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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. Encapsulation Studies. 96 modules encapsulated with Sylgard 186 Modules tested before/after encapsulation

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UCSB Encapsulation Studies

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  1. 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

  2. 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

  3. 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. 1 hour 2 hours

  4. 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)

  5. 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)

  6. 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

  7. 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

  8. 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

  9. 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. cold

  10. 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

  11. 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.

  12. 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

  13. 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

  14. 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.

  15. 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

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