Cooling rate, pad finish effects on mechanical behavior of SnAgCu alloys

Microstructure of lead free solder joints possesses a complex morphology owing to nonequilibrium solidification [16]. We reported the creep and monotonic behavior of Sn-Pb, Sn3.8Ag0.7Cu, Sn3.0Ag0.5Cu, and Sn1.0Ag0.5Cu alloys in our earlier research [2, 3, 10, 12, 18]. The behavioral impact of the microstructrual changes due to pad finish and cooling rate on micron-scale SnAgCu joints were studied to a limited extent in the available literature [9, 17]. Critically, the intermetallics formed during assembly vary in composition and structure due to reflow cooling rate and pad finish. In this paper, we study the impact of cooling rate and pad finish on three alloys formed by the mixing during assembly of: Sn3.0Ag0.5Cu (SAC305) paste with Sn1.0Ag0.5Cu (SAC105), Sn3.0Ag0.5Cu and Sn3.5Ag (SA3.5) ball (33-66% ratio of paste to ball material by volume). We examine the effect of pad finish on the microstructural disintegration near the pad and demonstrate its effect on the monotonic behavior observed. In addition to pad finish, we investigate the role of faster reflow cooling rate on the mechanical response by evaluating 1 °C/min and 6 °C/min reflow cooling profiles. For this, a comprehensive series of monotonic tests 24 in all on all the three alloys with Electroless Nickel Gold (NiAu), Organic Solderability Pad (OSP) and Solder on Pad (SOP) pad finishes were conducted. SOP/SOP pad finish has the highest saturation stress while that of OSP/OSP has the lowest for all the three alloys. It was observed that at higher cooling rates, there is a greater resistance to deformation. In particular, cooling rate has significant effect on mechanical behavior of joint with OSP/OSP pad finish.