Tension-plate review 991202
version 2
Tension-plates
Björn Lundberg
Elementary Particle Physics
Lund University, Sweden
Overview
The tension-plate (TP) is a printed circuit board (PCB) that sits on the end of the barrel-TRT straw-detector. The TP collects the signals from the straw-wires and distributes them to the read-out electronics and supports the mechanics for the read-out electronics.
The board will be two-layered, one layer will be a continous ground-plane facing the read-out electronics and the other with signal-traces and component pads facing the module.
There will not be any thermal relief around pinholes that are connected to ground. Thermal relief is normally done to simplify heating when soldering but also has the disadvantage to increase resistance and inductance because of the smaller contact areas.
Figure 1) Pad wihout thermal relief vs with thermal relief
All exposed copper areas should be gold-plated using 5 mm gold over 50mm nickel.
Since the whole detector should be gas-tight, the TP should not have any via-holes so the PCB should carry all signals on only one side. It would be possible to use a multi-layer board with blind via-holes that don't go through all layers or 'plugged' vias where drill-holes are filled with epoxy, but those solutions would be both more complex and expensive.
The PCB will be 1.0mm thick and made out of laminated fibreglass, so called FR-4.
Unfortunately, normal FR-4 contains bromine as a flame-retardant, but it is at least 50% more expensive as well as 2 months longer delivery time to get a bromine-free material.
Figure 2) Inner TP with stamp-card placement and carbon-fibre frame superimposed (blue)
Mechanics
The tension-plate is mounted directly at the end of the barrel on top of the high-voltage plate and its mechanical shape and dimension are defined by the barrel-module.
The distance between straw-to-straw is 6.8mm and the straw diameter is 4mm. Above each straw is a drill-hole where a brass eyelet is mounted and the sense-wire is crimped into place. The sense-wire is 30mm in diameter and is first fixated into the module using a twister to center it in the straw.
The straws are grouped in 8 according to how the high-voltage is distributed and each groups signal-return goes through a high-voltage capacitor to the ground-plane on the tension-plate. Because the TP sits over the HV-plate and since the HV-capacitor must be serviceable, the HV-capacitor goes up through a plastic sleeve where it is connected to a socket on the TP.
As far as possible, the routing on the TP keeps the grouping of the straws and routes the signals into the same ASDBLR-chip. However on TP#3, the density gets so high in several areas that the crosstalk between the traces may be a problem. Because of this, the layout is optimised for signal-integrity and some groups (~10%) have one or more straws going to a different ASDBLR-chip than the others. There have been several discussions whether or not this is a serious issue and the prototype TP for the outer module has its straws swapped. Measurements on this module are currently under way at Indiana University and early indications show that the effect of straws decoupling through different HV-caps has a neglible effect.
However, because of the objections raised on the possible problems, three versions of TP#3 exist:
1) One with strict connections HV-group to ASDBLR but with vias and traces on ground-plane layer as well as on the signal-layer.
2) One with the worst offending straws swapped and thereby reducing parallel traces but also breaking HV-group dependence for ~10% of the groups.
3) One without any regards to which HV-group the straws belong.
Because of a misunderstanding, it was actually the third version which was manufactured and tested at Indiana.
The signals from the sense-wires are distributed through 24W protection resistors and pin-sockets to the ASDBLR inputs on a read-out daughter-boards (called stamp-card). The stamp-cards have a modularity of 16 channels per board and have a footprint of two rows of 9 pins and two rows of 2 pins and measures 16.5x19mm.
This modularity has been chosen since it's the maximum modularity that can be common to all three designs.
Larger boards would be possible but this would require at least three different designs and extreme care must be taken to allow the same board to fit both ends of the barrel.
From a servicing point-of-view it would also be a disadvantage to have a number different spare types whereas the current stamp-card is common to all TPs and only requires one service component.
Manufacturing would also be more expensive because of increased number of start-up costs.
Figure 3)
Barrel frame structure.
Each concentrical layer is divided into 64 triangles where 2 triangles represent a module and thus a tension-plate.
There are also 2 holes in each tension-plate for gas-inlet and gas-outlet to the module itself.
Each tension-plate is mounted below a carbon-fibre frame, which effectively divides the area for the mounted read-out electronics into two triangular halves.
The module in the inner ring is simply called inner module or module #1 and the tension-plate is called inner TP or TP#1. Accordingly, the other tension-plates are called middle TP or TP#2 and outer TP or TP#3.
Each module is read out at both ends of the barrel and the modules have the same TP at each end but one end is the mirror image of the other.
This gives a total of 6 different board types to produce. Each concentric layer is divided into 32 sectors so we have a total of 3x2x32modules to produce + a number of spares.
The inner module contains 329 straws _ 329 eyelets (and resistors), 42 high-voltage capacitors and 21 stamp-cards, divided in two groups of 10 and 11 by the frame,
the middle module contains 520 eyelets and (resistors), 65 HV-caps and 33 stamp-cards (15+18) and
the outer module contains 793 eyelets and (resistors), 100 HV-caps and 50 stamp-cards (23+27).
Figure 4) Module end where tension-plate sits on top of HV-plate
Electronics
The signal from the sense-wires goes through a protection-resistor to the input of the ASDBLR-chip.
The ASDBLR-chip is an Amplifier-Shaper-Discriminator with BaseLine Restoration and contains 8 identical channels. It also functions as a 2-bit ADC on the output that is fed as differential ternary current to the DTMROC-chip.
The DTMROC-chip is a Drift-Time Measuring Read-Out Chip that can process data from 16-channels and sends its processed data through one differential dataline.
Two ASDBLR-chips and one DTMROC-chip are mounted together on a daughter-board with the size of a postage-stamp, thereby the name stamp-card.
There are currently two generations and one future generation of the stamp-cards, all with the same tension-plate footprint :
The 'old' series stamp-cards consisting of two separate stamp-cards, one for 2xASDBLR and one for 1xDTMROC where the DTMROC-card is piggyback mounted onto the ASDBLR stamp-card. The ASDBLR-card has one 8-channel chip on each side of the stamp-card. This series of card is optimized for the ASDBLR'96 and DTMROC'96/98.
The 'new' series stamp-card is a unit with two solid PCB-areas connected together with a flex-PCB. The solid areas correspond to the ASDBLR stamp-card and the DTMROC stamp-card whereas the flexible PCB replaces the intermediate connectors between the boards and thereby
increases reliability due to the removal of connectors. The two board-halves are the folded around the flex-cable and mould into position to make it similar in appearance to the 'old' card. Since the ASDBLR'99 is roughly half the size of ASDBLR'96, it's possible to put both ASDBLR-chips on the side facing the DTMROC. This card-series is optimized for the ASDBLR'99 and DTMROC'99 and is also known as the flex-card. Protection-resistors may fit on flex-cards in stead of tension-plate, see below.
The ASTRAL stamp-card where the ASDBLR and DTMROC-chips have been integrated into one chip. This is a possible solution for the future.
On top of the stamp-cards is a connector that connects to a roof-board or alternatively a flex-cable, which distributes power and signals to the stamp-cards one of the two triangles on the tension-plate.
The whole barrel read-out electronics thus consist of a sandwich with HV-plate, tension-plate, stamp-card (1 or 2) and roof-board.
Figure 5) 'New' stamp-card with 2xASDBLR+DTMROC and flexible tail.
Protection resistors (Rp)
The inputs on the ASDBLR-chip need a resistor to protect against possible HV flash-over and excessive signal amplitude. The value has been determined to be 24W.
The resistor size should be 1206 (ie 3.2x1.6mm) and is validated to withstand the required voltages, unfortunately this resistor is too big to fit on the tension-plate between straws so the smaller size 0805 (2.0x1.25mm) is chosen instead, however not validated.
The new flex-card design allows the input protection resistors to be placed on the daughter-card in stead of the tension-plate since the new ASDBLR99-chips are roughly half the size of the old ASDBLR96-chips and since the intermediate connectors are dropped in favour of the kapton interconnection.
These circumstances, together with the possibility to use TP designs without HV-grouping makes it possible to drop the resistors on the tension-plate.
Advantages with TP-design without Rp:
· Easier mounting and acceptance - less solder-joints and cheaper
· Increased reliability due to removal of components from inaccessible location
(the resistors will be facing the module and therefore not serviceable)
· Simpler tension-plate layout, overall shorter trace-length
· 0805 resistors NOT validated
Disadvantages:
· Delayed design schedule but sped up production schedule
· Layout problems due to protection resistors moved to flex-card and 0805 resistors used there as well, however changeable on flex-cards but fixed for ever on TP's
· No possibility to use HV-grouping
Plots for the different implementations on tp#1 can be found in the appendix section.
The 'Schedule' and 'Quality control' chapters below include implications of using and dropping protection resistors.
A decision on the issue must be taken during the review
Cooling
The calculated power-requirement per channel is estimated to approx. 40mW for ASDBLR and 60mW for DTMROC giving a total of 100mW per channel. This means that the inner TP will dissipate roughly 30 Watts, the middle TP 50 Watts and the outer TP 80 Watts. The total power for one end of the barrel-TRT would then be in the order of 5kW for the front-end electronics alone spread over a surface of 3.4m2. The maximum working temperature of the read-out chips should ideally be below 60ºC and definitely below 85ºC.
This power requires cooling, and two different schemes have been envisioned depending on the read-out electronics:
The 'old' series of stamp-cards requires that a cooling-plate is placed on top of the TP and cooling-tubes with water will run along between the stamp-cards. This scheme will transport the heat from the chips through the stamp-card PCB:s, through the stamp-card pins into the TP pin-sockets, through the TP PCB to the cooling-plate. This scheme is to somewhat unpredictable in the thermal conductivity between chip-to-board, pin-to-socket and socket-to-PCB-to-cooling-plate and therefore not 100% accurately in the simulations
The 'new' series of stamp-cards allow a radically different approach to the cooling in that a metal-tab is inserted between the folded two halves of the flex-card before they're mould into place. This metal-tab then connects to a metal cooling plate that lies on top of the stamp-cards with a slot for the roof-board connector. This approach has the added benefit the metal-tab between the folded flex-card halves also acts as a ground screen to protect the ASDBLR-inputs from DTMROC noise. The thermal conductivity is also optimized since it allows the chips to be mould with a high thermal-conductivity mould onto the metal-tab and therefore remove the dependence of the PCB for heat transport.
Manufacturing
Figure 6) Panel with two inner TP:s, std + mirror. Left PCB with milled outline and break-off tabs. Right PCB with drawn outline but solid part of panel.
The tension-plate itself will be manufactured in 40 complete sets of each of the 6 different types,
32 for the modules, 4 for spare modules and 4 spare TP's for servicing modules.
The PCB's can be made as panels containing both the standard and mirror version of the TP.
This will simplify mounting of components since panels normally are rectangular compared to the romboid shape of the TP's. The TP's can be milled out of the panels and only fastened using small breakaway tongues where the remaining extra material can be filed off after the TP has been mounted and broken away from the panel.
If the precision of the milling during manufacturing isn't sufficient, the TP's can be milled out from the panels later in our own workshop.
Components and mounting
The components on the TP are:
Gold-plated brass eyelets for the fastening of straw-wiresn
Surface mounted protection resistors, 24W 0805 0.125W (if used)
Pin sockets for stamp-cards
Pin sockets for HV-capacitors
The different steps in the mounting procedure are
1) mounting and soldering eyelets
2) mounting and soldering stamp-card pin-sockets
3) mounting and soldering HV-cap pin-sockets
4) visually inspect every solder joint so every joint is gas-tight
5) visually inspect every eyelet and socket for solder inside
6) measure resistance between every eyelet-pin-socket pair to 24W (<0.5W in case no Rp)
7) measure resistance between every eyelet and ground to > 20MW
8) measure resistance between stamp-card ground sockets and ground planes to be <0.5W
9) measure resistance between HV-cap pin-sockets to be <0.5W
10) replace and repair as needed between every step above