Bonding of large substrates by silver sintering and characterization of the interface thermal resistance

Low-temperature silver sintering technology, which has been proven to be a promising die-attach solution, was extended to bonding large substrates. Strong bonding strengths for substrates greater than 25 mm × 50 mm were achieved by sintering a nanosilver paste at temperatures below 270oC with less than 5 MPa pressure. To characterize thermal performance of the substrate-attach interface, we applied a transient thermal technique with cumulative structural function analysis. Using self-heating and the temperature-sensitive threshold voltage of a power device, we measured transient thermal responses of the device placed at various locations on the bonded structures. Each transient thermal response was used to find cumulative structural function, a relation between cumulative thermal capacitance and cumulative thermal resistance from the device junction to the ambient. Two-dimensional maps of the interface thermal resistances were obtained from the structural function plots. We found that for well-bonded substrates, the average specific thermal resistance contributed by the sintered silver interface was between 5.20 mm²K/W and 5.78 mm²K/W with a variation of 4.7% to 6.0%.