Effect of intermetallic compounds on reliability of Sn-Ag-Cu flip chip solder interconnects for different substrate pad finishes and Ni/Cu UBM

Twin demands on Pb-free solder interconnects for process compatibility and the decreasing pitch size of the bumps introduce enormous stresses on the joint quality of the solder-Under Bump Metallurgy (UBM) and the solder - pad interface. The joint quality is governed by the IMC formation. The objective of this study is two fold. First is to compare the near eutectic Sn-3.8Ag-0.7Cu lead-free solder with standard eutectic (63Sn-37Pb) lead based solder for IMC formation on the UBM-solder pad and solder-substrate pad interface. And second is to investigate the lead free flip chip solder joint for different surface pad finishes. The paper focuses on 4 different surface pad finishes namely, Electroless Nickel Immersion gold (ENIG), Organic Solderability Preservative (OSP), Direct Immersion Gold (Cu/Au), and Direct Immersion Silver (Cu/Ag) and its flip chip reliability. The UBM studied has Al as the adhesion layer, Ni (7% V) as the diffusion barrier layer and Cu as the wetting layer. Die shear, ball shear and electrical continuity are carried to evaluate the solder bump integrity and the failure mode. IMC formation at the interfaces is observed with optical and Electron microscopy. The results obtained in this paper will provide understanding of the fracture, shear strengths and electrical resistance of the solder bumps in flip-chips. Results correlating IMC growth and morphology for different thermal aging hours with electrical resistance and ball shear would be discussed in the paper.