Effect of Large Amplitude Thermal Cycles on Power Assemblies Based on Ceramic Heat Sink and Multilayer Pressureless Silver Sintering

The effect of large amplitude thermal cycles between −50°C and 180°C on power assemblies based on pressureless sintering of successive silver layers on an Aluminium Nitride (AlN) heat sink is investigated. In order to evaluate the impact of various adhesion layers on AlN as well as the relative density of the current track layers, various configurations with silver cermet, TiNiAg and WNiAg adhesion layers and track layer relative density of 81% and 66% were tested. Results show that the use of silver cermet as adhesion layer and a track layer with a high relative density of about 81% and a closed porosity presents the best behaviour with a low delaminated area ratio (16%) and acceptable shear stress value (7 MPa) after 1000 thermal cycles. For this configuration, the surface analysis performed after the shear tests on 1000 cycles aged samples show that the fracture takes place at the interface between the die attach layer and the device as well as the interface between the die attach and the track layer. However, for all other tested configurations, the fracture surface analysis performed at the end of the cycling tests highlights that the interfaces between the adhesion layer and the sintered track layers or the AlN ceramic are the weakest points of the assembly. The relationship between cracks observed after the aging and the adhesion layer nature, the track layer density, the die attach thickness as well as the void presence in the die attach is discussed. Finally, main guidelines allowing achieving device to heat sink assemblies able to withstand large amplitude thermal cycles are proposed.