ucsb encapsulation studies

UCSB Encapsulation Studies

UC Santa Barbara Based upon a 6 week study by F. Garberson

in collaboration with

A. Affolder, J. Incandela, S. Kyre and many others

UCSB Encapsulation Study, CERN - March 15, 2005 - incandela

Encapsulation Studies

• 96 modules encapsulated with Sylgard 186• Modules tested before/after encapsulation

No channel failures were found

• For thermal cycling, an environmental chamber was leased. Cycles were run in 3 modes: SEVERE:

-30 C to 50 C at 45 min/cycle and then tested VERY SEVERE

-40 C to + 60 C or -20 to +80 C at 65 min/cycle and re-test. EXTREME

-40 C to +80 C at 95 min/cycle and re-test. Between 6 and 50 thermal cycles in all cases


Humidity• Humidity not well controlled

Environmental chamber does not have a dedicated dry air supply

The heater allows outside air to enter

– Humidity spikes upwards and condensation appears at beginning of every heating cycle

At the end of every set of cycles, modules are held at high temperatures until humidity drops.

– Modules come out dry

• Overall a more severe test.

Thermal Cycles from -30 to +50 C

-30

-20

-10

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70

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idit

y /

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rees

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Relative Humidity

Temperature

1 hour 2 hours


Severe Thermal Cycles

• 96 modules tested At least 7 cycles each between -30 C

and +50 C at 45 min per cycle

• 5 had new failed channels after cycles One had 9 new 2 sensor opens, three

others had 1 new 2 sensor open, and one had 1 new 1 sensor open.

– 2 bond lift-offs visible

• In total, 0.03% channels were affected Note that a few opens were found in

the PA itself– Not underneath encapsulant – It appears that micro-cracks or

scratches in the PA opened up with multiple thermal cycles (possibly due to residual humidity freeze cycles - expand the cracks)


Very Severe Thermal Cycles

• An intermediate range Thermal cycles intended to be

extreme, were not quite as extreme as expected because the temperature ramp intervals were too short.

• 9 modules cycled -40 C to +60 C at 65 minutes per cycle 3 modules had total of 5 new 2

sensor opens (2SO): 0.11% of channels.

• 46 modules cycled -20 C to +80 C at 65 minutes per cycle 15 modules had 60 new opens:

0.24% of channels 3 APVS on 2 modules fail: 1.5% of

channels (discussed later)


Extreme Thermal Cycle

• 34 modules cycled -40 C to +80 C at 95 minutes per cycle

• 16 modules had total of 58 opens: 0.29% of channels

• Notes of possible concern: 1 APV with dead FE


4 Modules Irradiated• Report from T. Affolder.

On the 4 SS4 modules (16 chips), 3 dead chips and 1 broken wire bond developed during the irradiation or afterward.

– One of the dead chips was NOT seen at Karlsruhe.

• Details: Module 5049:

– Chips 3 & 4 dead. – No charge inject response, noise consistent with a dead or

saturated chip. Fluence=3.7E14 Module 5102:

– Chip 2 dead. Same symptoms as above. Fluence=3.9E14 Module 5071:

– One new 1 sensor-to-sensor open. Fluence=4.8E14 Module 5050:

– No new problems. Fluence=5.3E14


APV Failures• Did the encapsulant cause the APV failures?

In chip failures of thermal-cycled and irradiated modules, there was less current drawn from the preamp than normal.

No indication that back-end bonds were affected. – Currents moved as expected when changing initialization of the readout circuitry.

• Encapsulant over FE bonds removed, bonds remade for 4 APV All 4 chips had the expected currents when varying the chip initialization. This points to the encapsulation shearing bonds to the FE somewhere.

warm


APV Failures

cold

• Did the encapsulant cause the APV failures? In chip failures of thermal-cycled and irradiated modules, there was less

current drawn from the preamp than normal. No indication that back-end bonds were affected.

– Currents moved as expected when changing initialization of the readout circuitry.

• Encapsulant over FE bonds removed, bonds remade for 4 APV All 4 chips had the expected currents when varying the chip initialization. This points to the encapsulation shearing bonds to the FE somewhere.


Opens• Overall results

Total of 106 two sensor opens and 30 one sensor opens

– 90% from Extreme cycles Many caused by bond liftoffs

– Notably on module edges where bonds are difficult

Most opens were not visible

• Hypothesis One issue may be that these

were old modules that had a number of wirebonding issues.

Decided to try a control sample of recent production modules

OB2 Opens

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OB1 Opens

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Control Sample• 9 Modules without encapsulant put through both Very Severe

and Extreme cycles, with electronic tests done in between. 6 with ST silicon and type 20 hybrids, 3 with HPK silicon

• 23 cycles from -30 to +50 C, 15 cycles from -20 to +80 C No modules damaged

• Modules were then encapsulated Repeat same set of cycles Again, no modules damaged

• Lastly they were extreme cycled: -40 C to +80 C in 95 minute intervals, and made it through a full 19 cycles (far more than any other set of modules). 8 of 9 were perfect 1 module had three new two-sensor opens.


All Results• Rates of opens

Severe cycles: 0.03% (old modules) 0.00% (new) Very Severe: 0.15% (old modules) 0.00% (new) Extreme: 0.30% (old modules) 0.02% (new)

• Locations of opens: (old modules) 80% pitch adaptor to sensor and 20% sensor-to-sensor No known Backend Hybrid wirebond breaks Several power bonds (see below)

• Chip Failures Irradiated OB2 modules (4 old ones)

– 3 dead chips due to broken power bonds Thermal cycles

– 4 dead chips due to broken power bonds


Conclusions

• In older modules - Encapsulation causes 0.03% to 0.3% of wirebonds to fail in SEVERE and EXTREME thermal cycles, respectively Most failures PA to SENSOR

• Wirebond failure drops to completely negligible level in a sample of newer modules

• Most serious concern is that power bonds are pulled due to location between APV and PA

• Notable non-problems No back end bonds failed in thermal cycles or irradiation Almost no SENSOR-SENSRO bonds failed either


Follow up studies• What happens to noise

performance of modules as a function of bias voltage with encapsulation? We see no change in noise

due to presence of encapsulant

• Can encapsulation be done in a dry environment? To some extent, yes, but not

so easy to do in an super low humidity environment

Should be considered in comparison with possible humidity/corrosion effects over time for non-encapsulated bonds.

OB2 DOFF encapsulation compare

1.75

1.77

1.79

1.81

1.83

1.85

1.87

200 250 300 350 400 450 500

volts

raw

no

ise

avg std. 1 encapsulated

5358 - not encaps, stand 3

5478 - not encaps, stand 3


Recommendations• Either:

Wirebond back end bonds of hybrids• If these bonds have something fall on them or short them, we

lose whole chips in many cases if they are not encapsulated. There appears to be no risk with encapsulation.

• Do not encapsulate any other bonds• Could conceivably encapsulate sensor-sensor without much

risk, but the gain is marginal and it would mean that all bonding centers would need encapsulation equipment.

• If only BE bonds encapsulated, then we would need encapsulation capability only at a few locations (possibly only CERN)

• OR• Drop encapsulation altogether

• May mean less work but maybe not, and• Would leave most control bonds prone to shorting or damage

ucsb encapsulation studies

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