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EFFECT OF SURFACE FINISHES AND INTERMETALLICS ON THE RELIABILITY OF SnAgCu INTERCONNECTS
McCluskey, F. Patrick
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Power semiconductor devices are used in a wide range of applications. In these applications, power semiconductor devices are required to handle large currents and as a result they tend to dissipate large amounts of heat. In addition, the device and their attendant packages must be capable of withstanding power cycling for many years. Traditionally, devices have used high lead die attaches for high electrical and thermal conductivity. Now, with the drive in industry to replace lead-contained solder with lead-free solder alternatives, there is a drive to assess lead-free solder to use as the die attach in power device packages. This dissertation assesses the reliability of Sn3.5Ag0.8Cu lead-free die attach under accelerated power cycling conditions, especially the effect of surface finishes from the die and the substrate on die attach reliability because of the thin die attach thickness (<100mm), which is expected to increase the influence of intermetallics formed at the interfaces on the joint reliability. The main part of the thesis is to evaluate the power-cycle reliability of Sn3.5Ag0.8Cu die attach in power MOSFET modules subjected to power cycling. Accelerated power cycling tests, failure analysis, thermal transient analysis and thermo-mechanical modeling were conducted. 3D thermal analysis correlated an increase in the package thermal impedance to the amount of crack propagation and determined that crack initiation is the limiting process under power cycling. In the experiments, die tilt was observed and die attach cracks always occurred near the middle of the bond line on the side with thicker die attach. This is not addressed in typical thermo-mechanical simulations on solder joint reliability. Such simulations predicted that the thinner side exhibits higher stress than the thicker side and were expected to be easier to fail. Microstructural characterization provided evidences that microstructure of die attach changes with thickness. First, a higher Ag3Sn concentration was observed in the thinner die attach due to dissolution of Ag from backside die. Second, a more uniform distribution of Ag3Sn precipitates exists in the thinner die attach due to faster cooling. So a thinner Sn3.5Ag0.8Cu die attach is more resistant to fatigue failure even under higher stresses.