Inhibition of cracking in Cu 6 Sn 5 intermetallic compounds at the interface of lead-free solder joint by controlling the reflow cooling conditions

The polymorphic phase transformation that occurs in Cu6Sn5 intermetallic compounds (IMCs) has the potential to generate stresses that lead to crack formation in soldered joints during multiple reflow cycles in the assembly processes and in service. In this paper the authors report a study on the effect of single and multiple reflow cycles on the growth of interfacial Cu6Sn5 and the incidence of cracking in that IMC layer in BGA balls reflowed with two different cooling conditions. The BGA alloy/substrate combinations studied were Sn-3Ag-0.5Cu (SAC305)/Cu and Sn-0.7Cu-0.05Ni-1.5Bi (SN100CV)/Cu. The cooling conditions were (i) direct-cooling in a conventional reflow profile, and (ii) an alternative reflow profile with three extended isothermal holding times of 30, 60, and 180 seconds at 140 °C during cooling. It is found that the alternative reflow condition has the potential to minimise cracking in the Cu6Sn5 IMC layer at the joint interface and thus improve the shear strength of the joint.