Mechanical Reliability Evaluation of Stripped and Replated Component Termination Finishes

Abstract

The termination finish of Small Output Integrated Circuit (SOIC) and Small Output Transistor (SOT) chip components were converted from Pb-free to Sn-Pb (backward conversion) and vice versa (forward conversion). The motivation for these conversions is due to a combination of factors such as the supply chain constraints on component availability, European Union’s (EU) legislation on “Restriction of Hazardous Substances” (or RoHS), and the growth of tin whiskers on matte tin finish components. The conversions were performed using a “Robotic Stripping and Solder Dipping Process”, and the mechanical reliability of the converted components was evaluated through lead pull testing. In this experiment, a 100% (all finishes are given in weight percentage unless otherwise specified) matte tin finish was first stripped and re-plated with an eutectic Sn-Pb finish. Then, components with 100% matte tin and eutectic Sn-Pb termination finish were replaced with a Pb-free Sn-3.5%Ag-0.5%Cu (SAC305) finish. Three Printed Circuit Board (PCB) surface finishes namely Immersion Silver (ImAg), Organic Solderable Preservative (OSP), and Electroless Nickel/Immersion Gold (ENIG), were evaluated with Sn-Pb and Pb-free processes. All the assembled boards were subjected to an initial analysis, which includes visual inspection with an optical microscope and X-Ray analysis. Subsequently, a time zero analysis was performed which includes cross sectioning, Scanning Electron Microscope (SEM) analysis and lead pull testing. The pull testing was performed on a Chatillon TCM 201-SS equipment. All the leads were pulled orthogonal to the surface of the PCB. After isothermal aging at 150°C for a time period of 10 days (240 hours), cross sectioning and pull testing were performed to study its effect on Intermetallic Compound (or IMC) growth and reliability.