In recent years, the most explosive technologies in electronic systems have demanded ever-higher functionality, ever-faster circuit speeds, and always increasing interconnection density. Electrolytic and electroless nickel/gold (Ni/Au) deposition process are used commonly to provide flat, solderable pad surface finish on area array (BGA or CSP) packages and printed wiring boards (PWB). The electroless nickel/immersion gold (ENIG) process is widely used which do not requires plating lines for electrolytic plating, better meets the fine pitch wiring requirements. However, ENIG deposition process may cause or contribute to catastrophic, brittle, interfacial solder joint fractures. ENIG plating has previously shown lower reliability at solder joints. This is because Phosphorous segregation at the interface Sn-Ni intermetallic and Ni layer caused poor adhesion at that interface, especially high phosphorous content (10∼15%) of the electroless Ni. There have been many studies verified that Ni3P formation is a major factor, which causes weaker joint strength and flat fracture surface. Owing to sodium hypophosphite (NaH2PO2) was used to provide electron and return Ni ion to deposit on Cu pad for ENIG plating, it is not dodge that phosphorous element exist at the Ni layer. Hence, ph value, temperature and loading factor (plating area/ plating tank volume) in the plating path are controlled to reduce phosphorous content (less than 10%) to avoid Ni3P formation. Furthermore, ENIG has a potential risk of black pad, because Porous Au plating layer caused the oxidation of Ni layer underneath the Au plating to occur solder joint failure and low shear forces after assembly. In order to overcome foregoing problem, a flip chip ball grid array (FCBGA) test vehicle is used to compare three kinds surface finish electroless Ni/Au, direct gold and solder on Cu pad in this study 63Sn/37Pb solder bump is reflowed onto these substrates. High temperature storage test (HTS) is used to evaluate thickness and structure of IMC to affect solder joint attachment reliability. Ball shear test is used to measurement joint strength at various HTS time. Optical microscopy (OM) and scanning electron mcroscopy (SEM) are used to observe failure modes after ball shear.
Reliability of Sn-Ag-Cu Solder Joint on ENEPIG Surface Finish: 1. Effects of thickness and roughness of electroless Ni-P deposit