How to upgrade a pixel detector: lessons from Phase-1 being applied to Phase-2 CMS Pixel Upgrade

The High-Luminosity Large Hadron Collider at CERN is expected to produce proton-proton collisions at a center-of-mass energy of 14 TeV, aiming to achieve an unprecedented peak instantaneous luminosity of 7.5E34 cm-2s-1, resulting in an average pileup of 200. To cope with these running conditions, the CMS detector will undergo an extensive upgrade: Phase-2. This upgrade includes the complete replacement of the CMS silicon pixel detector, introducing improvements such as increased radiation resilience, finer granularity, and capability to manage increased data rates among other changes. This is, however, the second time CMS has replaced their pixel detector. The differences and similarities between the Phase-1 and Phase-2 upgrades of the Inner Tracker of CMS will be outlined. Specific lessons learned from the operation of the Phase-1 detector will be highlighted, along with how this experience has informed the approach of the design and assembly of the Phase-2 Inner Tracker as preproduction of modules is approaching.