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CMS Pixel Detector
Miscellaneous
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Phase 1
Phase 2
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Layer 1 Replacement
Layers 2-4
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| Layer 1 Replacement Elog |
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Message ID: 11
Entry time: Wed Sep 18 15:45:45 2019
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| Author: |
Matej Roguljic |
| Category: |
Module assembly |
| Subject: |
HDI sparking under HV test |
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HDI 8010 passed all test except the HV. Under the HV test, some sparks could be heard and even seen by eye on the right TBM (closer to the HV pad!). Testing it again, showed that it was indeed damaged by the spark. Sparking craters could be seen with the microscope between pads 4 and 5 of the TBM.
The same thing happened a month later, on HDI 8012. Again, it passed all the test until HV when sparks appeared and damaged the TBM, and, curiously enough, the damage was on the same position on the TBM as with 8010.
HDI 8011 was put under the HV test as well, however, this one didn't have any wires bonded to the TBMs because gold plating was missing on several of the wire bonds. Sparking could be heard, but we couldn't located where the damage happened. There is one potential candidate, shown on the photo attached to this log entry.
On 18.09. a possible explanation for the sparking was thought of. Wolfram and Matej put one of the "faulty" HDIs under HV test and noticed that sometimes a spark could be seen between the HDI handle and the aluminum base plate! It looks like the HV pin rests near the border of the HV pad and the ending of the HDI. Because of that, there is a discharge from the HV pin to the HDI handle. The discharge connects to the ROC wire bond pads closest to the HV pad providing a way to affect the TBM! This is further strengthened by the fact that the damaged TBM pad is connected to the first four ROCs closest to the HV pad. To test this, we put kapton tape on the HDI handle, near the HV pad and tested the HDI again and there were no sparks.
The HDIs to be used for L1 replacement are slightly longer to prevent HV jumping to the sensor. Incidentally, this might also solve the sparking issue. If the issue is not solved on the longer HDIs, HDI holders will have to be further isolated in the region close to the HV pad. |
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