Jim: I'll try not to be too long winded!!! However ... we have done extensve testing regarding the effects of neg vs pos etchback on both rigid and flex multilayers. Bottom line ... pos etchback is ALWAYS preferred. Neg allows separation at the PTH barrel. Excessive etchback (pos) can result in plating folds which is not desireable. The 55110 guidelines for pos etchback are pretty good. With multi-layer rigid flex there is another problem. The use of acrylics in the flex sections, when extended into the rigid sections (standard flex multi-layer), can result in excessive Z axis expansion during thermal cycling (acrylic CTE is higher than the other materials and it is hydroscopic). This results in barrel cracking and / or separation. Pos etchback will help ... but the solution is to eliminate or reduce acrylic (10% max). A number of PCB fab houses have now made this their standard product. As far as reliability ... we have run Rel. Enhancement Tests and traced failures to butt joints (no etchback) or single point contacts (minimal pos. etchback). Negative etchback was a DISASTER! (Note: etchback is only one of a number of contributing factors to PTH failure. Another one is the method / occasion of rework. The thermal stress to the PTH can be excessive if too many rework cycles with too much heat in a localized area is applied. Another is the type of copper, HTE is preferred.) Our experience suggests that up to 8 layers is relatively safe if designed properly and fab'd with HTE Cu and pos etchback. As the layers increase above 8 all factors become more and more critical. I've really only scratched the surface ... we have lots more info if you need it. I hope I've been of help. Vince Giardini Parker Hannifin, GESD (516)231-3737