And you mention only TC: the coefficient of hygroscopic expansion at constant temperature between, say, 25% and 75% RH (allowing time for stabilisation) is likely to be as much again per %RH. Just the copper has a TCE of 16.6 µm/m/°C and that's inorganic. Organic materials have other molecular characteristics which render them mechanically unstable and not always predictable. Brian Werner Engelmaier /* wrote: > Wow Jan, > You have: "PCBs approx 450mm long, 50mm wide, 1.7mm thick, 12 layers, 17u > copper inner > layers, Isola IS420 material, boards are produced with longest dimension in > warp direction in PCB production panel" â��and you expect a 75um tolerance after > subjecting the PCB to board drying and leadfree reflow. > > Just consider that in PCBs you are marrying 4 different materials [laminate, > prepreg, woven glass-fiber reinforcement, copper] together all of whom have > significantly different physical properties, particularly at different T's and > in different directions. > > The laminate (which is fully cured) and the prepreg (not fully cured) have > different resin contents, go through glass transition (which changes molecular > structure and is not fully reversible, only nearly so), start to decompose at > SAC solder T's (actual weight loss, which is irreversible). > > The glass-cloths is in your PCBs laid up with the warp direction in the > longest dimension in the PCB production panel making your PCBs quite non-uniform in > the x and y directions [the normal practice is to alternate warp and fill > directions on alternate layers to equalize the different properties in the warp > and fill directionsâ��even with that, it is not uncommon to measure CTE-values > different in x and y by as much as 2 ppm/C]. > > The copper layers are mostly etched away in irregular [in the sense of > physical behavior] patterns. > > â��and you expect a 75um tolerance after subjecting the PCB to board drying and > leadfree reflow?! > > > > Werner > Future workshops: > Pb-Free Soldering Processesâ��Survival, Quality, Reliability, July 17/18, Thal, > Switzerland > Pb-Free Soldering Processesâ��Survival, Quality, Reliability, July 21/22, Bad > Segeberg, Germany > Pb-Free Soldering Processesâ��Survival, Quality, Reliability, August 18, O > rlando > Reliability Issues with Lead-Free Soldering Processes, September 22, > Schaumburg > Failure Mode and Root Cause Analyses Reliability (Fatigue, Brittle Fracture, > ENIG), September 22, Schaumburg > Solder Joint Reliability: Parts 1 & 4, Oct. 9, Moscow, Russia > Solder Joint Reliability: Parts 1 & 4, Oct. 17, Timisoara, Rumania > Solder Joint Reliability: Parts 1 & 4, Oct. 22, Tallinn, Estonia > > > > ************** > Gas prices getting you down? Search AOL Autos for > fuel-efficient used cars. > (http://autos.aol.com/used?ncid=aolaut00050000000007) > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 15.0 > 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 > ----------------------------------------------------- > --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------