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48 The PCB Magazine • June 2015 Table 2 shows the service temperature rating we have tentatively assigned for these products. L shows higher values than some might have predicted but this rating is consistent with experience in flex applications. Clearly A has a much higher service temperature than the UL rating would suggest. The table shows that RTI ratings are not a good method of predicting ser- vice temperature. (Service temperature is main- ly determined by peel strength; RTI is done on samples with no copper foil.) When the flex clads were tested at 250°C in an air oven per the IPC service temperature test, the copper foil became so brittle that the peel strength could not be measured. Therefore, the actual service temperature of A, X, and D could be 250°C, but we have not been able to mea- sure peel strength because of copper oxidation. A quick test to add ENIG to the copper surface proved unsuccessful because the nickel made the copper too brittle. To measure service tem- perature of high temperature flex clads above 225°C will require development of a new tech- nique. Flexible Bondply and Coverlay Testing Results UL does not assign a MOT rating for flex- ible bondplies and coverlays. In some cases, it is possible to get a MOT rating for a combined package (clad and coverlay for example). The IPC service temperature test method was adapt- ed to test bondplies and coverlays. The bond- plies and coverlays are laminated to copper to create clads and then tested as if the samples were clads. Our testing so far has shown wide variation in service temperature ratings of bondply and coverlays, suggesting that the test Table 2: IPc service temperature rating vs. ul rating (Degrees c). FeAtuRe FLExIBLE CIRCUIT MATERIALS FOR HIGH-TEMPERATURE APPLICATIONS continues