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Effect of sintering atmosphere on the shear properties of pressureless sintered silver joint

Sintered silver is a possible replacement for high performance and Pb free die attach materials in power modules and traditional microelectronic packages because of its high melting temperature, high thermal conductivity and good thermo-mechanical properties. However, reliable sintered silver joints can only be easily formed on Ag or Au plated surfaces during pressure sintering. Here we present the initial results of two pressure-less Ag pastes (i.e. micron-sized flakes and nanoparticles) that can sinter on copper (Cu). These sintered Ag joints exceeded the minimum die shear strength as per MIL-STD883E; nano-Ag preferred the N2-5%H2 environment to produce die shear strength of 17 MPa on the H2 cleaned Cu while micron-Ag joint produced die shear strength of 14 MPa on the Cu substrate when sintered in ambient air. Fractography of sintered nano-Ag joints showed that the silver particles have necked to different degrees in N2 and N2-5%H2. Heavy oxidation caused by ambient air sintering prevented bonding of nano-Ag on the Cu oxide, resulting in a flat fratography. Similar flat fractograph was also visible for micron-Ag sintered in N2 atmosphere because of residual binders prevented the sintering on the Cu substrate. In the presence of ambient air, the oxidative combustion of the binders and capping agents de-oxidized the Cu substrate to allow sintering onto the Cu. Cross section of this micron-Ag joint on Cu substrate confirmed the joint formation in ambient air during pressureless sintering. These results showed that different binders and capping agents influence the sintering properties differently to produce reliable sintered Ag joints.