Speaker
Description
During LHC Long Shutdown 3, ALICE will replace its innermost three tracking layers with wafer scale stitched Monolithic Active Pixel Sensors(MAPS) bent around the beam pipe. Each 27 cm-long sensor is connected via wire bonds to a custom three-layer Flexible Printed Circuit(FPC). These FPCs deliver power, transmit up to 40 high-speed(10 Gb/s) data and control signals. Designed for a half-cylindrical geometry and 100 µm ASIC pitch, they were optimised through simulations and electrical tests. Custom tooling enables precise bending and wire bonding. This contribution presents the design, integration, and assembly process of the ITS3 FPCs, highlighting technical challenges and solutions.
Summary (500 words)
During LHC LS3, the three inner-most vertexing layers of ALICE will be replaced by three layers of Monolithic Active Pixel sensors (MAPS) of 27 cm length, bent around the beam pipe and thinned to 50 µm. The layers are held in place by carbon foam instead of rigid structures and air-cooled, thus presenting a material budget of 0.09% -X0 per layer only. Depending on the layer, each sensor contains 3, 4 or 5 identical, independent sensor segments of 27 cm x 1.95 cm size.
The electrical connections between the sensors, the power supplies, readout and control are achieved by a stack of three independent Flexible Printed Circuits (FPCs) of two layers, split into identical and independent parts connected to one segment each, adapted to match the half-cylindrical shape and the variable sensor ASIC pitch of 100 to 150 µm on the sensor side. It connects two analogue and three digital supplies to the sensors for a total of 2.5 A, up to 40 differential 10 Gb/s readout signals and up to 40 control and clock signals of 5-160 MHz.
The selection of the polyimide base material, the manufacturing rules, the power integrity and signal integrity optimisation were guided by simulations, prototyping, electrical measurements with Vector Network Analysers (VNA) and Time Delay Reflectometers (TDR). The layout of the FPC and the locations of the sensor ASIC pads were aligned to facilitate their integration.
During manufacturing and final assembly of the FPC, mechanical, physical, and electrical parameters are verified at several steps of the production. The bending of the FPC into a cylindrical shape for each layer required the development of custom tools. The FPC assembly and passive components placement are done manually due to the non-conventional shape.
The connection between the sensor and the FPC is made when both are aligned on a mandrel to ensure mechanical stability. The wire bonding is achieved with a wire bonding machine adapted to the cylindrical shape of the sensor and FPC. The displacement of the wire bonding head in lateral direction is replaced by a rotary movement of the mandrel controlled by an external stepper motor.
This contribution will describe the design, the optimization by simulation and test, the integration of the ITS3 FPCs, the assembly process and the electrical interconnections to the cylindrical shaped FPC. It will also discuss challenges, findings and lessons learnt.