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36 The PCB Design Magazine • August 2014 or so, and must possess loss characteristics low enough to support succeeding generations of increasingly faster line cards without attenuat- ing signals beyond budget as frequency climbs. These passive interconnects that tie together the press-fit connec- tors, into which the modules for line communications plug, can stave off the need for redesign as data rates in- crease, provided the Df of the laminate is low enough to provide a wide safety margin. Line cards to support 100-Gb/s Ethernet commu- nications also call for PCB materials with loss charac- teristics lower than that of Megtron 6. Such cards could have upward of 20 layers with very dense routing, many thousands of plated holes packed very tightly, and many expensive, highly inte- grated devices packaged in BGAs that have com- plex contact matrices on a tight pitch. These cards can involve multiple lamination cycles during fabrication and their assembly entails the high temperatures required for lead-free sol- der. Beyond the transits through the assembly oven and wave soldering, some cards could re- quire rework to remove and replace a BGA, for example, and that repair would involve heat to remove the device and then another heat cycle to attach the replacement. Such designs and circumstances are ripe for CAF growth unless a laminate is thermally very stable and robust, the bond between resin and fabric is void-free from the start, and care is taken during drilling and desmear to preclude wicking. There is also the possibility that the thermal cycling can change the Dk and Df of a material, thereby compro - mising signal integrity. Cisco Systems and the other manufacturers of high-speed network equipment have developed extremely stringent internal procedures and stan- dardized test vehicles for qualifying laminates to ensure the materials will survive conditions far more severe than would ever be encountered dur- ing manufacture. These are tall hurdles. Materials that pass would also be prime candidates for a wide spectrum of other high-speed digital appli- cations, provided the price is right. The Isola representatives pointed to two laminates, the second of which was just introduced in late June 2014, Tachyon and Tachyon- 100G, respectively, which they recommend for build- ing router backplanes, line cards, and PCBs for other very high-speed digital ap- plications. The two lami- nates have identical elec- trical characteristics, in- cluding a Df of 0.002 and a Dk of 3.02 that is invariant up to 40 GHz. Tachyon-100G was introduced to target very high-speed line cards (think 100-Gb/s Ethernet) because of its thermal stability, in particular a very low co-efficient of expansion in the Z-ax- is, suiting it especially to such high-layer-count constructions. Both the materials use spread glass along with very low-profile copper foil (2 µm Rz surface roughness) to help minimize weave-in- duced differential skew, cut signal rise times, and reduce jitter and intersymbol interference. The materials come in a wide range of pre- preg and core thicknesses and are processed in the same manner as typical FR-4 laminates. They can be used as either a core or prepreg in hybrid FR-4 builds. Any materials with the sort of dielectric and thermal performance as described are welcome additions to a PCB manufacturer's catalog of laminates, especially since they do not involve the complications inherent in processing PTFE- based materials. I'll provide comparisons with other laminates in the near future. PCBDESIGN design for manufacturing Amit Bahl directs sales and marketing at sierra Circuits, a PCB manufacturer in sunnyvale, California. he can be reached by clicking here. Such designs and circum- stances are ripe for CAF growth unless a laminate is thermally very stable and robust, the bond between resin and fabric is void-free from the start, and care is taken during drilling and desmear to preclude wicking. " " HIGH-SPEED NETWoRKS DRIVE NEW MATERIAL CHoICES continues