Porosity Evolution of Ag-Sintering at Die Attach

Abstract

Die-attach for high power devices such as IGBT is facing increasing challenges of service temperature, thermal conductivity, electrical conductivity, and reliability. Conventional high-lead solder materials are reaching their limitation of performance. Recently, sintering of Ag-paste has been advocated as a technology to meet this new challenge. In general, this technology prints the Ag paste on top of a direct bond copper substrate, followed by drying, die placement, and heating under a pressure up to 50 MPa. The high pressure is applied in order to avoid formation of high porosity within the Ag joint. The need for high pressure during sintering requires the acquisition of expensive equipment, and inevitably, results in low through-put. Some Ag pastes incorporate polymeric ingredients to avoid the need for pressure, at a cost of high-Ag porosity and a low tolerance for high service temperatures. In this study, a novel Ag-paste without any polymeric inclusion has been developed for the pressureless sintering die-attach process. The porosity of sintered joints was lower toward center of die, due to the venting route factor. The porosity was also lower with a higher bondline thickness, due to a reduced tension in the joint. Thermal aging time has negligible effect on porosity, presumably due to balanced stress between tension and sintering shrinkage. The maximum service temperature of Ag sintered joint is about 470°C, versus 230°C for high-Pb joints.