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58 The PCB Magazine • December 2014 extent of signature pass-through. Depending on the design copper above or below, however, the signal pass-through is not validated as irrel- evant to the final "light switch" at the end. This is where problems can arise at the CM. There can be a void above or below the signal pass- through that allowed a signal validation during ET, but now that components are installed the via or feed-through cannot validate the signal even though the board is functional. Does this mean the board was not 100% tested? The an- swer is no. In a Class II board no violation has occurred. In a Class III board the void should have been detected if the test point was avail- able. One must remember in a Class III scenario that the test point must be available and not just because the design shows it. Manufactur- ing processes such as via fill or via cap to the opposing side must be taken into consideration as mask may be pressed into the via from the opposing side, filling the barrel and actually insulating the ability to accurately probe the landing pad or annular ring. These all must be considered from the OEM/design side to ensure that a class III test can be performed if indicated. 100% test is always achieved for end-to-end "light switch." For enhanced redundancy test- ing the OEM must be aware that design options such as via fill and capping can put an "owl's nest" on the redundant connections that may not allow the enhanced test. todd Kolmodin is the vice president of quality for gardien Services uSa, and an expert in electrical test and reliability issues. to read past columns, or to contact Kolmodin, click here. ELECTRICAL TEST COvERAGE continues Figure 1: optimization.

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