IMC Growth Mechanisms of SAC305 Lead-Free Solder on Different Surface Finishes and Their Effects on Solder Joint Reliability of FCBGA Packages at Different Thermal Aging and Temperature Cycle Conditions

Intermetallic Compound (IMC) growth behavior of lead-free solder plays an important role in Ball Grid Array (BGA) solder joint reliability for Flip Chip BGA (FCBGA) packaging application. Growth mechanism of IMC is reported based on diffusion model and thermal treatment such as Accelerated Thermal Cycling (ATC) and isothermal aging exposure also contributes growth rate and morphology of lead-free solder IMC. Among the lead-free solder alloys, Sn-3.0wt.%Ag- 0.5wt.%Cu (SAC305) solder is a promising substitute for Sn-Pb because of its good mechanical properties and wettability with current surface finishes. After thermal exposure, BGA solder joint reliability is degraded due to IMC formation and its growth. In this study, two different thermal treatments, ATC and isothermal aging, and two different pad surface finishes, Solder on Pad (SOP) and Electroless Ni Immersion Gold (ENIG), are considered in terms of IMC growth rate and its mechanical solder joint reliability. SOP finished interface forms thin ε-phase Cu3Sn layer and scallop-like η-phase Cu6Sn5 layer, in contrast, ENIG finished interface forms thick (Cu,Ni)6Sn5 IMC layer and it prevents overall IMC growth. Different surface finished test vehicles are evaluated in ATC test at 0ºC to 100ºC temperature range and Ni diffusion layer shows longer solder joint fatigue lifetime than non-diffusion barrier interface based on micro cross-section and dye penetration analysis results. In isothermal aging test at 100ºC and 150ºC, aging temperature and time are valid factors to decide mechanical shock reliability. Interfacial fractures are found at 100ºC aged test vehicle due to easier crack propagation at interface between thin Cu3Sn layer and scallop-like Cu6Sn5 layer based on SEM microstructure analysis results. Finally, this investigation proposes how to improve solder joint reliability and prevent interfacial fracture for SAC305 lead-free application.