Interface Reactions and Shear Strength of Lead-Free Sn-3.5Ag Solder with Ni-W-P Metallization

Conventional electroless Ni-P metallizations have been found to be inadequate for providing long term protection of the underlying metallization due to their P content, which leads to complicated interfacial reactions. The use of lead-free solder accelerates interfacial reaction because its Sn content is higher than that in conventional Pb-Sn solders. It is believed that codeposition of W into the Ni-P can effectively retard Ni-P precipitation and Ni crystallization, and thus enhance the solder joint reliability. In this study, the microstructure and shear joint strength of electrolytic Ni, electroless Ni-P and electroless Ni-W-P metallizations were investigated. All 3 metallizations were subjected to reflow soldering followed by thermal aging, and finally ball shear tests. It was found that electrolytic Ni and electroless Ni-W-P behave similarly in terms of IMC thickening rates and ball shear strength, and both have outperformed Ni-P.