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August 9, 2012 FDC meeting


  1. Cathode corrosion
  2. Production Construction Tracking (Dave)
    • Cathode production
    • Fixing cathodes
    • Second package refurbishment
  3. Engineering (Bill)
  4. Electronics (Chris, Nick)
  5. TDR, page 132-145 (Lubomir)
  6. Other


Participants: Bill, Dave, Chris, Nick, Vlad, Eugene, and Lubomir

Cathode corrosion

- Lubomir: a test with cathode foils, Viton/EPDM O-rings and water was done (at Blue Crab dirty area). First, the O-ring samples were pressed against the foils with a lead brick, also using G10 and aluminum plates to equalize the pressure. After a couple of days, water (from the tab) was put on the samples. Corrosion appeared immediately on the next day: very well pronounced on the EPDM sample and resembling the same dark traces that we had on the cathodes of the first and second packages (see first picture above), also some corrosion but with lighter color on the Viton sample. Olga did SEM analyses on the two sample foils at the places of the O-rings: second link above. Generally, the experiment was not done in a clean environment and tab water was used, so you see a lot of dirty stuff like chlorine. At the same time sulfur is visible in almost all the areas of the EPDM sample, while for the Viton sample sulfur is present in much smaller areas visible with high magnification. We started the same experiment inside the clean room using distilled water; expect results next week.

- Three other test experiments are running. One with production cathodes and a wire frame that started more than two weeks ago. Second one with similar samples using the small-scale prototype, running since yesterday when we were able to fix the HV. The third one is the Vlad's experiment using the vacuum chamber (picture linked) with same samples (EPDM, Viton with/without Apiezon) with same gas, but no HV, with some water inside to create moisture, all this heated with a lamp (only during the day) up to 80degC. It started yesterday; we will open the chamber on Monday.


- Dave: cathode #61 (type-3) that was supposed to be the first for package #2 showed a problem: the Mylar peeled off on the second day after gluing it. The surface is tacky. Bill will consult with the manufacturer of the Hysol epoxy and we will do several experiments to test it. The frame was re-used, Casey sanded it with #220 as he did for the frames that were converted to type-2 from type-1, but using different sanding tool. Tomorrow we will see if the surface will be still tacky and will re-sand it in the same way as for the converted frames.

- We discussed the change/instability of the resistance of the cathode contacts (present for several percents of the total number of channels). We measure the resistances every time we put a cathode in the stack. It is more prominent for the older cathodes and we are not sure if it is a results of some kind of aging, or simply due to more handling and deforming of the cathode frames. Vlad is looking if the bad channels are just statistical fluctuation. Looking with a microscope on the conductive tape he counted that we have ~76 silver balls per pad (2.5x0.5 mm^2), but only ~20 of them are big. Assuming Poisson distribution, in about few percents of the contacts one could expect a resistance that is twice bigger than the mean one. Vlad will do measurements of the resistance of the contacts on one (or more) cathode to see if it can be explained just statistically.

- Most likely the z-axis conductive tape technology will be a concern for the coming few years, therefore we discussed the conventional way of soldering to the 2micron copper. Anatoly and Casey soldered two connectors successfully, the question is what is the yield. Bill: according to Mark one has to control the heat on the copper very well, he has seen melting the copper. Another important question is, what if for example one has to replace a broken connector on the daughter-card. Unlike with the conductive tape technology, un-soldering the rigid-flex is not possible. Anatoly tried it; the only way one can do it is by cutting the flex channel by channel but then there's a good chance to cut part of the copper traces. Eugene: can we replace the connector while the daughter card is on the cathode. Chris and Dave will try to test if that's possible. Certainly these discussions will continue.

- Cathode #2 (type-3), one of the first cathodes, showed high resistance on some of the channels in two connectors, in total 8-10 channels, that could not be fixed. We decided to try to fix these channels by soldering a wire between the top of the flex pad and the copper trace just outside of the spacer ring (we have about 3mm trace there), as Anatoly suggested. Anatoly and Chris will give it a try.

- We decided to continue with the above tests and postpone for one more week the installation of the second package. We are flushing the third and the fourth packages (in parallel) with nitrogen since last Friday when we got a dewar.


- Bill will work on designing a tool for the cathodes to keep them flat during the installation. He made one some time ago that works with vacuum, but it can be used only at one side of the cathode.


- Roger talked to Allflex, they will find out how much material we have there. Roger will ask for quotation but we have to decide for how many foil sets. Eugene proposed to have two options: 1) the minimum, which is 20 sets (if we are to replace all the cathodes from the first two packages, i.e. 24, and we have 14 spares we will need 10 more sets, plus 10 spares) or 2) 50 sets which close to the total material there.

- According to Dave we can re-use a high percentage of the old daughter cards (rigid-flex assemblies), therefore we will ask for a quotation only for 20 cathodes, times 9 cards on each: ~200 cards.


- Lubomir added few subsections in the TDR, we should have a preliminary version for the next meeting.