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50 The PCB Magazine • June 2015 method has many issues with reproducibility. Our internal testing shows service temperature for coverlays that are well below the accepted performance in the field. This suggests that the present method does not measure the true capa- bility of coverlays and bondplies. Some of these issues are caused by the pro- cess of microetching sheets of copper foil, which get laminated to the coverlay or bondplies to form a clad. This obviously is not easy to pro- cess. The other issue is the coverlay sample that is aged in the oven is just coverlay with cop- per foil on one side. Previous testing has shown that adhesion loss between copper foil and the dielectric is driven by the presence of oxygen. The multiple layers of copper, bondply and cov- erlay will reduce the amount of oxygen present at the copper interface, which will prolong the life of the flex circuit at higher temperatures. We are designing a new internal method to measure service temperature for coverlays that captures all of the possible failure modes. This method makes a simple flexible circuit with coverlay(s) laminated over circuit lines designed for bend testing. Temperature aging of these cir- cuits should show reduced bend performance if any of the three possible failure mechanisms occur: loss of copper adhesion, loss of adhesion between dielectrics (such as clad and coverlay) or increase brittleness of the dielectrics (espe- cially of the coverlay). The number of bends that the test circuit can survive will decrease with any of the failure modes. The data in Table 3 shows bend performance after aging at 180°C. The copper clad laminate used for this test was an all-polyimide clad. Listed in the table are the coverlays tested. The percentages are based on the ratio of bends after 1000 hours of temperature aging to the number of bends with no aging. For this evaluation, the coverlays passed if the bend performance after thermal aging was at least 50% of bend perfor- mance with no aging. The results show that some coverlays actual- ly show good service temperature performance. The L coverlay has been known to survive in the temperature range of 150–180°C depending on the circuit design and environment. The bend Table 3: Percentage of bends after thermal aging, 1000 hours. FeAtuRe FLExIBLE CIRCUIT MATERIALS FOR HIGH-TEMPERATURE APPLICATIONS continues