High-Speed Networks Drive Material Choices

by | Aug 19, 2014 | 0 comments

One third of the entire human population uses the Internet, according to the International Telecommunication Union, the United Nations agency for information and communication technologies. And access to the network is burgeoning, especially in developing countries. Not only is the sheer number of people online soaring, but demand for video services and other data-intensive applications is compounding burdens on network bandwidth.

Likewise, data intensive business applications are straining the bandwidth of corporate IT infrastructures. Every sector of the electronics industry is being driven by the need to provide greater network capacity, yet improve the efficiency of network communication equipment in terms of bits per second per watt. This includes the manufacturers of laminates for printed circuit boards.

Cisco Systems presented a roadmap for the company’s laminate requirements through 2015. The company’s top priority for high-end network routers and line cards is a material that has half the Df of Megtron 6 at 10 GHz and requires no unusual processing during PCB fabrication, as do PTFE laminates, which are difficult to drill, desmear, and plate. It also emphasizes resistance to CAF and thermal reliability for lead-free assembly operations for such a material.

I recently met with Leena Guila, who manages product marketing to OEMs and other customers for Isola, and her colleague Michael Miller, senior manager for OEM marketing. Close relationships with laminate suppliers are imperative for PCB manufacturers such as my company, which specializes in prototype fabrication and assembly. We had a far-ranging discussion, but concentrated on the material requirements for high-speed digital systems, especially network routers and line cards. They differentiated between the properties needed for high-speed network line cards and those for router backplanes.


PCB Material Design Guide

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.

Backplanes for carrier-class routers comprise many layers, may span distances of 30 inches or so, and must possess loss characteristics low enough to support succeeding generations of increasingly faster line cards without attenuating signals beyond budget as frequency climbs. These passive interconnects that tie together the press-fit connectors, into which the modules for line communications plug, can stave off the need for redesign as data rates increase, provided the Df of the laminate is low enough to provide a wide safety margin.

Line cards to support 100-Gb/s Ethernet communications also call for PCB materials with loss characteristic 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 integrated devices packaged in BGAs that have complex 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 solder. Beyond the transits through the assembly oven and wave soldering, some cards could require 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 compromising signal integrity.

Cisco Systems and the other manufacturers of high-speed network equipment have developed extremely stringent internal procedures and standardized test vehicles for qualifying laminates to ensure the materials will survive conditions far more severe than would ever be encountered during manufacture. These are tall hurdles. Materials that pass would also be prime candidates for a wide spectrum of other high-speed digital applications, 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 building router backplanes, line cards, and PCBs for other very high-speed digital applications. The two laminates have identical electrical characteristics, including 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-axis, suiting it specially 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-induced differential skew, cut signal rise times, and reduce jitter and intersymbol interference. The materials come in a wide range of prepreg 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.


Material Selector by Sierra Circuits


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