Understanding thermally induced stresses in ATLAS ITk Pixel modules

The stresses induced by differences in the coefficients of thermal expansion in the module stack can lead to stress on the solder bumps that is sufficient to cause the bump to break. FEA simulations have been made to understand the stresses and how they can be mitigated. FEA simulations highlighted the importance of the copper content of the electrical hybrid and the support on which the modules are glued. Experimental measurements are required to verify the FEA simulations. The first step in these was the development and thermal cycling of simple daisy chain structures that were made with the correct bump-bond geometry, had a copper layer to simulate the electrical hybrid and were mounted on CFRP to simulate the local supports. The daisy chains were mounted in a system that allowed the bump-connectivity to be evaluated during the thermal cycling. Results from thermal cycling of the daisy chains will be presented and compared to the FEA predictions.