Thermal cycling reliability of hybrid pixel sensor modules for the ATLAS High Granularity Timing Detector

The reliability of bump connection structures has become a critical aspect of future silicon detectors for particle physics. The High Granularity Timing Detector (HGTD) for the ATLAS experiment at the High-Luminosity Large Hadron Collider will require 8032 hybrid pixel sensor modules, composed of two Low Gain Avalanche Diode sensors bump-bonded to two readout ASICs and glued to a passive PCB. The detector will operate at low temperature (-30 °C) to mitigate the impact of irradiation. The thermomechanical reliability of flip-chip bump connections in HGTD modules is a critical concern, particularly due to their characteristically lower bump density (pixel pitch dimensions of 1.3 mm× 1.3 mm). This paper elaborates on the challenges arising from this design characteristic. Finite element analysis and experimental testing were employed to investigate failure modes in the flip-chip bump structures under thermal cycling from -45 °C to 40 °C and to guide the module redesign. The optimized design demonstrates significantly enhanced robustness and is projected to fulfill the full lifetime requirements of the HGTD.