Method of fabricating a high-layer-count backplane
View Patent ↗The disclosed board fabrication techniques and design features enable the construction of a reliable, high-layer-count, and economical backplane for routers and the like that require a large number of signaling paths across the backplane at speeds of 2.5 Gbps or greater, as well as distribution of significant amounts of power to router components. The disclosed techniques and features allow relatively thick (e.g., three- or four-ounce copper) power distribution planes to be combined with large numbers of high-speed signaling layers in a common backplane. Using traditional techniques, such a construction would not be possible because of the number of layers required and the thickness of the power distribution layers. The disclosed embodiments use novel layer arrangements, material selection, processing techniques, and panel features to produce the desired high-speed layers and low- noise high-power distribution layers in a single mechanically stable board.
1. A method of fabricating a multi-layer circuit board, the method comprising:
fabricating a plurality of high-speed core layers, each comprising a dielectric core of a first dielectric material with a patterned reference plane on one side and a plurality of patterned high-speed differential trace pairs on the opposite side;
fabricating at least one power core layer, comprising a dielectric core of a second dielectric material with a patterned power plane on at least one side, the patterned power plane having a thickness at least equivalent to the thickness of three-ounces-per-square-foot copper;
stacking the high-speed core layers and the at least one power core layer together with other layers, including b-stage dielectric layers of the first and second dielectric materials, the stacked layers arranged such that
at least two patterned power planes exist, separated by at least one layer of the second dielectric material,
each trace-pair side of a high-speed core layer abuts a b-stage layer of the first dielectric material,
each power-plane side of a power core layer abuts a b-stage layer of the second dielectric material,
a transition from the first dielectric material to the second dielectric material occurs across a reference plane, and
where two high-speed core layers are adjacent, the trace-pair side of one high-speed core layer faces the reference plane side of the other high-speed core layer;
laminating the stacked layers together; and
forming a large plurality of plated thru-holes distributed throughout the circuit board, the plated thru-holes electrically connecting the reference plane layers, the power plane layers remaining electrically isolated from each other and from the reference plane layers, within the circuit board.
2. The method of claim 1 , further comprising forming each dielectric core using at least two sheets of dielectric material.
3. The method of claim 1 , wherein fabricating each high-speed core layer comprises forming, on the same side of the core as the high-speed differential trace pairs, a thieving pattern that maintains a minimum lateral separation from all high-speed differential traces on that layer, the minimum separation at least five times the spacing between the two traces of a high-speed differential trace pair.
4. The method of claim 1 , further comprising treating the outward-facing conductive surfaces of each high-speed core layer to roughen those surfaces to approximately the same roughness as the roughness of the inward-facing conductive surfaces.
5. The method of claim 4 , further comprising treating the outward-facing conductive surfaces of each power core layer to roughen those surfaces.