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46 The PCB Magazine • June 2015 FLExIBLE CIRCUIT MATERIALS FOR HIGH-TEMPERATURE APPLICATIONS continues materials this is mainly polyimide films. The RTI is determined by loss of tensile strength (mechanical property) and dielectric strength (electrical property) with long-term thermal aging. The RTI tests only measure the degrada- tion of the base film properties. The samples are tested declad (without copper) so it does not capture copper adhesion loss aspects of thermal aging. So RTI by itself is not a good measure of the capability of a copper clad laminate. The UL MOT ratings are assigned for cop- per clad laminates and primarily measure the loss of copper adhesion to the adhesive layer. The MOT uses accelerated heat aging. MOT is mainly used for rating copper clad laminates. The UL MOT cannot be higher than the lowest of the mechanical or electrical RTI. A new IPC test method has been developed to measure service temperature (IPC-TM-650 2.6.21B) and was originally created specifically for flexible copper clad laminates. It was later revised to allow measurement of service tem- perature of bondplies and coverlays. The test measures loss of copper adhesion and dielectric strength with high-temperature aging, but does not use accelerated aging; it measures property loss after 1000 hours of aging at the target tem- perature. The test method is still new and not much data has been reported from this test un- til now. Flexible Clad Testing Results We have completed testing of flexible copper clad laminates with the IPC service temperature test, and after testing at multiple temperatures, we determined a tentative service temperature rating for most of our clads. The test measures peel strength after 1000 hours of aging. If the final peels are more than 50% of the starting peels then the material achieved a rating at that temperature. Table 1 shows the percent drop in peel strength after 1000 hours for multiple clads and aging temperatures. A, L, T and F are all commercial products. X and D are experimen- tal polyimide films that were laminated to form copper clad laminates. The three all-polyimide films all show very high service temperature (A, X and D). For all three films, copper foil was laminated directly to the polyimide, so no thermoset adhesive is present in these constructions. The samples that used adhesives to bond the copper foil to the polyimide core all had lower performance (L, F and T). Based on all the testing done so far, Table 1: Percent of original peel strength after 1000 hours of thermal aging. FeAtuRe