Improvement on interfacial structure and properties of Sn–58Bi/Cu joint using Sn–3.0Ag–0.5Cu solder as barrier

Sn–58Bi solder recently attracts much attention for low temperature soldering, but is also restricted by the degradation on Sn–Bi solder joint from the interfacial Bi embrittlement. This study introduced a thin solder layer with the composition of Sn–3.0Ag–0.5Cu (SAC) on Cu pad to improve the interfacial structure and shear properties of Sn–58Bi solder joints under soldering and the subsequent aging conditions. SAC solder layer with the thickness about 5 μm was coated on Cu pad using Hot Air Soldering Leveling (HASL). During soldering condition, the thickness of interfacial intermetallic compound (IMC) layer in Sn–58Bi joints increased with the soldering times, and there was no obvious difference on the bare and SAC-coated Cu. However, during aging condition, the growth on interfacial IMC layer was effectively retarded by the SAC coating compared to their bare Cu counterparts. SAC-coated samples that were reflowed with varying soldering times and then aged also showed a thinner IMC layer. The retarding effect on IMC growth from SAC-coated Cu pad was attributed to the reason that SAC solder on Cu produced an initial Cu₆Sn₅ layer with fewer scallops, fewer grain boundaries between scallops, and therefore fewer diffusion pathways for Cu atoms to diffuse into the solder and react to grow the intermetallic layer. Ball shear tests also showed that aged SAC-coated joints displayed higher shear strength and through-solder ductile failure while Sn–58Bi on bare Cu pad displayed lower shear strength and brittle failure through the interfacial IMC layer. Therefore, SAC Pb-free solder can be recommended as the surface finishing on Cu pad to improve the performance of Sn–58Bi joints because it can be easily realized by HASL method in electronic industry.