Effects of Solder Reflow Peak Temperature on Sn3.5Ag to Cu-OSP Surface

This study investigate the effects of the reflow peak temperature on IMC(intermetallic) between Sn3.5Ag solder ball with Cu-OSP (organic solderability preservatives) pad. The morphologies of Cu6Sn5 grains formed at the interface between Sn-3.5Ag (wt.% unless otherwise specified) and OSP pad were studied in this work. Reflow profiles were performed for 40 sec at peak temperatures of 503 K, 508 K, 513K and 518K. Morphologies of interfacial Cu6Sn5 grains were observed by scanning electron microscopy (SEM). The results showed uniform IMC layer and smaller grain size at peak temperatures of 503 K and 508 K. However the IMC layer grew sharply and became prism type at peak temperatures of 518K. SMT Reflow profile was set up in order to simulate the SMT process. It was conducted to evaluate the mechanical strength of IMC. The IMC layer thickness increased after 2X reflow(T0 reflow + SMT reflow) for all group samples, and the continuity of the IMC layer samples with 503K and 508K was better compared to other groups. The prismatic IMC at 513K and 518K after 2X reflow was more obvious. IMC crystal shear strength was tested by ball shear, HSS(high speed shear) and scrape test. In-line process data including solder ball shear strength, solder ball shear strength after 2X reflow and HSS strength data can all meet specification and all failure mode is normal. The solder ball shear data of T0 and after 2X reflow samples showed no significant difference. The mean value of HSS data of samples with 508K was higher than other groups. Scrape test result showed no crack ball for all 4 groups at T0. After 2X reflow, samples with 503K and 508K perform better than the samples with 513K and 518K. Based on the IMC and mechanical performance, the peak temperature of 508 K was selected as the optimal profile. BL-Drop test and BL-TC were performed for samples with 508K and the result passed. The 1st drop failure was at 58 drops and the failure mode was not at component side. First failure of BL-TC was at 2009 cycles.