Effect of reflow time on interfacial reaction and shear strength of Sn–0.7Cu solder/Cu and electroless Ni–P BGA joints

Abstract

The interfacial reaction and shear strength between the eutectic Sn–0.7wt.% Cu solder and two different kinds of ball grid array (BGA) substrates (Cu and Au/Ni–P/Cu) during reflow at 255°C for up to 30min were studied. With the solder joints between the Sn–0.7Cu solder and Cu substrates, a Cu6Sn5 intermetallic compound (IMC) was formed at the interface. In the case of the electroless Ni–P substrate, the IMC formed at the interface was mainly (Cu, Ni)6Sn5. Also, a P-rich Ni (Ni3P) layer was observed as a by-product of the Cu–Ni–Sn reaction. The Ni3P layer was between the (Cu, Ni)6Sn5 IMC and the electroless Ni–P layer. The thickness of the Cu6Sn5, (Cu, Ni)6Sn5 and Ni3P layer was found to increase with the reaction time. The growth of the Cu6Sn5 IMC layer for Cu substrate was faster than that of the (Cu, Ni)6Sn5 IMC layer for electroless Ni–P substrate. The Ni content in the (Cu, Ni)6Sn5 IMC layer formed at the interface increased with increasing reaction time. In the ball shear tests, the shear strength value did not change much as a function of reflow time. In all samples, the fracture mainly occurred in the bulk solder. However, for the Sn–0.7Cu/electroless Ni–P joint reflowed for 10min, the fracture occurred in ductile and slightly brittle mode.