thesis Scope and Organization ... 4
2. Theoretical Background
... 72.1 Introduction ... 8
2.2 System-in-Package ... 8
2.2.1 Advantages and Disadvantages ... 10
2.2.2 Applications ... 10
2.3 Low-Temperature Processing Materials ... 11
2.3.1 Substrate Materials ... 11
2.3.2 Joining Materials ... 12
2.3.3 Encapsulating Materials ... 13
3. Packaging Concepts for WSN Nodes
... 153.1 Introduction ... 16
3.2 Goals and Requirements ... 16
3.3 Component Choices ... 17
3.3.1 Antenna ... 17
3.3.2 Battery ... 18
3.3.3 Sensor ... 20
3.3.4 Other Electronic Components ... 21
3.4 Node Design ... 21 3.4.1 Practical Approach ... 22 3.4.2 Research Approach ... 23 3.5 Fabrication Process ... 24 3.5.1 Practical Approach ... 24 3.5.2 Research Approach ... 26
3.6 Electrically Conductive Adhesives as a Replacement for Ultrasonic Welding and Soldering ... 27
3.6.1 Theoretical Background ... 29
3.6.1.2 Encapsulants ... 30 3.6.2 Strength ... 32 3.6.2.1 Samples Preparation. ... 32 3.6.2.2 Experiment Setup ... 32 3.6.2.3 Results ... 33 3.6.3 Resistance ... 35 3.6.3.1 Samples Preparation ... 35 3.6.3.2 Experiment Setup ... 37 3.6.3.3 Results ... 37
3.6.4 High Humidity and Elevated Temperature Test with Bias ... 39
3.6.4.1 Samples Preparation ... 39
3.6.4.2 Experiment Setup ... 40
3.6.4.3 Results ... 41
3.6.5 Conclusions ... 42
3.7 Inkjet Interconnections on Top of Batteries ... 43
3.7.1 Theoretical Background ... 44
3.7.1.1 Adhesion Between Two Materials ... 44
3.7.1.2 Conductive Inkjet Printed Inks ... 46
3.7.2 Samples Preparation ... 46
3.7.3 Experiment Setup ... 47
3.7.3.1 Battery Foil Surface Preparation for Optimized Adhesion .. ...48
3.7.3.2 Adhesion Test Method ... 49
3.7.3.3 Electrical Characterization Method ... 49
3.7.3.4 Silver Migration - Temperature-Humidity-Bias Test ... 50
3.7.4 Results ... 51
3.7.4.1 Optimized Adhesion of Substrate ... 51
3.7.4.2 Electrical Characteristics ... 55 3.7.4.3 Adhesion Characteristics ... 56 3.7.4.4 Macrostructure ... 57 3.7.4.5 Microstructure ... 57 3.7.4.6 Silver Migration ... 58 3.7.5 Conclusion ... 59
4.3 Packaging Concept ... 65
4.4 Fabrication Process ... 67
4.4.1 Materials ... 68
4.4.2 Flow Chart ... 69
4.4.3 Critical Points ... 71
4.5 Inlaying of the chips in the PCB material ... 72
4.5.1 Theoretical Background ... 72
4.5.2 Measurements ... 72
4.5.3 Conclusions ... 76
4.6 Lamination of the microfluidic channels ... 77
4.6.1 Dry Film Resists: Theoretical Background ... 77
4.6.2 Dry Film Resist on Rogers Material ... 77
4.6.3 Dry Film Resist on the Silicon Chips ... 81
4.6.4 Conclusions ... 84
4.7 Inkjet printed electric interconnections ... 86
4.7.1 Theoretical Background ... 87
4.7.2 Reliability ... 88
4.7.3 Towards Smaller and Smaller... ... 90
4.7.3.1 80 mm Diameter Nozzle ... 90 4.7.3.2 40 mm Diameter Nozzle ... 90 4.7.4 Conclusions ... 92 4.8 Performance ... 93 4.8.1 Leakage Test ... 93 4.8.2 Functionality Test ... 95
4.9 Discussion and Conclusions ... 98
5. Conclusions and Future Work
... 1015.1 Conclusions ... 102
A. Accelerated Testing and Failure Detection
... 105A.1 Introduction ... 106
A.2 Accelerated Testing ... 106
A.2.1Temperature Cycling ... 106
A.2.2Thermal Shock ... 108
A.2.3Moisture Resistance ... 109
A.3 Failure Analysis Techniques ... 111
A.3.1Cross-sectioning ... 111
A.3.2Decapsulation ... 112
A.3.3SEM ... 113
A.3.4X-Ray ... 114
A.4 Failure Mechanisms ... 115
A.4.1Corrosion ... 115 A.4.2Delamination ... 117 A.4.3Cracking ... 117
Bibliography
... 119Summary
... 129Samenvatting
... 133Acknowledgement
... 137List of Publications
... 141Introduction
11.1
Low-Temperature and Low-Cost Packaging
Technologies: Smaller and Smaller...
Packaging of electronic circuits consists on grouping a set of elements and establishing interconnections between the different elements to form an electrical circuit; all these elements are presented as a unit and thus it can be seen as a black box. One of the main functions of packaging is to provide a suitable operating environment to the circuit; moreover, it provides mechanical support and protection from the environment, power distribution, signal distribution and heat dissipation [1].
Three of the main factors affected by packaging are the cost, the performance and the reliability of a system, thus the evolution of packaging originates from the need to optimize in cost, performance and reliability.
A package should satisfy the application requirements and constraints. Therefore, this work is limited to the packaging issues related to the aimed applications.
The main element enabling the creation of electronic circuits and systems is the interconnect; without the interconnections it would be impossible to establish communication between the elements that form a circuit/system.
Back in time, those interconnections were made by soldering the terminals of the components or by plugging them; later, with the origination of transistors, the size of these components was dramatically reduced and enabled the use of printed circuit boards providing a higher degree of integration [1]. Automated processes were born reducing costs and soon after that, photolitography techniques appeared enabling an even higher degree of integration at even lower costs [1].
Integration became a necessity and nowadays, technologies as System-in-Package (SiP) allow a very high degree of integration by interconnecting all kinds of components (antennas, optical elements, microprocessors, memories, etc.), built with different technologies, and grouping them within a single package.
Integrating several components fabricated with different technologies brings with it other issues as the compatibility of the different materials, the thermal stress than can be handled by the different components, etc. Special focus is given to those problems during this work.
All this evolution lead to technologies as Wireless Sensor Networks (WSNs). A WSN is a network composed of nodes, the nodes at the time contain sensors to monitor environmental variables like temperature, humidity, pressure, location, etc. in different locations at the same time. The nodes communicate the obtained data to a main location or share it with nearby nodes.
A WSN node is composed by an antenna and a radio chip to provide communication skills, one or more sensors to provide sensing skills, a microcontroler to provide processing capabilities and a battery to provide electrical energy to the system.
WSNs are the perfect example of a technology that requires the System-in-Package packaging approach to reduce size and costs but at the same time to increase performance and reliability. This work is partially focused on low-temperature and low-cost SiP solutions for WSN nodes.
1.2
Motivation and Objectives
New technologies create new needs. In the areas of logistics and asset management those new needs will be created by the technology proposed in the PLEISTER (Packaged Label Electronics Including Sensing Talkative Enhanced Radio) project, founded by STW (Stichting voor de Technische Wetenschappen). This thesis is the product of working on packaging solutions for the PLEISTER project.
In the vision of the PLEISTER project, a group of Wireless Sensor Network nodes can form a self-contained network capable of monitoring itself; that is to say, if a node joins/leaves the network, this will be noticed automatically and reported to other nodes. Furthermore, the network can sense and monitor status information as location, temperature, humidity, among others.
The ability to track & trace assets (as capital goods, food, etc.) and to identify objects and persons by using the PLEISTER nodes enables a new approach towards inventory management and track & trace allowing even theft protection (or kidnapping) plus monitoring environment conditions during transport.
The elements necessary to build such active tags are nowadays available in the market as sensors, batteries, embedded systems, antennas, etc. It is the goal of this work to combine all those multiple devices, from bare die to surface mount devices (SMDs), battery and antenna, in a single small size and low cost package.
The goals and challenges of this work are:
• To scale down current solutions so that a tag can have the size of a stamp and yet meet and even improve current state-of-the-art requirements for WSNs. The minimal package size is determined by the component occupying the largest surface area, which is expected to be either the antenna or the battery.
• To provide an economic competitive device. The tags must be low-cost and thus technologies like silicon and ceramic sub-strates must be replaced by foils for instance, or by creating the interconnect layers directly on top of the battery, that is to say, the battery is used as the substrate.
• To enable automatic mass production of the nodes. Automatic mass production must be possible with the proposed solutions. Special focus should be given to the reliability and life time of the nodes.
• To integrate sensors in the package. Some sensors like tempera-ture and humidity sensors require that their active area is exposed to the environment and thus the packaging approach should provide the flexibility of open paths to the environment. • To integrate different technologies like battery, electronics and
antenna into one package. The compatibility of the different process steps is a challenge due to thermal stress; for example, most of the batteries cannot withstand temperatures higher than 80 °C because of their electrolyte.
In addition, the methods proposed and developed within the frame of the PLEISTER project are extended for their application on packaging solutions for microfluidics.
1.3
Thesis Scope and Organization
This thesis treats the research and development of new integration methods for System-in-Package solutions. Accelerated testing is part of the methodology to research the proposed methods in order to determine the compatibility of the materials used. The integration methods developed are mainly low-temperature integration methods and intend to be low-cost as well.
Chapter 2 presents the theoretical framework which consists on the definition of System-in-Package as well as its advantages and disadvantages. Furthermore, this chapter presents an insight into the materials used for fabrication processes at low-temperatures.
Chapter 3 presents SiP packaging solutions for Wireless Sensor Network nodes within the frame of the PLEISTER project. The goals and requirements of such nodes are presented as well as the components used to build the node. Two packaging approaches are proposed; a conventional packaging approach which consists of using methods and technologies already available in the market to build the nodes and a research approach, which consists in developing new integration methods. The experiments used to investigate the new integration methods are explained and results are presented. Two of the main developments of this chapter are the use of electrically conductive adhesives (ECAs) to replace ultrasonic welding and soldering, techniques nowadays used to attach batteries to circuits; and the use of batteries as substrate by inkjet printing the interconnection circuits directly on top of them.
Chapter 4 presents SiP packaging solutions for integrating chips into microfluidic systems. The goals and requirements of the microfluidic systems are presented. Furthermore, the packaging concept is explained and the approach to develop it is divided in three main developments: the inlaying of chips in a printed circuit board (PCB) material by using a non-conductive adhesive (NCA), the fabrication of microfluidic channels on top of the PCB+Chip combination by using TMMF dry film photoresist, and the electronic interconnection between the chips and the circuits on the PCB enabled by inkjet printing. The performance of a device built with the proposed technology is presented.
Chapter 5 presents the conclusion of the work presented here and suggestions for further research.
An important part for the development of this work is accelerated testing and failure detection. Therefore, Appendix A presents a summary of accelerated tests, failure analysis techniques and failure mechanisms relevant for the thesis.
Theoretical Background
22.1
Introduction
The renaissance of SiP at the mid 90s was allowed due to the low-cost and high-performance substrate materials development and the massive demands on high performance along with compact products.
During the development of this thesis, new interconnection methods for SiP are proposed, developed and researched. Therefore, this chapter gives an insight in the advantages and disadvantages of SiP technology as well as low-temperature processing materials.
2.2
System-in-Package
SiP is defined in [2] as “a single module containing multi-chips and passive components to provide all the needed system and/or sub-system level functions”. It allows integrating various functional dies fabricated with different process technologies onto one substrate [3]. A SiP design process has an interdisciplinary character due to the disciplines that it involves as mechanics, electrionics and thermodynamics.
Figure 2-1 illustrates different SiP approaches. According to literature [5], the objectives of a SiP are:
• To provide alternate and cheaper solutions with respect to Sys-tem-on-Chip (SoC).
• To provide higher levels of integration and better electrical per-formance.
• To reduce the overall assembly size and weight achieving cost effectiveness.
2.2.1 Advantages and Disadvantages
SiP is a technology that is based on IC back-end packaging while SoC is based on IC front-end processes [3].
The following list enumerates the advantages of using System-in-Package instead of System-on-Chip:
• Relatively shorter design cycle, R&D time and time to market. • Less routing complexity on the system board.
• Contains off - the - shelf components allowing a modular design [5] by integrating components of different materials and proc-esses into the system [3]; therefore, it is easier to upgrade. • Better electrical performance could be achieved in most cases
[5] since each part of the electronic system is fabricated in the technology that promotes the best performance.
According to literature the disadvantages of using a SiP approach instead of a SoC approach are:
• Relatively immature infrastructure for SiP compared with con-ventional single - chip - package technology.
• Multiple voltages may be required but multiple power and ground planes make it too expensive [6].
• Testing is the biggest bottleneck to SiP development [3].
2.2.2 Applications
SiP has been adopted by the industry due to its comprising miniaturization, electromagnetic insulation, noise reduction, low power consumption, total cost reduction and circuitry simplification in high speed applications.
In the RF/Wireless area, SiP technology is widely applied due to its advantages in size, performance, and time to market. Some of the applications are the RF cellular phone and bluetooth [2].
Concerning sensors, SiP use has been recently increased in biometric sensors (e.g. finger print sensors), CMOS image sensors, and MEMS sensors (accelerometers). Some of the sensors are being integrated into portable devices like PDAs [7] and smart phones.
Talking about high speed digital applications, the performance improvement that SiP brings due to the decrease in ground bounce results in a much lower bit error rate. High speed and high volume Dynamic
Random Access Memory (DRAM) modules for servers and computers [5] are a clear example of this.
In addition, some other applications of SiP technology are:
• Digital camera, video, etc. [2]. • Power supply [2].
• Non volatile memory storage NAND flash cards such as Com-pact Flash, Secure Digital, and Multimedia Card (MMC) [5]. • Code storage and execution package for portable devices, cell
phones and PDAs using SRAM [5].
• Small, odd form medical and specialty applications such as hearing aid, and implantable heart pacers [5].
2.3
Low-Temperature Processing Materials
2.3.1 Substrate Materials
The main purpose of a substrate material is to provide the electrical interconnections between the components, as well as to cool down the components.
The substrate is selected on the basis of its electrical, mechanical and thermal characteristics. Epoxy - glass laminates are one of the most used circuit board laminates, nevertheless flexible substrates were introduced in the mids 90s [8] and offer a wide potential for highly complex folded packages and 3D modules [9].
Some of the advantages of this kind of substrates are their low cost, mechanical flexibility and light weight as well as low dielectric constant thus good electrical insulation [8]. Furthermore, it is possible to use them in three dimensional packages since they could be bent and curved into flexible shapes [9].
On the other hand, flexible substrates require a specially engineered tooling to fixture the substrate for assembly due to the typical assembly loads [10].
Fig. 2-2 Flexible substrate (a) and FR-4 (b)
Figure 2-2 shows (a) an inkjet printed structure on a flexible substrate (polyamide - PI) and (b) an inkjet printed structure on FR-4. The flexible substrate gives the posibility of playing with the spatial dimentions in a more effective way.
2.3.2 Joining Materials
The purpose of joining materials is to create an electrical path between the components and the interconnection lines as well as to provide mechanical stability.
Those materials are critical to the overall reliability of the electronic system [11]. Some of these materials are soldering alloys and adhesives.
In the case of soldering, tin - lead alloys, which used to be the most commonly used in the electronics industry, are banned nowadays. The soldered joint properties depend on the materials being joined due to the formation of different type and amount of metallic compounds [12].
On the other hand, adhesive materials are used in bonding surface mounted components to printed circuit boards, bonding semiconductor devices to substrates, and for structural joints in chassis and housings. Polyurethanes, silicones, polyimides, acrylics, cyanoacrylates, epoxies, and modified epoxies are commonly used adhesives in electronic packaging [11].
The main advantage of adhesive bonding as a joining process is that many dissimilar materials can be joined as metals, ceramics and polymers; furthermore, adhesives can be cured at low temperatures, as low as room temperature in some cases.
2.3.3 Encapsulating Materials
The purpose of an encapsulant is to protect semiconductor devices and interconnections from moisture (which is associated with electro oxidation and metal migration), ozone, ultraviolet radiation, corrosion and mechanical damages. Furthermore, they act also as dielectric insulators. A key feature to achieve this task is the covering of interconnects and the die without voids and damages [1].
For advanced semiconductor packaging, liquid encapsulation techniques have been extensively used; nevertheless, transfer molding technology has been used for traditional semiconductor devices when a huge volume of production is required [12].
These materials provide protection for the device from subsequent processing or the intended service environment. Other functions of those materials are facilitating heat transfer from components or damping mechanical vibrations and maintaining electrical isolation.
Epoxies are widely used as encapsulants because of their relatively low shrinkage, good mechanical strength, excellent adhesion and resistance to many chemical processes and application environments. Epoxies are relatively rigid matherials.
Packaging Concepts for
WSN Nodes
33.1
Introduction
In the coming years Wireless Sensor Networks (WSNs) and active tag technologies will bring new concepts in logistic processes and asset management.
It is the vision of the PLEISTER (Packaged Label Electronics Including Sensing Talkative Enhanced Radio) project that a group of designated active tags can jointly form a self-contained ad hoc network capable of monitoring itself. Joining or leaving the group is noticed immediately and reported to external entities. Moreover, status information as location, temperature, humidity, among others is sensed and monitored.
To perform those tasks during a long period of time, the nodes should have a power source with a long life time, for instance a Lithium ion rechargeable battery. The nodes should also facilitate a flexible charging process that requires a low number of plugs.
Furthermore, the nodes should be small and easy to mount on any surface. In this way, the WSN nodes can be used, for example, to track assets that are being transported by boat to Asia, among other applications. This chapter treats System-in-Package packaging solutions for Wireless Sensor Networks nodes within the frame of the PLEISTER project.
3.2
Goals and Requirements
In the context of the PLEISTER project, each active tag should have communication and sensing capabilities as well as its own energy source. The packaging approach of each node should meet the following requirements, previously mentioned in Chapter 1:
• A tag must be cheap.
• A tag must have a total maximum size of 2 cm x 2 cm x 1 cm. • The operation frequency has to be 2.4 GHz.
• The tags must meet the restriction of hazardous substances (RoHS) compliance.
• The packaging design should provide an environmental opening for sensors to allow a direct contact with the environment. • The application temperature range of a tag is from -20 to 50 °C. • The battery application temperature range is from 0 to 60 °C.
In addition, the power source for each node is a shapeable Li-ion battery in research and development by Philips. Such battery brings two important constrains to the fabrication process; its electrolyte starts to decompose at 80 °C and the leads that contact the battery to the outside world are made of nickel and aluminum, the last one impossible to solder.
The difference in the application temperature range between the node and the battery originates from the battery characteristics. The battery is designed to operate at temperatures in the range of 0 to 60 °C, however, it can also be stored at -20 °C. Further details about the battery are described in Section 3.3.2.
3.3
Component Choices
This section explains the different components that form the node and the criteria followed to select them.
The components explained here are the antenna, the battery, the sensor and the Nordic chip, which is a wireless system on-chip solution.
3.3.1 Antenna
The objective of the antenna is to optimize the energy delivered from the transmiter to the receiver.
In omni-directional antennas, the power is radiated uniformly resulting in a spherical pattern. The antenna used for the PLEISTER nodes is a patch antenna. This kind of antenna has an apple-like radiation pattern and presents the maximum gain along the XY-plane gradually degrading as the observation point moves towards the Z axis.
The antenna size should match the size requirements presented in Section 3.2; therefore, the area of the antenna should not exceed an area of 2 cm x 2 cm. Furthermore, the antenna is designed to work within the industrial, scientific and medical (ISM) band (2.4 - 2.48 GHz).
The antenna is made of a square copper patch on FR-4 material. The metallic patch is fed via a coaxial probe which acts as radiating element, shorting pins are added to create a current path through the ground. In addition, there is a tuning coil in the back side of the ground plane because the antenna is too small for the required wave length.
Fig. 3-1 Antenna
Figure 3-1 shows the antenna used for the node.
3.3.2 Battery
The battery used to provide energy to the wireless node is a rechargeable shapeable Lithium battery. Such battery is formed by a stack of electrodes separated by a Rivet polymer and protected by an aluminum foil further called the battery’s packaging foil.
To communicate the chemistry inside the battery with the outside world, aluminum (Al) and nickel (Ni) leads are used [13]. Figure 3-2 shows the structure of the battery. The stack of electrodes has a copper lead connected to the negative electrode and an aluminum lead connected to the positive electrode, the aluminum lead is welded to another aluminum lead and the copper lead is welded to a nickel lead. The whole structure is packaged with the battery’s packaging foil, leaving some of the aluminum and nickel leads unprotected to contact the inside chemistry with the outside world.
The use of this kind of battery limits the fabrication process of the node in three different ways. The first limitation is that the electrolyte of the battery starts to decompose at 80 °C.
The second limitation is that the leads that connect the battery to the outside world are made of Ni and Al; being Al a difficult material to work with due to its oxide layer. To overcome this problem, battery manufacturers attach additional Ni leads to the Al and Ni contacts by means of ultrasonic welding. Such Ni contacts can be soldered to the Printed Circuit Board (PCB).
Fig. 3-3 (a) Composition of the battery’s package foil, and (b) fabricated battery2
1. N.B. Palacios Aguilera et al. Limitations of Gluing as a Replacement of
Ultra-sonic Welding: Attaching Lithium Battery Contacts to PCBs. Proceedings of the 6th
International Microsystems, Packaging Assembly and Circuits Technology (IMPACT) 2011, 19-21 October 2011, Taipei, Taiwan, pp. 251-254.
The third limitation is the melting point of one of the layers of the packaging foil, the polypropylene (PP) layer, which melts at approximately 160 °C [13]. As a consequence, the battery’s packaging foil cannot be subjected to temperatures higher than 155 °C and the battery itself, after fabrication, cannot be subjected to processes at temperatures higher than 80 °C. Figure 3-3 shows the cross-section of the battery’s packaging foil which has a 25 µm thick polyethylene therephthalate (PET) layer at the outer surface.
The battery can operate at temperatures from 0 °C to 60 °C; furthermore, it can be stored at -20 °C. Depending on the size and design of the battery, it can provide capacities in the range of 10 mAh to 2.9 Ah.
The battery used for the nodes has a size of approximately 20 mm x 20 mm x 3 mm and it has capacity of approximately 70 mAh, enough for providing energy to the nodes for 7 days before it needs to be re-charged. Furthermore, the voltages provided by the battery are in the range of 3 V to 4.2 V.
More details about the fabrication of the battery can be found in [13].
3.3.3 Sensor
The sensor used in the nodes is the SHT21 from Sensirion. This sensor is capable of sensing humidity and temperature. The size of the sensor’s foot print is 3 mm x 3 mm and its height is 1.1 mm.
It consuimes an energy of 3.2 uW per measurement, when using the minimum resolution (8 bits) [14]. The relative humidity (RH) operating range is 0-100 % with a response time of 8 seconds. The temperature operating range is -40 to 125 °C with a response time from 5 to 30 seconds [14].
3.3.4 Other Electronic Components
Another crucial element in the node is the Nordic chip nRF24LE1E. This chip is an ultra-low power wireless system on-chip solution that adds functionality to the node.
This on-chip solution contains a 2.4 GHz transceiver, a microcontroller compatible with the 8051 microcontroller, 16 kB flash program memory, 1 kB RAM data memory, a 16-32 bit multiplication/ division co-processor (MDU), a 6-12 bit analog to digital converter (ADC), among others [15]. The footprint size of the chip is 5 mm x 5 mm and the height is 0.85 mm.
The power consumed by the chip is always the sum of the current drawn from all the different modules active at a determined moment. During deep sleep mode, the drawn current is 0.5 µA; during standby mode, 1 mA; during active mode, 2.5 mA. The maximum current drawn by the transceiver in TX mode is 11.1 mA and in RX mode, 13.3 mA. The ADC when busy requires 1.5 mA, and the random number generator uses 0.5 mA [15].
3.4
Node Design
Accorging to Section 3.1 and Section 3.2, each node of the WSN should contain at least the following elements: antenna, battery, small embedded systems, wireless network devices and sensors.
Two approaches are considered to develop a package that integrates all these components and meets the requirements previously mentioned: practical approach and research approach. The practical approach uses technologies already available in the market and the research approach researches and explores packaging technologies not available in the market yet.
Since the battery and the antenna require a cetain minimum area to meet the requirements enlisted in Section 3.2, being the biggest elements in the node, the packaging approaches focus in using efficiently the space in the XY-plane, that is to say, there is enough space to put the electronic components in one layer instead of stacking them; however, the battery, the antenna and the layer of electronic components are stacked.
3.4.1 Practical Approach
The practical approach consists of integrating the node elements by using and combining already existing and standard technologies.
The antenna is fabricated on FR-4 material. Another PCB element contains the interconnection lines for all the devices that form the node.
Figure 3-4 shows the practical approach concept. It consists of the battery attached to the PCB with the electronic devices and this at the same time, is attached to the antenna, then the elements are molded in order to protect the components from the environment. Furthermore, the access for the sensor is made by a through hole in the antenna.
Fig. 3-4 Practical approach concept
Table 3-1 shows the critical points of this approach and how to address their solution.
Table 3-1. Critical points for the practical approach CRITICALPOINT HOWTOSOLVEIT...
Connect the battery to the
electronics Ultrasonic welding and soldering of the battery contacts Antenna Create the antenna with multilayer tech-nologies to integrate the RF part on it Opening for the sensors Through hole in the antenna Assembly of all the
3.4.2 Research Approach
The research approach proposes new methods of interconnection that can be applied to the nodes in order to optimize space, size and resources as well as to reduce costs.
In this approach, the battery is the main core for the development of the packaging method since it is one of the biggest elements and serves as the mechanical support for the whole node.
This approach researches the use of the battery as the substrate for the whole system by inkjet printing the interconnection lines directly on it. Figure 3-5 shows the concept of inkjet printed interconnection lines on top of the battery.
Fig. 3-5 Concept of ink-jet printed interconnection lines on top of the battery3
By inkjet printing the interconnection lines directly on the battery, costs, space, time and materials are reduced by eliminating the use of a substrate to fabricate such interconnection lines.
3. N.B. Palacios Aguilera et al. Reliable inkjet-printed interconnections on foil-type
Li-ion batteries. IEEE Transactions on Device and Materials Reliability, vol PP no
Furthermore, the use of electrically conductive adhesives (ECAs) as a replacement for ultrasonic welding and soldering to electrically connect the battery to the system is studied. This approach intends to eliminate the high temperatures required by soldering during the fabrication process of the nodes.
Table 3-2 shows the critical points for this approach and suggestions on how to address them.
This thesis treats the use of the battery as a substrate to inkjet print the interconnection lines on it. The critical points are left for future work with exception of the use of ECAs to ellectrically connect the battery and other components.
3.5
Fabrication Process
The packaging solution is developed from two points of view; the practical approach, which consists on integrating the elements that form the System-in-Package using already available and reliable tested technologies; and the research approach, which consists on developing new methods and technologies to create the SiP; the second approach requires a lot of reliability tests before it can be implemented in a production line.
3.5.1 Practical Approach
In Section 3.4.1, Figure 3-4 shows the concept of the practical approach. Table 3-2. Critical points for the research approach
CRITICALPOINT HOWTOSOLVEIT...
Connect the battery to the
electronics Glue the battery contacts by using an electrically conductive adhesive Antenna Inkjet printed antenna directly on the battery Opening for the sensors Use a dam material around the sensor and then dispense a fill material Assembly of all the
PCB, as well as the the antenna. The environmental access for the sensor is integrated in the antenna as a through hole. Furthermore, the battery, antenna and PCB are attached together and molded.
The basic process steps to fabricate the package are:
• The antenna is fabricated with a through hole that acts as the opening for the sensor allowing environmental access to it. • The electronics interconnection is fabricated on another PCB. • The microchips and other elements are attached to the PCB
using standard connection technologies (flip chip, wire bonding, surface mount, etc.) according to each components capability. • Three 3D-printed posts are glued on the battery’s surface. A
UV-curable adhesive is used. These 3D-printed posts will allow keeping the PCB and the battery parallel during the encapsula-tion process. The same method is used to keep the antenna and the PCB parallel as well. Figure 3-6 shows the 3D-printed posts. The printer prints a base for mechanical support of the printed structures. The printed structures are removed from the base before using them.
Fig. 3-6 3D-printed posts used to keep paralel the components of the node during encapsulation
• A high viscosity non conductive adhesive (NCA) and a small mold made with PDMS are used to protect the area around the sensor by creating a wall around it. This wall avoids that the encapsulation material flows into the sensor space and blocks the opening for the sensor. Figure 3-7 shows the PCB with the electronic components; the PDMS wall for the sensor is illus-trated also. The wall is glued to the antenna first and then to the PCB with the electronic components.
• The battery nickel contacts are soldered to the PCB. Commer-cially available batteries are already equipped with welded nickel contacts.
• The 3D-printed posts are also glued to the PCB by using a UV-curable material.
• The components are placed in the mold and a low-viscosity encapsulation material is dispensed.
• Trapped air is removed by using vacuum.
Fig. 3-7 PDMS wall to protect the sensor’s active area from the encapsulation component
• The encapsulation material is cured at room temperature or at 80 °C in a Nitrogen atmosphere.
3.5.2 Research Approach
As mentioned in Section 3.4.2, the core for the development of this approach is the battery. Two topics are developed during this approach: the use of the battery as a substrate by inkjet printing the interconnection lines directly on it and the use of an electrically conductive adhesive as a replacement for ultrasonic welding and soldering to attach the battery contacts to the interconnection lines. Those topics are treated with detail in Section 3.6 and Section 3.7. The rest of the points are left for future work; however, they are mentioned in the fabrication process.
To develop a node with this approach, the following steps are performed:
• Inkjet print the antenna on the battery’s foil.
• Attach the components with an ECA that cures at maximum 80°C or other low-temperature method.
• Fabricate the battery.
• Create a dam around the components by using a high-viscosity UV curable adhesive material. Such dam is also created around the sensor.
• Protect the components and interconnection lines against the environment by dispensing a low-viscosity UV curable encapsu-lation material inside the dam. The active area of the sensor is not covered with a low-viscosity encapsulation material.
3.6
Electrically Conductive Adhesives as a
Replacement for Ultrasonic Welding and
Soldering
This section presents in detail the study of electrically conductive adhesives as an option to directly perform the electrical connection of Li-ion batteries to electronic systems. This option enables the replacement of ultrasonic welding and soldering, techniques nowadays used to attach the batteries to the systems.
The experiments performed to study the performance of electrically conductive adhesives (ECAs) as an alternative to ultrasonic welding and soldering of shapeable Li - ion batteries consist of several steps:
• 1) The selection of an ECA.
• 2) The performance of the ECA compared to ultrasonic welding with no ageing process.
• 3) The performance of the ECA compared to ultrasonic welding after a thermal shock test.
• 4) The performance of the ECA compared to ultrasonic welding after a moisture resistance test.
• 5) The performance of the ECA under elevated temperature and high humidity conditions with an electrical signal present.
To determine such performance two parameters are measured: contact resistance and strength of the bond.
In practice, bonds are usually not directly exposed to the environment but protected by an encapsulant; therefore, encapsulated bonds are studied to determine the best combination of bond method and encapsulation material. In this case, the strength of the bond is not measured for obvious reasons.
In order to measure the electrical performance of the unprotected bonds, aluminum-brass contacts and nickel-brass contacts are used. The bond area is 5 mm x 3 mm in all cases. To study the protected contacts, a different experiment is designed using PCBs and glob top materials.
A PCB with silver (Ag) finished contacts is used to create the glued bonds and measure their contact resistance. Aluminum and nickel leads are used to prepare the specimens. Such leads are the same as the ones used to contact the battery's chemistry to the outside world. The leads have a width of 2 mm. The ECAs used to glue the leads to the PCBs are listed in Table 3-3. The properties listed in the table are relevant for the understanding of this work.
The glob top materials used to protect the bond area are listed in Table 3-4. The properties listed in the table are relevant for the understanding of this work.
The thermal shock test performed to the samples is explained with detail in Appendix A. The temperatures used for this test are -20 °C and Table 3-3. Electrically conductive adhesives and their properties [16, 17]
ECA RESIN BASE SCURING CHEDULE IONIC CONTENT RESISTIVITY DELO Dualbond IC343 Modified polycarbamin acid derivative 3 h 80 °C Cl- <10 ppm 1.4x10 -4 Ω cm Silductor 6310 Silicone 30 min150 °C K+Na+Cl- < 2 ppm 5x10 -4 Ω cm
Table 3-4. Properties of the glob top materials [18, 19, 20] GLOBTOP RESIN BASE CURING
SCHEDULE IONIC CONTENT
Loctite 3129 Epoxy 30 min80 °C Cl- < 171 ppm Vitralit 1650 Epoxy UV-A 60 mW/cm30 s 2 K+Na+Cl- <5 ppm
80°C. The moisture resistance test performed to the samples is explained with detail in Appendix A.
The temperatures used for the reliability tests were selected according to the temperature range of the application described in Section 3.2.
3.6.1 Theoretical Background
3.6.1.1 ECAs
Electrically conductive adhesives consist of usually an epoxy resin that contains particles made of a conductive material. Such conductive material is typically silver; gold, nickel, copper and carbon are other metallic materials that can be used. The most common use of silver filled epoxy resins is as a replacement for solder.
Silver is the most cost effective material. Pure silver particles’ composites show improved conductivity when heat aged, exposed to heat and humidity, or thermal cycled. The silver particles’ oxide layer presents good electrical conductivity [21]; however, adsorption of moisture is a problem of adhesive polymers and it could reduce its strength and cause migration of the silver particles [22].
ECAs can be classified as anisotropic and as isotropic adhesives. An isotropic conductive adhesive (ICA) conducts equally in all directions and does not require of a pressure applied over the area to be glued while curing [21]. Anisotropic conductive adhesives (ACAs) contain conductive particles (z-axis conductive) dispersed in a non conductive adhesive (NCA) mass; once the adhesive film or paste is applied, a pressure has to be applied over the substrate side during curing so the conductive particles are trapped between the conductive contacts forming a conductive path between the contacts [22].
Furthermore, adhesive resins can be thermoplastic, thermosetting or elastomeric materials [23]. The adhesives used during the development of this work are isotropic thermosetting materials, therefore the curing process for this kind of materials is explained here.
Thermosetting materials experience a chemical reaction when heated, forming a cross-linked network. Once the material is cured, the cross-linked network is locked in place. The more reactive sites in the hardener, the more tightly cross-linked the cured material is and therefore presents a better thermal, chemical and moisture resistance [23].
Fig. 3-8 Anisotropic conductive adhesive before curing (top) and after curing and applying pressure (bottom)
3.6.1.2 Encapsulants
Electronics have too be protected from the environment to avoid damage, for example, corrosion caused by a very humid atmosphere. Such protection can be a metallic, a ceramic or a plastic material.
Most of the commercial products are packaged with plastic materials, which have several advantages with respect to metallic and ceramic materials. Those advantages are [24]:
• Lower design and manufacturing costs • Less weight
• Smaller size
• Their dielectric constant is lower than that of ceramics
Encapsulants should have sufficient mechanical strength, good adhesion to the components, chemical resistance, electrical resistance, high thermal stability and moisture resistance in the temperature range of use. However, three of the main concerns of plastic materials are their ionic impurities, their glass transition temperature and their coefficient of thermal expansion [24].
One of the encapsulant materials treated in this work is glob top which is usually made of epoxy resins. Epoxy resins contain ionic contaminants as the chloride ions, from the epichlorohydrin used in the epoxidation of the resin, and the bromine ions, result of the flame retardants incorporated into the resin. Potassium is another ionic contaminant [24].
The ionic contaminants become a big problem when moisture reaches the inside electronics. Chloride ions break the protective oxide layer of the aluminum and electrolytic corrosion occurs at an accelerated rate [25]. The time to failure in plastic encapsulated materials can be related directly to the chloride and other halides in epoxy materials.
Nowadays, encapsulants contain as low as 10 ppm of the corrosion inducing ions [24]. According to [26], materials used for encapsulation of integrated circuits should have less than 4 ppm of water extractable chloride or other corrosive elements.
Glob top materials are dispensed as fluids over the devices to be protected, and then they are cured. The curing process happens when the polymer's macromolecules form cross-networks through chemical linking. The goal of glob tops is to cover the die and bonds with a minimum fluid thickness avoiding the encapsulant to run everywhere.
Furthermore, epoxy resins are typically available as two part materials consisting of a resin material A with fillers to adjust viscosity and other properties, and a hardener system B that is used to cure the resin system.When both parts come into contact, an irreversible exothermic chemical reaction starts, the heat of the reaction raises the temperature of the materials, and the viscosity is reduced. When the viscosity is reduced, the resin flows easier enabling a better and faster wetting of parts and surfaces and therefore better adhesion. The heat from the reaction of both parts also accelerates the cross-linking of the system. When the system is cured, the mix thickens to a non-flowing gel, stiffens and hardens into a solid [27].
The resin shrinks before the curing process is complete; some of the volatiles in the fluid materials boil off during the curing process and when the material returns to room temperature it shrinks. Adding filler reduces the shrinkage.
In the case of a one part system, the curing process is initiated by heat, humidity from air, or UV-light.
3.6.2 Strength
Lap shear tensile strength is measured to determine the maximum stress the bond can withstand.
3.6.2.1 Samples Preparation.
The specimens used for these measurements consisted of two materials bonded with either an ECA or ultrasonic welding. The dimensions of each metal stripe are 30 mm x 5 mm and the area of the bond is 15 mm2 in both cases, glued and welded bonds. The metals used to create the bond are aluminum-brass and nickel-brass.
Prior to the gluing process, the surface of the metals is prepared with sand paper, steel wool, ethanol and acetone.
Each test condition is repeated four times.
3.6.2.2 Experiment Setup
A setup to perform lap shear tensile strength measurements is configured as Figure 3-9 shows. This setup is used to determine the strength performance of the bonds. The device used is the Deben Microtest Module.
Fig. 3-9 Strength measurements setup4
By means of software, the setup starts by slowly increasing a tension in the right direction in Figure 3-9, beginning with 0 N, until the specimen is broken or the force reaches the limit of the load cell (in this case 200 N). This results in a graph of extension (mm) against force (N)
from which a graph of strain versus stress (MPa) is obtained by performing basic calculations.
3.6.2.3 Results
Table 3-5 enlists the average tensile strength measured for fresh samples as well as aged samples.
In the case of the Al - Brass specimens, the fresh glued bonds show to be stronger than the ultrasonic welded samples. Figure 3-10 shows that the aluminum broke before the glued bond. In the case of the ultrasonic welded specimens, the bond broke before the aluminum.
Fig. 3-10 Broken Al before the glued bond failed5
4. N.B. Palacios Aguilera et al. Gluing as an Alternative to Solder Flexible
Batter-ies for its use in System–in–a–package: Preliminary Results. Proceedings of the 11th
Electronics Packaging Technology Conference (EPTC) 2009, 9-11 December 2009, Singapore, Singapore, pp. 550-555.
Table 3-5. Average measured tensile strength of the bonds in MPa NOAEGING THERMAL SHOCK MOISTURE RESISTANCE
Ni-Brass Glued 8.695 2.398 1.69 Ni-Brass Welded 4.405 3.188 3.578 Al-Brass Glued No failure 1.92 (2/4 samples-failed) Open Circuit (corrosion) Al-Brass Welded 1.948 2.02 1.768
In the case of the Al-glued samples, the strength is not measured since the bond was already broken after the moisture resistance test due to corrosion.
Figure 3-11 shows, in the left upper side, the surface of the brass after the moisture resistance process and, in the right upper side, the surface of the glue on the side of the brass after the moisture process. The lower part of the figure shows the aluminum and the glue on the side of the aluminum after the moisture process. According to this figure, it is clear that the corrosion effect is caused by the combination of Brass-ECA-Al and humidity.
Fig. 3-11 Moisture process effects in the Al-Brass glued bond6
Brass and aluminum are non-noble materials; the electrically conductive adhesive contains silver particles, a noble material. The epoxy resin of the ECA adsorbs humidity providing a physical path between the noble and the non-noble metals. When the humidity is adsorbed by the epoxy resin, it acts as an electrolyte promoting corrosion. For a deeper explanation of the corrosion phenomena, please refer to Appendix A.
5. N.B. Palacios Aguilera et al. Gluing as an Alternative to Solder Flexible
Batter-ies for its use in System–in–a–package: Preliminary Results. Proceedings of the 11th
Electronics Packaging Technology Conference (EPTC) 2009, 9-11 December 2009, Singapore, Singapore, pp. 550-555.
6. N.B. Palacios Aguilera et al. Gluing as an Alternative to Solder Flexible
3.6.3 Resistance
During previous experiments it was determined that the current-voltage characteristics for all the combination of materials studied behave linear; therefore, an average of the contact resistance is used as the resistance value from now on. Such value is the result of the average of 3 specimens' voltage measured at a certain current.
3.6.3.1 Samples Preparation
In this section, the gluing and the ultrasonic welding method used for the study of the protected contacts are explained. The bond area is, in all the specimens, 2 mm x 2 mm.
Each test condition is reproduced 3 times, that is to say, three specimens of Al welded contacts protected with Vitralit 1650 are prepared, three specimens of Al glued contacts protected with Loctite 3129 are prepared, and so on.
To prepare the glued specimens, the process shown in Figure 3-12 is followed. The PCB is cleaned and degreased with ethanol. A mask of Scotch tape is placed on the PCB's surface (b), leaving uncovered the connection pads; the purpose of this mask is to have exactly the same amount and dimensions of glue per bond. Subsequently, the ECA is dispensed over the open area (c) and the excess of glue is removed with a razor (d). The Scotch tape mask is removed (e).
The aluminum and/or nickel leads are mechanically brazed with sand paper and steel wool; they are cleaned with ethanol and acetone, and finally placed on the glue (f). The specimens are cured in an oven with air flow according to the curing schedules specified in Table 3-3.
For the ultrasonic welded samples, the Ni-Ni specimens are welded twice using an energy of 70 J. The Ni-Al specimens are welded once with an energy of 35 J. The energies and how many times a sample is welded are determined by measuring the strength of bonds welded at different energies.
Fig. 3-12 Glued specimens preparation7
After preparing both, the glued and welded contacts, the bond area is protected with glob top material. The glob top materials used to protect the contacts are the UV curable high purity material Vitralit 1650 and the heat curable material Loctite 3129.
7. N.B. Palacios Aguilera et al. Limitations of Gluing as a Replacement of
3.6.3.2 Experiment Setup
A 4-point configuration is used to sense the voltage variations according to the current variations and to determine the contact resistance. An HP 34401A Multimeter is used to measure the current, an HP3478A multimeter is used to sense the voltage and a Power Supply EST 150 Delta Elektronika is used to generate the current.
To study the protected contacts, the contacts are prepared and the contact resistance is measured (A), then the contacts are protected with glob top material and the contact resistance is measured once more (B). The moisture resistance test is performed and the contact resistance is measured a third time (C). The different contact resistance measurements are compared to draw conclusions. The test is considered a pass for the specimens that present a change of less that 20% in the contact resistance measurements of the aged samples with respect to the fresh samples [28, 29].
3.6.3.3 Results
Unprotected Samples. Table 3-6 shows the average contact resistance for
the fresh specimens. Furthermore, it lists the change in resistance with respect to the fresh specimens after the thermal shock and the moisture resistance tests.
Protected Samples. The results are divided in two tables. Table 3-7
presents the measurements from the contacts protected with Vitralit 1650. Table 3-8 presents the results of the contacts protected with Loctite 3129. The numbers presented in both results' tables are an average of the 3 samples tested for each condition. Furthermore, notice that the measurements that pass the reliability tests in Table 3-7 and Table 3-8 are bolded. Aeging test in both tables refers to the unbiased moisture resistance test.
Table 3-7 does not show the contact resistance measured after the curing process of the Vitralit material because the distance of the UV source is optimized in a way it does not cause an increase of temperature at the bond and thus a change of contact resistance.
Table 3-7 shows that the welded contacts protected with Vitralit do not fail. The glued contacts fail.
Table 3-8 shows that the welded contacts can fail after applying Loctite 3129; however, they pass the reliability test with respect to the measurement (B) made after applying the glob top material, showing that there is a problem during the glob top process; the contact resistance after
Table 3-6. Contact resistance of unprotected bonds MATERIAL AND PREPARATION METHOD FRESH SAMPLE (mΩ ) AFTER THERMAL SHOCK TEST (%) AFTER MOISTURE RESISTANCE TEST (%) Ni-Brass Glued 10.1 127.8 -12.2 Ni-Brass Welded 3.2 85.0 89.2 Al-Brass
Glued 22.4 114.0 CircuitOpen Al-Brass
Welded 1.7 111.1 106.9
Table 3-7. Contacts protected with Vitralit VITRALIT 1650 ALUMINUM FRESH SAMPLE (mΩ ) ALUMINUM AFTER AGEING TEST (%) NICKEL FRESH SAMPLE (mΩ ) NICKEL AFTER AGEING TEST (%) Welded 31.1 -0.8 26.0 -3.8 Glued (DELO) 59.3 115.6 70.2 42763.0 Glued (Silductor) 129.2 12054.0 84.2 26764.0
Table 3-8. Contacts protected with Loctite 3129 LOCTITE 3129 ALUMINUM FRESH SAMPLE (mΩ ) ALUMINUM AFTER GLOB TOP (%) ALUMINUM AFTER AGEING TEST (%) NICKEL FRESH SAMPLE (mΩ ) NICKEL AFTER GLOB TOP (%) NICKEL AFTER AGEING TEST (%) Welded 17.0 25.2 5* 25.3 -8.3 -0.4* Glued (DELO) 79.6 -26.9 91.2* 84.8 142.7 4295.0* Glued (Silductor) 72.5 170.2 1052.0* 79.2 67.1 2573.0*
applying the glob top should not change with respect to the contact resistance of the fresh sample.
The glued contacts fail in all the cases.
In Table 3-8, for the numbers with the symbol *, the percentage of change is calculated with respect to the contact resistance measured after the glob top step. In all the other cases, the percentage of change is calculated with respect to the measured contact resistance of the fresh specimen.
Table 3-9 shows that the Al welded and glued contacts do not fail after applying Loctite 3128, if a Nitrogen (N2) atmosphere is used during the curing step.
In this case (Table 3-9), welded and glued Al samples are used since Al is the most problematic material; a glob top with similar properties than Loctite 3129 is used.
3.6.4 High Humidity and Elevated Temperature Test with Bias
3.6.4.1 Samples Preparation
The moisture resistance test, previously mentioned in this chapter, is performed to glued Al and Ni contacts simulating the voltage provided by the battery in both polarities: non-inverted and inverted. Furthermore, the currents drawn by the node are also simulated.
Table 3-10 indicates the polarities of the contacts when they are configured as non-inverted polarity and as inverted polarity.
The Al and Ni contacts are glued to a PCB with Ag plated contacts. Table 3-9. Loctite 3128, Nitrogen atmosphere
LOCTITE 3128 CUREDIN N2 ATMOSPHERE ALUMINUM FRESH SAMPLE (mΩ ) ALUMINUM AFTER GLOB TOP (%) Welded 49.0 -5.2 Glued (DELO) 57.9 -11.6
3.6.4.2 Experiment Setup
To simulate the battery, a voltage of 4 V is provided between the Al and the Ni contacts; furthermore, currents are forced to flow, simulating the currents required by the whole node. Four different currents are used for testing the configuration with both polarities: 200 µA, 1 mA, 10 mA and 20 mA. The contact resistance is measured during curing, during the test and at the end of the test.
Fig. 3-13 Configuration of the experiment
Figure 3-13 shows an eschematic of how the measurements are performed.
Table 3-10.Plolarities of the simulated battery contacts NON-INVERTED INVERTED
Al +
3.6.4.3 Results
Figure 3-14 shows the results for the Al contacts and Figure 3-15 shows the results for the Ni contacts.
The Al contacts behave better when high currents are drawn by the node using the inverted polarity; however, when using the right polarity, their reliability is bad, their contact resistance increases more than 100% (and in one case it decreases almost 100%) after the test with respect to the resistance of the recently cured contacts. According to Ohm’s law, the final measured resistance of those samples with the respective drawn current results in a voltage drop of less than 200 mV.
Fig. 3-15 Results Ni contacts
The Ni contacts present lower contact resistances with respect to the Al contacts; nevertheless, their contact resistance increases more than 100% after the test with respect to the resistance of the recently cured contacts. According to Ohm’s law, the final measured resistance of those samples with the respective drawn current results in a voltage drop under the 40 mV.
The results show that the resistance is always lower than 100 Ω., however, whether this is or not a problem for determined application depends on the drawn current.
3.6.5 Conclusions
The contact resistance of the glued bonds is higher than the contact resistance of welded bonds; when welding, the oxide layer of the metal is completely removed during the process while, during gluing, the oxide layer is incompletely removed due to the immediate contact of the surface with oxigen prior to gluing and after removing the oxide.
For an application in which the fabrication process is limited to low temperatures and welding is not possible, gluing may be the right option. The reliability of glued contacts is not acceptable when they are used for long terms because of the high increases in contact resistance; as time passes, more and more humidity is adsorbed by the adhesive resin resulting in corrosion and therefore in a high contact resistance. When the Li-ion battery’s power is present, the glued contacts may be a considerable problem, depending on the currents required by the aimed application.
Moreover, the chloride ions concentration in low-temperature curing adhesives is not low enough as to eliminate corrosion problems; therefore, corrosion cannot be completely avoided.
3.7
Inkjet Interconnections on Top of Batteries
The battery is one of the biggest elements in the package and can provide mechanical support to the whole node; therefore, it is interesting to investigate if this battery could serve as an interconnect substrate for the electronics. Because of these two reasons, inkjet printed interconnections on top of the battery are studied by printing test structures and applying high humidity and elevated temperature stresses on them.
For these purposes, the battery’s packaging foil described in Section 3.3.2 is used as the substrate material.
Furthermore, polymers usually have low surface energy values [30]; this results in poor wettability of the ink on the substrate. In general, for inkjet printing applications, good wettability is desired for well defined structures.
It is well known that plasma-chemical treatments result in an increase of the surface energy of polymers [30, 31, 32, 33, 34]; moreover, they provide a dry, clean, low temperature and fast processing environment [34]. The hydrophobic surface of PET substrates can be made hydrophilic using plasma treatment.
According to [35], the interfacial adhesion in metal-polymer systems not only depends on the wettability of the polymer but also on the morphology. Plasma treatments enhance the mechanical interlocking between substrates and inks [35]. Therefore, the use of an Oxygen (O2)
based plasma to improve the surface energy of PET is also studied in this section.
3.7.1 Theoretical Background
3.7.1.1 Adhesion Between Two Materials
Wetting is defined, according to [26], as a material, for instance a silver ink or an adhesive, spreading over and making intimate contact on a molecular scale with a substrate, by developing physical forces at the interface (van der Waals, Lewis acid-base interactions). For this to occur, there should not exist any weak boundary layer at the interface that can interfere and become a weak link in the formation of the physical forces.
Wetting can be measured by the contact angle that a drop makes with the substrate surface. Figure 3-16 indicates where the contact angle is. If the drop spreads over the surface with a small contact angle, equal or close to zero, wetting occurs [36].
Fig. 3-16 Contact angle of a water drop on the battery’s packaging foil with out a plasma treatment
Along with the surface energy, the topology of a substrate also affects the bond strength or coating on the substrate; this phenomena is explained by the mechanical interlocking theory. Mechanical interlocking refers to the interlocking of the adhesive/ink into the cavities, pores and asperities of the solid surface as the main factor determining the strength of the adhesion [37]. According to this theory, a better adhesion can be achieved by improving the surface morphology and the physicochemical surface properties of the surface and the adhesive/ink.
Increase of interfacial area due to surface roughness and the wetting conditions together, allow penetration of the liquid into pores and cavities.
In most metal surface preparation processes, mechanical abrasion provides mechanical interlocking and increased area over which the forces
If the substrate has a low surface energy, surface roughening could contribute to air pockets being trapped in the irregularities on the surface [26]. Therefore, chemical or physical surface treatments are generally employed on low surface energy substrates to increase the critical surface tension and make them more hidrophilic, thus improve wetting. The critical surface tension was defined by William Zisman and it is when the liquid completely wets the surface (contact angle = 0). In other words, if a graph of cos θ against the liquid surface tension is traced, the critical surface tension happens when the line intercepts cos θ = 1 [36].
Furthermore, surface treatments also remove contaminants or weak boundary layers. Conventional methods of surface treatments are [26]:
• Abrasion
• Primer. The surface is coated with a dilute solution chemically similar to the adhesive.
• Chemical etching
• Flame treatment. The surface is treated with a controllable gas flame to oxidize surface layers.
• Corona discharge. The surface is altered by ionized particles generated by high voltage electrodes.
• Gas plasma. The surface is altered by gas plasma formed by electric current passing through a gas medium at a specified fre-quency.
Mechanical methods roughen the surface while solvents remove contaminants and additives. Mechanical abrasion is usually preceded and followed by solvent cleaning. On the other hand, oxidative methods as flame treatment and chemical etching introduce functional groups and change the surface topography [26].
Without doubt, plasma treatments are very attractive since apart from removing contaminants they can affect the surface energy by adding functional or polar groups and by forming functional groups that allow covalent bonding between the elements. In addition, this treatment has a minimum impact on the environment [26, 36, 38].
In other words, plasma treatment attacks several factors that influence the adhesion; by removing the contaminants from the surface it contributes to mechanical interlock; by introducing functional groups, it increases the surface energy of the substrate and it modifies the surface chemistry allowing covalent bonding between the liquid and the substrate [36, 38].
3.7.1.2 Conductive Inkjet Printed Inks
During the last years, inkjet printing has been of wide interest for electronic applications. The strong point of inkjet printing technologies is that material is deposited only where needed, compared with other techniques as lithography, the material costs are highly reduced.
There are inks based on metal nanoparticles, metallo-organic decomposition, and metallo-organic complexes [39]. A good ink is compatible with the substrate and is easy to work with, providing good resolution and requiring of low maintenance.
Silver nanoparticle inks are the matter of subject for this work. Nanoparticle inks are composed of a liquid vehicle that can be an organic solvent or water, and dispersed or dissolved metallic nanoparticles [39]. The liquid vehicle determines the properties of the ink and the metallic nanoparticles, the functionality [39]. Furthermore, the inks require of a stabilizing agent to avoid aggregation and to provide reproducible functionality.
To achieve a conductive structure, the ink needs to be sintered by means of heat, UV-light or chemicals. Nowadays, most of the conductive inks are sintered with heat.
Sintering refers to the combination of particles growth and grain boundary migration. During the sintering process, the liquid vehicle and stabilizing agents evaporate and the particles come close to each other. Once the particles are close to each other, they coalesce and the grain size increases [35].
In the case of UV-light, the mechanism is triggered by absorption of light by the metallic layer, the absorbed light results in heat and the liquid vehicle is evaporated [39].
3.7.2 Samples Preparation
To study the inkjet printed interconnections on top of the battery, the battery’s packaging foil previously described in Section 3.3.2 is used as the substrate material, the test structures are printed on it. Such packaging foil is a commercially available battery foil from Sumitomo Chemical Co., Ltd. with a thickness of 132 µm.
Two different inks are used to print the test structures. The first ink (Ink A) is a silver nano-particle based dispersion ink with a mean
