Wearable devices have the potential to transform multiple facets of human life, including healthcare, activity monitoring, and interaction with computers. At the same time, a number of technical and adaption challenges hinder widespread and daily usage of wearable devices. Recent research efforts have focused on identifying these challenges and solving them such that the potential of wearable devices can be realized. In this monograph, the authors guide the reader through the state-of-the-art of wearable devices, detailing the challenges that researchers and designers face in achieving wide-adoption of the technology throughout society. The authors also identify the application areas where these devices are most likely to gain acceptance. They point the way to overcoming these challenges by detailing the recent advances in providing physically flexible designs, the energy management for such designs and finally consider some of the security and privacy aspects of wearable devices such that user compliance can be improved. This monograph serves as a comprehensive resource for challenges and solutions towards self-powered wearable devices for health and activity monitoring.

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Self-Powered Wearable IoT Devices for Health and Activity Monitoring

This paper describes the fabrication opportunities that Printed Circuit Boards (PCBs) offer for electronic and biomedical engineering. Historically, PCB substrates have been used to support the components of the electronic devices, linking them using copper lines, and providing input and output pads to connect the rest of the system. In addition, this kind of substrate is an emerging material for biomedical engineering thanks to its many interesting characteristics, such as its commercial availability at a low cost with very good tolerance and versatility, due to its multilayer characteristics; that is, the possibility of using several metals and substrate layers. The alternative uses of copper, gold, Flame Retardant 4 (FR4) and silver layers, together with the use of vias, solder masks and a rigid and flexible substrate, are noted. Among other uses, these characteristics have been using to develop many sensors, biosensors and actuators, and PCB-based lab-on chips; for example, deoxyribonucleic acid (DNA) amplification devices for Polymerase Chain Reaction (PCR). In addition, several applications of these devices are going to be noted in this paper, and two tables summarizing the layers’ functions are included in the discussion: the first one for metallic layers, and the second one for the vias, solder mask, flexible and rigid substrate functions.

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Printed Circuit Boards: The Layers’ Functions for Electronic and Biomedical Engineering

Replacement of gold (Au) wires by copper (Cu) wires is being widely studied owing to the advantages of copper over gold. Researchers in the past have correlated reliability issues in the Cu wirebonds with higher ionic contamination in the epoxy molding compound (EMCs). New EMCs with 5ppm contamination or less are being developed to address this issue. Green EMCs often have very high filler content which reduces coefficient of thermal expansion (CTE) and increases the modulus (E) of EMC. Higher CTE mismatch between EMC and Cu wires can cause accelerated fatigue failures. In this paper, a novel approach is presented to address this issue. Quad flat no leads (QFN) packages bonded with Cu wires were molded with variety of EMCs. The parts then were scanned using X-ray CT system and the scanned data was converted into FE platform. This 3D model includes all the details and represents true geometry of the package. Thermal cycling analysis was performed on the packages to study effect of different EMCs on the reliability of Cu wirebonds. Plastic equivalent strain $( \varepsilon _{\mathrm {p}})$ was extracted in each case to assess the damage accumulated on the critical wire due to thermal loading. The new methodology presented in the paper allows true assessment of the accrued damage using the true 3D geometry.

Effect of Green EMCs on Fatigue Reliability of Molded Cu Wirebond System

This article presents an overview of testing methods applied to flexible hybrid electronics (FHE). It starts by briefly summarizing and discussing the main properties of substrates, chips, and materials commonly used for flip-chip bonding of FHE. It also describes general accelerated testing methods and how they are related to the current progress in FHE testing. Stressors, such as temperature cycling, temperature/humidity, bending, stretching, and torsion, have been commonly applied to FHE. The application of those stressors and the corresponding failure modes, mechanisms, and factors that impact reliability is summarized and discussed. Methods for testing the reliability of FHE devices, such as radio frequency identification (RFID) tags, have been developed, but there are no industry-wide standards yet to address all aspects of this emerging technology.

A Review of Methods for the Reliability Testing of Flexible Hybrid Electronics

Recent developments in stretchable and flexible sensing and processing technologies enable a wide range of wearable devices. These devices can pave the way to medical applications ranging from health and activity monitoring to diagnosis and treatments of movement disorders. However, recent studies show that this potential is hindered by both adaptation challenges that affect the end users and technology challenges faced by developers. This paper first summarizes the challenges faced by wearable devices targeting health and user activity monitoring applications. Then, it reviews recent research progress towards addressing these challenges in energy harvesting, energy management, flexible system design, and test areas.

Special Session: Physically Flexible Devices for Health and Activity Monitoring: Challenges from Design to Test

Flexible electronics in wearable applications may be subjected to flexing, bending, stretching in addition to exposure to temperature and humidity. Presently, there is a general lack of test protocols for the reliability assessment and survivability assurance of the flexible substrates. Flexing and bending in operation may be accrued under stresses of daily motion. Flexible substrates often use serpentine patterns to accommodate high stretch in the neighborhood of 25-100 percent. Flexible copper traces subjected to cyclic mechanical bending result in stretching of the outside layers and simultaneously compressing the inner layers. Cyclic bending may also result in formation of wrinkles on the flexible substrate causing delamination. Meaningful accelerated test protocols and acceleration transforms relating test-performance to operational reliability are needed. In this paper, a new test protocol has been developed for replicating the stresses of daily motion for flexible substrates. The conventional-fabricated flexible circuits have been studied. Test coupons have been designed to include the common trace geometries encountered in flexible electronics applications. The effect of cyclic mechanical bending, exposure to human body temperature has been studied and analyzed using Flex-PCBs. Flex-PCBs have been subjected to cyclic bending and the failure modes were analyzed as a function of bend radius, bend angle and number of cycles to failure. Extremely tight bend radius and bend angle larger than 90° have been studied. Further, in order to monitor the resistance of copper traces and develop life prediction model, Data Acquisition unit has been used. This prognostic health monitoring technique captures the increase in resistance of copper traces with the growth in fatigue due to cyclic mechanical bending. The study addresses the need for life prediction models of flexible copper traces on flexible polyimide substrate.

Development of Test Protocols for the Flexible Substrates in Wearable Applications

Flexible batteries in wearable applications can serve a diverse array of functions including logic, control, memory and biometric sensing. The power sources may be operate under sustained exposure to human body temperature superimposed on ambient temperature extremes, mechanical loads under stresses of daily motion, and various depths of charge-discharge cycles. There are significant technical challenges in meeting the power-needs of wearable electronics including maximizing capacity in thinner form-factors, flexibility and robustness under the operating and usage environment. In this research study, the combined effects of deep-charge, shallow-charge, distinct bending load(s) and operating temperatures have been characterized for Li-Ion batteries. Thin flexible battery cells were cycled through multiple charge-discharge cycles under simultaneous bending loads plus thermal stresses. Output parameters such as efficiency, power, capacity and charge-discharge time have been analyzed for battery state assessment. The sensitivity of capacity degradation to a number of charge-discharge parameters is quantified with a regression model.

Effect of Shallow Cycling on Flexible Power-Source Survivability under Bending Loads and Operating Temperatures Representative of Stresses of Daily Motion

Flexible printed circuit boards lack the structural stiffness of the rigid printed circuit counterparts. Thermo-mechanical deformation in flexible printed circuit assemblies may be very different from that in rigid board assemblies. The double-sided board used for the experiment is of BGA 256–144 combination with dummy components, A-PBGA256–1.0mm-17mm and ACABGA144–1.0mm-13mm. The three-dimensional measurements of deformation and strain have been visualized on the geometry of the solder joints in the package. Digital volume correlation (DVC) method has been used to find the displacements and strains in interconnects of operational electronics. The x-ray microscopic computed tomography (μCT) system has been used to generate the 16-bit digital volume data. The x-ray detector has ability to image the x-ray attenuation of x-rays through the object. Reliability testing of SAC 305 solder interconnects has been performed on doublesided flexible circuit board using x-ray μCT by heating the package to 100°C. 3D-finite element models have been developed to ascertain the degree of error in the model prediction from non-destructive experimental measurements in reflow and thermal cycling.

Thermo-mechanical Deformation in Flexible-Board Assemblies During Reflow and Post-Assembly Usage

Combined effects of distinct bending load(s) and operating temperatures can significantly attenuate the life of flexible Li-Ion batteries in foldable wearable electronics. Present day technologies call for battery applications, which require exposure to bending stresses, human body and varying ambient temperatures. Current health monitoring techniques and test standards for flexible power sources are still in the nascent stages. Flexible power sources such as Li-Ion batteries may undergo multiple charge-discharge cycles during operation, therefore development of a hardware testbed, which mimics the operating conditions in their areas of applications, is needed. In this research study, the combined effects of distinct bending load(s) and operating temperatures have been characterized for Li-Ion batteries. Battery cells were cycled through multiple charge-discharge cycles under simultaneous bending loads plus thermal stresses and output parameters such as efficiency, power, capacity and charge-discharge time have been analyzed for battery state assessment. State-of-the-art Li-Ion batteries have been tested under multiple operating temperatures of 50°C and 25°C combined with distinct bending angles of 15° which are imposed only during the discharge cycles in order to replicate real world applications. The charge and discharge rate was varied between 0.5C to 1C. The hardware testbed takes battery measurements at regular intervals and enables cells to be in either the charge or discharge regime. The main goal of this research study is to observe and document the survivability of Li-Ion batteries when subjected to simultaneous distinct ambient temperatures and bending loads in foldable electronics as function of the above mentioned battery parameters.

Jason Marsh

Papers

Development of Test Protocols for the Flexible Substrates in Wearable Applications

Effect of Shallow Cycling on Flexible Power-Source Survivability under Bending Loads and Operating Temperatures Representative of Stresses of Daily Motion

Thermo-mechanical Deformation in Flexible-Board Assemblies During Reflow and Post-Assembly Usage

Flexible Power-Source Survivability Assurance under Bending Loads and Operating Temperatures Representative of Stresses of Daily Motion