Kinetics of intermetallic growth between lead-free solders and silver-palladium metallizations
Kinetics of intermetallic growth is studied between two lead free solders 3.5 wt% Ag-96.5 wt% Sn, 4 wt% Ag-95.5 wt% Sn-0.5 wt% Cu and two Dupont metallizations 6138, 7484 and pure palladium. The wt% Ag:Pd ratio in Dupont 6138 and 7484 is 19 and 3 respectively. Diffusion couples are prepared by solder reflow on (1) low temperature cofired ceramic (LTCC) substrates having Ag-Pd metallizations and (2) pure palladium foils. These reflowed samples are then annealed at different temperatures. The Ag-Sn and Ag-Sn-Cu and Du Pont 7484 diffusion couples are annealed at temperatures of 100, 130 and 156 °C. The Ag-Sn and Du Pont 6138 samples are annealed at temperatures of 125, 156 and 176 °C. Ag-Sn/Pd and Ag-Sn-Cu/Pd samples are annealed at 156, 175, 195 and 210 °C. At different time intervals samples are taken out of the furnace and cross section analysis is done to determine the intermetallic thickness. For the times and temperature ranges investigated is this study diffusion controlled intermetallic growth is observed for (1) all the metallization and solder diffusion couples (2) Ag-Sn-Cu/ Pd diffusion couples. For Ag-Sn/Pd sample diffusion controlled growth is observed for temperatures 175, 195 and 210 °C. At 156 °C the rate controlling mechanism changes to reaction control. This variation in rate controlling mechanism is correlated to the change in the intermetallic phase morphology that forms in the Ag-Sn/Pd diffusion couple at 156 °C. Activation energies and reaction rate constants for diffusion controlled intermetallic growth are determined for the different sample schemes.