Study of drop-performance improved lead-free solder by PCB pad finish

This study aimed to evaluate the drop reliability using Ag content and metal additives in a Sn-Ag-Cu system composition. In this study, the main compositions of the solder ball were Sn1.0Ag0.5Cu and Sn0.5Ag0.5Cu, respectively, and Ni and Co elements were additions a additive element. The PCB plating was used Cu-OSP, immersion Sn, electroplated Ni/Au, and ENEPIG (electroless Ni electroless Pd immersion Sn). The drop properties of Sn0.5Ag0.5Cu and Sn1.0Ag0.5Cu-Ni were improved by about 20% and 15%, respectively compared to that of the Sn1.0Ag0.5Cu solder ball. The Sn1.0Ag0.5Cu-Ni solder ball improved by about 15% compared to the Sn1.0Ag0.5Cu solder ball. These results are because the absorption of drop impact is increased due to the modulus and the yield strength of the solder ball are decreased by lower Ag content and Ni additives. Generally the interface IMCs between the Cu-UBM and Sn-Ag-Cu solder ball is a Cu₆Sn₅. But the Cu₆Sn₅ IMC is changed to (Cu,Ni)₆Sn₅ IMC, and the growth rate of the IMC is decreased by a Ni additive. The interface IMCs between Ni-UBM and the solder ball were (Cu,Ni)₆Sn₅, regardless of solder composition and additives. In this study, Sn0.5Ag0.5Cu-Ni solder composition was shown the best drop property that was improved about 35% compared with the Sn1.0Ag0.5Cu solder ball in all test conditions.