LHCb Silicon Tracker hybrids

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LHCb Silicon Tracker hybrids

• overview LHCb
• ST Detectors IT TT
• hybrid design
• procurement testing
• words on vendor issues
• conclusions

Frank Lehner U Zurich LECC Heidelberg Sep 14,
2005
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Detector overview
IT ladder

• IT-detector module
• one (10 cm) or two (20 cm) sensor ladders, 320 or
  410 ?m thick silicon with pitch 200 ?m
• hybrid at ladder end, supported on aluminum
  balcony
• TT-detector module
• full length 160 cm
• 500 ?m thick silicon with pitch 180 ?m
• two half-modules w/ 7 sensors each are mated to
  full length
• full module has at both ends hybrid stacks (out
  of detector acceptance)
• hybrids supported on carrier pieces
• further readout via 5 m long cables to service
  boxes (digitization, see talk by A. Vollhardt)
• Total electronic channels 280k
• IT 336 readout units w/ 3 chips each
• TT 280 readout units w/ 4 chips each

TT half module
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Beetle chip
size 5.5 x 6.1 mm2

• common development for LHCb silicon detectors
• designed by ASIC lab Heidelberg NIKHEF to meet
  LHC requirements
• fabricated in 0.25 ?m CMOS radiation tested up
  to 40 MRad
• 128 channel low noise preamplifier w/ CR-RC
  shaper
• analogue pipeline, programmable latency up to 4
  ?s
• L0 trigger rate 1.1 MHz
• multiplexed deadtimeless R/O within 900 ns
• input pads four row staggering, effective pitch
  40 ?m

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IT hybrid

• three Beetle chips on flex (polyimide) PCB 130 x
  77 mm2
• opening in bottom solder resist _at_ beetle
  positions for thermal and GND contact
• ceramic pitch adapter
• flexible pigtail
• connector single row 60-pin, pitch 0.5 mm
• PCB mounted on aluminum heat spreader
• 386 hybrids needed (incl. spares)
• 15 as pre-series delivered

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TT hybrid

• four Beetle chips on flex PCB 67 x 70 mm2
• stack of up to three hybrids at both ends of a TT
  full module
• three variants/flavor with different size
  necessary long/medium/short
• lowest level aluminum nitride carrier
  pitchadapter to silicon
• upper two levels copper plate pitch adapter to
  flexible interconnect
• robust dual row 80 pin connector w/ 1 mm pitch
• 322 hybrids needed (incl. spare)
• 15 as pre-series delivered

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Pitch adapters

• pitch adapters (PA) as fan-ins from 200 ?m
  silicon strip pitch to 40 ?m?narrow Beetle input
  pitch
• all three types of PAs for IT TT
• sizes from 77 x 11 mm2 to 94x24 mm2
• 0.25 mm Alumina in thin film
• metal TiW (50 nm)/Au (2 ?m)
• Al wire bonding very satisfactory
• no severe degradation after accelerated aging
• fan-in structure is mirror image of fourfold
  staggering of Beetle
• Trace width/separation
• less than 10 ?m in limited areas
• 30-40 ?m trace width elsewhere

Vendor RHe Microsystems GmbH
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Flexible PCBs

• four layers (from top routing/routing/GND/VDD)
  flex PCB polyimide 50 ?m, Cu 18 ?m
• sizes 67 x 60 mm2 (TT) up to 130 x 77 mm2 (IT)
• Au/Ni surface for Al bonding
• min. trace width/separation 75 ?m/75 ?m
• solder resist on top (laquer), polyimide bottom
  layer
• only one active component (Beetle)
• passive components (all SMD) Rs, ceramic Cs,
  PT-1000, B2B connector
• design core successfully used since 1st
  testbeam 2001
• prototypes, pre-series and series PCBs
  successfully produced at Optiprint AG

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Hybrid assembly

• all assembly steps done at single company (RHe
  Microsystems)
• procedures, tolerances, acceptance criteria
  defined in specs document
• manual chip placement, accurate 20 ?m
• aluminum wire bonding
• 25 ?m wire for chip PA bonds
• 30 ?m wire for flex PCB - chip
• regular pull tests performed
• thermal cycling 10x from -20ºC to 60ºC, 5
  minutes at each endpoint
• test stand set up at vendor
• test of basic digital and analog functionality of
  all beetles on hybrid
• repair (chip replacement) still possible

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Hybrid assembly

• full documentation of which chip goes to what
  position on hybrid from wafer maps (-gt hybrid
  database)
• excellent documentation of pull tests, electrical
  tests repair work

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Hybrid testing pre-assembly tests chips

• chip testing on uncut beetle wafer at MPI
  Heidelberg
• setup wafer prober needle card DAQ
• test of full functionality of analog and digital
  performance
• power consumption
• I2C addressing tests
• control logic
• sequential pipeline scans
• pulse shape scans
• ()
• tight quality acceptance criteria
• typical yield 90
• 3x more good chips than needed

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Hybrid testing pre-assembly tests PCB PA

• flex PCB tests at vendor (Optiprint AG)
• automatic optical inspection
• electrical test for contact
• only perfect PCBs accepted, no bad PCB found up
  to now (gt100)
• specs (bond pad width, thickness etc.) defined in
  specs document
• bond tests on lot basis (at hybrid assembly
  company)
• pitch adapters (RHe Microsystems GmbH)
• bond pull tests
• electrical test with needle card (on spare pads)
• complementary optical inspection
• several shorted traces allowed prior to repair
• laser repair of shorts final spec ? 1 short or
  interrupt
• 70 w/o defect at all remaining have up to 3
  repairs

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Tests during hybrid assembly

• test stand set up at vendor (RHe Microsystems
  GmbH)
• short and important test of basic analog and
  digital functionality
• test performed before and after thermal cycling
• 10x cycles 20C lt-gt 60C, 5 min. at each endpoint
  standard
• test is done after assembly but prior to
  chip-pitch adapter bonds
• easy detection of missing bonds
• also full chip replacement still
  possible/feasible at this stage
• so far no sign of infant mortality of 43 (43)
  thermally cycled TT-hybrids

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Hybrid burn-in test

• extensive hybrid testing program after delivery
  to MPI Heidelberg
• characterize, document and grade hybrids
• start with visual inspection
• detailed R/O test
• at start/middle/end of 48h long burn-in run at 1
  MHz rate
• R/O tests similar to Beetle chip testing wafer
• tests in box at room temperature, no thermal
  cycles
• up to 16 hybrids (any combination of IT TT
  hybrids) can run at same time
• analysis output linked into database

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Hybrid burn-in test

• full digital and analog tests (similar to chip
  wafer tests)
• I2C addressing, power consumption etc.
• raw noise baseline subtracted noise
• mapping of pipeline - relative pipeline cell
  gain 5 variation
• pulse shapes timing characteristics
• ()
• check power-on startup
• run at 1 MHz trigger rate check Beetle
  synchronicity
• check HV contacts monitor HV at 600V check for
  sparking effects during burn-in
• check PT-1000, VDD, VDDD sense lines
• yield 84 out of 92 hybrids passed on first go -gt
  analysis of failed hybrids not yet concluded

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Problems

• IT hybrid has openings under Beetle chip
  positions in bottom solder resist
• filled with silver epoxy
• connection to common GND
• thermal conductive path to heat sink
• problems on some prototype and pre-series hybrid
• openings too large problems with insulation
• potential risk of GND/VDD short repair with HV
  insulating cover
• fixed for series production

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vendor issues I

• choice of right vendors is a big issue for
  small-scale projects with design features that
  are at cutting edge
• we experienced large difficulties in hybrid
  prototyping phase which caused lots of delays
  fortunately, they were no real show stoppers of
  the project
• pursued initially approach of two different
  companies
• ceramic pitch adapter (PA) production in thin
  film technology
• hybrid assembly, SMD loading bonding
• problems with coordinating quality standards and
  agreeing on common specs among companies like
  pull strength on PA etc.

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vendor issues II

• vendor qualification process of first company
  (dedicated PA production) suffered significantly
  from low quality and large delivery delays of
  ceramics
• mainly due to overestimation of own abilities at
  company
• partially due to design/layout that could have
  been made simpler
• second (dedicated SMT) company lost interest in
  continuing after successful prototypes
• due to low quality ceramics/PA
• due to technical difficulties (4x staggered
  chip-PA bonding)
• luckily, we could identify a single company
  capable of PA production, SMT and wire bonding
  in-house (RHe Microsystems GmbH)
• developed tight relationship customer - company
  from day one
• worked/cooperated together on several design
  modifications to simplify fabrication process
• rapid successful prototypes on very short time
  scale
• proof of principle achieved, smooth transition to
  series production

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vendor issue III

• our lessons
• start with vendor qualification as early as
  possible
• be prepared for back up solutions
• work closely together with companies
• what can they do? what is achievable at
  reasonable yield/cost? what do they recommend?
• what can be changed/modified in the
  design/process to make it simpler/fail safer?
• cultivate a tight connection close relationship
  
• understand early if the company shows real
  interest in the project. This interest should go
  beyond commercial interests

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Summary

• hybrid assembly outsourced to single company
  capable of pitch adapter production, SMT mounting
  and aluminum wire-bonding
• hybrid pre-series (15) for IT TT detectors has
  been delivered just recently
• thorough hybrid testing program is installed
• 84 out of 92 tested hybrids passed our testing on
  first go
• testing not yet fully concluded - remaining
  hybrids being debugged
• expect to give go-ahead for series production
  very soon