We are testing prototypes with garden variety 140C FR4 from PCBexpress. Production models will be high temp 175C or 200C FR4. Polyimide (polyamide is nylon, I think) has been the standard in my industry for a couple of decades, but I hate the stuff. It is especially bad about absorbing moisture and contamination.
Performance of the present PCB is ENTIRELY related to cleaning. In fact performance improves after prolonged "cooking" in the test oven. The PCB I am testing now has turned a pretty brown color (actually looks like polyimide), but is working great. So whatever decomposition products are being generated sure are not hurting things. The well browned PCBs may actually perform better than the new ones. Boiling the PCB in DI water is one technique I an using at present. Just letting the PCB soak for a few hours in DI water after an ethanol (denatured alcohol) scrub to remove flux residue works almost as well as the boiling procedure. What I am struggling with now is the best standardized cleaning procedure. Of course, I do not know how much cleaning the production PCBs will require, but I suspect some prior to potting. ----- Original Message ----- To: "Multiple recipients of list CHIPDIR-L" <[EMAIL PROTECTED]> Sent: Sunday, March 02, 2003 12:23 PM > Syd, I think you raise a number of issues. My studies way back in the early > 1970s showed FR4 started out gassing and changing composition at 130C. Some of > the breakdown products were very acidic. Perhaps this is a decomposition issue > rather than an ionic contamination issue. Are you using high temperature > laminates or ordinary FR4? > > BTW much later studies in the late 90s showed that polyamide boards were > slightly lower in leakage at room temperature but considerably better at high > temperatures. I don't recall at how high a temperature I tested. > > "Syd Levine (AnaLog)" wrote: > > > These are getting baked after washing as part of the testing routine. These > > boards have to be able to go to 150C, and really need to go to 175C. Would > > soaking them in the distilled water help get the ionic residue off em. > > I'm inclined to believe that long term soaking is not a good idea. Most > contaminates go into solution quickly so soaking is probably not necessary. A > lot would have to do with the quality of plated through holes (PTH) in the PCB. > If the holes are rough the plating may have voids or fissures that could trap > contaminates in the glass fiber within the PCB. When the board is heated these > fissures will spread the contamination back on the board surface. > > There is another issue. Ions conduct but so do metals and carbon, silicon etc. > Do you know for sure you have an ionic contamination issue? > > Still another issue. DI water is free of ions but not necessarily free of > organic compounds. Distilled water is free from both ions and organic material > but very quickly turns acidic when exposed to air due to CO2 absorption. I used > DI water in combination with alcohol to remove both ionic and non polar > compounds. > > I used a three step cleaning process. After cleaning with all ordinary PCB > cleaning techniques I did a high presser wash with DI water to remove the polar > contaminates. Then I chased the water with ultra pure isopropinal to suck out > any remaining water and remove the non polar contaminates. Finally an overnight > bake out at 108C. All handling was done with rubber gloves (no talc please) and > forbad assembly by assemblers who smoked tobacco. > > The banding of smokers was the result of spectrographic research I discovered > showing fingerprints on disk wafers made by assemblers wearing rubber gloves. > Nothings is as good as you think it is when you go for ultraclean. > > My procedures resulted in a successful process but I was not dealing with high > temp or even high voltage. BTW my assembly department was not very happy with me > and would shortcut my "silly" procedures at every opportunity. > > Fred Townsend > > > I > > don't seem to be able to get rid of all of it. > > > > > BTW do you bake your PCBs after you wash them? > > > > > > Fred Townsend > > > > > > > > > "Syd Levine (AnaLog)" wrote: > > > > > > > I discoverd my problem, I think. I have been trying to use grocery > > store > > > > (Wal-Mart) distilled water to wash these dern PCBs. At 150C, the ionic > > > > residue is causing trouble. Where can I find good DI water in bottles? > > I > > > > hate to invest in a machine when I need maybe a gallon or two per week. > > I > > > > suspect I need water that is close to the chip making standards (18 > > > > megohm/cm). > > > > > > > > Syd H. Levine > > > > AnaLog Services, Inc. > > > > Phone: 270-276-5671 > > > > Telefax: 270-276-5588 > > > > E-mail: [EMAIL PROTECTED] > > > > URL: www.logwell.com > > > > > > > > -- > > > > Author: Syd Levine \(AnaLog\) > > > > INET: [EMAIL PROTECTED] > > > > > > > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > > > > San Diego, California -- Mailing list and web hosting services > > > > --------------------------------------------------------------------- > > > > To REMOVE yourself from this mailing list, send an E-Mail message > > > > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > > > > the message BODY, include a line containing: UNSUB CHIPDIR-L > > > > (or the name of mailing list you want to be removed from). You may > > > > also send the HELP command for other information (like subscribing). > > > > > > > > > -- > > > Author: Fred Townsend > > > INET: [EMAIL PROTECTED] > > > > > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > > > San Diego, California -- Mailing list and web hosting services > > > --------------------------------------------------------------------- > > > To REMOVE yourself from this mailing list, send an E-Mail message > > > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > > > the message BODY, include a line containing: UNSUB CHIPDIR-L > > > (or the name of mailing list you want to be removed from). You may > > > also send the HELP command for other information (like subscribing). > > > > > > > -- > > Author: Syd Levine \(AnaLog\) > > INET: [EMAIL PROTECTED] > > > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > > San Diego, California -- Mailing list and web hosting services > > --------------------------------------------------------------------- > > To REMOVE yourself from this mailing list, send an E-Mail message > > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > > the message BODY, include a line containing: UNSUB CHIPDIR-L > > (or the name of mailing list you want to be removed from). You may > > also send the HELP command for other information (like subscribing). > > > -- > Author: Fred Townsend > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). > > -- Author: Syd Levine \(AnaLog\) INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
