PCB007 Magazine

PCB-Apr2014

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32 The PCB Magazine • April 2014 The material damage is monitored in an IST coupon by measuring changes in capacitance 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. HDI PWB RELIABILITY continues 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 typically on a 0.5 mm (.020") grid. In conclusion, the use of HDI PWB reliability in lead-free applications is a dual-edged sword. The copper interconnections are more prone to a breakdown and the material is more prone Figure 9: staggered vs. stacked microvias.

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