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20 The PCB Magazine • April 2017 In general, as with any tradeoff in design, the products with the lowest loss and in some cases the lowest dielectric constant are the more dif- ficult to process. From a fabricator's view, if you can become one of the first to master the pro- cessing of these new materials, you will get first choice of end users. From an end-user (OEM) view, always ask materials suppliers which fab- ricators they would recommend. Most materi- als suppliers work hard with a select group of fabricators to make sure that have some proven facilities to recommend to early end users. Table 2 shows a comparison of these properties. Suppliers The primary suppliers of high-speed flex materials are mainly either existing flex suppli- ers, or high-speed rigid board material suppli- ers. The primary high-speed material develop- ers in North America are DuPont, Rogers, and Taconic. Some low-loss polyimide and LCP suppliers are based outside the U.S., such as Panasonic and Ventec International Group. Recently some of these materials are becoming available in North America and Europe through distributors. Another option to consider is mixing prod- ucts from different material suppliers. Flex cir- cuits have been tested at material suppliers and made at fabricators mixing a clad from one company and a bondply or coverlay from an- other company. Again, make sure the compat- ibility has been tested, and the processing opti- mized before ordering new flex circuits. Copper roughness does affect signal loss at high speeds. We did not discuss this in this paper because the rolled-annealed (RA) copper foils that are the standard for flexible circuits are smoother than the traditional electrodepos- ited (ED) coppers used in rigid boards. Many of the high-speed rigid materials are now made with low roughness copper foils. The smoother RA copper has been a high-speed advantage for flex clads before the high-speed dielectrics were available. The North American suppliers of these new high-speed flex materials have been very good about making sure that the Flex Material Speci- fications (IPC-4204 and 4203) and the Flex Processing Specification (IPC-6013) have been updated for these new materials. In some cases, this required updating materials specifications sheets for 4204 and 4203. In other cases, this meant submitting new specification sheets. The Flex Performance Spec (6013) has already been updated in several sections for both thermoplas- tic dielectrics and fluoropolymer dielectrics. In most cases this just involved updating IPC-6013 with wording from IPC- 6018, the performance specification for high-speed rigid boards. Summary Materials to make high-speed flexible cir- cuits are now available from many material suppliers. In deciding which materials to test or use, remember the tradeoffs the suppliers made in categories we discussed: electrical properties, mechanical/flex properties, and ease of process- MATERIAL CHOICES FOR HIGH-SPEED FLEXIBLE CIRCUITS Table 2: Property comparison of high-speed flex material types.