78-Layer Orthogonal Backplane PCB: Design and Manufacturing

A backplane has always been the largest and most demanding board in a system, and the move to 112G and 224G PAM4 signalling has pushed it into a class of its own. An orthogonal backplane carries the line cards and the switch fabric cards at right angles to each other, which shortens every channel, and doing that at 78 layers involves problems that simply do not exist at 20 or 30 layers.

Why Orthogonal Architecture Is Used

In a conventional backplane the line cards plug in parallel and the fabric cards sit at one end, so a signal may cross the entire board twice to reach its destination. The channel is long, the insertion loss is high, and the routing between cards competes for the same layers as everything else.

Rotating the two card families ninety degrees relative to each other removes most of that distance. A line card connects to a fabric card directly through a shared connector field rather than travelling to the end of the shelf and back, which shortens the channel, reduces the number of layer transitions and opens up the airflow path through the chassis. For systems running 400G, 800G and beyond, that combination is difficult to achieve any other way. The differential routing principles involved are the same ones described in differential pair routing.

What 78 Layers Contains

The layer budget is allocated by function. Roughly 30 to 40 layers are signal layers carrying the high speed differential channels between the connector fields. Another 20 to 25 are ground planes, which is a large number because each high speed layer needs a reference plane close to it on one or both sides, and because the planes also provide the return path that keeps the connector launch transitions controlled.

Ten to fifteen layers are power planes, split into the various rails the system requires and arranged so that each has a low impedance path from the source to the point of load. The remaining layers are shielding and mechanical balance. The whole stack is built symmetrically about the centre, because a board 10 to 18 mm thick will not stay flat through repeated lamination cycles if one half of it expands differently from the other. The via structure that connects this many layers is discussed in via and stack selection.

Materials for 112G and 224G Channels

Material selection is driven by the loss budget. At the Nyquist frequency of a 112G PAM4 channel, the target for the whole channel is often less than 28 dB, and a significant part of that is consumed by the board itself. That rules out standard FR4 and pushes the design towards ultra low loss laminates with a dissipation factor below about 0.005 and a dielectric constant in the region of 3.0 to 3.5.

Several families are in volume use for this class of board, differing mainly in loss, dimensional stability and price. The lowest loss materials are also the most expensive and the hardest to process, and because a 78 layer board consumes a large amount of material, the difference per square metre becomes a substantial part of the project cost. Dimensional stability matters as much as loss, because layer to layer registration across 78 layers and a 600 mm panel leaves very little room for material movement.

Glass transition temperature is the other threshold. Above 180 degrees Celsius is normal for this class, partly for assembly and partly because sequential lamination subjects the sub-assemblies that make up the stack to repeated thermal cycles before the board is finished.

Signal Integrity Requirements

Impedance is controlled to a differential value, usually 85 or 100 ohms, with a tolerance of about five percent. That tolerance applies after lamination and drilling, not on the drawing, so it constrains the dielectric thickness control and the etch process as much as the layout. Backdrilling is effectively mandatory on a board of this thickness, because a via stub that is a fraction of a wavelength long will produce a reflection that closes the eye at the top of the band.

Crosstalk is managed by spacing, by layer assignment and by the orthogonal routing pattern itself, which keeps aggressor and victim channels travelling in different directions for most of their length. Ground planes between high speed layers provide additional isolation, and the general spacing rules described in crosstalk control apply with less margin than usual.

The connector interface is part of the channel. A backplane is only as good as its launch, and the press fit connector footprint has to be designed so that the impedance through the connector field, including the vias and the antipads, matches the routing. That is usually verified with a test coupon that reproduces the launch rather than the trace.

Manufacturing Difficulty

Building 78 layers requires four to eight separate lamination cycles, each of which subjects the partly built stack to heat and pressure. Registration between layers is held to roughly 50 microns, which is a demanding figure on a panel that may be 600 mm or more across, because the material itself moves during each cycle and the movement accumulates.

Drilling is the second obstacle. Some holes have an aspect ratio above 15 to 1, so the drill has to reach deep into a thick stack without wandering, and the plating has to throw copper uniformly into a barrel that deep. Copper balance has to be maintained layer by layer to keep the finished board flat, because a thick stack with uneven copper will bow and the bow cannot be corrected after lamination. Sizes above 600 mm also need presses and drilling equipment that not every fabricator owns.

Inspection and Qualification

Inspection on this class of board is not a final step but a continuous one. Automatic optical inspection is run after each imaging operation, X-ray confirms registration through the stack, cross sectioning verifies plated copper thickness and lamination quality, and electrical test confirms every net. Reliability testing adds thermal stress and conductive anodic filament or insulation resistance testing on coupons, which is how a qualification programme demonstrates that the 78 layer construction will hold up over a service life measured in years.

Class 3 of the high reliability manufacturing standard is normally specified. The acceptance criteria are stricter on annular ring, on plated barrel integrity and on inspection documentation than the standard class, and the traceability requirement means a coupon from the actual build has to be retained and identifiable.

Design for Manufacture

Keep the stack symmetrical, which reduces warpage and is the single most effective measure available on a board this thick. Control the via strategy deliberately: backdrilled through vias for signals that traverse the whole stack, and buried or blind structures only where the fabrication plan supports them, because each additional lamination cycle adds cost and yield risk.

Confirm material availability before the design is frozen. Ultra low loss laminates are produced in limited width and have long lead times, and a 78 layer board consumes a large quantity of panel area. Run the signal integrity analysis early as well, because finding a crosstalk or eye margin problem after the stack is built means a redesign, not a layout tweak.

Applications and Cost

This class of backplane appears where bandwidth per rack is measured in terabits: AI training clusters with large accelerator meshes, high capacity data centre switching, core routers and optical transport equipment, and high performance computing systems. The common thread is a system that needs many high speed channels between many cards, in as little space as possible.

Cost reflects all of the above. Prototype quantities are typically priced in the thousands of dollars per board, falling as volume rises but remaining an order of magnitude above a conventional multilayer board. Material, layer count, backdrilling volume, impedance requirements and the level of testing all move the number, and the lead time is measured in weeks rather than days because the sequential lamination and the material procurement dominate.

FAQ

Why is an orthogonal backplane better than a parallel one? It places the line cards and the fabric cards at ninety degrees, so the channel between them is shorter. That reduces insertion loss and crosstalk, allows more channels in the same rack, and produces a cleaner airflow path through the chassis.

Is backdrilling always required on a board this thick? For 112G and 224G channels it is effectively standard practice. The stub of a through via left behind after drilling is long enough on a thick board to cause a reflection that degrades the channel, so the stub is removed by controlled depth drilling.

What laminate is used for a 78 layer backplane? An ultra low loss material with a dissipation factor below about 0.005 and a dielectric constant near 3.0 to 3.5, chosen for both its electrical performance and its dimensional stability through repeated lamination cycles.

How long does it take to build one? Prototype builds typically run six to ten weeks and production eight to fourteen, dominated by material procurement and by the four to eight sequential lamination cycles the stack requires.