I've been looking and not finding anything, so I'll try posting the question here... We're doing some investigation of PWB constraint, and as part are doing some thermal expansion coefficient measurements. On the reference sets, (bare polyimide PWB, 6 & 12 layer unmounted and 6 & 12 layer mounted on aluminum) we're seeing what looks like a hiccup in the TCE curve at about 15 degrees C. (The TCE runs are from -40 C to 100 C). The setup has strain gages mounted on the PWB and a titanium silicate reference slide, then is being heated in at 2 degrees per minute. From -30 to 15 degrees there's a longitudinal TCE of about 18 ppm/C and a transverse TCE of about 15 ppm/C. From 20 to 90 C the longintudinal TCE is 19 and the transverse is about 16 ppm/C. But there's this pronounced dip in the range of 15 to 20 C. It doesn't show up on the other boards, which are rigidly bonded to low TCE substrates, just the soft bond to aluminum and the boards with no bonding at all! There is soldermask on the panels, any chance that that might have a glass transition at about 15 C? Thanks, Steve Axdal [log in to unmask] ################################################################ TechNet E-Mail Forum 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ TechNet E-Mail Forum 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################