MRB on SLAC NCR 226 MCM Encapsulation Delamination R.P. Johnson January 3, 2005

MRB on SLAC NCR 226MRB on SLAC NCR 226MCM Encapsulation DelaminationMCM Encapsulation Delamination

R.P. Johnson

January 3, 2005

January 3, 2005 MRB, SLAC NCR 226 2

ContentsContents

1. Description of the nonconformance

2. Overstress analysis and additional testing

3. Suspected root cause

4. Impacts to inventory

5. Corrective action

6. Effectiveness of corrective action

7. Recommended disposition


1. Description of Non-Conformance1. Description of Non-Conformance• Item: MCM part number LAT-DS-00898 and LAT-DS-00899.• Nonconformance: >40 MCMs have been rejected at SLAC

because they each have >15 channels disconnected between the ASICs and pitch adapter or >0 disconnected bias traces. This is ~15% of MCMs tested.

• Some additional loss of channel connections has occurred on MCMs already integrated onto trays.

• The vast majority of the loss of channels is from delamination of the epoxy encapsulation from the Kapton pitch-adapter flex circuit.


Number of Disconnected ChannelsNumber of Disconnected Channels

• 34% have zero disconnected channels

• 79% have <9 disconnected channels out of 1536 (0.5%)

• 85% have <16 disconnected channels out of 1536 (1%)

• 10% have more than 50 disconnected channels!

• The worst case has 750 disconnected channels (50%)!

1

10

100

1000

0 100 200 300 400 500 600 700 800 900 1000

Disconnected Channels

MC

Ms


Location of Disconnected ChannelsLocation of Disconnected Channels

• The disconnected channels are most likely to occur toward the MCM center.

0

50

100

150

200

250

300

0 500 1000 1500

Strip Number

Dis

co

nn

ec

ted

Ch

an

ne

ls p

er

64

S

trip

s


Time Distribution of the ProblemTime Distribution of the Problem• Before July 1 the disconnected channels are few and are consistent

with being mostly from cracked traces.• After July 1 we started seeing MCMs with huge numbers of missing

connections.• This time dependence is not understood. Teledyne claims that there

was no corresponding change to process or personnel.

0

100

200

300

400

500

600

700

800

2-Apr 2-May 1-Jun 1-Jul 31-Jul 30-Aug 29-Sep

Delivery Date

Dis

conn

ecte

d C

hann

els


Delamination During MCM MountingDelamination During MCM Mounting• 4 Tower-A MCMs suffered visible

delamination of the encapsulation while mounting the MCMs onto trays at G&A.

• These were MCMs with essentially zero broken wire bonds when tested at SLAC.

• In 3 of the cases the MCM was delivered to SLAC before July 1.

• The delamination was generally near the MCM end, in the controller-chip region.

• This effect may have a similar root cause to NCR 226, in terms of poor encapsulation adhesion.

• However, the problem was eliminated in Tower B by

– Modified procedure to attach the MCM to the jig.

– Less adhesive for MCM mounting; hence less force on the MCM from the jig.

Visible gap

142 consecutive wire bonds are broken!


Mounting MCMs onto TraysMounting MCMs onto TraysMounting MCMs onto TraysMounting MCMs onto Trays• The MCM is clamped with the pitch-adapter

pushed against a straight jig, to ensure a straight top edge for wire bonding.

• Custom pins clamp the rest of the board to the jig and also have a thin washer to guarantee the adhesive bond line.

• The jig precision aligned to the tray, gluing the MCM to the tray with a venting pattern of 3M 2216 epoxy.

MCM

Tray Panel


2. Overstress Analysis & Additional Testing2. Overstress Analysis & Additional Testing

• Preproduction MCMs used for qualification did not show any problems with wire bond breakage after 220 thermal cycles from 30C to +85C.

• This suggests that it is possible to make the encapsulation work, so that a design change may not be necessary if process problems can be solved.


Thermal Cycles of Flight MCMsThermal Cycles of Flight MCMs• All MCMs get 21 thermal cycles from 30C to +85C.• We have data before and after thermal cycles for only 21 MCMs.• The large-scale breakage appears to occur during these cycles.


CSAM (acoustic microscopy) Test ResultsCSAM (acoustic microscopy) Test Results

• High frequency (~40 MHz) sonar reflected images show in red delamination of the black Hysol epoxy encapsulation from the Kapton pitch adapter.

• SN 600, delivered May 23, has no disconnected channels (zero broken wire bonds). Nevertheless, the left-hand end shows substantial delamination.

• SN 11046, delivered September 1, has 362 disconnected channels, all on the right-hand 1/3 of the MCM. In that region the delamination approaches 100%, as dose the percentage of broken wire bonds.

Chip 0


3. Suspected Root Cause3. Suspected Root Cause

• The delamination occurs during thermal and/or mechanical stress.• However, good bonding to the pitch adapter may be enough to

withstand the stress, so the bonding process is suspect.• Thermal Strain:

– Height of the vertical cliff (raised strip on the PWB): 0.55 mm

– Temperature range from cure (125 C) to the thermal-cycle low (30 C): 155 C

– CTE mismatch between the PWB and encapsulation along the cliff: 37 ppm

– Strain: 37×106 ×155 = 0.57%

– Relative displacement if free: 0.0057×0.55 mm = 0.003 mm = 3 microns

Polyimide-glass PWB: Arlon 35N

Encapsulation:

Hysol FP-4450

CTE in plane 16 ppm 18 ppm

CTE out of plane 55 ppm 18 ppm


X-Ray Cross Section of an MCMX-Ray Cross Section of an MCMX-Ray Cross Section of an MCMX-Ray Cross Section of an MCM

Flex Circuit

Internal Cu Planes

ASIC and Conductive Glue

Wire Bond

Encapsulation FillEncapsulation Dam

Fiberglass

The “Cliff”


Stress AnalysisStress Analysis

The FEM shows the expected stress at the cliff, where the CTE mismatch is.

But it is not quantitatively conclusive, since we do not know at what stress this bond will fail.

If the bond along the cliff failed, then we would expect the pitch adapter to start to peel away from the encapsulation starting at the corner.

cliff

Pitch adapter

Corner


Destructive Physical AnalysisDestructive Physical AnalysisChip 12 Section, bond 30Chip 12 Section, bond 30S/N 775S/N 775

Copper trace

Circuit board

Wire bond fracture

DelaminationDelamination

• The wire bond is lifted up by the delamination.• The foot remained welded to the copper trace, so the bond broke.• In the case of broken bonds, the delamination extends all the way

to the dam, as seen in the CSAM images

25 m Al wire

Diane did not see evidence of delamination on the “cliff”.


Silicone ContaminationSilicone Contamination

• NASA QA noticed last autumn that the Teledyne Kapton masking tape had a silicone-based adhesive.

• This was found at the time of the production stop, so as far as we know, all MCMs built to date used this masking tape.

• Prior to soldering, the pitch adapter was covered 100% by that tape.• The tape remained on in the reflow oven and during vapor

degreasing afterwards.• The pitch adapter was wiped with acetone and alcohol afterwards,

and plasma cleaning was done before wire bonding.• NASA found evidence of silicone residue at the encapsulation-pitch-

adapter interface during DPA of MCM S/N 775.

• From experience, even a thin residue of silicone can prevent proper bonding of epoxies, so this is the leading hypothesis for the failure of the encapsulation.


4. Impacts to Inventory4. Impacts to Inventory

• 72 MCMs have been mounted on Tower-A and Tower-B trays.• Enough MCMs remain for 3 more complete towers.• Part of a 6th tower could also be equipped.• Changing the requirement from <16 broken wires to <9 broken wires

would eliminate an estimated 18 MCMs, including at least one tall. This may still allow 5 towers to be equipped.

Short Tall TotalFlight MCMs in Italy available for use 68 7 75MCMs in rework at Teledyne (returned to vendor) 19 1 20MCMs ready for burn-in at SLAC 0 0 0MCMs needing rework at SLAC (pitch-adapter debonding) 31 4 35MCMs at SLAC needing final test/inspection 9 1 10MCMs on hold that could be used if we relaxed standards 12 2 14NCR MCMs that may or may not be fixable 6 1 7MCMs on flight trays in Italy (Towers A and B) 64 8 72MCMs on non-flight trays in Italy (mostly Tower-0) 36 4 40Non-flight MCMs in electronics & FSW group 36 1 37Other rejected MCMs, classified non-flight 47 6 53Total MCMs 328 35 363MCMs still available for use (sum of the first 5 rows) 127 13 140


5. Corrective Action5. Corrective Action

• MCM screening at SLAC– 100% test of all signal and bias

traces on the existing stock following thermal cycles and burn-in. Reject all MCMs with >15 dead or disconnected signal channels or with >0 disconnected bias traces.

• MCM mounting at G&A– Mount the MCM to the jig very

carefully, working from center outward.

– Reduce the amount of adhesive, so that less pressure is required on the MCM and pitch adapter to squeeze it down.

– Visually inspect for delamination using a microscope before committing SSD ladders to the tray.

Pitch-Adapter Test Fixture


Corrective ActionCorrective Action

• New Teledyne production– Eliminate the tape with silicone adhesive from the production line

(done).

– Procure tape with acrylic adhesive and test that it really is acrylic (done).

– Clean the MCM tooling with acetone and use a water-break test to ensure that the tooling surfaces are not contaminated (done).

– Implement the 100% test of the pitch-adapter connections at the MIP-3 stage (the second test fixture has been procured).

– Develop a back-up plan using fiberglass covers in place of encapsulation, in case the encapsulation continues to fail (in progress).

• Environmental testing– Change the Tower qualification test lower limit from 30C to 20C and

the acceptance test lower limit to 15C.• This change was made because of ladder issues, but it also helps reduce

stress on the MCMs.

– Change the MCM test range to 25C+60C, still with 20 cycles. Burn-in remains +85C for 168 hours.


6. Effectiveness of Corrective Action6. Effectiveness of Corrective Action• The worst delaminations on Tower-A were caused by stress from

MCM mounting. Those delaminations propagated up to 50% during thermal testing.

• New procedures prevented breakage during mounting for Tower-B.• The SLAC screening was mostly effective, but in a couple of cases

small existing regions of broken wire bonds expanded by up to 70%.• Vibration had no effect in any case.

Tower MCM SN SLAC G&A T-Cycle Vibe T/V

A 506 0 156 174 177 178

A 634 4 21 22 22 22

A 1100 2 128 138 140 128

A 11351 0 34 36 36 51

A 11343 1 4 6 6 9

B 11392 7 13 17 not yet not yet



The table shows the wire-bond breakage history for all flight layers with >8 disconnected channels.


Effectiveness of Corrective ActionEffectiveness of Corrective Action

Flight Tower Quality:• Tower A:

– 3 of 36 SSD layers have >2% disconnected channels in the MCM.

– Other problems exist in connections within some ladders on heavy trays (subject of a different NCR).

– Only 5 of 36 SSD layers on the tower have <98% efficiency for detection of a minimum-ionizing particle.

– The average efficiency over all layers is 98.7%, so while some individual layers do not meet our goal of 98%, on average we satisfy our goal.

– Monte-Carlo studies show negligible impact on science even with all 16 towers having this degree of inefficiency.

• Tower-B:– After tray thermal cycles and testing of stacked trays, the worst case is

a 1.8% loss of MCM channels in a single layer.

– Assuming little change in tower environmental testing, we can anticipate a tower with all layers having >98% efficiency and an average efficiency >99%.


Effectiveness of Corrective ActionEffectiveness of Corrective Action

• New MCM production– We have no results yet, but our goal is to eliminate wire-bond breakage

from delamination of encapsulation. (There probably will be some residual loss of channels from cracked pitch-adapter traces, but that problem has not been observed to increase during environmental testing.)

– We are setting up a plan to bond and wire bond non-flight chips to some PWBs immediately, followed by encapsulation and then CSAM before and after thermal cycles for verification.

– The first batch of ~25 MCMs will undergo qualification testing:

• Complete MCM thermal-cycle and burn-in routine, followed by the 100% electrical test.

• Send at least 2 MCMs for CSAM. We assume that they will have no broken wire bonds.

• Thermal cycle at least 2 MCMs 200 times, followed by the 100% electrical test.


7. Recommended Disposition7. Recommended Disposition

• Scrap (use for EGSE) MCMs with >15 bad channels (dead or disconnected) or >0 disconnected bias traces.

• After this screening, use the remaining stock of MCMs for flight towers A, B, 1, 2, and 3, subject to success in all other tests and inspections.

• Continue using the MCM mounting procedures that were successful in Tower-B trays.

Documents

MRB on SLAC NCR 226 MCM Encapsulation Delamination R.P. Johnson January 3, 2005