Effect of pad surface finish and reflow cooling rate on the microstructure and the mechanical behavior of SnAgCu solder alloys

Abstract

The microstructure of Pb-free solder joints fabricated with SnAgCu alloys possesses a complex morphology owing to nonequilibrium solidification of Sn. The microstructure also depends on variables such as pad surface finish as well as process conditions during solder reflow. Specifically, the Ag3Sn intermetallics formed during the assembly vary in composition and structure due to pad finish as well as reflow cooling rate. An understanding of the behavioral impact of the microstructural changes due to pad finish and reflow cooling rate on micron-scale SnAgCu joints is an important need at the present time. In this paper, we study the impact of pad finish and cooling rate on three alloys formed by the mixing during assembly of Sn3.0Ag0.5Cu paste with Sn1.0Ag0.5Cu, Sn3.0Ag0.5Cu or Sn3.5Ag balls (with 33% of paste material and 67% ball material by volume). We examine the effect of pad finish on the microstructural disintegration near the pad and correlate the microstructural change to the observed behavior under constant strain rate loading. For this, a comprehensive series of constant strain rate 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. We also investigate the role of faster reflow cooling rate by evaluating the mechanical behavior of the solder joints resulting from 1°C/min or 6°C/min reflow cooling profiles. The NiAu–OSP pad finish yielded the highest saturation stress during constant strain rate tests while the saturation stresses of OSP–OSP, SOP–OSP combinations were comparable to each other. Also, joints formed using reflow profiles with higher cooling rates exhibited a greater resistance to deformation. In particular, the cooling rate has a significant effect on mechanical behavior of a joint with OSP–OSP pad finish.