Developing high performance and climatically reliable Hearing Aids

This PhD project is the outcome of collaboration between Celcorr (Centre for electronics corrosion) research group at DTU and Widex A/S (hearing aid manufacturer) as a part of Industrial PhD, supported by Innovation Fund Denmark. The project is motivated by the need to understand the effect of climatic conditions and contamination on the corrosion-related failures and overall climatic reliability of hearing aids. Hearing aids are miniature size devices and during operation, are subjected to various levels of corrosive contamination from the atmosphere and human body along with varying temperature and humidity conditions. All these factors are known for causing failures in electronic devices and their components in the form of SIR reduction, ECM migration and many more. The part of the research conducted in this thesis focuses on developing a detailed understanding of various failure modes, mechanism and its causes through root cause failure analysis of failed hearing aids from different markets throughout the globe. The comprehensive knowledge acquired from field failure analysis of hearing aids was used to further investigate factors causing the corrosion, development of test methods to replicate field failures, and finding optimum corrosion protection strategy for hearing aid devices. The project's overall aim is to develop a more robust strategy for hearing aids in terms of humidity-related issues based on indepth understanding of the failure mechanisms and various factors involved.

Chapter 1 introduces the climatic reliability issues related to hearing aid devices and presents the motivation behind the current PhD project. Chapter 2 provides a comprehensive literature review discussion on the potential factors causing the formation of a water layer on the PCBA (print circuit board assembly) surface and its influence on corrosion reliability of electronics. Furthermore, different types of device and component level test methods used for reliability evaluation of electronics were presented and finally the detail discussion on the corrosion protection of electronics by conformal coating is discussed. A short summary of the literature review and overall objective of the thesis is provided at the end of the literature review.The test results comprise of three appended papers (submitted to an international journal) and two research chapters.

Chapter 3 and 4 (paper 1 and 2) constitutes the statistical and root cause failure analysis of hearing aid devices from tropical, Europe, USA and Japan markets. The result consists of detailed information about the failure mechanisms and its causes for different device components. The most prominent failure cause was potassium hydroxide (KOH) residues coming from the leakage of Zn-air batteries (ZAB’s), which are used as a power source for hearing aids. Therefore, subsequent chapter 5 (paper 3) focuses on investigating the synergetic effect of temperature, humidity and exposure duration on the rate of electrolyte leakage from ZAB’s. Chapter 6 presents different corrosion test methodology and setup developed for mimicking the failures from the field and implementation of the acceleration factors in the tests that were identified from the field failure investigation conducted in chapters 3 and 4. Final chapter 7 investigates the performance of different potential conformal coating candidates under exposure to simulated field climatic conditions and KOH contamination.

Overall, a high amount of corrosion was observed in all the field-failed devices across different markets. Microphones showed the highest failure percentage among all the hearing aid components, while other components like hand solderings, battery contacts, and LED showed high susceptibility and failure probability due to corrosion. The failure cause in the tropical region was dominated by the leakage of electrolyte from ZAB, whereas failures from Europe, USA and Japan markets were dominated due to high amount of sweat and salt ingress into the device.

The leakage of ZAB is influenced by climatic factors (high temperature and humidity) and the duration of its exposure to the conditions. Among the various hearing aid batteries tested, the temperature effect had a more pronounced impact on the amount of KOH released from the batteries. KOH residues showed a very high hygroscopic behavior with deliquescence occurrence at ~ 50% relative humidity (RH) and its impact on the corrosion failure of electronics was evaluated by electrochemical impedance spectroscopy analysis using interdigitated pattern (SIR comb pattern).

Finally, the performance of different conformal coatings as a corrosion protection strategy was evaluated using a Test PCBA board. Test methods consisting of EIS and DC leak current test using a conformal coated interdigitated pattern on the Test board under exposure to climatic cycle (humidity/temperature cycle) and KOH contamination were used for the study. Among all the coatings, acrylate polyurethane-type conformal coatings showed the most promising results with good adhesion and high resistance to water absorption and degradation due to KOH contamination.

Overall outcome from the work is discussed in chapter 8, and major conclusions are listed in chapter 9 along with future perspectives.