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PCB-Apr2017

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14 The PCB Magazine • April 2017 MATERIAL CHOICES FOR HIGH-SPEED FLEXIBLE CIRCUITS 3.0–3.5). The old assumption was that the issue was the polyimide. It was known that the earli- est polyimides did have high moisture absorp- tion. However, as Table 1 shows, the dielectric losses of the standard flexible circuit adhesives (acrylics and epoxies) are much higher than the polyimide film. In fact, some more recent polyimide formulations have very low dielec- tric loss, which means that some all-polyimide dielectric clads are quite good for high-speed circuits. But not all polyimides are the same; some are higher loss than others, and there is a correlation between dielectric loss and moisture absorption for most polymers but clearly this is true for polyimides. A good initial rule is that dielectrics that have high moisture absorption will have high loss [1, 2, 3]. Table 1 also shows some other dielectric choices that have been used recently to create high-speed flex materials. Fluoropolymers, the mainstay of high-speed rigid boards, have now been incorporated into flex materials by sever- al material suppliers. Fluoropolymers have the lowest dielectric constant of any polymer class and have very low dielectric loss. The issue is that pure fluoropolymer films do not have good enough mechanical proper- ties to be used alone. Some suppliers have tried using special woven glass with fluoropolymers for flex applications. Some have added fillers to improve properties as is common in rigid high- speed materials. And one supplier is using a polyimide core with fluoropolymer as the outer layers and as the adhesive. All these modifica- tions help with both the flexible/mechanical properties and in some cases with processing ease. However, these modifications increase the dielectric constant and dielectric loss, so the pure polymer properties cannot be achieved in actual products. These new materials are in many cases very good materials for high-speed flex [4,5] . Liquid crystal polymers (LCPs) also have good dielectric properties including very low loss and very low moisture absorption. LC poly- mers are used in both clad and the bondply. The fluoropolymer constructions are also used in both clads and bondplies [6] . The primary clads that are being used for high-speed flex circuits are based on fluoropoly- mers, liquid crystal polymers, and low loss all-polyimide constructions. Bondplies and in some cases coverlays are available in both the LCP and fluoropolymer constructions. How- ever only recently have low loss coverlays and bondplies been made with all polyimide con- structions. In fact, low loss coverlays have been the most difficult to develop and are only very recently available. The expansion of materials for high-speed rigid boards has been led by new low-loss ther- moset adhesives. Variations of these low-loss ad- hesives are now becoming available in flexible circuit materials and this should provide more options for bondplies and coverlays. While tra- ditional adhesives used in flex are high-loss, this does not mean that all thermoset adhesives are high-loss. The dielectric constants of almost all the polymers in Table 1 are fairly low and in a fairly narrow range as they exist in actual flex materi- als. And although there are some advantages of lower dielectric constant, the dielectric loss is the most important dielectric property when choos- ing materials for high-speed flex. The spread in dielectric loss is much wider that dielectric con- stant for the full range of flex circuit materials. When discussing dielectric loss and high- speed controlled impedance circuits, two fac- tors must be considered: the speed of the circuit design (1 Ghz or 20 Ghz) and the length. All all-polyimide clad has been used for 20 Ghz cir- cuits at 1" long signal lines. This would not be possible at 10" in most cases. So, the best mate- rial for each application depends on both the speed and the length. Table 1: Comparison of dielectric properties of flex polymers (data measured at 1 GHz).

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