A Novel Bi-Free Low Temperature Solder Paste with Outstanding Drop-Shock Resistance

Durafuse™ LT paste was designed as an alternative low temperature solder targeting at enabling hierarchy design for portable devices with good temperature cycling and superior drop-shock performance. It is formulated with the mixed solder powder technology to allow the paste reflowable at around 200°C or above. Durafuse™ LT formed solder joints with a melting temperature above 189°C and a plateau type reflow profile was recommended. Impact of reflow profiles on both drop shock and temperature cycling performance was investigated. The drop number increased with increasing peak temperature up to 210°C, presumably due to an improved homogeneity of solder joint. It also increased with increasing plateau time at peak temperature for temperature below 210 °C. At 210 °C, the drop number decreased with increasing plateau time, presumably due to increasing IMC thickness at pad interface. Homogeneous solder joints were more resistant to drop failure than mixed alloy system, and could be attributed to better homogeneity for the former system. Regardless of reflow profile impact, Durafuse™ LT showed at least two-order-of-magnitude higher drop shock resistance than Bi-Sn-Ag solder. Using the optimized reflow profiles, Durafuse™ LT outperformed SAC305 in drop-shock test. For LGA with homogeneous solder joint, Durafuse™ LT exhibited a characteristic life about 2.5X of BiSn0.4Ag. For chip resistor reliability in TCT, no significant difference in reliability could be discerned when comparing ENIG with OPS. Durafuse™ LT was slightly lower in reliability than SAC305. Higher peak temperature resulted in a better reliability for Durafuse™ LT. The chip size appeared to be an insignificant factor.