The reliability, as tested by thermal cycling, of printed wire boards (PWB) are established by three variables; copper quality, material robustness and design. The copper quality was most influential and could be evaluated pretty accurately by microsectioning methods to determine the general quality of the board. Next in the hierarchy of influence came the robustness of material, and the tertiary influence was design. With the advent of Removal of Hazardous Substances (RoHS) causing lead to be removed from solder and the increase in thermal excursions for assembly and rework to 260C, materials tend to be as much a problem as copper quality in the robustness of PWBs.
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