Epoxy Flux Prevent Hot Tear at VIPPO Solder Joints

VIPPO design enabled a better signal quality and speed, but it also suffered side-effect caused by this design. At VIPPO location with or without a solid-Cu-hole-fill, the coefficient of thermal expansion (CTE) typically around 17ppm/°C which is quite lower than that of non- VIPPO pads nearby, which is around 45ppm/°C. For double-sided PCB, at ramp-up stage of the second reflow, the difference in CTE of neighboring pads would result in excessive tension on the joint on VIPPO pads, and consequently resulted in Hot-Tear (HT) before reaching melting point of solder joints. Epoxy Flux (EF) was developed to eliminate HT problem. EF dip process did not work due to insufficient EF volume to bond BGA strongly to PCB to nullify the ΔCTE factor. EF dispense process wet well, exhibited low voiding, and allowed sufficient EF volume to fill the gap between BGA and PCB, thus enabled a strong bonding force to nullify the ΔCTE factor, and consequently eliminated the HT defect. The EF developed had the epoxy stress balanced around the solder joints and avoided joint-drifting and solder-extrusion problem. EF dispense process was recommended to be used without solder paste when the BGA was to be assembled. It cured fully in the reflow process, and was non-tacky on touch, and also met IPC SIR requirement. Commercial underfills tested did not work on preventing HT from happening, due to the joint-drifting and solder-extrusion problem. Edge bond or corner bond was also expected to be unable to prevent HT defect. Compared with other polymeric reinforcement materials, EF dispense process minimized the process steps hence was a lower cost valid solution than other reinforcement approaches.