de Haas!!Arie!!a.p.dehaas@phys.uu.nl!!NIKHEF Utrecht!!R. van de Graafflab!!PO Box 80.0000!!3508 TA Utrecht!!Netherlands!!ALICE!!TRACK!!Electronics for trackers!!Very low-mass Microcables for the ALICE SSD detector!!990602026

A. P. de Haas, C.J. Oskamp, A. van den Brink, P. Kuijer Nikhef Utrecht V. N. Borshov, S.K. Kiprich, V.M. Ruzhitsky SRTIIM Kharkov, for the ALICE collaboration
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The ALICE Inner Tracker (ITS) silicon strip layers will use kapton/aluminium microcables instead of traditional wirebonding throughout the front-ends. The silicon strip detector (SSD) consists of two concentric layers, each layer is made up of more than 30 one metre long carbon-fibre spaceframe ladders. On each ladder are mounted, depending on the layer, 23 or 26 front-end modules in an overlapping fashion. Ladders also overlap in order to eliminate dead area in the layers. In total there are 1770 modules with 1536 strips giving aprox. 2.5 million channels.Heavy-ion physics dictate a very low-mass design. A front-end module consists of a double-sided strip detector and a double-sided hybrid on which the 12 (2x6) 128 channel readout chips are mounted. The detector and the hybrid are decoupled mechanically, the detector is mounted on the ladder with high accuracy while the hybrid is connected to the cooling pipes running along the ladder. These pipes are loosely coupled to the ladder structure to accomodate differences in thermal expansion.  The hybrids are mounted in a different plane as the detector, so the connection between the chips and the detector must be made with flexible microcables, at the same time eliminating the need for fan-outs, as this can be done in the cables. The cables are made of 12 um. Thick kapton foil with 14 um. thick aluminium traces, width 40 um. and 88 um. pitch on the chip side and 95 um. on the detector side. The traces are directly bonded to the bonding pads through bonding windows in the kapton foil using a low-power, low-temperature and low bonding force (typically 10-15 grams) single-point TAB-bonding process with specially designed bonding wedges. The kapton foil is glued to the silicon surface to increase the mechanical strength. As the chip input pitch of 44 um. is too small for the cable design, half the traces run straight from the first row of bondpads and the other half folds around to connect to the fourth row. The cable also includes the output connections of the chip to the hybrid, this side of the pattern has a fan-out which fits in a 22 pin ZIF micro- connector. In this way it is possible to test bonded chips before mounting them on the hybrid. The hybrid is 0.3x10.5x68 mm., just big enough to fit 12 chips and associated SMT components and the two cooling clips connecting to the pipes. To save material the read-out and power bus connecting the chips and components consists of a multi-layer microcable using the same technology. The via's in this cable are made by bonding traces in different layers to each other through bonding windows. Here the kapton foil is 20 um thick and the aluminium 30 um. and trace widths range from 70 um to 2 mm. Only the part of the cable containing the bondpads for connecting the chips and the solder/glue pads for the SMT components is glued to the hybrid, the rest of the bus is floating free in the air. The bus has short "umbilical string" on one side which is bonded to the ladder cable. This is a similar cable of variable length (depending on module position) which runs along the ladder to the endcap module at the end of the ladder. This cable also ends in a ZIF connector, so again a completed front-end assembly can be tested before mounting it on the ladder. Working in close collaboration, the NIKHEF UTRECHT and SRTII Kharkov groups have designed, produced and tested these cables, as well as researched postioning and assembly methods. A double-sided strip detector using prototype cables has been installed in the NA57 experiment at CERN in 1998.
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The ALICE Inner Tracker (ITS) silicon strip layers will use kapton/aluminium microcables (12/14 um thickness) exclusively for all interconnections to and from the front-end chips and hybrids, completely eliminating traditional wirebonding.  Benefits are increased robustness and an extra degree of dimensional freedom.  Utilising a low-power, low temperature and low-force (10-15 grams) single-point TAB bonding process, aluminium traces are directly bonded through bonding windows in the kapton foil to bond pads on the chips and the hybrid. The same technique is also used to interconnect these microcables to create multi-layer bus structures with "bonded via's". A double-sided strip detector using prototype cables has been installed in the NA57 experiment in 1998. 
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