Metallurgically and mechanically reliable microsilver-sintered joints for automotive power module applications

Conventional soldering methods with Pb-containing and Pb-free solder alloys have been extensively used in the manufacturing of the power modules. However, other reliable bonding methods that replace these conventional solders must be considered, because the solder alloys have an insufficient reliability at high temperatures above 200 °C. In this study, Ag-sintering methods with microsized Ag powders are evaluated as a replacement of the conventional soldering method for power module applications in electric vehicles. Ag paste was fabricated with particle sizes in the range of 2–3 μm; the metal content of the paste was 90%. The effects of the sintering parameters were investigated, including pressure, temperature, times of the sinter-bonding reactions, and joint strengths. The sinter-bonding reactions of mechanical contacts, neck-growth in-between Ag particles, and the interdiffusion between the Ag paste and metal layers increased with increasing sintering pressure and resulted in the formation of stable and dense Ag sinter-bonded joints. In this study, rapid Ag-sintering processes completed within 10 min were realized at 250 °C, and stable Ag-sintered joints with high-shear strengths were obtained at 250 °C and 10 MPa. The sinter-bonding pressure and temperature strongly affected the metallurgical sintering reactions and mechanical strengths of the Ag-sintered joints. We conclude that the sintering temperatures above 250 °C or the pressure-assisted processes above 2 MPa can form strong Ag/Ag sinter-bonded joints using microsized Ag paste.