PCB007 Magazine

PCB-June2015

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44 The PCB Magazine • June 2015 FeAtuRe Abstract Many opportunities exist for flexible circuits in high-temperature applications (automo- tive, military, aerospace, oil and gas). Flex cir- cuits in these applications have been hindered by a lack of materials that can survive higher temperatures. Some materials, especially some thermoset adhesives, break down over time at higher temperature, becoming brittle or losing adhesion to copper. Polyimides tend to perform much better under high temperature. The other issue is the lack of good test meth- ods to verify that flex materials can survive higher temperatures. Several methods for test- ing copper clad laminates exist, but there are very few for coverlays and bondplies. We will discuss different test methods for measuring high-temperature capability including the new IPC service temperature test. We will also report on test results for various flexible materials and our recommendations for the best flexible ma- terials for high temperature applications. This by Sidney Cox DuPOnT cIrcuIT anD PacKaGInG MaTErIalS Flexible Circuit Materials for High-Temperature Applications will include development work on new flex ma- terials for high-temperature applications. Introduction More applications require flexible circuits that must survive high-temperature environ- ments. These include automotive applications near the engine, oil and gas down-hole pumps, and aerospace applications near jet engines. There have been limited test methods to deter- mine what temperatures flexible materials can survive. The damage caused by high-temperature en- vironments will mainly fall in three categories: loss of adhesion between copper and dielectric, loss of adhesion between dielectric layers, and embrittlement of the dielectric layers. At the highest temperatures the copper would also be- come brittle, but in most cases the flexible cir- cuit dielectrics fail first. Thermoset adhesives seem to be most sensitive to embrittlement es- pecially compared to polyimide films which are much more resistant to high temperatures. UL has two different temperature ratings. The RTI (relative thermal index) is based on the temperature aging of base dielectrics. For flex

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