Large Area Precision Cathode Boards for ATLAS Muon Upgrades

The largest phase-1 upgrade project for the ATLAS Muon System is the replacement of the present first station in the forward regions with the so-called New Small Wheels (NSWs) during the long-LHC shutdown in 2019/20. The NSWs will be equipped with eight layers of small-strip thin gap chambers (sTGC) arranged in multilayers of two quadruplets, for a total active surface area of more than 2500 m^2. Large area circuit boards for the sTGC quadruplets have trapezoidal shapes with surface areas up to 2 m^2. To retain the good precision tracking and trigger capabilities in the high background environment of the high luminosity LHC, each sTGC plane must achieve a spatial resolution better than 100 μm to allow the Level-1 trigger track segments to be reconstructed with an angular resolution of approximately 1 mrad. The precision cathode plane has strips with a 3.2 mm pitch for the precision coordinate and the cathode plane on the other side has large area pads for triggering. The position of each strip must be known with an accuracy of 40 µm along the precision coordinate and 80 µm along the beam. On such large area detectors, the mechanical precision is a key point and then must be controlled and monitored all along the process of construction and integration into quadruplets and wedges. The cathode boards are produced in industry by either CNC machining or chemical etching of copper plated FR4 boards. An insulating pre-preg layer is pressed on top of the copper readout elements. Material flow of the underlying FR4 boards during the cathode board manufacturing processes of CNC machining or etching and pressing have been found to have significant impact on the placement of the copper read-out elements. Production of the sTGC detectors is well underway, including the assembly of the 3 metre long wedges maintaining 100 μm position accuracy for installation into ATLAS. We will describe the technological innovations, production challenges in industry and sTGC construction sites, measuring and tracking the dimensional precision of the circuit boards, and alignment of the chambers in ATLAS.