ATLAS ITk Pixel Module Bump Bond Stress Analysis

The upgrade of ATLAS for the high-luminosity LHC (HL-LHC) will among many detector components replace the tracking detector with an all-silicon tracker (ITk). The outer layers are composed of strip modules while the innermost 5 layers of ITk are composed of hybrid pixel modules mounted on carbon local supports. The large temperature ranges during operation and the heterogeneous nature of the system means that thermally induced stress is present in the module bump bonds. This paper presents a model using finite element analysis of the pixel module to estimate the maximum stress in the bump bonds. Experimental results are shown of bump strength from lap-shear measurements. Finally, detailed module characterisation is presented of module bump failure due to thermal cycling. Bump bonds are demonstrated to survive 100 cycles over the design thermal cycling range with less than 0.1% thermally induced bump bond disconnects.