Preparing Silver–Copper pastes in accordance with percolation theory for die attach bonding

Abstract

The Si die and Cu substrate were successfully bonded with self-developed Ag–Cu pastes through two processes: tube furnace annealing at 250 °C for 30 min in an Ar atmosphere and hot pressing at 250 °C and 5 MPa for 30 min in air. The microstructures, mechanical properties, thermal conductivity, and resistivity of the bonded samples were analyzed and then compared with those of commercial Ag paste. For the samples subjected to tube furnace annealing, the critical Ag volume of the Ag–Cu paste was 35%, shear strength was 5.17 MPa, and resistivity was 9.22 μΩ-cm according to percolation theory. For the samples subjected to hot pressing, the critical Ag volume of the Ag–Cu paste was 75%, shear strength was 8.88 MPa, and resistivity was 12.88 μΩ-cm. The critical Ag volume could not be evaluated on the basis of percolation theory because the thermal conductivity increased gradually with the Ag content, and no saturation trend was observed. The thermal conductivity of the 75 vol% Ag–Cu paste produced through hot pressing was 241 W/mK, which was considerably higher than the commercial Ag paste (thermal conductivity: 48.417 W/mK; electrical resistivity: 400 μΩ-cm), and the cost of 25 vol% Ag particles was successfully reduced.