A thin board with no innerlayers will grow with moisture absorption, with the degree depending on the material. Thus you can get shrinkage by heating it and driving off the mositure. A thick multilayer made of typical epoxy/fiberglass and copper voltage planes won't budge much with moisture so you have to look elsewhere for sources of shrinkage. However, if a panel isn't 100% cured coming to assembly, an above Tg temperature exposure could cause a few hundred ppm shrinkage. For practical purposes the board could be coming to you sufficiently cured to be consider fully cured, but with high temperature you could still raise the Tg a few degrees more and impart a little further cure that results in noticeable panel dimensional change. It's also possible residual stresses from fabrication are coming into play and causing the shrinkage. I know this can happen if you bake a board in an oven for a while - I don't know if the time/temperature exposure of your assembly process is enough to make this a plausible explanation. If this is happening, you can learn to expect the change and live with it (by adjusting your dimensions at assembly) or have the board house bake the board above Tg and have it shrink in their house instead of yours. They'd probably have to go back and re-size their innerlayers to give you a board at print dimensions that would hopefully now be stable through further heat excursions. As far as I know specs are written around the dimensions (and flatness) of the panel as delivered. Hidden residual stresses or lack of total cure that cause the panel to shrink (or warp) in assembly aren't considered. In some cases it might make sense to spec the board to be oversized as received so that it ends up the right size when assembled. Jonathan Whitcomb IBM Endicott 607-757-1326 [log in to unmask] fax 757-1156 Dept T49G bldg 257-2 1701 North St. Endicott NY 13760 aisle H office X013 pager 2930 ---------------------- Forwarded by Jonathan C Whitcomb/Endicott/IBM on 08/26/99 02:20 PM --------------------------- Dave Hoover <[log in to unmask]> on 08/26/99 11:46:48 AM Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond to Dave Hoover <[log in to unmask]> To: [log in to unmask] cc: Subject: Re: [TN] PCB material shrinkage Tom, Are these doublesided or multilayer PCB's???? Tell us alittle about the board. Groovy -----Original Message----- From: Tom Oliver [mailto:[log in to unmask]] Sent: Thursday, August 26, 1999 8:03 AM To: [log in to unmask] Subject: [TN] PCB material shrinkage Hello My first post and first question to the experts. I am new to the electronic game most of my experience has been in mechanical inspection and test. My question: is there a rule of thumb on material shrinkage on board material or a spec. Our boards are coming in with all the dimensions at nominal but after wave they have shrunk down to the low limit and sometimes below low limit. Most of our tolerances are +/- .003. This is giving us headaches with interference problems when we put other components on the boards. Is it normal for the boards to shrink. Our arrays are approx. 4" x 12" with 25 separate boards on each array. Thx's in advance Tom Oliver Eagle Comtronics [log in to unmask] ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. 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If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ############################################################## ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################