# Silver Sintering for Silicon Carbide Die Attach: Process Optimization and Structural Modeling
## 3 Results  
### Reliability Assessment for Pressure Sintering Optimization  
The porosity of the sintering layer is measured after the sintering process, as depicted in Figure 5. Voids in the sintered layer appear as black points inside the silver material. This experimental output quantifies how the porosity decreases with higher sintering pressure: for the lower sintering pressure, \(p_{1}\), the fraction of voids is 20%, for the intermediate value, \(p_{2}\), the fraction of voids is 12%, and for the maximum pressure, \(p_{3}\), the fraction of voids is 8%. The experimental measurement error is within the 0.5%. SEM analyses are performed on cross-sections of virgin parts and after 3000 TS, as reported, respectively, in Figures 6 and 7. The time-zero pictures show that silver sintering joints are free of crack. Such as in Figure 5, even if with less contrast due to different image acquisition setup, Figure 6 highlights that the void percentage is lower for the maximum pressure \(p_{3}\) according to the reduced black spots, which also, in this case, are representative of the sintering porosity. The investigation after reliability stress shows the presence of fractures in the three scenarios. From a qualitative standpoint, it is observed that the crack length is less critical considering \(p_{3}\). Looking to the \(p_{1}\) and \(p_{2}\) results, fractures propagate from the left to the right side while cracks are less enhanced considering the highest pressure \(p_{3}\). The same behavior is also observed for other sample cross-sections, which are not reported for the sake of brevity. Cross-section cracks' length is quantified for the three scenarios. Placing equal to 1 the cracks' length for \(p_{1}\), it decreases to 0.61 and 0.46 for \(p_{2}\) and \(p_{3}\), respectively. On the basis of this evidence, the sintering process has been set considering the sintering pressure \(p_{3}\). Figure 5: Quantification of porosity fraction by SEM for the three sintering pressures. For the samples assembled using the \(p_{1}\) value, the evolution of sintering degradation during the TS test was monitored with no destructive SAM focused on interface between AMB substrate and silver sintering. The SAM output is presented in Figure 8 at four different steps: after assembly (t\({}_{0}\)) and after 1000, 2000 and 3000 shocks. The progressive delamination is highlighted by the reduction in contrast between chips' imprints and substrate increasing the number of cycles. Figure 6: SEM analysis on sample cross-sections after the sintering process for the three considered sintering pressures. Figure 7: SEM analysis on sample cross-sections after 3000 TS for the three considered sintering pressures. The effect of sintering degradation on thermal performance is highlighted for \(p_{1}\) case by the monitoring of body diode drop voltage \(\Delta V_{F}\), as well as reported in Figure 9. In this graph, it was pointed out that \(\Delta V_{F}\) and indeed the temperature swing after power pulse application increase during liquid TS. Considering that the thermal path involves sintering layers, the degradation of thermal behavior could be explained by sintering delamination. The continuous physical monitoring of silver layer degradation represents a pillar for the development of numerical model to reproduce silver sintering aging during reliability stress. As mentioned in Section 2.3, the experimental curves retrieved for the quite-bulk sample (with less than 3% of void fraction) are assumed as a starting point for the finite element simulation of mechanical response of sintering layer with a certain porosity percentage. Numerical outcomes are plotted in Figure 11. Coherently with assumptions, it is possible to observe that the \"bulk\" curves of Figure 11 are the same that \"25 \({}^{\circ}\)C\" and \"200 \({}^{\circ}\)C\" of Figure 10. FEA simulations highlight and quantify how the porosity inside the sintering material reduce the layer stiffness resulting in lower elastic modulus and yield values.