Improving robustness and performance of Li-ion systems for hearing aids

This Industrial PhD project was a collaboration between the CELCORR research group at DTU and WS Audiology, supported by Innovation Fund Denmark. The objective of the project was to enhance our understanding of the various factors that influence climatic reliability and improve the robustness of Li-ion systems used in hearing aids. Hearing Aids (HA) are low-power microelectronic devices used worldwide. The ongoing trend in the HA industry has been to move towards Li-ion battery technology replacing conventional zinc-air batteries. The corrosion reliability of HA devices is a critical issue due to their exposure to harsh user environments, including body fluid contact, high humidity, and temperature. The combined effect of higher operating voltage (4.3V) and design miniaturization poses new risks for corrosion reliability.

Chapter 1 introduces the climatic reliability challenges in hearing aid devices and the motivation behind the current PhD project. Furthermore, it presents the scope and structure of the PhD project. Chapter 2 reviews literature on the main risk factors influencing corrosion in electronics, such as humidity, water layer formation, and contamination. Furthermore, discussing the different failure mechanisms, along with testing methods and corrosion protection with conformal coatings. Lastly, providing insights into the current reliability challenges related to Li-ion batteries as well as degradation mechanisms and testing methods. Because of the nature of the thesis “Materials and methods” chapter is not included, but it is part of each individual chapters/papers. A short summary of the literature review and the overall objective of the thesis is provided at the end of the literature review. The four following chapters, i.e., appended papers, form this thesis's body and results section.

To fully understand the environmental reliability issues of the Li-ion powered HA, it is crucial to investigate field performance by conducting root cause failure analysis on the field-returned failed devices. Therefore, the study in Chapter 4 investigated the field failures in two different HA models (open vs. glued model). The analysis used a systematic FMEA-based approach to identify and understand failure modes, mechanisms, and potential causes using various analysis techniques (e.g., LOM, SEM&EDS, and ICP-OES). Furthermore, statistical analysis was conducted based on the repair data created by the global service center to reveal the failure percentage and time to failure of different components.

Chapter 5 addresses the electronic corrosion mitigation strategies by evaluating the performance of various vapor-deposited conformal coatings and comparing them to the currently used liquid-based conformal coating, which has been shown to provide insufficient protection for the components on the hearing aid FPCBA. Chapter 6 investigates corrosion on the galvanic charging contacts and introduces a method for accelerated corrosion reliability testing of these charging contacts. Chapter 7 analyzes failed lithium-ion batteries used in hearing aids, using various electrochemical techniques and imaging methods. It aims to connect the corrosion failures found on the hearing aid's FPCBA with the identified issues in the batteries.

Based on the results, Passive components, solder connections, and galvanic charging contact showed the highest failure percentages among the field failed HA devices. The most common causes of corrosion failures were high levels of chloride ions, high humidity, flux residues, and insufficient coating protection. A conformal coating study showed that vapor deposited conformal coatings significantly improved corrosion protection compared to liquid-based reference coating, with longer time to failure and reduced failure rates. The findings highlight the effectiveness of vapor deposited conformal coatings against humidity and sweat, suggesting their potential for enhancing the reliability of microelectronic devices like hearing aids. The corrosion reliability study of charging contacts identified that artificial sweat, humidity, and potential bias were the primary factors leading to serious corrosion failures on charging contacts. The plunger in the charger was the most vulnerable point for corrosion due to thin gold plating and 5V potential bias. Lastly, the failure characterization of the field-failed batteries showed a significant reduction in the discharge capacity compared to new batteries, even after less than two years of use. This accelerated degradation is likely due to external factors, such as corrosion on the device's circuit board, leading to higher discharge rates and increased stress on battery components. Furthermore, high discharge rates (2C to 10C) and deep discharges (below 3V) accelerated irreversible damage, aligning with field observations of rapid capacity fade.

Chapter 8 discusses the general outcome of the PhD work, and chapter 9 summarizes the overall conclusions. Chapter 10 prove some future perspectives to the work.