Hi Steve, As a longtime user of a microblaster and a former conformal coating process engineer, your question falls well within my realm of experience. Here are my questions and recommendation: The thing you have not mentioned is the configuration of the part that is conformal coated with Type UR. Is the part flooded with material beneath the belly of the part, essentially potting it to the board? Hopefully it is only coated with the standard thin coating (.002 +/.001" thick) so that all you have to do to remove it is cut away the urethane filleting and desolder the leads? The hazardous decomposition vapors resulting from overheating the urethane can be collected using a small activated carbon benchtop filter system. Just remember that the coating breaks down approximately at 250 F so rapid decomposition occurs if you use a 700 F tip so do not leave the tip in contact with the urethane for any length of time. I usually have used a chisel shaped Metcal tip at 630 F with considerable success to do this. The conformal coating on the part does not normally need to be removed typically nor does the conformal coating on the pwb. Reinstall the new parts, clean, dry, and recoat with the same urethane material. If the part is buried beneath urethane and there is too much material to cut through the fillets, then the microblaster process might be used. Baking soda works well in the microblaster to cut the urethane but the risk is that the microblaster can go right through the board epoxy as if it were the urethane so you cannot direct it at the board surface. You have to always direct it at the part (which will be scrapped of course) or else you will have problems. If the pwb is copper clad in the area of question, it becomes a much easier task as the blaster pressures can be set so that it will cut through the organic materials and not the copper or metal below. The sodium bicarbonate medium will certainly remove the urethane and then it is removed with an aqueous wash afterwards. I have never been able to measure any charge from ESD so the only problems are the removal of the medium and the hazard of cutting into the board. It is very repeatable and I have actually performed control depth micro blasting to expose traces that are four and five layers in, but that is a different story. Hope this helps. Phil At 02:43 PM 6/4/2003 -0400, Steve Gregory wrote: >Hi All! > >First I would like to thank everyone for all the dialog about ISO and >Prototypes. There has been some extremely good discussion! > >My next subject has been talked about recently, but I would like to get a >little more detailed about some things. > >We are about to embark on a task up-grading a bunch of assemblies (1200+) >that have been coated with urethane conformal coating. The upgrades will >consist of removing just a handfull of parts, and replacing them, maybe >adding a jumper wire or two, and replacing part number labels with new rev >ones. So we're going to spot remove and spot re-coat. > >The -7721 lists a preferred removal method order table, and it lists >Thermal as number 1, Grinding / Scraping number 2, Solvent number 3, and >Micro Blasting number 3. > >I've searched the achives and picked-up a bit of info, and there was a >thread asking about what chemical to use removing urethanes from plastic >SMT parts. I also did a search of the 'NET for some info, which prompts me >to ask some questions. > >Why would Micro-blasting be listed as one of the least desirable methods? >I understand the ESD issue, but from what I learned in looking at two >companies that I found that make this equipment (Crystal Mark and CCR >Company), they address the ESD issues. Are their claims over stated? >There's a *.PDFdocument that CCR has up on the web, that makes for some >interesting reading. Go to: > >http://www.ccrco.com/study.pdf > >In that document, they state that using a thermal method is hazardous >because the coating will produce toxic gases when burnt. I called >Conathane and they said that if it is overheated, there is some cyanide >given off. But I would think that only happens when you actually burn and >char the coating, true? I've been playing around with a hot air pencil and >a orange stick, and that works with no charring or burning, but that's >dead slow. > >I really don't want to use chemicals because it is so messy and takes >forever, and we're only going to be doing a handful of places on each board. > >So what method would you use if you had 1200 boards staring you in the >face, with just a handfull of places on each board? > >-Steve Gregory- > >P.S. I'm looking into the possibilities of leasing a micro-blasting >system, unless somebody says that's the worst thing I could do... >--------------------------------------------------- >Technet Mail List provided as a free service by IPC using LISTSERV 1.8e >To unsubscribe, send a message to [log in to unmask] with following text in >the BODY (NOT the subject field): SIGNOFF Technet >To temporarily halt or (re-start) delivery of Technet send e-mail to >[log in to unmask]: SET Technet NOMAIL or (MAIL) >To receive ONE mailing per day of all the posts: send e-mail to >[log in to unmask]: SET Technet Digest >Search the archives of previous posts at: http://listserv.ipc.org/archives >Please visit IPC web site http://www.ipc.org/html/forum.htm for additional >information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 >ext.5315 >----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------