Water film formation on PCBA surface - Investigation of aspects contributing to premature corrosion failures and safety measures for electronics reliability improvement

Abstract

This PhD work was supported by the Innovation Fund Denmark through the IN-SPE (Innovation Consortium for Sustained Performance of Electronics) and by CELCORR (Centre for Electronic Corrosion) through the CreCon (Consortium for Climatically Reliable Electronics). The project is motivated by the climatic reliability issues that are faced by the electronics industry, which are related to the intermittent and permanent corrosion failures induced by the water layer formation on the PCBA surfaces. The demand for deep understanding of the water film build-up phenomenon on the electronic surfaces led to the investigations of synergistic effects of the related factors such as: materials’ characteristics, manufacturing/process effects, process cleanliness, PCBA design, potential bias, and climatic variations such as changes in humidity and temperature. The assessment of the impact of these parameters was performed under accelerated testing (condensing conditions) and under the conditions relevant for the electronics operation, while also simulating the geographical changes in climate parameters worldwide. Number of material characterization and electrochemical testing methods were used for understanding the relationships between various parameters in determining the water film build-up on electronic surfaces and associated failures on a PCBA surface. Chapter 1 provides an introduction to the climatic reliability issues and a motivation of the current PhD project. Chapter 2 reviews various factors influencing the water layer formation on the PCBA surface (e.g. humidity, surface characteristics, ionic contamination, temperature etc.) and the effect on corrosion reliability. Additionally, common failure mechanisms related to the mentioned factors are described together with the climatic reliability test methods. Chapter 3 summarizes the literature review and the objectives of this PhD project. Chapter 4 summarizes the materials and test methods employed in this thesis. The results of investigations are presented in the form of individual research papers published in peer reviewed journals or in the form of draft intended for publication in the journals. Chapter 5 contains a summary highlighting the objectives and conclusions of each paper. The research results are summarized in 7 papers constituting chapters 6-12. Paper 1 (chapter 6) investigates the chemistry and morphology effect of the PCBA surface on the water film formation. Papers 2 and 3 (chapters 7 and 8) focus on the thermal decomposition of no-clean solder flux residues under simulated soldering conditions and related reliability issues. Papers 4-6 (chapters 9-11) investigate the interactions between no-clean solder flux residues and humidity under various temperature conditions, and the effects on the electrical performance and corrosion occurrence. Paper 7 (chapter 12) presents the effect of PCBA cleanliness under isothermal and non-isothermal conditions developed between the PCBA surface and outdoor, as well as under typical climatic profiles, and the related failure occurrence in electronics. Finally, chapters 13-15 provide the overall discussion, conclusion, and suggestions for future work. Overall, the investigations showed the importance of PCBA laminate/solder mask surface chemistry and topography on the water film build-up and related failure occurrence due to the electrochemical process that may occur under humid and condensing conditions. Results show that with an optimized materials properties and surface characteristics, an extension of time-to-failure might be obtained upon the device exposure to transient condensing conditions. The presence of ionic post-production contamination on the PCBA surface is often inevitable due to the thermal characteristics of a conventional wave soldering process and the use of no-clean flux technology. The acidic activators used commonly in the flux formulations do not decompose easily under the soldering conditions and often remain on the PCBA surface after the manufacturing process in the form of acids or anhydrites, highly susceptible to interact with moisture. The process-related residues reduce the upper humidity boundary for water film formation on the PCBA surface and enhance the moisture adsorption processes. The contamination effect is more pronounced if the residues comprise of hygroscopic species or mixtures of contaminants, and under high temperatures, where the water film thickness is higher and favours the significant SIR reduction and the electrochemical migration. The types and importance of flux chemistries are investigated in detail, which shows the importance of activator chemistry in the no-clean flux system in determining the final residue compositions after the thermal treatments, its amount, and humidity-related corrosion failures in electronics. The electrical properties of the water layer formed under transient conditions originate from the characteristics of the conductor bridging and electrochemical process at the electrodes (faradaic reactions and ion transport through the water layer), which were studied in detail in chapter 12. The extent of water film build-up is strongly dependent on the ambient humidity level, differential temperatures between ambient and PCBA surface, rate of the temperature change, cleanliness of the PCBA, and the PCBA design and enclosure characteristics. Even under potentially non-condensing conditions, the formation of a significant amount of water film can occur if highly hygroscopic residue remains on the PCBA surface due to the altered (reduced) humidity threshold for failures. The extent of water layer build-up is dictated by the ionic nature of the residues. Under condensing conditions, the presence of highly hygroscopic flux residues accelerate the formation of a continuous water layer and conductor bridging, leading to the risk of failure occurrence. The presence of thermal mass attached to the PCBA enhances the local moisture condensation and prolongs the wetting time of electronics, and this effect is further enhanced if hygroscopic residues remain on the PCBA surface.