Next-generation backplane production technology

5Next-generation data rates are placing higher demands on backplane design, assembly, and test. New production techniques are required to deal with these evolving data-rate realities.

A new generation of high-speed protocols are here, with the next generations already on the horizon. The exciting world of high speed has arrived; before we know it, today’s technology will be superseded.

The next generation of PCIe, Ethernet, and Infiniband, to name a few, will add more complexities to the signal channel. We are seeing data rates increase exponentially. The “evolution of backplanes” has begun.

What is changing as backplanes evolve, and what are the challenges seen during this evolution? There are obvious upgrades required for the hardware and we must also gain new understanding of the high-speed channel, but we must also consider the production and test environment, which consists of connector technology, engineering knowledge, PCB materials and fabrication, simulation capabilities, automated production and assembly test, and production-level signal-integrity testing.

Signal integrity: Not the end of the road

We all understand that connectors and PCB materials need upgrading, along with engineering knowledge and expertise, to design at high speeds. PCB fabrication is also a challenge due to backdrills, aspect ratios, copper roughness, tolerances, and impedance control.

Signal integrity is an essential discipline required to define the parameters of the channel for performance at high speed; as such, a huge amount of time and effort is spent on signal integrity, quite understandably. But what’s next, when the new generation of PICMG products are in production? Our 30 years of experience in backplane design and assembly tells us that the production of high-speed backplanes is as important as signal integrity to ensure that all the engineering work is not undone during the production phase of the backplane.

Higher data rates require smaller compliant pins and via holes. New connectors include compliant pins as short as 1.10 mm, pressed into a 0.37 mm plated hole. This size shrink puts the emphasis on ensuring these pins are assembled correctly and do not compromise the signal. (Figure 1.)

Figure 1: Higher data rates require smaller compliant pins and via holes.

The assembly technology of yesterday will struggle to process the new generation of compliant pins because of the use of manual-press machines and limited test technologies. Designers need to have 100 percent confidence that all compliant pins are correctly positioned in the via hole. This is the challenge for the new production environment.

The importance of production and test

The final stage of ensuring that the backplane will function and perform as designed is the production and test phase. Days, weeks, and months of engineering, signal-integrity simulations, PCB design, and post simulation are spent tuning the PCB design for optimal performance. Even so, months of engineering work can be quickly undone if the connector pin is not inserted into the PCB hole correctly.

With hundreds of pins in a connector pin field, how can we detect one bent pin if we cannot physically see it? A pin that is not inserted correctly can cause a system failure, which needs to be detected and fixed before the backplane leaves the production environment. (Figure 2.)

Figure 2: X-ray image of bent compliant pins.

Bent pins and the signal

The smallest variance in design and production can have the biggest impact on performance at high data rates. A production-induced fault, or bent pin, will have clear effects on the signal including, but not limited to, impedance drop, insertion loss, return loss, and mode conversion. (Figure 3.)

Figure 3: Small variances, such as bent pins, will have large effects on performance at high data rates.

The effects of a bent pin are not only limited to the immediate effect on the signal. What happens over time if the bent pin is undetected? PICMG-compliant systems are often deployed in the harshest of environments. In these high-stakes environments there is the risk that a bad pin can, over time, cause damage to the connector and PCB due to shock and vibration. This situation can cause wear on the protective surface of the PCB and potentially short to another copper feature, or the pin could break away from the connector and become a floating object that could cause a short between two other compliant pins.

Enabling the evolution of technology

Engineers, connector vendors, and PCB vendors have all stepped up and provided a path to higher data rates. The last link in the chain to ensuring that systems make the successful evolution into the next generation of products is the production environment. The investment and infrastructure must be in place to facilitate the production.

What equipment and processes do we need to be able to successfully assemble next generation backplanes? How do we process the backplane to ensure the integrity of the new generation of compliant pin? Unless you have visited a dedicated backplane production facility that is capable of producing backplanes up to 56G PAM4, then you might be surprised by the technologies involved. These include light-assisted connector placement, prepress-compliant pin engagement, automatic connector press machines, automatic pin scan optical inspection, automatic 100 percent X-ray inspection and signal-integrity testing, automatic electrical test, hi-pot power testing, and quality assurance by onsite signal-integrity specialists. (Figure 4.)

Figure 4: 100 percent production-level signal-integrity testing on every signal can identify missing or incorrect backdrills in the PCB that are otherwise invisible. Such testing will identify and eliminate PCB fabrication faults that otherwise go undetected.

The use of automated press machines enables designers to program the machine to recognize anomalies during the press process. Each connector has a set profile and an expected force to correctly press the connector. If the recorded force during assembly differs from the profile, then the assembly is flagged and inspected for any potential bent pins.

Consider production from the very start

In addition to the equipment, there is also the expertise and knowledge required to introduce a product into and through the production environment. This process often starts at the early stages of a project when signal-integrity simulation is complete and PCB layout begins:

  • Design for Manufacture
  • Design for Assembly
  • Design for Test
  • FAE product support.
  • Detailed and concise PCB manufacturing instructions.
  • PCB fabricator control – correct drill size and plating, the correct drilling processes, predrill and multipeck drilling, hole T/P, and backdrill stub control.

As an example, to avoid plating knee within the plated hole for the compliant pin, it is recommended that the drill process should always be from the press-fit side of the PCB to ensure clean entry of the compliant pin into the plated hole. Clear instructions must be given to the PCB fabricator to control the integrity of the plated hole. They do not know what the hole is used for or the data rate of the PCB.

Regular audits and spot checks should be performed with PCB vendors to ensure quality is maintained and will not adversely affect the signal.

There is a huge amount of expertise and knowledge required to process high-speed backplanes. We see identical comparisons to our experiences during the years of evolution in the IT/datacom markets where data rates also increased exponentially. All the engineering and production expertise, equipment, and infrastructure invested to facilitate the IT/datacom evolution are now required to successfully produce PICMG-compliant backplanes and products.

Production and test are often the forgotten links in the chain to ensure signal integrity. Any chain is only as strong as the weakest link. Let’s not forget the importance of production – a vital link in the evolution of technology.

Gary Routledge is Engineering Manager, EMEA & APAC, for Amphenol Backplane and Systems Integration.