A Novel Approach to Ultra-Low Temperature Interconnection: Double-Sided SLID Process Using SAC BGAs and Eutectic SnBiIn

We present a preliminary study on ultra-low temperature interconnections using a double-sided solid-liquid interdiffusion (SLID) process that combines solid SAC305 solder balls with liquid eutectic SnBiIn solder paste (Field's metal, Sn-16.5Bi-32.5In) on bare Cu and Ni (ENIG) metallizations. This approach enables the lowest possible reflow temperature at both the package and card levels (simulated card assembly), ensuring complete Indium homogenization due to its higher diffusivity compared to Bismuth, while achieving near-homogeneous Bismuth distribution. The study systematically optimizes SLID parameters, including solder paste volume (varied using different mask sizes), different reflow temperatures and times, to achieve complete homogenization. Experimental results show that optimal SLID conditions—140°C for 5 minutes with a 700 µm mask—ensure full Indium diffusion into SAC, while Bismuth remains near-homogeneous. To evaluate reliability, aging studies are conducted under Telcordia conditions at 85°C for 500h and 1000h, analyzing interfacial and bulk microstructural evolution on Cu and interfacial evolution on Ni (ENIG). Mechanical properties by shear testing exhibit stable performance despite revealing complete Cu pad consumption. On the other hand, preliminary analyses using ENIG pad metallizations shows that Ni acts as an effective barrier to suppress Cu dissolution, thereby recommending its use to mitigate potential interfacial degradation. These findings establish the feasibility of a double-sided SLID process using SAC BGAs and SnBiIn solder for next-generation ultra low-temperature interconnections, offering superior diffusion characteristics, reduced Bi segregation, and enhanced long-term mechanical reliability compared to conventional low temperature soldering methods