Implementation of Electrochemical Impedance Spectroscopy (EIS) for validation of humidity robustness of PCBA design elements

The motivation of this PhD project is the issue of climatic reliability testing of electronic devices with increasing complexity, with a focus on automotive electronics. Today’s methods for assessing humidity and temperature induced failure modes are based on techniques that impose a DC potential to the Printed Circuit Board Assembly (PCBA) test specimen. While these approaches detect the occurrence of corrosive failures like dendritic growth due to electrochemical migration (ECM), they fail to distinguish the steps leading up to the failure. For evaluating the reliability of electronic assemblies, this impedes detailed knowledge on the cause-effect relationship of all influencing factors. The main focus of this work is to assess Electrochemical Impedance Spectroscopy (EIS) as an alternative non-destructive testing method, overcoming the drawbacks of DC testing. The primary purpose of the EIS approach is to trace the formation of a thin water layer on the surface of a PCBA. As EIS can distinguish between different resistive and capacitive processes, and states on the PCB under certain temperature and humidity conditions, water film formation can be monitored. This water film is the prerequisite for ECM to occur and the cause for various other moisture related PCBA failures and thus, understanding and controlling water film buildup can significantly improve the design for reliability already in the early stages of electronics development.

Chapter 1 provides an introduction into the topic of climatic reliability testing and scope of the work conducted within this PhD project. In chapter 2, the basics of automotive electronics development are summarized and the relevant humidity related aspects are covered. In addition, humidity related failure mechanisms in the PCBA context are described and the methodologies of climatic reliability testing are explained. Chapter 3 summarizes the literature review in the light of the purpose of the PhD project. Chapter 4 summarizes the materials and methods used as part of the different activities in the project. Various results from the research work are represented in the form of manuscripts, meant for publication in international journals. Chapter 5 summarizes each of the papers with the most important objectives and findings, before the complete papers are attached in the following chapters 6-9. In Paper 1, the water layer buildup on a PCB laminate surface is traced by EIS measurement and related to the occurrence of dendrite shorting upon using a DC based approach. In Paper 2, a measurement approach of alternating EIS and DC testing is used to provide a detailed allocation of sequential water layer buildup and Electrochemical Migration. Paper 3 deals with the localization of enhanced condensate formation on PCBA surfaces due to a temperature profile or the presence of contaminants. Impedance Spectroscopy and Time Domain Reflectometry are employed to assess the influence of these parameters. In Paper 4, an extensive evaluation of the EIS technique in terms of feasibility to determine climatic reliability is attempted. This includes a review of literature information as well as comprehensive evaluation of the results from Papers 1-3 and additional evaluation with the point of view of feasibility of using EIS as a complementary approach to DC testing. Chapter 10 and Chapter 11 provide a general discussion and conclusion of the work and in Chapter 12; suggestions for future work are given.

Overall, the results from the investigations show the importance of appropriate climatic reliability testing methods. It was demonstrated that EIS as an alternative approach to the conventional DC-based SIR (Surface Insulation Resistance) method is a powerful method for assessing humidity robustness of PCBAs. The EIS method is not only non-destructive to the test specimen, but it was also shown that it can detect critical humidity states prior to signal changes in the DC. By comparing with the conventional SIR measurement approach, contrasting both methods clearly proved the general feasibility of using EIS as predictive tool for corrosion failures. A comprehensive understanding regarding the contribution of the PCBA design parameters, like surface properties and contamination impact, as well as climatic test conditions individually and combined could be obtained. It was shown that delivering a cause-effect relationship of the failure mode by EIS measurement and evaluation of resistive and capacitive system properties is possible. The condensation process and buildup of a water film and thus the prerequisite for corrosive processes like ECM can be determined by EIS threshold values without actually producing the failure. In this scope, a long time future investment in the EIS technique as a standardized method for PCBA humidity robustness evaluation is advisable.