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34 The PCB Design Magazine • December 2014 between ground layers in the coupon. First, measure the capacitance between two flooded ground planes in the coupons in the as-received state and then again after the coupon has un- dergone preconditioning, and at the end of test. If there is significant material damage then a -4% or greater drop in capacitance is seen. To confirm the drop in capacitance is indicative of material damage, one or two of the coupons are subjected to a microsection to check for the presence of material damage. The major types of material damage found are adhesive delamination, cohesive cracks, and crazing. Adhesive delamination is typically be- tween two laminated surfaces like the b-stage, c-stage and copper interfaces. On occasion, ad- hesive delamination is seen between the glass bundles as a group and epoxy of the dielectric. This type of failure is found typically on a 1 mm (.040") grid or greater. The most common type of material damage is the cohesive crack, which is a crack that goes through the b-stage, c-stage and glass bundles. The cohesive failure is a breakdown of the ep- oxy system due to high temperatures of assem- bly. This type of failure is found typically on a 0.8 mm (.032") grid. Crazing is the separation between glass fi- bers and the epoxy system. It looks like silver sheen on the glass bundles due to an envelope of air around the glass fiber. Crazing provides a pathway for conductive anodic filament (CAF) formation. This type of failure is found typical- ly on a 0.5 mm (.020") grid. Figure 9: staggered vs. stacked microvias. feature HDI PWB RELIABILITY continues