Hello, We assemble a six-layer two-sided SMT board which has been troublesome lately because of increased bow/twist problems. The increased bow/ twist appears to coincide with our recent changing of suppliers for this PWB. Unfortunately, I cannot say this definitively because I don't have historical bow/twist data - only qualitative perceptions from our SMT line operators. Another difference we noticed between the old supplier and the new supplier is the old supplier's boards tended to be on the high end of the thickness spec (.065"-.067") whereas the new supplier's boards tend to be on the low end of the thickness spec (.057"-.059"). This led us to speculate whether there might be some correlation between the material chosen (core laminate and pre-preg) and the bow/twist properties. Our documentation doesn't specify what to use for core laminate or pre-preg so it's quite possible the two suppliers chose different core-laminate and pre-preg material. I'm looking for some feedback: What build-up would you use to fabricate a six-layer .062" board with 1 oz copper inner layers and 2 oz copper outer layers (after plating)? Do you think the build-up could have any influence on bow/twist? (Also, the copper distribution on all layers appears fairly balanced.) We're going to cross section some of the old supplier's boards to determine their build-up. The new supplier has the following build-up: 1 oz copper foil 7629 B-stage .015 copper-clad core 7629 B-stage .015 copper-clad core 7629 B-stage 1 oz copper foil I believe the 7629 B-stage is .008" thick. Would there be any advantages to using two sheets of 1652 B-stage which is .005" thick per sheet? Since the new supplier's boards are on the low end of the thickness spec we could add the additional .006" per board, I think, without a exceeding our board thickness specification. Please e-mail replies to [log in to unmask] Thanks! Mark Lettang