Speaker
Description
The ALICE ITS3 upgrade at CERN replaces the innermost vertex detector layers with six self-supporting, half-cylinder MAPS sensors.
This concept introduces new electrical and mechanical challenges addressed by a custom flexible printed circuit (FPC). The FPC distributes eight 10.24 Gb/s signals, control lines, and five power supplies for 24 segments, enables a semi-cylindrical transition, and bridges millimeter- to micrometer-scale pitches. Built from three laminated sub-circuits over six layers, it is shaped via high-temperature, high-pressure molding. A Python-controlled, motorized bonding tool enables precise micro-wiring on curved surfaces. This work presents the FPC’s design, production, and integration into the ITS3 system.
Summary (500 words)
The ITS3 upgrade of the ALICE experiment at CERN replaces the innermost three layers of the vertex detector during the Long Shutdown 3 (2026–2030). This will transition from 432 ALPIDE chips measuring 15 x 30 mm, mounted on carbon frames, to only six self-supporting MAPS sensors from 58.7 to 97.8 x 265 mm2 in a half-cylinder configuration for all layers. Each sensor is built up by 3-5 independent sensor segments on the same silicon sensor.
Such major change in mechanical architecture introduces new constraints, both electrically and geometrically, including a flexible printed circuit (FPC) that connects the sensors to the readout and power electronics.
The FPC meets three main challenges:
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Ensuring the distribution of eight high-speed signals at 10.24 Gb/s and eight clock and control signals, as well as delivering 2.5 A for five power lines serving the digital and analog sections independently for each of the 24 ITS3 segments.
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Enabling a semi-flexible geometric transition between flat connectors and a half-cylindrical silicon structure.
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Performing a pitch conversion between the millimeter-scale world of electronic boards and the micrometer-scale world of CMOS sensors.
To meet these goals in terms of readout speed and geometric form, we designed, manufactured, and characterized this new type of half-cylindrical, multilayer flexible printed circuit made of specific materials, while maintaining compatibility with industrial manufacturing techniques.
The circuit consists of a laminated assembly of three flexible sub-circuits, for a total of six electrical routing layers over a length of ~26 cm. This method allows for high flexibility in distributing impedance-matched signals, while ensuring isolation between data lines, digital power, and analog power supplies.
The multilayer structure was assembled and laminated onto specific molds by curing at 200°C under 8 bars of pressure in an autoclave, to ensure precise maintenance and reproducibility of the half-cylindrical shape.
The multi-level design also simplifies pitch adaptation between the final flexible printed circuit and the curved detector. It is achieved through a three-step stair-like structure in the micro-bonding area between the two elements, with a final pitch of 100 to 150 μm.
Since micro-bonding machines are traditionally designed to work with flat elements such as printed or integrated circuits, connectors or supports, we developed and adapted a motorized mechanical tool that enables all micro-bonding to be performed along a cylindrical path directly on a half-layer in its final shape.
To significantly simplify and, most importantly, avoid any errors during the connection of several thousand micro-wires, we semi-automated and synchronised the rotational axis of the mounting jig with our existing micro-bonding equipment. This computer-assisted tool was developed in Python and also allows conversion of the machine’s flat coordinate program into the circular coordinates required by this new cylindrical setup.
This contribution will describe the FPC manufacturing process and its electro-mechanical integration in ITS3.
