Effects of multiple reflows on interfacial reaction and shear strength of SnAgCu and SnPb solder joints with different PCB surface finishes
The effects of multiple reflows (1, 2, 4, 6, 8, 10 numbers of reflow) on interfacial reaction and shear strength of Sn3.8Ag0.7Cu (SAC) and Sn37Pb (SP) joints were systematically investigated using electrolytic Ni/Au and organic solderability preservative (OSP) finished pad. In the case of OSP finished substrate, the well-known intermetallic compound (IMC) Cu6Sn5 was observed for both solders. While for the Ni/Au finished substrate, the IMC Ni3Sn4 and (CuNi)6Sn5 were formed on the interface of SP and SAC joints, respectively. With the increasing number of reflows, the thickness of IMCs on all reaction couples increased gradually and the IMC grain coarsened. The IMC growth kinetics showed that the interfacial IMC Cu6Sn5 was controlled by grain boundary diffusion and that the IMCs (Ni3Sn4 and (CuNi)6Sn5) near Ni layer had interactive diffusion control of grain boundary and volume diffusion. Moreover, IMC growth at Cu interface was faster than that at Ni interface, irrespective of the solder type. Shear test results showed that the shear force did not change much until eight numbers of reflow for each reaction couple. The ductile fracture indicated that the shear strength of solder joints could not be significantly influenced by the thickness and morphology of the interfacial IMC.
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