ATLAS ITk pixel module bump bond stress analysis

The upgrade of ATLAS for the high-luminosity LHC (HL-LHC) will among many things replace the tracking detector with an all-silicon tracker, known as the ITk. The five inner layers of the ITk will use hybrid pixel detector modules, known as the pixel system. The pixel system is designed for a maximum radiation dose of about 8 MGy and particle fluence of 1016 1MeV Neutron equivalent per cm2, without any safety factors. All but the first layer will use a quad module, made up of 4 front-end (FE) ASICs, bump bonded to a sensor consisting of ~150,000 50x50m pixels. The ASIC, designed by RD53, is ~20x20mm2 and the overall area of the Quad module is ~40x40mm 2. The pixel modules are built in two steps: the first is the hybridization in industry, when the FE ASICs are flip-chipped to a sensor tile, producing a so- called bare module; the second step is the module assembly, that is, the gluing of a flexible PCB (flex) onto the bare module. The flex connects the FE ASICs and sensor to the on-detector services. The modules are parylene coated for protection against high voltage breakdown. The modules are then glued to the bare local supports, that are of different designs depending on the location in the pixel system. The bare local supports are made from low CTE materials with high stiffness and are cooled. The adhesive used to fix the module to the local support has a high modulus. This results in module-local support system that is a heterogenous object consisting of materials with a range of CTEs and moduli, most notably copper in the PCB traces and CFRP of the local support. To guarantee long term operation in the high radiation field the lowest coolant temperature of -45 oC is required. This temperature both reduces shot noise and prevents thermal run-away and reduces the impact of the radiation damage on the sensor current. A maximum operational temperature of +40oC is imposed by the interlock system. To ensure that the module can operate for 10 years over the operational temperature range, the module is designed to withstand 100 cycles over the temperature range of +60oC to -55oC to account for assembly, shipping, and accidents during operation. The wider temperature range used for design validation ensures that the operational conditions of order of 150 cycles between +25 oC and −45oC are met with some safety margin. The large temperature ranges and the heterogenous nature of the system means that thermally induced stress is present in the module bump bonds. This paper presents finite element analysis of the pixel module to estimate the maximum stress in the bump bonds. Experimental results are shown of bump strength from double lap-shear measurements and compared to simulation. 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 disconnects.