Minute of Meeting on EMI Taping of Towers
There was a lot of discussion about whether it was really needed and who would do it that I can't really capture here. There will have to be further discussion of this at management meetings. For the moment it is clear that tower A will be done at SLAC; that it would be best if Plyform could but the foil on the top tray, and that the foil and tape should really go through the tower thermal vac test to make sure there are no problems with it.
The technical discussion in the last 20 minutes of the meeting is easier to summarize. Below is a list of concerns that were expressed and responses that have been taken or who is going to take action:
There are action items for NORDBY and HIMEL.
There are resistors on the cable that will get covered with aluminum tape. They are only protected with conformal coating. This is not enough to be sure the tape won't short them out. General agreement with this concern. We will protect them with kapton tape. Nordby will add this to the draft procedure.
The ground layer of tape on the outside of the cable could cause electrical change. Himel checked with Johnson about this after the meeting he was not at all concerned about this over such a short distance. Note that after the tape is applied there will be an electrical test.
Abrading the aluminum could cause conductive dust which could cause shorts. This shouldn't be done on the completed tower. Himel will check with NASA to see if the abrasion is really necessary. If it is, we may be able to vacuum or clean afterward. The problem the abrasion is supposed to solve is that aluminum has a nonconductive oxide that can prevent good electrical contact.
Cleaning with acetone is a problem because it dissolves the solithane and the adhesive on the tape. Can its use be eliminated from the procedure? Himel will check with the same NAS person to see if it is absolutely necessary. Use of Isopropyl alchohol, IPA is not considered to be a problem
It was suggested that the tape on the vertical corners be applied in many short pieces instead of one long piece. Nordby will see what works best on the EM tower.
There was concern that the aluminum would come off the sidewalls if the tape is pulled off to remove a wrinkle. Himel stated that he had tried this on a coupon without any problem. There was still concern that the EM towers aluminum wasn't stuck on as well. Nordby will check when testing the taping procedure on the EM tower.
Tape on the top covers places the CMM touches and will mess up its measurements. Nordby thought those touch points were only used for rough alignment. He will check to make sure.
Finally at the tracker technical meeting on Thursday the 16th the use of electrodag 504 was suggested instead of aluminum foil. Himel looked this up after the meeting. He found the manufacturer's web site: http://www.achesonindustries.com/electronic_materials/shielddata.asp with other model numbers of conductive paints (but not 504). They are paints intended for EMI shielding and most of them are intended to be used on plastics. Some are intended for metal. Normally a 1 to 2 mil layer is sprayed on. Spraying conductive paint on a completed tower seems to Himel like it is too likely to create conductive dust that can get where we don't want it. He's also dubious about getting a good (and permanent) seal across the cracks between the sidewalls. If someone wants to pursue this further, please do so.
Tom Himel snail mail
SLAC MS 66
email firstname.lastname@example.org 2575 Sand Hill Rd