Mechanical Aspects of VPX and VPX-REDI Enhance Functionality and Ruggedness
October 07, 2008 | BY: Ivan StraznickyDownload PDF
The VPX standard brings not only enhanced and denser modern I/O, but also improvements for cooling that enable more functionality and robust ruggedization.
Recent design wins involving VPX (VITA 46/48) single board computers and VPX Gigabit Ethernet Switch engines, particularly for the military's high-profile Future Combat Systems (FCS) program, illustrate the rapid rate of adoption and success that the new VPX standard has experienced in the short couple of years since the first products were made available. VPX, in some ways the "next-generation"VME board architecture, was, like VME, defined and standardized by the VITA Standards Organization (VSO). It leverages much of the goodness of the venerable VME standard, such as the 6U form factor and support for the VME electrical specification for backward compatibility, while adding significantly greater bandwidth, ruggedization and ESD protection, among other enhancements. VPX is well on its way to establishing itself as VME for the 21st Century for high-end aerospace and defense applications.
To appreciate the attributes of VPX and VPX-REDI, the complementary standard defined by VITA-48, it is important to understand the mechanical aspects at the plug-in module level, in addition to the electrical characteristics. One of the reasons that VPX is becoming popular so quickly is that it is both revolutionary and evolutionary. It is revolutionary in that it provides a vast amount of high-speed differential I/O for new serial fabrics such as Serial RapidIO, PCI Express and 1/10Gbit Ethernet, as well as high-speed I/O interfaces for video, storage and sensor capabilities. And it is evolutionary because it maintains support for the VME databus providing compatibility with legacy boards. From a mechanical perspective, VPX and VPX-REDI also maintain support for the standard 6U and 3U form factors long preferred by the military for rugged applications and support for both PMC and the newer XMC mezzanine cards. 6U VPX modules offer an optimal balance between functional density and ruggedness, while 3U VPX offers the advantage of compactness for space-constrained applications. The 3U VPX form factor also delivers a significant amount of I/O for a small card, essentially dedicating the entire P2 connector location to I/O.
One of the key mechanical attributes of VPX and VPX-REDI is the introduction of a high-speed connector. This new connector, the 7-row version of the Tyco MultiGig RT2, has been modified specifically for VPX, with changes such as a shortened length to fit onto typical conduction cards, and a thicker gold contact plating for better harsh environment resistance. The new connector set also includes hardware that provides safety grounding in addition to alignment and keying (Figure 1). The same connectors and alignment hardware are used in both 6U VPX and 3U VPX form factors along with PMC and XMC support.
The two mating halves of the VPX connector are quite different from traditional pin-in-socket connectors. The backplane connector (Figure 2) houses an array of two-pronged contacts for each signal and is press fit into backplane printed wiring boards (PWBs). The daughtercard connector uses mini-PCBs in a housing in order to get good high-speed performance. A side benefit of this design is that there are no metal pins to bend or break. The PCB wafers themselves come in three styles for power, differential or singleended signal needs, and each has built-in ESD protection.
The VPX standard also has a provision to populate certain board locations with optical or RF connectors for even greater signaling speeds, as well as the other advantages that optical and RF technologies deliver. The standard defines that connectors in the P5 and P6 locations (at the bottom end of the 6U connector stack) can be substituted by optical and RF connectors.
VPX has undergone arguably the most extensive harsh environment testing ever given to a COTS standard connector, and the RT2 connector has more than proven itself. Several rounds of testing have been performed in the last few years, the most extensive of which was the VITA 46 testing conducted in 2005.
That testing used a 6U conduction test vehicle to represent the worst dynamic case for the connector, and the environments tested were those typically required of a rugged, deployed military application. The connector passed all of the qualification tests, notably random vibration, salt fog, dust & vibration and ESD. Other testing that has been successfully passed by the connector on representative cards includes customer-specific vibration tests and product qualification tests run by board vendors such as Curtiss-Wright. As a result of this successful testing, customers are confident in VPX modules and the types of cards now being released that span the range from rugged air-cooled 6U cards to ultra-rugged conduction-cooled 3U cards.
VPX's companion mechanical standard, called VPX-REDI, represents a group of specifications written to substantially improve functional density on VPX cards (Figure 3). To that end, VPX-REDI introduces two new pitches, a 0.85 and 1-inch pitch. VPXREDI's 1-inch pitch increases useable area and volume, which allows for a doubling of backside height for such functional increases as PWB thickness and the use of taller components. It also allows for a cover for line replaceable modules (LRMs). The top side height increase can be used for both taller and/ or thicker cooling components and for covers. In addition, the 1-inch pitch frees up 5-6 square inches of real estate previously off limits due to PMC I/O keep-outs.
VPX-REDI enables a variety of improvements. Beginning with aircooling, the additional topside height of the 1-inch pitch means that taller heat sink fins can be used, which substantially increases heat transfer coefficients. For conduction-cooling, thicker metal can be used to reduce conduction thermal resistance, and the option of a secondary side retainer reduces the length of the conduction heat path, both of which result in higher allowable power on the module and/or lower device temperatures.
VPX-REDI also introduces standardized liquid flow-through (LFT) cooling, which has been used successfully for years to cool very high power electronics on military platforms. The liquid is brought in from the chassis through quick disconnect (QD) connectors, routed across the module to provide highly efficient cooling, and exits the module through another QD connector. VPX-REDI also introduces air flow through cooling, which is similar to LFT in that it flows a coolant across channels above the electronics, but uses air instead of liquid.
All these improvements increase functional density. Using power as a rough measure of functionality, Curtiss-Wright has been able to develop cards with over 160W of power, and cool them at the extreme conditions of 71°C inlet air or 85°C card edge for air- and conduction-cooling, respectively. These milestones represent an increase of over 50% compared to previous generations of these products. Even more impressive is the future that liquid flow-through cooling promises. In cooperation with Parker-Hannifin, Curtiss- Wright has developed a 6U LFT prototype that has been tested to over 650W with 55°C inlet PAO coolant.
Another prominent feature of VPXREDI is its standardization of LRMs. LRMs enable the Two Level Maintenance (2LM) objectives that are being advanced by the armed forces for substantial cost savings and logistics improvements. Implementation of an LRM involves a number of factors, starting with placing covers over the electronics to protect against handling or ESD damage. Safety grounds also need to be added to prevent electrical shocks from reaching maintenance personnel, and the connector itself needs to protect the electronics from ESD pulses. This is done on the VPX connector through leading-edge ground strips that conduct electrostatic discharges away from sensitive signal contacts. Testing of this approach shows discharges as high as 15 kV resulting in very low signal voltages, on the order of a volt.
VPX and VPX-REDI bring more than just a lot of high-speed I/O to the table. The space and cooling improvements result in much higher functional density, and the standardization of LRMs for 2LM fills a big gap for COTS modules. COTS board vendors and customers alike are taking notice and developing and fielding VPX products to greatly increase performance of rugged military systems.