The Large Hadron Collider (LHC) at CERN is the largest accelerator in the world. ATLAS, one of the two general-purpose experiments at the LHC, is contributing to achieve a better understanding of the fundamental laws of nature through a vast physics program that spans from high precision measurements to search of New Physics (NP) beyond the Standard Model (BSM). Starting in 2029, the LHC will increase its luminosity through its upgrade to the High- Luminosity LHC (HL-LHC), in which the large increase in cumulated statistics will enable the search of rare processes that may provide new hints of NP. In order to address all the challenges linked to the high-density particle environment expected at the HL-LHC, several upgrades to the ATLAS detector are required.

The Inner Tracker (ITk) is the new all-silicon tracker of ATLAS for the HL-LHC. Crucial for vertexing and for the reconstruction of the charged particle trajectories, the ITk requires of high radiation tolerance and high granularity due to its proximity to the collision point. Comprising a Pixel and Strip detectors, the ITk provides fine position resolution in a compact design. With a total area of 13 m², the ITk Pixel detector covers a large tracking volume by means of nearly 10,000 individual pixel modules arranged in five different layers. Currently in the pre-production and production stages, there is an on-going research program within the collaboration to optimize the production processes of all the different components, from modules to mechanical supports and electrical services.

This thesis presents the work done on the quad pixel modules for the ITk Pixel detector. Before their integration into the local support mechanical structures, the pixel modules undergo several assembly steps, from the bare module assembly (bump-bonding of the sensor to the readout chips) to the cell-loading and pigtail connection. At each different stage the modules are subject to a set of extensive measurements as part of their Quality Control (QC) program. At the cell-loading stage the module is bonded to a highly thermal conductive material used to dissipate the heat generated by the front-end electronics. Thus the tests after cell loading are mostly focused to evaluate the good thermal performance of the assembly. The pigtail connection is a critical operation due to the complex pigtail shape and the need of using very specific mechanical jigs tailored to this application. Testing several modules after pigtail assembly is important to confirm that no damages occurred during this delicate assembly step.