I was hoping you would get some answers, because it is a question I have never heard before (just when you think you've heard them all, eh?) I am no expert, but my first thought was that laminate bow would be very hard to control because the weave is all going in the same direction. In multilayer boards they are interleaving laminates with the weave one way and then the other so they "balance out" when they all get squished together... BUT... the laminate thicknesses you mentioned are the thickest I've ever heard of, which doesn't leave a lot of room for cross-weaved layers bonded together. When you say laminate do you mean just the B-stage material that separates the core copper clad stuff? Maybe the copper clad is more stable, and the laminate being pliable just conforms? Anyway, the real question you asked was "should we require the laminate vendor to tighten the bow and twist specification?" and my answer would be "I doubt it" since we are getting good boards that meet spec. If you are having trouble getting flat boards, either talk to your fabrication vendor about better construction (balanced stackups) or look for a better fabricator best wishes, Jack -----Original Message----- From: Shirley Xiao [mailto:[log in to unmask]] Sent: Tuesday, April 25, 2000 5:20 PM Subject: Resend: Bow & Twist Specification Sorry to bother everyone again. I guess few response to this topic maybe becasue of Easten Holiday. Shirley Xiao Dear All: What I have on hand is a old revision of IPC-6012(1996). section 3.4.4 Bow and twist: "The printed board shall have a maximum bow and twist of 0.75% for boards that use surface mount components and 1.5% for all other boards. Panels which contain multiple printed boards which are assembled on the panel and later separated shall be assessed in panel form." From the PCB user point of view, what is the maximum bow and twist percentage that the placement machine can tolerate? While in IPC-4101 section 3.8.4.3 Bow and twist laminate materials (table 8 bow and twist, mm per 30.5 cm), 3.0 mm per 30.5 cm which is equivalent to 1.0% of bow and twist (for laminate thickness of 0.79-1.67 mm double sided with panel size from 200 mm to 300mm). My question is the requirement for laminate is even loose than that of printed circuit board. Is it reasonable or should we require the laminate vendor to tighten the bow and twist specification? Any comments will be appreciated. Shirley Xiao ############################################################## 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 ##############################################################