A novel bonding technique to bond CTE mismatched devices

Thermal expansion mismatch between electronic devices and their substrates induces stresses in the assembly during bonding and operation. These stresses in extreme cases cause cracking of the electronic device during bonding. For GaAs devices back-side bonded to a high conductivity artificial diamond substrate using Au-Sn solder, analytical and numerical analyses were conducted to determine the bonding stresses in the GaAs die. Bonding experiments were conducted to study the effect of varying cooling rates on die failure. Experimental data demonstrate that only smaller sized dice (1 mm×1 mm and 2 mm×2 mm) survived bonding without cracking. This observation was corroborated by analytical and numerical studies which show that the stress induced in the larger dice exceeded the strength of the material. Slow (≈2°C/min) but continuous cooling from the bonding temperature did not improve the survival rate of the dice significantly. A new cooling scheme was developed utilizing the high temperature creep properties of the Au-Sn solder. This cooling scheme incorporates solder layer creep at high temperatures to relieve the stresses in the attached die. Bonding experiments with sizes up to 10 mm×10 mm and thicknesses down to 4 mils were performed with a 100% survival rate of the dice