By Marc Charbonneau, Bill Kenny, Bruce Peardon and Tony Roberts
Multigigahertz Internet backplanes preserve signal integrity all the way through and across the board.
Small motherboards are the backplanes found in personal computers and equivalent machines, but the coffee-table-size backplanes that go into network-level, fabric-switching products (see Figure 1) are faster, larger and a great deal more complex. They operate at multigigahertz data rates, and provide interfacing bandwidth between the World Wide Web and all the computing power that drives it (see Figure 2).
Certain manufacturing abilities allow today's backplanes to operate successfully at multigigahertz data rates. Some of these abilities include maintaining uniform characteristic impedance over a large area; drilling 40,000 holes through 50 board layers with accuracy; and copper-plating the inside diameter of those holes.
Design considerations for multigigahertz backplanes include trace geometry and routing, connectivity, electromagnetic compatibility, thermal management, board layer minimization and circuit card partitioning. Redundancy is important. Schematic capture, pad sizes, signal layer count and impedance levels are also design issues.
Characteristic Impedance
At multigigahertz data rates, backplane traces are actually microwave transmission lines. Characteristic impedance should be 100 W ± 10 percent for balanced, differential-pair lines, and 75 W ± 5 percent for single-ended lines.
Variations beyond these specifications can cause severe reflections at signal connection points. Any given board may require a mix of the two. Because the entire backplane is covered with conductive traces, specified characteristic impedance tolerance must be maintained over every square inch of the backplane thickness, which could be as much as 0.400".
For in-process and final product characteristic impedance testing, an array of test samples (coupons) are strategically placed around and built into the backplane's area, then cut out and tested individually in a laboratory setting. For electrical testing, a huge "bed-of-nails" tester verifies connectivity among all the backplane nodes.
Backplane Fabrication
 Figure 2. Multigigahertz backplanes support as many as 192 high-speed signal inputs on one side that connect to high-speed circuit boards on the other side. |
Backplanes are fabricated as multilayer stackups of etched copper foil and epoxy-impregnated glass fabric. The current industry-standard resin, which is soaked into the glass cloth before processing, is flame-retardant FR-4, or an even higher-performance material.
Once cured, the fiberglass is solidified and stable. A layer of copper is attached to both sides, and the trace artwork is etched into the copper cladding. Another layer of fiberglass material separates and insulates the copper layers from each other.
Another critical point in the backplane fabrication process is the manufacturing ability to maintain a total envelope of ± 0.003" layer-to-layer registration accuracy throughout all 50 board layers of the backplane thickness. "Post-etch-punch" is a technique that aids this registration.
During fabrication, a production machine with high-resolution video cameras that identify fiducial target patterns on the board processes each board layer. This machine uses the fiducials to adjust the X-Y stage position of each layer until it is individually correct. Then, a punch-platen descends and punches six to 10 common reference holes through the layer. A multipin laminating plate is used to stack up the punched layers into proper sequence and registration. Applied heat and pressure then fuse the final stack into a single, multilayer backplane board.
Skin Effect
At multigigahertz frequencies, electrical signals do not propagate uniformly within the cross-sectional area of copper traces, but rather concentrate themselves at the surfaces (top and bottom) of the copper and avoid the middle. This phenomenon, called "skin effect," restricts the available cross-sectional area for the flow of energy, increases ohmic impedance, increases signal losses and perversely attenuates the signal.
The textbook mitigation of skin effect is to provide more surface area — to widen and flatten the copper traces so the signal energy has greater surface area along which to propagate. This lowers the trace's impedance and helps preserve signal integrity. However, manufacturers cannot just continue to use wider, fatter traces on the backplanes when at the same time, they are trying to achieve just the opposite — concentrate everything into smaller and smaller spaces. It is a tradeoff and a design challenge because pin-to-pin connector spacing and dense card placement both limit available space for trace routing.
Backdrilling Solution
In order to provide greater bandwidth through the backplanes, more board layers are needed to accommodate more signals. More layers mean thicker boards. Thicker boards mean longer plated through holes (PTHs). Longer PTHs mean greater attenuation of the signals because PTHs behave like frequency-attenuating, low-pass pi filters (see Figure 3A and B). Thus, the greater the bandwidth required, the more layers required, the longer the PTHs and the more parasitic loading. This is a quite a design challenge.
Engineers have recently teamed up to develop a simple solution to help solve this design issue — simply drill out the back of the board beneath the PTH. This replaces the original dielectric material with air, beneficially reducing the length of the PTH and slashing its parasitic loading effect (see Figure 3C).
The next challenge is to drill 40,000 holes in a single backplane and yet achieve an acceptable yield. The through-hole plating capability is currently running about at a 13:1 (length:diameter) aspect ratio. In order to achieve the smallest hole size to save space, it works out that the optimum drill diameter is only 0.022". Comparatively, a human hair is about 0.005".
Peck drilling is the answer to this. Instead of trying to run the drill bit all the way through the backplane in one shot, this technique is to drill into the board, then withdraw the bit to clear the chips, then continue drilling and withdrawing, and so on. This way, even though it takes more time, a much greater manufacturing yield is achieved — all the through holes come out clean and perpendicular, with no burrs or debris.
Conclusion
The wide scope of expertise and equipment required to design and build multigigahertz backplanes of this caliber is unique. An early technical partnership between customer and vendor engineering staffs is crucial for the success of complex backplane designs. These joint engineering efforts help assure robust, manufacturable, cost-effective designs capable of meeting the insatiable demands of today's and tomorrow's networking and telecommunications customers.
MARC CHARBONNEAU is a Senior Field Applications Engineer, and BILL KENNY is a Senior Sales Engineer, Teradyne Connection Systems, 44 Simon St., Nashua, NH 03060; (603) 879-3000; Fax: (603) 879-3050; Web site: www.teradyne.com. BRUCE PEARDON and TONY ROBERTS are Members of the Technical Staff, Lucent Optical Networks Group, 1600 Osgood St., North Andover, MA 01845; (978) 960-6995; Fax: (978) 960-1964; Web site: www.lucent.com.
SPEC SHEET
End Applications:
Computer networks, telecommunications, wireless, voice/image/data systems
Related Products:
Network fabric-switching products, routers, multiplexers, loop relay and ATM machine devices
Main Point:
Demand for Internet bandwidth has driven serial data rates in fiber optic data systems well into the gigahertz range. The mission-critical backplanes between these high-speed optical networks and the high-speed computers that drive them must not cause a bottleneck.
Backplane performance issues address characteristic impedance, high-frequency skin effect, and board layer count vs. parasitic loading. Close cooperation between backplane vendors and customers is a must for successful design.
