Hideo Nakako

Thermal cycling lifetime estimation of sintered metal die attachment

Sintered metals are considered to be high temperature die bonding materials for SiC power devices. Thermal cycling tests have shown that sintered copper is expected to achieve excellent lifetime properties, especially as compared to sintered silver. However, the mechanism behind this has not yet been confirmed. To clarify the mechanism, it is necessary to know the basic strengths of sintered metals, such as their stress-strain relations and fatigue properties. In this study, we measured the stress-strain relations and fatigue properties of sintered metals and estimated thermal cycling lifetime using these data and finite element analysis (FEA) to clarify the mechanism. The stress-strain relations were determined using three-point bending tests and FEA. The fatigue properties were measured using micro-fatigue testing equipment. Strain of the sintered layer was calculated by FEA and compared with the fatigue testing data. The estimated thermal cycling lifetime of sintered copper was longer than that of sintered silver, which is consistent with experimental trends. We analyzed the results and found that the long thermal cycling lifetime of sintered copper stems from the lower plastic strain of sintered copper layers due to the higher yield stress of sintered copper.

Effect of Manufacturing Process on Micro-Deformation Behavior of Sintered-Silver Die-Attach Material

Effects of pressurization and thermal-processing conditions on the deformation properties of sintered-silver sheets, excluding the contribution of the voids, (hereinafter, referred to as “micro-deformation properties”) were evaluated. Three types of thin-plate specimen sintered under the different conditions were prepared and these micro-deformation properties were compared. The first type was pressurized at 10 MPa for 10 min in the atmosphere at 573 K, the second type was annealed at 573 K for six hours before pressure processing and pressurized at 10 MPa for 10 min in the atmosphere at 573 K, and the third type was not pressurized and sintering in the atmosphere at 573 K for 10 min. The micro-deformation properties of these specimens were estimated by tensile testing and finite-element analysis (FEA) using a model that reproduces the microporous-structure. The microporous-structure models were made from serial cross-sectional images obtained by focused-ion-beam scanning electron microscope. These estimation results indicate that the micro-deformation properties of sintered-silver sheets do not depend on pressure, but on thermal-process conditions. The estimated micro-deformation properties were compared with those determined by nanoindentation tests and the validity of the estimated micro-deformation properties was confirmed. A deformation property of non-pressurized specimens was predicted by using the estimated micro-deformation properties and the microporous structures obtained from the pressurized specimens. The predicted deformation properties corresponded to experimental one. It is therefore concluded that if the deformation properties and microporous-structure of a sintered-silver specimen under a certain condition are determined, it is possible to predict the porosity dependency of deformation properties by FEA.