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

Abstract

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.

Power, readout and service hybrids for the CMS phase-2 upgrade

“…The PCB is a four-layer flex with 12 μm thin copper and polyimide layers, stiffened with a CF stiffener and compensated on the opposite side to keep the circuit flat after lamination [3]. In order to have one version of the circuit for the three different spacings of the PS module, three different aluminum nitride (AlN) spacers were designed to be glued under the hybrid, one for each of the PS module versions (Figure 10).…”

“…The latter is not only simpler and cheaper to produce, but also can achieve a much smaller thermal resistance. This is the case because of the lack of the compensator layer (which is a thermal insulator) necessary for the CF stiffened version [2], and of the addition of blind thermal vias between the last two layers (vias not possible in CF). The only disadvantage of this stack-up is the CTE mismatch (∼16 ppm/K) with the CF baseplate (200 μm thick support upon which the hybrids and the sensors are glued) of the PS module, which requires the use of flexible glue for the adhesion of the hybrid to this CF baseplate.…”

“…It has two fine-pitch connectors on flexible tails to connect to the PS-FEHs on the left and right sides and one small connector to connect to a powering tail that brings in directly the 1.25 V from the PS-POH. The PCB is a four-layer flex with 12 μm thin copper and polyimide layers, stiffened with a CF stiffener and compensated on the opposite side to keep the circuit flat after lamination [2]. In order to have one version of the circuit for the three different spacings of the PS module, three different aluminum nitride (AlN) spacers were designed to be glued under the hybrid, one for each of the PS module versions (figure 10).…”

Test system for the service hybrid of the 2S module for the CMS Phase-2 Outer Tracker upgrade

“…Final hybrid prototypes were developed and produced in parallel with their corresponding test cards [4]. Prototype 2S SEH test cards were used to qualify a total of 26 SEH prototypes.…”

A High Throughput Production Scale Front-End Hybrid Test System for the CMS Phase-2 Tracker Upgrade

“…The new design of the CMS Outer Tracker is based on two main types of modules, the stripstrip (2S) and the pixel-strip (PS) modules [1]. These modules require state of the art High Density Interconnect (HDI) front-end hybrids assembled with fine pitch flip-chip front-end ASICs, connectors and passives [2]. All together 27900 2S type hybrids and 26960 PS type hybrids are required for the module constructions.…”