PhD Defence at DTU Mechanical Engineering

PhD Defence 18th June: Climatic Reliability of Electronics: Early Prediction and Control of Contamination and Humidity effects

Wednesday 27 May 15

Vadimas Verdingovas from DTU Mechanical Engineering defends his PhD "Climatic Reliability of Electronics: Early Prediction and Control of Contamination and Humidity Effects" Thursday, 18th June at 13:00. The defence takes place at the Technical University of Denmark, in Building 308, Meeting room no. 38. Professor Dr. Rajan Ambat is principal supervisor and Dr. Morten Stendahl Jellesen is co supervisor.

Abstract
This PhD project is motivated by the demand for deeper understanding of the effects of humidity and contamination on corrosion related issues and overall reliability of electronic devices. The directions for the research subtopics were to a significant extent guided by the climatic reliability issues the electronic companies are currently facing; however, the information available today is limited and incoherent. Work in this thesis focus on the synergistic effects of process related contamination, humidity, potential bias, and PCBA design related aspects, while various tests methods suitable for electrical measurements are attempted and compared with standardized test methods. The focus is also on the methods for corrosion prediction on a PCBA layout with the aim of profiling corrosion prone regions or simulating the possible humidity effects on a circuit design assuming parasitic circuit due to water layer formation on the PCBA surface.

The chapters 2-5 review the factors influencing the climatic reliability of electronics namely humidity interaction with the PCBA and ionic contamination on the surface, common types and sources of ionic contamination in electronics, the test methods and techniques, and failure mechanisms related to climate and contamination. Chapter 6 summarizes the materials and experimental methods employed. Results of various investigations in this thesis are presented as individual research papers published or in the draft form intended for publication in international journals. Prior to the appended papers, chapter 7 provides a short summary of appended papers with important results and discussion.

The results are summarized in 8 papers, presented in chapter 8-15. Papers 1-3 investigate interaction between ionic contaminants i.e. NaCl, flux residues, WOAs and humidity, and their effects on leakage current, corrosion and electrochemical migration. Paper 4 compares the two types of ionic contamination i.e. NaCl and flux residue in terms of their impact on leakage currents and probability for electrochemical migration, while Paper 5 is focused on electrochemical migration of tin under pulsed voltage. Paper 6 contains a corrosion study of Au-Al wire bonds and Au-Al intermetallics in iodine environments. Paper 7 presents a novel method developed for profiling of tin corrosion on the PCBA surface. Paper 8 focuses on analysing the feasibility of corrosion prediction by circuit simulation using the combined empirical information from above works assuming parasitic circuit formation. Finally, chapter 16 provides an overall discussion, conclusions, and suggestions for future work.

Overall the investigations clearly showed the importance of deliquescence RH and solubility of ionic contaminants in determining SIR and electrochemical migration. This is related to the increase of water layer thickness as a medium for ion transport. Under thin water layers, the concentration of ionic contaminant, bias voltage, and local pH formation are the factors influencing formation of tin dendrites. Investigation of electrochemical migration under pulsed voltage has shown that the reduction of duty cycle prolongs the time to failure. Under these test conditions the precipitation of tin hydroxides is more favourable, and less metal ions get reduced at the cathode. Corrosion study of Au-Al wire bonds in iodine environments has shown that the Al metallization and Al rich intermetallic phases are most susceptible to corrosion. The failure mechanism involves oxidation of Al via formation of Al iodides and consequent formation of Al oxides and/or hydroxides. The method developed for tin corrosion profiling using tin ion indicator gel provides a clear indication of tin corrosion and tin ions movements on the PCBA surface, giving valuable information for corrosion prediction or failure analysis.

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https://www.mek.dtu.dk/english/nyheder/Nyhed?id=%7BEBEFF3F6-FD66-4713-9C63-AB06DA17D163%7D
10 JULY 2020