Thermal Cycling Reliability of Lead-Free Solders (SAC305 and Sn3.5Ag) for High-Temperature Applications

Abstract

Applications with temperatures higher than the melting point of eutectic tin-lead solder (183°C) require high-melting-point solders. However, they are expensive and not widely available. With the adoption of lead-free legislation, first in Europe and then in many other countries, the electronics industry has transitioned from eutectic tin-lead to lead-free solders that have higher melting points. This higher melting point presents an opportunity for the manufacturers of high-temperature electronics to shift to mainstream lead-free solders. In this paper, ball grid arrays (BGAs), quad flat packages, and surface mount resistors assembled with SAC305 (96.5%Sn+3.0%Ag+0.5Cu) and Sn3.5Ag (96.5%Sn+3.5%Ag) solder pastes were subjected to thermal cycling from -40°C to 185°C. Commercially available electroless nickel immersion gold board finish was compared to custom Sn-based board finish designed for high temperatures. The data analysis showed that the type of solder paste and board finish used did not have an impact on the reliability of BGA solder joints. The failure analysis revealed the failure site to be on the package side of the solder joint. The evolution of intermetallic compounds after thermal cycling was analyzed.