The Determination and Utilization of AuSn Solder Creep Properties to Bond GaAs Dice to Diamond Substrates

Abstract

The stress in gallium arsenide dice larger than 2mm × 2mm conventionally bonded to diamond substrates using eutectic gold-tin solder is larger than the dice’s fracture strength leading to fracture during bonding. This paper discusses a novel bonding technique to reliably bond large gallium arsenide devices to diamond substrates using eutectic gold tin solder. In this technique, the cooling process after bonding is controlled to induce creep in the solder layer. Creep deformation in the solder layer relieves part of the stress in the die and thereby prevents die cracking. An analytical model describing the stress relaxation in the die due to solder creep deformation was developed. Due to the lack of published creep properties of AuSn solder, they were experimentally determined. Optimal cooling curves were generated using the relaxation model to reliably bond different sizes and thicknesses of GaAs dice to diamond substrates. Bonding experiments indicate that all the bonded dice cooled using conventional techniques cracked after bonding and that those cooled using the generated cooling schemes survived. Previously, these creep properties were boot-strapped from the published values for other solders. The experimentally obtained properties deviate substantially from those previously boot-strapped. However, it is interesting to note that the implied cooling schemes were similar.