MRB on SLAC NCR 226Excess of Disconnected MCM Channels

R.P. Johnson

November 9, 2004

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1. Description of Non-Conformance

• MCM part number LAT-DS-00898, SN 11378 was found to have 151 channels disconnected between the amplifiers and the pitch adapter.

• This MCM had zero non-conductive cracked traces (from the visual inspection) and zero dead amplifiers.

• This problem was found Friday, November 5, at SLAC while testing 42 MCMs using the new electronic pitch-adapter test fixture.

• Earlier the same week 1 MCM already attached to a tray and SSDs at G&A was found to have about 300 disconnected channels.

• Also, at G&A and Pisa 2 trays were found to have a disconnection of 1 or more bias lines, which may be from the same root cause.

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Inspections and Tests• A new fixture designed at SLAC solves our old problem of not being

able to test the connections to the amplifier inputs until SSDs are attached. This became a critical problem when the MCM mounting changed from screws to gluing.

• Initially our assurance was based on– Wire bond quality control at Teledyne

– Visual inspection of all wire bonds and bond feet at MIP-2 at Teledyne

– Sample testing of several preproduction MCMs by probing each pitch-adapter trace.

• After an MCM on Tower-0 was found with 25 cracked traces, we did the following:– Visual inspection for cracks in pitch-adapter (and manual probing with a

DVM for continuity) traces before shipment to Italy. This appears to be quite effective, albeit with a rejection rate of 14%, requiring no more than 8 such cracked, nonconductive traces.

– Tested with 200 thermal cycles whether the incidence of non-conductive cracks gets worse (it does not).

– Stepped up existing efforts to get an electrical test method developed.

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The Electrical Test Method• Use a conductive, flexible strip (zebra strip) to short circuit 100% of

the pitch-adapter traces to ground.– Bias HV is turned off, to protect it.

– AVDDA must be turned on, but the resistance of the zebra strip is high enough that it does not draw enough current to cause any problem.

• Run a charge-injection scan and measure the response of each channel.– Channels with inputs shorted to ground will look dead.

– Channels not connected to the pitch adapter will look alive, unless the amplifier is already dead for some other reason.

• So, if a channel measures good in the normal electrical test, with the input floating, AND it measures dead with the zebra strip applied, then we are certain that it is a good, connected channel.

• If a channel measures good with the zebra strip applied, then either– it is disconnected from the pitch adapter, or

– the zebra strip did not make contact with the pitch adapter (false negative).

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Pitch-Adapter Electrical Test Fixture

CONNECTOR SAVER

TEST FIXTURE

ZEBRA CONNECTOR

GROUND LEAD

STORAGE CASE BASE

MCM WITH PITCH ADAPTER

SOFTWARE/ELECTRICAL FIXTURE

Designed by Peter Amnuaypayoat

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Disconnected Ch. vs. Cracked Traces

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Disconnected (electrical)

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Visual inspection and test data from 41 MCMs. Note that the number of disconnected channels is probably a little bit overestimated, due to occasional false negatives given by the test fixture. (False positives are not possible, unless the connection is intermittent.)

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Disconnected Ch. vs. Cracked Traces

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Cracked Traces

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sVisual inspection and test data from 41 MCMs.

The 42nd MCM had 151 disconnected and 0 cracked traces.

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Bias Disconnects

• The pitch-adapter electrical test fixture will not detect problems with the bias traces.

• Our visual inspection rejected MCMs with even 1 cracked nonconductive bias trace, but this does not catch all problems.

• Up to now we relied on– 100% electrical test of the PWB.

– Visual inspection of the bias resistor.

– Wire-bond QC and visual inspection at MIP-2 at Teledyne.

• Still, there are 2 cases of MCMs in Italy with bad (sometimes intermittent) bias connections.

• Therefore, we are now doing the following procedure on all MCMs before shipping to Italy:– Turn the bias voltage down to 2.5 V, and turn on the MCM power.

– Using a voltmeter, probe each of the 16 bias traces past the bend region, and verify that the appropriate voltage (2.5 V or 1.5 V) is seen.

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2. Overstress Analysis & Additional Testing

• We tried out the test fixture on the 4 nonflight MCMs that we recently passed through 200 thermal cycles from 30C to +85C.– 3 were not bad, with the worse case being 15 disconnected channels.

– 4th had 60 to 80 disconnected traces, depending on the test trial. The results were variable, and we found clear cases of false positives. The pressure from the fixture was causing reconnections of some of the failed traces.

• This experiment would be more interesting if we had a measurement before the thermal cycles as well as after.

• A more relevant test will be to check the connections at MIP-3 (i.e. at Teledyne) and then again after thermal cycles and burn-in at SLAC.

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3. Suspected Root Cause• We suspect failure of the wire bonds where they connect to the pitch

adapter. They are not visible (black encapsulant), but we have indirect evidence:– Cracked traces in the visible region of the pitch adapters are eliminated

by our thorough visual scans.– Our visual inspection at MIP-2 ensures that the wire bonds were there

and were initially connected (we couldn’t fail to see 151 disconnected or missing bonds!).

– The MIP-2 visual inspection also rules out that any significant number of the disconnected channels could be due to pitch-adapter cracks close to the cut edge.

– The very low failure rate of wire bonds on the power and digital I/O GTFE connections nearly rules out wire-bond failure on the ASIC side of the bond.

– The pitch-adapter substrate is the more difficult material to wire bond to.

• So, we believe that either the bonds failed during encapsulation or later during thermal cycling. The intermittent ones must have failed after encapsulation in order to have the foot still so close to the pitch-adapter trace.

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Impacts to Inventory

• The 1-in-42 (2.4%) occurrence of large numbers of disconnected bonds is not a big problem for MCM availability.

• The 14% loss due to >8 cracked traces is a big problem. At that rate we have to review again whether we will have enough parts to complete the build, given that this is not the only source of loss of MCMs (e.g. Tower-0). I suggest that we consider raising the maximum number of bad channels from 8 to 15. This is the limit used at the tray level and corresponds to a worst-case inefficiency of 1%.

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5. Corrective Action

• Test all MCMs with the PA test fixture both at MIP-3 and before shipping to Italy. We need to manufacture one more fixture.

• Review and possibly improve the Teledyne wire-bonding controls. The present procedure calls for the following:– Plasma clean in Tepla (Recipe #1) prior to wire bonding.

– At the start of each shift, wire bond the test pattern on a single die on the wire bond set up PCB. Perform 100% destructive pull test. During actual wire bonding, perform Non destructive Pull Testing (NDPT) on every 3rd unit. NDPT test on 2 die, 30 wires on a single PCB only.

• Update and re-release the burn-in and test procedure LAT-TD-02367.

• Verification plan:– Test our remaining MCMs with the PA fixture asap.

– Consider doing extra thermal cycles on some non-flight MCMs, with tests before and after.

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6. Effectiveness of Corrective Action

• The PA test fixture is almost guaranteed to find the missing connections, the only exception being cases where the pressure from the fixture results in a temporary reconnection. This will not happen on all or even most of any large number of missing connections on a given MCM.

• There is some concern that the tray-level thermal cycles could produce more disconnections.– The burn-in procedure thermal cycles are more severe in terms of

number of cycles (20 vs. 4) and range (30C to +85C vs. 30C to +55C).

– This would be the reason to try some more extensive thermal cycle testing on non-flight MCMs.

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7. Recommended Disposition

• Scrap (use for EGSE) the MCM that is the subject of NCR 226.• Return all MCMs to SLAC that have not been tested with the PA

fixture and bias-circuit hand probing.• Begin mounting to trays those MCMs that passed the PA and bias

probing tests.