When it comes to developing new backplanes, designers have to confront a number of challenges, from new high speed architectures and interconnect techniques to the use of innovative materials. But they also have to address more mundane factors concerning prototyping and manufacturing, whether that is achieving target performance levels; maintaining signal integrity and interoperability; minimising cost but optimising manufacturability, or ensuring that both lifecycle reliability and TCO targets are met.
For engineers, the resulting design will often call on them to balance these competing requirements as one will inevitably impact on another. Finally, there is a need to future proof the final backplane design.
When it comes to developing modern backplane designs, the number one issue is, and is likely to remain, the need for speed as Justin Moll, vice president, US market development with Pixus Technologies, explains: “Most of the backplane specifications that we work with are progressing to much higher speeds. AdvancedTCA is, for example, moving beyond 40Gbit/s and specification work to 100Gbit/s has already started. MicroTCA is completing specification work to 40Gbit/s and, while OpenVPX is also moving to 40Gbit/s, it is also incorporating optical contacts and coax contacts for RF via VITA 66 and VITA 67 respectively.”
According to Moll, the move to RF is a growing trend. “VITA has created specifications to incorporate RF contacts into a VPX backplane (per VITA 67) and the designs are using new connectors. Vendors will need to familiarise themselves with them and be able to have the right test and simulation setups in place,” he suggests.
Typically relevant standards when it comes to backplane specifications include: VITA 65 for OpenVPX, PICMG 3.0 for AdvancedTCA, and MicroTCA.0 for MicroTCA.
New graphics, video, GPU, FPGA and telemetry cards, as well as other high-performance subsystems, are having an impact on backplane design and, along with advanced single board computers and mission-critical applications, are combining to constantly ‘raise the bar’ when it comes to delivering higher system performance.
According to Moll: “The higher speeds we are seeing are certainly more challenging and will require different design techniques, higher grade materials and more simulation/characterisation.
“These faster speeds often require more spacing between the signals to prevent crosstalk and to ensure clean signals.
“This will often mean more layers and more expense. Furthermore, the PCB material is moving from FR-4 equivalents to lower dielectric materials, such as Nelco4000-13SI and Megtron-6. As a result back-drilling the vias is often required in thicker backplanes, which will increase costs.”
Moll believes that finding the right balance to keep the costs low is ‘certainly a challenge for backplane companies’. Cost is an issue, but things need to be taken into account.
“For example,” he continued, “the longer the backplane, the greater the cost due to the need for lower dielectric materials to offset the line losses. Usually, when there are more signals between connectors, more layers are required which often results in thicker backplanes which then require back-drilling.
“Higher power requirements are also adding to the backplane thickness.”
Don't try to do it yourself
When it comes to backplane design, Moll believes that in the main, the short answer is ‘don’t try to do it yourself, especially when it comes to a high-performance design’.
“It’s part art and part science,” he suggests. “An experienced team knows the best routing techniques and ‘tricks of the trade’ to save time and effort. Designers should not only focus on the signals, but also on the power rails and other signals, such as system management. Having proper power interfaces for today’s systems and interfaces for system management is becoming increasingly important.”
Greater demands for customisation have resulted in a growing number of companies offering services to help companies with their designs.
Harting Integrated Solutions, for example, provides a backplane customisation service. According to Robert Pulman, the company’s European product applications manager: “We are seeing a growing trend towards more customisation which is being driven, in part, by the fact that there are simply more standards which tend to be more application specific.
“Ours is a custom and semi-custom business model, which means we can take a specific standard and customise it for a particular customer.”
Harting has developed new test fixtures for ATCA backplane testing
Pulman says the company’s main markets have tended to be embedded, broadcast and telecoms, but he notes increasing demand from the rail and defence sectors.
Whether an application requires a standard form factor or a highly complex, high-speed backplane, the company can provide design services, assist with prototype assembly and deliver high volume production.
“There has certainly been an increase in demand for high speed backplane systems whether for 10G, 40G or 100Gbit/s applications,” Pulman suggests. “As a result, we are seeing demand increase for certain types of standards, particularly CompactPCI and VPX. VPX is robust and can support much higher speeds; it may not be the cheapest, but it certainly provides high levels of performance.”
According to Pulman, standards provide the cheapest route to the high speed designs the market is calling for. “The rules are set and we simply reapply those rules with different customers,” he explains.
The validation process for backplane designs usually features a number of stages, according to Moll.
“These include: step 1, design simulation; step 2, post layout simulation; and step 3, TDR measurement on a real backplane with the aid of calibrated test paddles. Measurements are typically done on the shortest and longest pairs and between pairs that have the highest probability of being aggressors. Finally, step 4 – when possible, load the backplane with test cards capable of transmitting and receiving data simultaneously. You need to determine the highest bit error rate.”
With higher speeds comes the need for computer simulation to optimise conceptual designs and to then ensure integrity and performance.
Companies like Harting use computer simulation to predict custom backplane performance and to significantly reduce the number of prototype spins.
“Simulation means a quicker design cycle and a higher probability that a backplane design will meet or exceed the design specifications and cost parameters,” explains Pulman. “It is becoming more important that you do simulation because the cost of amending a layout can be prohibitive.”
After a thorough inspection of incoming components, a backplane will be assembled in a prototype facility, usually in conjunction with a production facility so there will be seamless transition into production.
“Be aware that not all PCB shops can manufacture with higher grade materials,” warns Moll, “drill bit wandering, etch control and layer registration are all becoming increasingly important in modern designs.”
When it comes to the backplane it is vital that the final design has sufficient robustness, performance, flexibility, and the capabilities necessary to be able to compensate for any variance that may occur due to different subsystems – the backplane’s performance remains a make-or-break factor in terms of the overall system’s success.
