The structure of the polyimide has a lot of twist turns and folds this gives the polyimide a lot more surface area then standard FR4. The polyimide chemical structure is also quite a bit hydrophilic (i.e. it likes water and water likes it) therefore it does accumulate water more readily then FR4. Hope this helps, "By the way no fair for the "AD penalty"... I can't go to APEX... :-( Alain Savard, B.Sc. A lot of English people call me "Al" and I am a Chemist... so I am an Al-chemist Chemical Process Analyst CAE Electronics Ltd. e-mail: [log in to unmask] -----Original Message----- From: Guenter Grossmann [mailto:[log in to unmask]] Sent: February 28, 2000 5:00 AM To: [log in to unmask] Subject: [TN] Polyimide Hi all Got a question for the alchemists. Why does Polyimide absorb so much water ? Have a great day Guenter ############################################################## 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 Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ##############################################################