August 16, 2012 FDC meeting
- Cathode production Construction Tracking (Dave)
- Cathode corrosion
- Status 
- Possible procedure changes
- Conductive tape technology
- Engineering (Bill)
- Cathode strong-back
- Other: epoxy tests
- Electronics (Chris, Nick)
- TDR, pages 132-145 (Lubomir)
Participants: Eugene, Bill, Dave, Simon, Vlad, Beni, and Lubomir
- Dave: working on 8 new cathodes to be used in the second package. The bottleneck is the card gluing since we have not decided if we want to change the procedures.
- Lubomir about the SEM results: On the wire samples some small crystals (~3um) are seen containing sulfur, Olga counted about 25 of these on 2cm wire length; too small quantity to account for the copper damage on the cathodes. On the EPDM surface - no conductivity and very low yield from the microscope - still one sees some sulfur on the used EPDM, but much less or nothing on the virgin EPDM and on the prototype EPDM. Dave will check if the virgin and used EPDMs are from the same batch.
- Results from the vacuum chamber (Vlad's) experiment: it was running from last Thursday (end of the day) till Monday morning, pictures linked above. One clearly sees damages on the EPDM sample, just a little on the Viton one and no damages are seen on the Viton+grease sample. The samples are now with Olga and will have SEM results next week. Bill proposed to make another sample with Viton+grease with curving shape to have more potentially damaged area and then measure if the resistance has changed. Vlad will check the resistance of the samples that we have now. On Tuesday we started the same test except there's no water inside the gas volume.
- As of now we see two solutions of the corrosion problem: using the technology that we used already for the third and fourth packages, i.e. Viton+grease, or in addition to that, covering big areas on the cathode including all the traces with Kapton glued with Hysol. Using Kapton foils doesn't seem to create leakage but we tested this on a flat Lexan sheet. We decided to use the standard technology for the second package, i.e. only Viton+grease. When we come to the first package (in about two-three month from now) we will discuss again the use of Kapton; there we may have to use it if we are going to re-use the cathodes from the former second package since some of the channels have to be fixed with conductive ink. Of course we will wait for the latest SEM results before continuing with this procedure. Note that one once you chose one of the two options you can't use later the other one, like you can't use epoxy if the surface is greased.
Conductive tape technology
- Microscope pictures from the damaged cathodes (package one) on which we have removed the rigid-flex assemblies (or daughter cards) are linked. On the card contacts one can see the silver balls, on the foil contacts the conductive tape is visible with some balls inside and also on the copper pads (the cathode side). We found it is important to count the number of dents (craters) on the copper pads after removing the conductive tape with heating and cleaning the place. Vlad did this for one full cathode. First he measured the resistances on all the channels, then the cards were removed and then he did the counting by classifying the craters roughly as big and small (factor of 2-3 in diameter). The distributions of the big/small/sum of the two are Poissonian with a mean value of the sum of ~5. By correlating the conductance (1/R) with the number of the balls one can estimate roughly that a big ball has about 1.2 Ohm on average and a small - ~3 Ohms. Then some correlation is visible between the conductance and the weighted sum of the number of the balls (first three histograms, in linear, log-y scale and as a profile). There are of course errors in counting the balls, but in average for most of the cases the resistance can be explained by the number of the balls, i.e. statistically.
- Another important result: all the first test samples using conductive tape technology were re-measured. On the first 8 cards all the channels showed resistance below 2 Ohms, and most of them being 0.6-1 Ohm. Among these ~half were with tinned card contacts, four were irradiated with ~1-10KRad, one was cooled with liquid Nitrogen. The last card (no special treatment) had many channels in the 10-100 Ohm region. When the samples were made we checked them with a testing card which tells you only that there's conductivity, we didn't measure the resistances. We suppose the last card was not glued correctly from the very beginning. These samples were made Feb-April 2011.
- We conclude: most likely the variations in the resistances we observe on the production cathodes are due to mechanical deformation of the cathode frames. The samples were made on small g10 plates and when handling them you don't have the leverage to deform them. The cathode deformations, both pringling and radial deformation may cause curvature at the contacts which can move the balls from their position.
- We discussed if (1) we are going to continue with the same conductive tape technology but making sure the cathode doesn't deform after the cards are glued to the cathode, or (2) we use conventional soldering or a new technology for bonding the contacts. After long discussions we decided to go with (1). Bill: not comfortable with the current procedure (actually nobody is), but making sure the cathodes don't move should help. Eugene: this question had to be discussed five years ago, now we should continue with the conductive tape technology, even if it turns out say 10 channels are not working it will not be a problem. The main argument to use the conductive tape is that you can repair it (Simon) or fix several channels using wires (Anatoly demonstrated that's possible). On the other hand, once the cards are soldered you can't re-solder them, although fixing some components on the cards is also possible. New technologies, cathode designs (Bill) were discussed but all these require at least a lot of R&D, followed by complete refurbishment of the cathodes and we are not in a position to do this now.
- Discussions how to make sure the cathode stays flat during all the operations after installing the conductive tapes, which includes: gluing the cards to the back of the cathode frame, moving the cathode from one place to another, storing the cathodes, cathode installation in the package, colling tube installation, testing the cathode channels, connecting the cables to the cards.
- After the meeting Bill, Dave, Lubomir and Casey discussed these issues at Blue Crab. Bill will make two strong-backs (lifting frames) with suction caps (use vacuum), to be used on the foil or the back side of the cathode for transportation. Casey will work on a table with rods to store the cathodes in flat position. Same clips that are used for grounding will be put on both sides of the cards attaching them to the wire frame, which will prevent radial deformation during tube installation, cabling and testing.
- Bill: Mylar gluing problem we had recently is due to humidity. Several tests were done showing much better results when using lamps for heating. Bill suggested using lamps from now on the improve the curing process.
- It turned out wrong Gerber files were used for the production of the extra 4 PCB sets ($9K). Eugene will discuss with the administration how to exclude such mistakes in the future; he suggested manufacturing another 4 PCB sets.
- Lubomir: the version linked above is almost complete; just sub-system section needs more work. Everybody is encouraged to make his remarks.corrections by the next meeting.
- Beni asked when the third package will be in 126 for testing. Need to install the grounds, all the parts are ready, Vlad will start working on that, then the cooling tubes have to be installed and finally Chris will do the HV connections - in about 2 weeks must be ready.