Inhibition on the growth of interfacial IMCs in Sn-15Bi molten solder with Ni or Co substrate under temperature gradient

Abstract

We reported to use different substrate pad to suppress the atomic migration and the growth of interfacial intermetallic compounds (IMCs) in Sn-15Bi molten solder joints under thermomigration, and investigated the thermomigration behaviors with copper as cold substrate but altering the hot substrate with copper, cobalt or nickel. A fin structure on Cu cold substrate was designed to magnify the thermomigration in solder matrix. Finite element simulation showed that Cu hot substrate exhibited the largest temperature gradient compared with Ni and Co. Under the combined effect from temperature gradient and chemical potential, more substrate atoms diffused through molten Sn-15Bi and enrolled into the interfacial reaction. Compared with the interfacial Cu₆Sn₅/Cu₃Sn IMCs in Cu/Sn-15Bi/Cu joints, (Cu,Ni)₆Sn₅ was observed at the interface in joints with Ni hot substrate, and (Cu,Co)₆Sn₅/(Co,Cu)Sn₃ were at the interface in joints with Co hot substrate. The thermomigration promoted the asymmetrical IMCs' growth in joints with the hot substrate of Cu or Ni, but compared with Cu hot substrate, Ni hot substrate can obviously retard the thermomigration induced interfacial IMCs' growth on the cold substrate. In Cu/Sn-15Bi/Co, the interfacial asymmetrical IMCs’ growth at the cold substrate was completely suppressed with fast growth of Cu-Co-Sn IMCs at the hot Co substrate because of the slow diffusivity of Co in Sn compared with Cu or Ni in Sn. Co substrate provides better thermomigration resistance than Cu or Ni.