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18 The PCB Magazine • April 2017 cuits. This product is new so the fabricator base is still small, but it is growing. The all-polyimide construction is by far the best options for high- speed circuits that also require high-tempera- ture capability [8] . All the choices of bondplies mentioned above can be used as coverlays except for the fluoropolymer/polyimide composites. As stated before, a low-loss coverlay is only important in microstrip applications. Many of the high- speed flexible circuits made so far with these new materials are striplines, where only a low- loss bondply is required. Processing Processing of many of the new materials has been the biggest delay in wide acceptance. This is to be expected when introducing new materials. Several of these new materials require high-temperature lamination. This includes the fluoropolymer/polyimide composite, the LCP materials, and the all-polyimide bondply. Many fabricators now have presses that will reach the required temperatures of 270°−310°C (520°−590°F). However, these higher tempera- tures require new press pads and good tempera- ture control. Fortunately, many new options for high-temperature press pads are becoming available. The limited press pad options did slow the acceptance of these materials but that should not be an issue from now on. Some of the fluoropolymer options and all the LCP options use thermoplastic films in both the clads and bondplies. To work well, the lami- nation temperature of the clad must be higher than the lamination temperature of the bond- ply. Lamination of the bondply at a tempera- ture too near the lamination temperature of the clad can cause circuits on the imaged clads to move during lamination (sometimes called swimming). This can be prevented by tight control of lamination temperature, but is best prevented by designing the materials with a wide spread in the lamination temperatures. The fluoropolymer/polyimide composites were developed with a 35°C differential in clad and bondply lamination temperatures. This is wide enough for most fabricators. The early LCP clad and bondply combinations had lamination temperatures that were too near each other which made processing exceptionally com- plicated because of the very tight temperature control required. Many of the newer LCP op- tions have been developed with a wider differ- ence between lamination temperatures. When comparing any new thermoplastic materials, this delta should always be a key question. Dimensional stability is another important consideration when looking at processing ease. First, movement of the clad after etching can be higher with some of these new materials. The all-polyimide constructions are typical of most flex used today. Some of the other dielectrics may have more movement and may require dif- ferent artwork compensation strategies. Howev- er, most of the material suppliers have managed this property well in their final products. The bigger issue is movement of the circuits during lamination of bondplies or coverlays. This is particularly an issue with thermoplastic dielectrics and/or if high-temperature lamina- tion is required. Even all-polyimide clads will expand more during lamination at 300°C versus the expansion at normal lamination tempera- tures of 190°C. This movement can be contained with proper lamination process control and ex- perience in the amount of movement expected. Drilling and plating must also be optimized for these new materials, particularly the fluo- ropolymer and LCP containing products. Both materials can be drilled and plated, but will re- quire careful optimization of the drilling pro- cess as well as the desmear and hole preparation process. Most materials suppliers can provide processing recommendations for lamination and drilling/desmear. " All the choices of bondplies mentioned above can be used as coverlays except for the fluoropolymer/polyimide composites. " MATERIAL CHOICES FOR HIGH-SPEED FLEXIBLE CIRCUITS