Silver Sintering Paste Rendering Low Porosity Joint for High Power Die Attach Application

Abstract Silver joints with ~10% porosity for die-attach have been achieved with specially engineered Ag sintering pastes, through combined pressureless sintering process plus thermal aging. The pastes developed in this work have the following advantages: 1) they can be used under pressure-less processing condition; 2) they are compatible with conventional reflow oven, resulting in a higher through-put as compared to that of stepwise heating oven; 3) they can be used under air reflow condition; 4) a highly reliable joint has been obtained as judged from the shear strength results from thermal aging and temperature cycling tests. The silver sintering pastes are versatile in bonding different metallization surface including Au, Ag and Cu. It can also be used in bonding large area (10mm × 10mm) dies. Shear test results under varied temperatures implied that the maximum service temperature of Ag sintered joints can be as high as 470 – 530 °C, depending on the shear strength pass criteria, and this is more than 250°C higher than that of high-Pb joints. Thermal aging test at 250°C for the joints generated on Ag-die/Au-DBC combination revealed that Ag continuously consolidates in the bulk phase resulting in the formation of larger pores with reduced numbers as compared to that of untreated samples. At the same time, it diffuses to sintered Ag/Au-DBC interface to form a dense Ag layer induced by alloying with Ni(Au) and Cu, which strengthened the bonding. A large bondline thickness is critical for obtaining highly reliable joints. The total porosity of the joint is found slightly decreased during the course of 3200h aging test. Temperature cycling at −55 °C to 200°C shows that the silver joints are stable for at least 1000 cycles.