stave assembly (a random walk through stave construction)

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  • Slide 1
  • Stave Assembly (A random walk through stave construction)
  • Slide 2
  • Outline I got asked to think of specifications for module mounting system build accuracy I think the intention was to try and review where to use build tolerances (stave, module placement, stave placement, cylinder placement) Found this hard so havent done it! Instead. I got a bit distracted I think the biggest issue with staves is not material, thermal stresses, flatness, bending stiffness etc for me that staves have a high intrinsic value and, unlike super-modules, have very limited possibility of re-work Began to worry that in cutting out pre-production aspects of programme in 30% cull we lost sight of ensuring the impacts of testing are considered at the development stage. Looked at assembly / testing of sub-components to see if the design and assembly process could be improved. Tim Jones - WP4 ( Meeting at ATC)10/01/20112
  • Slide 3
  • Introduction Consider the manufacture and testing of staves as an integrated programme of work with the aim of developing a specification for the assembly process. Originally this was motivated by the need to develop a set of requirements for the accuracy of module placement which in turn would allow the specifications for the module mounting system to be drawn up. It became apparent that perhaps a more holistic approach should be considered. The testing of components, sub-assemblies and finally the complete stave are considered as key parts of the process. Such considerations may have implications for the design of the stave, bus tape and modules and their associated transport and storage containers. Tim Jones - WP4 ( Meeting at ATC)10/01/20113
  • Slide 4
  • Definitions Hybrid: A readout circuit with a number of FE ASICs, auxiliary controllers and power regulation circuitry. Supplied in the form of an A4-sized panel with perhaps 8 hybrid circuits. Module: A module consists of a silicon wafer and two hybrids. Wire bonds connect the strips on the sensor to the input channels of the FE ASICs. Supplied in the form of one module in a PCB frame. Core The mechanical support upon which 24 modules are mounted (12 per side). The core consists of a carbon-fibre/honeycomb sandwich with embedded cooling. Each side of the core is covered with a copper/kapton bus tape which connects each hybrid with an end-of- stave (EOS) interface board. Stave A completed assembly of a core, 24 modules and two EOS interface boards. Tim Jones - WP4 ( Meeting at ATC)10/01/20114
  • Slide 5
  • Current SCT Large number of small elements (modules) were manufactured and thoroughly tested. Modules were then individually mounted onto relatively few large-scale structures (cylinders and disks) and attached to their final service interconnects. The collaboration became used to shipping large numbers of modules around the world safely in large batches (typically a few 10s). At the macro-assembly sites movements of large scale structures were local (within construction labs) and well controlled or were well prepared for (e.g. transport to CERN) with the assembly of custom transport infrastructure and trial runs. Tim Jones - WP4 ( Meeting at ATC)10/01/20115
  • Slide 6
  • Staves Staves (or sectors in the endcap) provide a form of local support containing, for the barrel staves, 24 modules. In the barrel section of the upgraded tracker there are approximately 500 staves. Note that in the 1997 SCT TDR local supports are shown in several drawings relating to the barrel. However, as we are all aware, the as-built detector abandoned local supports. Wonder why?? Staves fall between single modules and large macro-assemblies in terms of scale and number. Too numerous to consider operations involving them to be one offs Too few (and too costly) to be considered as a disposable item which can simply be discarded if it fails to meet specifications. Despite the numerous benefits the stave design brings, we have to accept that the fully glued stave is not (easily) re-workable and therefore not the ideal choice for this intermediate scale structure. Tim Jones - WP4 ( Meeting at ATC)10/01/20116
  • Slide 7
  • Considerations The ideal level of testing for a completed stave should be zero. Unnecessary testing risks damage to an object which is not readily repairable. Failures identified at this stage cannot easily be rectified. Staves have limited re-work capability. Removal of modules from the core will not be a routine operation or at least cannot be separated from the core in a useable state. (i.e. they get broken during removal). The possibility of damage to neighbouring modules would be a serious consideration in any decision to remove a failed module. Might be possible to replace (or at least stack) individual ASICs and wire-bond them to the hybrid and the sensor at any stage in the assembly process (hybrid, module or stave). Non re-workable interface connections made when modules are mounted onto the stave should be minimised:- All wire bonded connections are re-workable. Thermal interface between the module and the stave core is non-repairable, without module removal, and that this is unavoidable. The HV contact silver epoxy adhesive joint Not testable until the glues securing the module have cured. Non re-workable without module removal. Tim Jones - WP4 ( Meeting at ATC)10/01/20117
  • Slide 8
  • HV Contact A copper/kapton tab is attached using silver epoxy to the back plane of the silicon wafer during module construction. The electrical connection between the tab and the module back-plane is then tested at module assembly stage (In fact, it may be convenient to attach the tab before the hybrids are attached). The bus tape is re-designed to accommodate the tab and provide an area where wire bonded connections can be made to electrically connect the tab to the hybrid filter circuit. Testing of modules can be completed fully before module mounting and, with the exception of the thermal interface, all module service connections are via (re-workable) wire bonding. Tim Jones - WP4 ( Meeting at ATC)10/01/20118
  • Slide 9
  • Hybrid Assembly Pre-requisites Hybrids are manufactured in panels of 8 and are delivered with all the discrete components attached. The manufacturers QA will, or is very likely to, ensure that all parts are electrically perfect. FE ASICs have passed some basic QA (e.g. Power OK, digital performance OK) and which should have proper front-end functionality. Hybrid Assembly FE ASICs are attached to the all hybrids in a panel in one pass. The complete panel is then wire-bonded: all back-end, inter-ASIC and service bonds to the PCB are made. Test bonds are made using a custom field in one corner of the panel. Tim Jones - WP4 ( Meeting at ATC)10/01/20119
  • Slide 10
  • Hybrid Testing Electrical QA Check the full functionality of every hybrid in every possible electrical configuration (eg. bypassing, powering etc..) and the analogue performance of every FE ASIC. Check that the HV filter circuits operate up to full voltage. Mechanical QA Check the position of every FE ASIC relative to the hybrid to ensure proper positioning Thermal Cycling Pass each hybrid panel through a number of thermal cycles extending to beyond the normal operating range (-40 C to +20 C). Tony Affolder advises that hybrid thermal cycling at the panel level does not stress the hybrid in the way which thermal cycling aims to promote. Therefore this is not useful, or even desirable, as cycling at this stage might lead to deformation of the hybrid flex circuits compromising their use in module assembly. Potential issue may be that hybrids will not have been tested at operating temperature (under stress) BEFORE they get attached to silicon to become a module. Burn-in Operate the panel at an elevated temperature for an extended period to promote infant mortality. Given the comments above concerning the benefits (and desirability) of thermal cycling, it may be that conventional burn-in at an elevated temperature is also dangerous. Perhaps this stage should be replaced with an extended test at say 35 C? Final Test Verify the performance of each hybrid Tim Jones - WP4 ( Meeting at ATC)10/01/201110
  • Slide 11
  • Module Assembly Pre-requisites Tested silicon wafer Tested hybrids (2) Assembly Gluing of two hybrids onto the silicon wafer, attaching a copper/kapton flying lead to the backplane and wire- bonding the FE ASCIC input channels to sensor strips. The gluing of hybrids is a once-only operation which cannot be undone. Modules are mounted into a test frame and wire bonds connect the hybrid to tracks on the test frame which provide the interface to the test system. Tim Jones - WP4 ( Meeting at ATC)10/01/201111
  • Slide 12
  • Module Testing Electrical QA Check the full functionality of every hybrid in every possible electrical configuration (eg. bypassing, powering etc.) and the analogue performance of every FE ASIC. Compare with bare hybrid data. Check for missing /noisy strips. Perform HV scan up to specified voltage. Check leakage current is within specified limits. Mechanical QA Check the position of each hybrid relative to the silicon to ensure proper positioning when mounted on the stave. Thermal Cycling Pass each module panel through a number of thermal cycles extending to beyond the normal operating range (-40 C to +20 C). Check the thermal performance of both hybrid/silicon interfaces after cycling. Check for missing/noisy strips which might indicate lifting front-end wire bonds. Long-term test Operate the module under power and high voltage for an extended period of time to check