Speaker
Description
For the High-Luminosity Upgrade of the Large Hadron Collider, the ATLAS detector will receive a new silicon strip tracker. The module design utilizes readout hybrid flexes and powerboard flexes glued onto the sensor surface. In total, approximately 13,000 end-cap hybrid readout flexes and 6,000 end-cap powerboards are needed to build both end-caps of the strip detector. This contribution summarizes the latest status of the mass production of hybrid flexes and powerboards performed at the University of Freiburg, focusing especially on the challenges encountered due to quality variations and design decisions.
Summary (500 words)
The ATLAS detector will be equipped with a new silicon strip tracker as part of the High-Luminosity Upgrade of the Large Hadron Collider. The module design utilizes readout hybrid flexes and powerboard flexes attached to the sensor surface. To ensure efficient detector operation, these strip modules must achieve a noise occupancy of 0.1% at a detection efficiency of at least 99% throughout the full runtime of the High-Luminosity LHC. The ITk strip detector comprises one barrel section and two end-cap sections. In the end-caps, the strips are oriented towards the beam axis, resulting in non-rectangular sensors. To optimally tile the individual segments of a disk, six different sensor geometries are employed. They are "stereo annulus" ring segments with radii ranging from 384 mm to 968 mm from the beam axis. The readout hybrid flexes and powerboards must conform to these curved sensors to facilitate efficient wirebonding of the readout ASICs to the sensor and to provide sufficient area for the supporting electronics.
The readout hybrids are equipped with between 6 and 12 ATLAS Binary Chips (ABCStar) as frontend ASICs and between 0 and 2 Hybrid Control Chips (HCCStar) to aggregate the data. To accommodate the geometric requirements of the sensors, 13 geometrically distinct variants are required in the end-cap region. The primary functions of the flex circuit are to distribute power to all chips and to connect 160 Mbit/s data, trigger, and control signals between the chips. Due to the absence of connectors, these flex circuits must be wire-bonded at multiple stages of the detector assembly process. This includes initial ASIC attachment, followed by multiple testing steps before and after bonding to the sensor, and finally to the local support structure. Surface quality and contamination have been major concerns during the initial phase of production.
The powerboards provide a DC-DC converter, which steps down 11V to 1.5V for the readout ASICs, an Autonomous Monitoring and Control chip (AMACStar), as well as high-voltage (HV) filtering and switching for individual sensor HV cutoff capability with a reduced set of HV cables. To accommodate the spatial constraints on the different end-cap geometries and the power requirements of the various module variants, four distinct powerboard geometries are required. The DC-DC converter employs a radiation-hard bPOL12V and a solenoidal air coil, due to the necessity of operating in a 2T magnetic field while being subject to spatial constraints, which precludes the use of a toroidal coil. This imposes stringent shielding requirements, as the powerboards are glued directly to the sensor surface. The extensive control capabilities and power delivery requirements dictate rigorous quality control procedures at both room temperature and operating temperature (-35°C). With up to 6W of power delivery per powerboard, a production rate of 100 units per week, and a 5-day testing protocol, the testing setup presents its own set of challenges.
This contribution summarizes the experience, current status, and challenges associated with the quality control of hybrid and powerboard flexes, as well as the quality control of fully assembled powerboards.