Evaluation of Au-Si Eutectic Die-Attach for Power Hermetic Packages in Space Applications

This study systematically evaluates the performance of Au-Si die-attach bonding through a combination of finite element analysis (FEA), cross-sectional material characterization, visual inspection, and mechanical testing. FEA thermal simulations demonstrate that replacing the resin thermoset die-attach with Au-Si bonding significantly enhances thermal performance across key metrics. In Device A, θJA decreased from 44 °C/W to 36 °C/W with either 14 μm or 7 μm Au-Si bond-line thickness (BLT). Device B exhibited a similar improvement, with θJA dropping from 26 °C/W to 20 °C/W. Notably, both Au-Si thicknesses delivered nearly identical thermal performance, indicating that thermal benefits plateau beyond a certain BLT. Mechanical FEA further reveals that Au-Si legs experience lower shear stresses than the resin thermoset, contributing to improved stress distribution. Die shear testing confirms robust mechanical integrity, with an average bond strength of 24.85 kg, which is significantly higher than the minimum specification of 2.5 kg. Cross-sectional SEM and EDX analyses confirm a dense die-attach layer with clean interfaces and Au-Si inter-diffusion. Despite these advantages, concerns arise from the higher normal stresses, approximately 250 MPa, observed in Au-Si joints compared to 90 MPa in the resin thermoset, which may increase the risk of interfacial delamination. Additionally, blister-like anomalies observed in Device B indicate silicon compatibility issues, highlighting the need for careful die-attach material selection, particularly in legacy die node designs.