This is a topic I've addressed in my book. The temperature rating is really cumulative. I can't give a direct approximation because of the difference between the way assembly houses assemble boards, but the Tg really applies for board assembly. The Tg should be more than the temperature of the solder flow...cumulatively. Every pass diminishes the board. for an example, if your board has a (unrealistic) Tg of 150F and the bath was 100F, the material would be suitable, but it the board must go through 2 passes for assembly you are in trouble. Double sided assembly is similar. An assembly house will take 2 passes of the board through the solder flow. I haven't seen a table or chart showing the amount of degradation in the material for each pass. Therefore we must use common sense methods and determine first the number of passes required for assembly, (getting a quote on assembly before specifying the material) and then determine the material. I think most use experience and "hard lessons" to determine these values. Chris Robertson Author "PCB Designer's Reference" [log in to unmask] ----- Original Message ----- From: Robert Tarzwell To: [log in to unmask] Sent: Tuesday, July 08, 2003 12:48 PM Subject: Re: [TN] Operating temperature of PCB Hi Michel even though most people feel that as long as the temp is below the T/g then everything is fine in PCB land, however some long term studies I did showed that in fact everything is not fine in PCB land. When you run a FR4 pcb hotter a couple of problems arise, first the hotter you get it the more it starts to carbonize, the longer you keep it at a high temp the worse things get. one study showed with as little as three months at 100 deg C lowers the dialectric voltage rating as much as 30 percent. the temp your talking about is right on the border line for FR4,even high temp stuff a switch to BT epoxy or polyimide may be in order , polyimide is 2 to 3 times the price but BT epoxy gives you a higher operationg temp with only 1.2 X the cost. The damage to the base lam is based on time at temp and max temp the hotter and the longer the more damage is done, I have seen even 80 deg c type temp turn a fr4 board brown over the years. I have advocated for years that 160 deg FR4 has a max long term operating temp of 100 deg C. i would suggest you talk with some of the laminate tech guys ( not sales type) and see what they say. robert tarzwell megadawn.com --------------------------------------------------- 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 ----------------------------------------------------- -----------------------------------------------------