Hi Ken, >(1)Can any one provide me the possible causes for the defects? The loading conditions for the innerlayer separation come from the excursions to soldering temperatures. As I previously wrote on TechNet: The reliability concern with any of this is basically this: (1) At soldering temperatures the epoxy is way above its Tg, is very soft, and should be considered a high-viscosity liquid as far as behavior is concerned. (2) For analysis as to what is going on around a PTH, the PWB can be considered an infinite plane in the x- and y-directions. (3) While the epoxy tries to expand in all directions, it is constrained in the x,y-directions by the glass reinforcements. (4) Thus, the primary expansion is in the z-direction at roughly 3 times the linear thermal expansion, because all the volumetric expansion is funneled into the z-direction. (5) The PTH hole, however, allows the epoxy to also expand into the space occupied by the PTH if the PTH barrel wall is thin enough to by compressed. That is why in cross-sections of PTHs with 1 mil of copper plating and less one sees the PTH barrel bowed inward between innerlayer lands, particularly in prepreg layers (higher resin content, less cure) and when there are larger distances between lands (no non-functional lands), and sometimes a space between the barrel wall and the receded epoxy. If there were no copper plating, the hole diameter would decrease on heating. (6) Thus, the PTH barrel in tension in the z-direction, with the largest stresses near the PWB center, the PTH barrel experiences compressive hoop stresses, innerlayer lands experience tensile stresses at their attachment to the PTH barrel, and innerlayer lands near the PWB surface experience bending moments due to land rotation resulting from the z-direction expansion of the epoxy surrounding the PTH barrel. (7) Thus, the higher the temperatures reached the worse the reliability threat. I have heard of hand soldering operations performed with irons in excess of 410C! Depending of the polymer glass transition temperature, that might be a delta-T of 285C above Tg. In your case, resin smear makes the interface to the inner layer copper foil weak, causing separations at less severe loading conditions. >(2)How much is a variation for this kind of defects within a same date code? Resin smear, assuming your process is just out of spec, will occur to some degree within all codes made prior to an adjustment to your processing. >(3)What about the reliability of my assembled boards? The depends on the severity of the application your PCBs will experience. While the failure probability for your PCBs will be higher than it could be, you may not experience any failures in benign applications within the life of the product. More severe operating conditions however, are likely to propagate partial inner layer separations to completion, and thus failure. While I am not aware of a book that I could recommend to you in this context, I give a workshop "Design, Manufacturing and Reliability Issues of Small-Diameter/ High-Aspect-Ratio Plated-Through-Holes and Vias" that deals with the underlying issues; also PC-D-279 "Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies" would be helpful. Werner Engelmaier Engelmaier Associates, Inc. Electronic Packaging, Interconnection and Reliability Consulting 23 Gunther Street Mendham, NJ 07945 USA Phone & Fax: 973-543-2747 E-mail: [log in to unmask] ############################################################## 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. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################