Sip Solder Paste with Superior Slump Resistance via Powder Cluster Mechanism

A patent-pending new solder paste material has been developed for assembly of SiP with outstanding slump resistance without compromising the paste volume printed, and exhibit acceptable solder joint service temperature. In the fine pitch solder paste using medium temperature solder powder (MTSP), low melting solder powder (LMSP) was introduced. At printing, the powder mixture behaved as regular fine-pitch solder paste. Upon heating, the LMSP melted and formed "Powder Cluster" with surrounding MTSP. The Powder Cluster resisted slump, thus avoided solder bridging, consequently enabled high yield of assembly. For both 58Bi42Sn and 52In48Sn LMSP systems used together with SAC305 in solder paste, the desired LMSP content was found to be no less than 10% w/w of solder paste for great slump resistance at 100°C, 150°C, and 200°C, and no more than 15% w/w of solder paste for 1
^st
sign of melting of reflowed solder to be no lower than around 179°C. The slump resistance achieved at 100°C which was lower than the melting temperature of both 58Bi42Sn and 52In48Sn was attributed to solid-state diffusion bonding mechanism. At LMSP content > 10%, slump increased with increasing BiSn LMSP content due to metal volume factor, due to density being higher for 58Bi42Sn than SAC. This was not true for InSn system. 52In48Sn was slightly more effective in suppressing slump than 58Bi42Sn, attributable to the lower melting temperature of 52In48Sn. Higher print thickness and higher heating temperature resulted in a higher slump rate, indicating choice of LMSP and MTSP would affect the potential of slump resistance. Print pattern was found to affect slump resistance through "Paste Crowdedness" factor, which can be used as a tool for assessing the potential of slump during stencil design phase. The overall metal load tested was found to be acceptable at 82% w/w and can be further optimized for better print and non-slump performance.