By Michael Walmsley, Global Product Manager, TE Connectivity
Today, designers of embedded computing systems for space applications can use a reliable, standardized backplane interconnect family that evolved from the ground up. Interconnect technologies that performed successfully in critical embedded electronics for defense — from armored vehicle vetronics to military radar avionics — have been leveraged for satellites and spacecraft.
For decades, designers for space applications used customized interconnect designs to ensure the reliability of embedded electronics exposed to the extremes of space. Problematic temperatures, vibration, outgassing, and other factors can catastrophically compromise interconnect systems and signal and power integrity. The high cost and long lead times of a custom interconnect solution were considered a worthwhile investment against failures that are either impossible to fix in space or extremely costly.
Robust industry-standard interconnects were available, but not a sure bet for space. For example, the MULTIGIG RT interconnect family from TE Connectivity (TE) is a printed-circuit-based, pinless, modular solution employed in a variety of computer, communications, medical, industrial control, and military applications. Using printed-circuit wafers instead of a traditional pin and socket contact system, the MULTIGIG RT connector eliminates the open pin field on backplanes and reduces the end user's exposure to field failure in card cage systems. The MULTIGIG RT 2 connector was qualified to the VITA 46 VPX standard in 2006 and has been implemented with OpenVPX architecture across many defense platforms.
Figure 1. TE's MULTIGIG RT 3 connectors are part of its portfolio of OpenVPX products, and they are capable of data transfer rates of 25 Gb/s and higher. MULTIGIG RT 3 connectors are backwards compatible with legacy MULTIGIG RT 2 and 2-R systems (photo courtesy of TE Connectivity)
But was that sufficient for space applications? Several hurdles needed to be cleared to convince space systems designers that MULTIGIG RT connectors were ready for space flight.
Reliability of compliant pin board termination
The space industry for many years required solder joints at connector-to-board terminations. There was skepticism that a press-fit compliant pin contact would have the same reliability as a solder joint. But compliant pins are designed to produce a gas-tight seal to the plated thru-hole in the board and will provide a very reliable termination without the process variability and cleaning required from soldering. Extensive testing has been performed at min-max board hole sizes and different PCB platings to fully qualify compliant pin designs. Today there are numerous space applications using compliant pin technology, and its implementation is growing.
Handling extreme vibration
Vibration is one of the most challenging conditions of space applications.
MULTIGIG RT 2 connectors serve as the basic building block for connection between a VPX plug-in module to the backplane. These connectors meet the VITA 46 VPX standard for rugged embedded computing applications. They were tested by TE to and met the requirements of VITA 47, which defines the environmental conditions and acceptance requirements for VPX modules, including temperature, humidity, vibration, and operating-shock ranges.
A special VITA committee — the VITA 72 study group — devised a vibration test that subjected a 6U VPX test unit to random vibration levels of 0.2 g2/Hz for 12 hours, a severe requirement compared to the original VPX standard. The MULTIGIG RT 2-R connector was developed and launched in 2013 to support these extreme vibration levels. MULTIGIG RT 2-R connectors incorporate an enhanced quad-redundant backplane contact system that doubles the points of contact between the receptacle contact and the wafer.
Another feature is the contact beams and the contact areas on the two beams are asymmetrically positioned and the contact-beam lengths differ. Consequently, each beam has a unique frequency mode when vibrating, to avoid both beams resonating simultaneously and breaking contact.
Additional aggressive vibration test requirements have evolved from the space industry and have been tested in configurations representing system applications, in order to support specific mission requirements.
Surviving temperature extremes
MULTIGIG connectors were qualified to -55 to 105˚C when initially qualified for VPX in 2006, which met the VITA 47 standard for plug-in modules. In direct response to requirements from space systems developers, MULTIGIG RT connectors have been tested by TE and survived -55°C to 125°C, including exposure to 1000 hours of heat at 125°C and 100 thermal shock cycles from -55°C to 125°C. The connectors and contact system have passed this higher temperature level.
Materials selections for outgassing, tin whisker mitigation
Outgassing, which is the release of gases trapped in a solid, can degrade performance of charge-coupled device (CCD) sensors in satellites, thermal radiators, or solar cells. In the closed environment of spacecraft, outgassing is a major concern. Unlike heavy polymer plug-in module connectors used in conventional backplane connector designs, MULTIGIG RT connectors incorporate air gaps, so less polymer is needed. The polymer reduction not only reduces weight but also outgassing. With MULTIGIG RT connector materials, total mass loss (TML) is less than 1.0 percent and collected volatile condensable materials (CVCM) is less than 0.01 percent, which meets NASA and European Space Agency (ESA) outgassing requirements.
Connectors that use pure tin or tin-rich finishes can experience the growth of so-called “tin whiskers” — thin, metallic filaments that can extend to create a short between contacts, printed circuit board pads, and other components. Tin whiskers can also lead to metal vapor arcs if the solid metal whisker vaporizes. The resulting plasma of highly conductive metal ions can form an arc capable of carrying hundreds of amperes. MULTIGIG RT connectors address tin whiskering by offering options of tin-lead electroplating compositions for the contact tails, allowing users to create a high-performance rugged product for applications outside the Earth’s atmosphere.
Evolution of the SpaceVPX standards
The Next Generation Space Interconnect Standard (NGSIS) project developed SpaceVPX—an open standard based on OpenVPX standards. SpaceVPX is the umbrella term for VITA 78.0 and VITA 78.1 (also known as SpaceVPX-Lite) standards, which cover fault tolerant interoperable backplanes and modules as well as SpaceFibre for data and control planes, respectively. SpaceVPX also covers connector implementation and pin assignments.
When VITA 78 and 78.1 were developed, there was a need for new MULTIGIG RT connector pinouts within slots to support utility and management plane redundancy. Redundancy of power distribution to space systems is critical — they can’t afford power supply failures where it isn’t possible to access for repair. Extensive testing was completed on current carrying capability on adjacent MULTIGIG power wafers within a specific connector module and new connector module wafer configurations were developed as required to support the utility management slots for SpaceVPX.
Supporting next generation speeds
Figure 2. A comparison of the full spectrum of MULTIGIG RT products from TE (photo courtesy of TE Connectivity)
The MULTIGIG RT 2-S and RT 3 connectors were launched in 2019 to support higher data rates. This continues to leverage the rugged pedigree of the MULTIGIG RT connector system into next generation systems.
The MULTIGIG RT 3 connector achieves 25 Gb/s signal rates and higher to support protocols such as PCIe Gen 4 and 100GBASE-KR4 Ethernet. This connector builds from the MULTIGIG RT 2 platform and utilizes the quad redundant contact system from MULTIGIG RT 2-R connector to support high vibration levels. With MULTIGIG RT 3, the PCB wafers in the plug-in connector are enhanced with higher speed laminates and optimized for signal integrity, and compliant pins for both backplane and plug-in connectors support smaller PCB hole sizes to reduce noise and improve signal integrity at the board termination.
MULTIGIG RT 2-S connectors maintains the RT 2-R PCB footprint but utilizes the higher speed wafers in the plug-in connectors. This boosts performance (channel margin) in 10Gb/s applications and supports protocols like PCIe Gen 4 running at 16Gb/s baud rate.
Evolving from the MULTIGIG RT 2 interconnect platform, which has been shown to be reliable in demanding military applications, the MULTIGIG RT family has gone through rigorous testing and design enhancements for ruggedness and speed which meet the challenging requirements of space. As a result, the MULTIGIG RT connector family gives designers an easy-to-implement, modular, standardized and cost-effective interconnect system that ensures the reliability of their embedded-computing applications for space systems — and a platform to build from for tomorrow’s systems.
Michael (“Mike”) Walmsley serves as a Global Product Manager for TE Connectivity’s Aerospace, Defense & Marine business unit. Mike has more than 30 years’ experience with TE Connectivity and previously worked with AMP Inc. Most of his career has focused on engineering and product management with an emphasis on new product development. For the last six years, Mike has been with TE’s Aerospace Defense & Marine Business unit as a senior product manager for high-speed backplane and RF connectors.