Prototyping of Hybrid Circuits for the CMS Phase-2 Outer Tracker Upgrade at the HL-LHC

High Density Interconnect hybrids are being developed for the CMS tracker Phase-2 upgrade for the HL-LHC. These hybrids are flexible circuits laminated to carbon fiber composite stiffeners and are assembled with flip-chips, passives and connectors. The wire bonding to sensors and the soldering requirements for these components requires an almost perfectly flat surface. A lamination process is proposed, focused on the compatibility with a lead-free reflow process. The stack-up of the hybrid was optimized to balance the forces induced by the Coefficient of Thermal Expansion (CTE) differences in the assembly. The proposed lamination process was applied to prototype 8CBC3 hybrid circuits.