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16 The PCB Magazine • April 2017 With any of these new materials, there will always be a tradeoff of the three areas we dis- cuss in this paper: electrical properties, flexible/ mechanical properties, and ease of process- ing. All material suppliers are trading off these three areas to reach the right compromise in their final products. Both fabricators and end users must understand the tradeoffs and make material choices with a clear understanding of the compromises made. Bottom line is do not pick materials just based on electrical proper- ties; flex properties and ease of processing are still very critical in functional high-speed flex- ible circuits [7] . Mechanical and Flex Properties Most traditional flex materials have been optimized for flexibility and ease of process- ing at the expense, in many cases, of dielectric properties. Of course, at the time of the devel- opment of these early materials, "high-speed" was well below 1 GHz. Finding polymers with low loss is just the first step in developing new flexible circuit materials. Fluoropolymer films by themselves do not provide strong enough mechanical properties to be used directly in flexible clads. The strategies use by material suppliers to support fluoropoly- mers are to use special flexible woven glass, par- ticle fillers, or a polyimide core. All help with improvement of mechanical properties. The woven glass and particle filler approaches will limit the flexibility of the clads or bondplies, and therefore will not be the best choice for ap- plications that require multiple flexing at sharp angles and/or small bend ratios. They should be fine for some flex applications. Liquid crystal polymers work well as free standing dielectric films for high-speed flex clads. The mechanical properties are good enough for most applications. They may not have the high flex capability of all polyimide constructions but should be good enough for most applications. All polyimide dielectric clads provide the best mechanical and flex properties of all the choices and if the flex requirements are very high then they are the first choice. However, even the lowest loss polyimide clads are still slightly higher loss than most fluoropolymer or LCP constructions. We will discuss more about mechanical properties of these clads in the section on ease of processing. The choices become more complicated when discussing bondplies and coverlays. Many high- speed clads can only be used with specific bond- plies. The fluoropolymer/polyimide composites are mainly used with bondplies made of similar fluoropolymer/polyimide composites. The LCP polymer-based clads are also primarily used with LCP-based bondplies. Both approaches use thermoplastic polymers films that require high- temperature lamination. Many fabricators now have high-temperature presses that can reach the temperatures required for these new mate- rials (270°−310°C). However, as we will discuss further in the next section, the learning curve for processing these materials is steep. The low-loss bondplies and coverlays that are compatible with the all-polyimide clads have only recently been available. One option is a fluoropolymer core coated with a low-loss thermoset adhesive. It has been tested with both all-polyimide clads and with some of the fluoropolymer clads. One advantage is that the thermoset adhesives are laminated at more standard lamination temperatures. The other option which is quite new is an all-polyimide bondply and coverlay. It was de- veloped initially for high-temperature applica- tions when combined with all-polyimide clads. However, this new bonding film is low loss (0.003) and in the same range as the best all- polyimide clads. This new bondply does require high-temperature lamination but for now is the only method to make all-polyimide flexible cir- MATERIAL CHOICES FOR HIGH-SPEED FLEXIBLE CIRCUITS " Fluoropolymer films by themselves do not provide strong enough mechanical properties to be used directly in flexible clads. "