Necking of low-bump-height solder induced by reactive wetting of solder on Au finishes in print circuit board

We report a necking reliability issue in low-bump-height solder induced by reactive wetting of solder on Au finish in print circuit board. During reflow and solid state aging, the solders diffused to the Au finishes on the print circuit boards and reacted with Au to from Au-Sn compounds. Because the solder volume is very limited, this reaction took a lot of percentage of solders and caused necking of the solder joints. On the other hand, the joints without the Au layers have less necking problem. The necking will have detrimental effect on the mechanical properties of the solder joints. Therefore, it will be a serious reliability issue for flip-chip solder joints with low bump heights and microbumps in 3D IC.