Void propensity in solder joints on a single copper pad with a grain size spectrum tuned by strain annealing

The voiding phenomenon at the interface has been proven to weaken the mechanical properties of solder joints. The aim of this study was to investigate the influence of grain size on voiding propensity. Two kinds of substrates were used: high purity oxygen free copper (HPOFC) substrate and electrolytic tough pitch copper (ETPC) substrate. Grain size and voiding propensity were characterized by introducing grain size number (N) and void density (D). Strain-anneal method was conducted on the same single copper substrate to carry out quantitative researches about the relationship between these two parameters. It turned out that no voids were detected on HPOFC substrate while considerable amount of voids formed on ETPC substrate no matter what kind of solder balls were used.