Reliability Improvement of Low-Temperature Sintered Nano-Silver as Die Attachment by Porosity Optimization

Thermomechanical stress caused by the coefficient of thermal expansion (CTE) mismatch leads to the formation of cracks, delamination, and warpage in the sintered nano-silver (S-Ag) layer, which eventually results in the fatigue failure of silicon carbide (SiC) semiconductor devices. Herein, the gradient porosity distribution method was proposed to simultaneously reduce the maximum thermomechanical stress and homogenize the stress distribution. The influence of different porosity distributions on the maximum thermomechanical stress and strain in the S-Ag layer was investigated by the finite element simulation. The gradient porosity distribution structure ( $S_{1 > 2}$ and $S_{1 < 2}$ ) effectively reduced the maximum thermomechanical stress and strain by 8.4% and 8.0% compared to the conventional uniform porosity structure ( $S_{1 = 2}$ ). S-Ag bonding samples with $S_{1 = 2}$ , $S_{1 > 2}$ , and $S_{1 < 2}$ structures were prepared to confirm the simulation results, which exhibited similar shear strengths. After 500 cycles of thermal shock test, the delamination rates of $S_{1 > 2}$ and $S_{1 < 2}$ bonding samples were 7.7% and 9.8%, which were 5.0% and 2.9% lower than $S_{1 = 2}$ bonding samples, respectively. Both finite element simulation and experimental verification results demonstrate the positive effect of gradient porosity distribution structure on alleviating the thermomechanical fatigue failure caused by CTE mismatch.