Sintered-copper Die-attach: Processing, Properties, and Reliability

Abstract:
Die-attach by copper-powder sintering has the potential to be lower cost than that by silver-powder sintering. In this study, we evaluated a low-cost copper paste consisting of a micron-size copper powder for die-bonding on substrates with bare copper surface. The bonding process was done under pressure of 0-5 MPa and at temperature of 230-300deg C holding for 5-30 min in a reducing atmosphere of forming gas (4% H2/N2). The effects of temperature-time-pressure sintering profile on die-shear strength, electrical resistivity, and microstructures were investigated. The oxidation-free, sintered copper bond-lines had electrical resistivity as low as four times of the bulk value, and strong bonding strength over 65 MPa for 3 mm × 3 mm die-attach and over 53 MPa for 5 mm × 5 mm die-attach. Reliability of the sintered-copper bonds was evaluated in a thermal shock test (-55~125deg C). The average die-shear strength had no significant change after 2000 thermal shock cycles; and, analyses of scanning acoustic microscopy and scanning electron microscopy showed dense bond-lines without defects, cracks or delamination, after the thermal shock test. Our findings support the claim that copper-sintering has the potential to be a low-cost solution for implementing the powder-metal sintering technology in manufacturing power semiconductor devices and or modules.