LT-TLPS Die Attach for High Temperature Electronic Packaging

Low temperature transient liquid phase sintering (LT-TLPS) can be used to form high-temperature joints between metallic interfaces at low process temperatures. In this paper, process analysis and shear strength studies of paste-based approaches to LT-TLPS are presented. The process progression studies include DSC-analyses and observations of intermetallic compound (IMC) formation by cross-sectioning. It was found that the sintering process reaches completion after sintering times of 15 minutes for process temperatures around 50°C above the melting point of the low temperature constituent. For the shear strength studies, test samples consisting of copper dice and copper substrates joined by sintering with a variety of pastes having different ratios of copper and tin have been assessed. A fixture was designed for high temperatures enabled shear tests at 25°C, 125°C, 250°C, 400°C, and 600°C. The influence of the ratio of the amount of high melting-point constituent to the amount of low melting-point constituent on the maximum application temperature of the sinter paste was analyzed. Ag20Sn and Cu50Sn pastes showed no reduction in shear strength up to 400°C, Cu40Sn pastes showed high shear strengths up to 600°C, and extended curing further increased the joint strength. It was shown that LT-TLPS can be used to form high temperature stable joints at low temperatures without the need of applying pressure during processing.