Interfacial Reactions, Microstructure and Mechanical Properties of Pb-Free Solder Joints in PBGA Laminates

Sn-based alloys have been developed as Pb-free solder candidates to replace the Pb-containing solders used in microelectronic applications. However, their high Sn content and high melting point often cause excessive interfacial reactions, namely, dissolution of surface finish layers and concomitant formation of intermetallic compounds at the soldering interface. These interfacial reactions can therefore influence the microstructure and mechanical properties of the solder joints and eventually their reliability. Hence, the choice of a proper surface finish layer in printed circuit boards is an important issue in successfully introducing the Sn-based, Pb-free solders. In this study, the effects of surface finish layers and multiple reflows on the BGA (Ball Grid Array) solder joints have been investigated. A Pb-free solder alloy, Sn-Ag-Cu has been employed as the solder ball material. Five types of surface finish including combinations of Cu, Au/Ni, Au/Pd/Ni on opposite sides of the BGA balls, have been investigated. Intermetallic compound formation was measured as a function of reflow cycle for each surface finish. The effects of the interfacial reactions on the microstructure and mechanical properties of the solder joints were also investigated as a function of surface finish and reflow cycle.