PCBA design strategies for increasing intrinsic humidity robustness and developing predictive measures

This PhD project was part of broader research projects called ELMAC (ELectronics MAnufactured for Climate) project and CreCon consortium (Industrial Consortium for Climatically Reliable Electronics) which focused on developing knowledge to combat climate effects on electronics resulting in corrosion issues and functional failures. The motivation of the research work that forms the body of the project is the need for holistically combining the existing understanding of PCBA surface water film formation under humidity exposure, and of hygroscopic residue effect arising from solder flux residues especially from reflow process, and extending it to the domain of various component and board design parameters. This is of great importance as PCBAs are becoming increasingly complex and miniaturized with higher component density and smaller components. This also included developing a generic electrochemical model based on secondary current physics to predict leak current formation on PCBA surface under condensing conditions. Investigation of these factors together created a deeper understanding of the synergy between PCBA design parameters, reflow soldering process and residue formation, along with solder mask surface effects on moisture absorption and water film formation, which lead to corrosion failures. Effect of no-clean reflow solder flux chemistry (activators, tackifier polymers, binder etc.) and component-specific (QFN, BGA etc.) PCBA design effects on climatic reliability was investigated. Experimental methods include both DC and AC electrochemical methods and materials characterization methods such as SEM, EDX, LOM and X-ray imaging.

Chapter 1 provides a general background to the work. Chapter 2 talks about the motivation and scope of the thesis. Chapter 3 contains the structure of the thesis. Chapter 4 reviews and summarises available and pertinent literature and provides an aim of the thesis work. Chapter 5 summarizes different materials, test PCB structures, and experimental methods employed in various research activities. Chapter 6 provides a short summary of appended papers showing key objectives and findings from each of the 4 appended papers that make up the body of research work. The full details of these studies are contained in chapters 7-10 which are presented as full papers, either published in journals after peer-review or as ready manuscripts to be submitted to peer-reviewed journals.

Among appended papers, Chapter 7 focuses on the effect of condensed water film formation morphology on PCBA surface on leak current formation and electrochemical migration including a preliminary COMSOL-based model to predict droplet morphology effects on electric field distribution. Chapter 8 provides an insight into the contribution of Weak Organic Acid (WOA) contamination to connector pitch distance as a driving force for dendrite formation due to ECM. Chapter 9 studies the impact of reflow solder flux system chemistry on PCBA climatic reliability, with a special focus on the binder/tackifier polymer choice and WOA activator. Chapter 10 carries forward the findings from Chapter 9 to characterize and investigate the interaction between PCBA component design parameters and reflow flux system properties. Chapters 11, 12 and 13 respectively provide an overall discussion combining findings from appended papers, major conclusions from the present study, and suggest avenues of future research.

The study of water droplet morphology (Chapter 7) yielded an important finding; complete coalescence of droplets between two oppositely biased connectors is essential for dendrite formation. The inter-droplet thin water layer provides a much higher resistance than the larger droplet, thus acting as a deterrent to mass transport. However, PCBA processing during manufacturing and assembly leaves residues, the largest contribution of which comes from soldering residues. The study of process contamination and PCB layout (Chapter 8) compared the effects of 2 types of WOA residues on leakage current and impedance of a realistic PCB layout, by way of measuring water film growth, leakage current and the path of dendrite growth (if applicable). This study confirmed existing knowledge of Glutaric acid being more aggressive than Adipic acid and added an understanding of alteration (shortening) of dendrite growth path to include PCB conductive pads that have interconnections to, and lay in the vicinity of the original pads in consideration. A protective/encapsulating effect of reflow solder flux residue (usually a hydrophobic polymer) was studied as well, albeit over a shorter exposure period. Modern reflow solder fluxes are almost always of the no-clean variety, which means the encapsulant/binder keeps large amounts of activator acids contained within the matrix, and on the board surface. The study of reflow residue interactions with humidity (Chapter 9) investigated the behavior of different polymer chemistries and WOA mixtures of 5 solder paste systems trapped under a lab-scale component, that could experience denaturation, spreading and moisture uptake to bridge connectors with a conductive water layer due to harsh climatic conditions. A key finding was that the nature of polymer determines the release of contained WOA mixtures onto the PCBA surface, which then determines the rate of reduction of Surface Insulation Resistance of the system due to electrochemical process. The lab-scale component however had a much larger standoff height and connector pitch distance, and a simpler connector layout than real components, which led to the study in Chapter 10 of reflow flux-realistic component design parameter interaction in humid conditions. This study chose low-standoff components with small connector pitch distances (QFN, QFP, FBGA and capacitors), and studied the combined effects of different component design parameters and 3 different solder paste chemistries in a harsh climate profile, based on the knowledge gained from the previous study. Principal observations were the trapping of residues under components due to low standoff height, and certain connector features like thermal lugs. Warpage and tilting of the components were additional issues experienced based on the ability of flux systems to adapt to thermal gradients generated during the reflow process. The findings showed that all these factors influence humidity related reliability issues.