Tracker MRB Wire Bond Encapsulation from MCM to SSD

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TrackerMRBWire Bond Encapsulation from MCM to
SSD

• R.P. Johnson
• U.C. Santa Cruz

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1. Description of the Nonconformance

• A large fraction of the wire bonds connecting MCM
  to SSD Ladders broke loose from the SSDs on both
  tray sides during thermal cycling of the 4
  heavy-converter trays now in Tower-0. In the
  worst case more than 1000 bonds were broken.
• This occurred during the normal test flow of 4
  thermal cycles carried out between ?30ºC and
  50ºC.

DAM Nusil 1142
Fill Nusil 2502
ladder
DAM Nusil 1142
TMCM
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Example Wire-Bond Failure
Lateral shift
On axis shift
The wires are broken at their weakest point, at
the ankle between the bond and its foot on the
aluminum pad. The wires were clearly shifted and
bent by movement of the encapsulation. The
encapsulation fill was observed to have lost
adhesion both to the dam and to the SSD.
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2. Overstress Analysis Additional Testing

• Note that the light-converter trays and the
  no-converter trays (including the bottom tray)
  did not suffer from this problem.
• Four light trays passed through 18 thermal cycles
  between ?30ºC and 50ºC without loss of even a
  single wire bond.

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3. Suspected Root Cause

• Thermal movement causes the encapsulation
  material to shrink and stretch. Eventually the
  bond between encapsulation-fill and the SSD/dam
  broke. Then the moving encapsulation carried the
  wire bonds with it, easily breaking them.
• Evidently the movement is greater in the case of
  the heavy-converter trays. They are the most
  asymmetric trays, with thick tungsten on one
  side. We have not had the time or resources to
  try to predict the failure from detailed FEMs.
• How close the light-converter trays are to
  similar failure is unknown, besides the fact that
  they can survive at least 18 thermal cycles.
• Silicone adhesives such as the Nusil can readily
  stretch and contract, even at ?30ºC, but they are
  known not to adhere particularly well to glass
  (i.e. the SSDs) without a primer, and there is
  probably no way to use a primer for this process.

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4. Impacts on Inventory

• This failure was one of several reasons that
  Tower-0 became non-flight.
• This used up a large fraction of flight spares of
  several parts and materials (particularly
  closeout material, MCMs, and SSDs).

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5. Corrective Action

• Several possibilities have been considered
• Try a different Nusil fill material (e.g.
  CV2500), and move the dam back to give more
  bonding area on the SSD.
• Using the CV2500 would require further
  development work.
• Several trays would have to be built and
  thoroughly tested before we could declare
  success.
• We had little confidence that this would work
  (would be grasping at straws).
• Use epoxy instead of silicone.
• Epoxy adheres much better to the SSDs and other
  materials, but it is also much more rigid than
  Nusil, so that stresses would be much greater.
• Hence there is no guarantee that it would work.
• It is very difficult to find a suitable material
• Room temperature or low-temperature cure (60ºC
  max). This is a very unusual requirement for
  wire-bond dam-and-fill, because in normal
  packages high temperature is desired anyway in
  order to drive out bubbles.
• A UV cure probably would not work, because the
  geometry would make it impossible to remove all
  shadowing.
• The viscosity has to be suitable for this
  process.
• Not to mention all the outgassing requirements.
• Almost certainly a big development effort.

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More Possibilities

• Spray a thin conformal coating onto the wires to
  avoid short circuits.
• Jerry Clinton said that he has seen this done
  large-scale in industry on wire bonds, using a
  silicone-based material.
• From the old beam-test tracker, we have
  experience with spraying epoxy into wire bonds
  (to encapsulate them), so we know that the bonds
  can withstand the force of a spray nozzle.
• But it is difficult to see how to mask the spray
  from the rest of the tray.
• In any case, this is again a development program
  that would delay the first tower by at least a
  few months.
• Dont encapsulate or coat those wire bonds at
  all.
• This was the approach taken with the beam-test
  tracker.
• Uncoated wire bonds have flown before (e.g. AMS).
• Note encapsulating the wire bonds in this corner
  joint is exceedingly difficult. It took GA a
  very long time to develop the existing process,
  and even then they could not prevent quite a few
  bubbles.
• Because of the long development time, the process
  was not ready in time for the EM tower and
  mini-tower. Therefore, this failed test was the
  first experience with thermal cycling this joint
  with encapsulation.

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Pros and Cons of No Encapsulation
Pro Con
Much less potential stress on the wire bonds during vibration and thermal cycling. Individual wire bonds are strong compared with their mass (0.03mg). John Ku conservatively calculated a resonance frequency gt20kHz, and the pull strength is gt5g. Risk of short circuits of the 16 bias wire bonds from floating debris.
Wire bonds can be repaired if damaged during handling. (The soft Nusil does not protect very well against handling damage, anyway.)
Significant savings in cost and schedule at GA.
Allows us to proceed with Tower A.
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6. Effectiveness of Corrective Action

• Risk of short circuits we cannot evaluate it
  quantitatively
• 16 bias wire bonds per MCM are the main issue.
• Shorting the AVDDA (1.5V) bonds to ground could
  disable the entire MCM and those above it (with
  the rest of the system protected by
  polyswitches), but there is no nearby ground. It
  would require a very long conductor (several mm)
  and a very good contact to the aluminum bond to
  carry enough current to short this supply out.
• Shorting the Bias HV (120V) to the nearby AVDDA
  would disable a single ladder. The other ladders
  would be protected by the series 270kohm
  resistors in each ladder bias circuit.
• Adjacent signal wire bonds (out of the 1536 on
  the MCM) would be unlikely to short together.
  The contact resistance would have to be small
  compared with the amplifier input impedance (a
  few kohm) to affect the performance. A
  light-weight particle sitting on the aluminum
  oxide would be unlikely to make good contact.
  Even if it did, we would just lose 2 channels.
• Individual signal wire bonds might get shorted to
  the SSD edge. This would disable only that
  channel and maybe cause the ladder bias to be
  lost or reduced. The combination of series
  resistance (gt270 kohm) and the input protection
  diodes would readily protect the ASIC.

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Effectiveness of Corrective Action

• Mitigation of Short-Circuit Risks
• The carbon-carbon closeouts are painted inside
  and out to avoid carbon dust.
• The tray facesheet edges are also painted after
  trimming to avoid any loose fibers.
• The sidewalls are covered with aluminum on both
  sides.
• Care was taken to avoid possible generation of
  metal chips from the copper heat strap
  installation. This was the only source of
  conductive chips seen in the EM testing, and the
  design has been modified since then to fix the
  problem.
• Conformal coating option
• This seems like a reasonable technical solution
  to avoid risk of short circuits.
• A negative aspect is that it would prevent, or
  make very difficult, wire-bond repairs in case of
  a handling incident.
• The main problem is lack of time to develop,
  test, and qualify the process.

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Effectiveness of Corrective Action

• Risk of damage during environmental testing
• There is no risk of vibration damage to good wire
  bonds, unless the two substrates are able to move
  with respect to each other. The MCM and SSD
  ladders are both securely bonded to the very
  rigid tray, so relative motion during vibration
  should not be an issue.
• Relative thermal motion of the MCM versus SSDs
  could in principle damage wire bonds. However,
  the relative motion in our thermal cycles amounts
  to angular changes of a few degrees at most.
  (20ppm CTE difference over ½ the tray width,
  times 50ºC, is about 0.2mm, compared with 3 or 4
  mm long wire bonds.) We believe that the danger
  of breakage is much less than it is in the
  presence of encapsulation.
• Although in the EM testing we did not have the
  sensitivity to notice breakage of a few wire
  bonds, we do know that there was no large-scale
  breakage of unencapsulated wire bonds during the
  environmental testing.
• Corrosion risk
• Is there any danger of there being an environment
  inside the Tracker that is corrosive to aluminum
  wire bonds?

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7. Recommended Disposition

• The Tracker recommends proceeding without
  encapsulation between MCM and SSDs. This is the
  only way to avoid additional large schedule
  delays, and the risk appears to be small.
• Ladders remain encapsulated between wafers.
  There has been no issue with that joint. It is ¼
  the length of the MCM-SSD joint, there is no CTE
  difference between the two sides of the joint,
  and the geometry of the encapsulation fill is
  very different and more simple than in the
  MCM-SSD joint.
• MCMs of course also remain encapsulated, with
  epoxy. We have had large-scale wire bond
  failures there (perhaps during thermal cycles).
  But the problem appears only on isolated MCMs,
  and the most severe thermal cycles are done
  before sending the MCMs to Italy.
• We could consider doing some conformal coating
  development work in parallel with Tower-A
  assembly, but that would require new resources
  outside of Italy.