Back Plane Design

Not too long ago we used to speak about data rates of 3.125Gbps and enthusiastically found solutions for the backplanes able to carry the data. Today its 10Gbps and above (the speeds are on the roadmap for fabrics across VXS and other high-speed architectures). With the high speeds the signal integrity becomes not only important but an absolutely necessity. When carrying 10Gbps per one differential pair, the length stands as a milestone. In fact, every discontinuity in the backplane is a milestone. The longer the path, the bigger the influence of the surrounding environment of the signal. The long length involves the reference planes with the return path and the higher risk of crosstalk along the signal plus the attenuation due to connector and dielectric losses. The high signal density and the topologies imply the usage of many layers to accomplish the routing, leading to thicker backplanes. The immediate effect is the stub effect, which becomes very problematic for maintaining signal integrity at speeds above approximately 5Gbps. There are other ways to improve the signal performance than adding layers and making the backplanes thicker. This includes the use of high-speed materials, design practice to reduce the stub effect, counter drilling, or other methods.
Diagram of the Engineering Flow
First, let us consider the design team -- the design engineer, the signal integrity engineer and the project manager. Each has a key role in a smooth design flow. (See below diagram for design flow) They have to give the customer the confidence that the backplane will function at the required parameters, without running prototypes.

blackplane design Flow

 After analyzing the requirements, the design engineer of Compact CAD will recommend a basic architecture and the necessary number of links over connectors that fit the form factor. PCB Design Engineers has to choose the type of fabric, the connectors, and the PCB material. For the design engineer, the routing skills are not enough anymore. A modern design engineer of Compact CAD has expert knowledge of signal integrity, signal propagation, power distribution, architectures, connectors, materials properties, and fabrication processes. Each engineer has to thoroughly know all the applicable standard specifications, and must be able not only to follow the SI recommendations, but proactively work to define the necessary geometries and layout guidelines. The design engineer is often one who has to interact with the customers, so a deep understanding of the whole picture is quite valuable.