Effect of under bump metallurgy and reflows on shear strength and microstructure of joint between Cu substrate and Sn-36Pb-2Ag solder alloy

Abstract

The interfacial reaction between Sn-36Pb-2Ag (numbers are all in mass% unless specified otherwise) solder balls and Ni, Ag and Ni/Ag electroplated on a Cu substrate was investigated and the joint bonding strength was measured using a ball shear tester. The intermetallic Ag(3)Sn was observed at the interface of the Cu/Ag/solder joint and Ni(3)Sn(4) was found at the Cu/Ni/solder joint. In the case of Cu/Ni/Ag substrate, the layer sequence was observed to be Cu/Ni/Ni(3)Sn(4)/Ag(3)Sn/solder. The Ag layer was completely consumed by formation of Ag(3)Sn but the Ni layer remained. Environmental tests showed that the Cu/Ni/Ag substrate retained better solder joint reliability than either Ni or Ag single plated Cu substrate. Two types of reflow profiles were tested and the specimen reflowed by a higher temperature profile showed a higher solder joint strength. Solder joint strength and microstructural change were observed with several reflow cycles in considering the real board mounting conditions. There was significant evolution of solder and joint microstructures with reflow cycles and it explained well the change of solder joint strength.