Effect of cooling rate on growth of the intermetallic compound and fracture mode of near-eutectic Sn-Ag-Cu/Cu pad: Before and after aging

Several near-eutectic solders of (1) Sn-3.5Ag, (2) Sn-3.0Ag-0.7Cu, (3) Sn-3.0Ag1.5Cu, (4) Sn-3.7Ag-0.9Cu, and (5) Sn-6.0Ag-0.5Cu (in wt.% unless specified otherwise) were cooled at different rates after reflow soldering on the Cu pad above 250degreesC for 60 sec. Three different media of cooling were used to control cooling rates: fast water quenching, medium cooling on an aluminum block, and slow cooling in furnace. Both the solder composition and cooling rate after reflow have a significant effect on the intermetallic compound (IMC) thickness (mainly Cu6Sn5). Under fixed cooling condition, alloys (1), (3), and (5) revealed larger IMC thicknesses than that of alloys (2) and (4). Slow cooling produced an IMC buildup of thicker than 10 mum, while medium and fast cooling produced a thickness of thinner than 5 pm. The inverse relationship between IMC thickness and shear strength was confirmed. All the fast- and medium-cooled joints revealed a ductile mode (fracture surface was composed of the beta-Sn phase), while the slow-cooled joints were fractured in a brittle mode (fracture surface was composed Of Cu6Sn5 and Cu3Sn phases). The effect of isothermal aging at 130degreesC on the growth of the IMC, shear strength, and fracture mode is also reported.