Conduction cooled CompactPCI boards ready for insertion

By Clarence Peckham Conduction cooling of circuit cards is a technology that has been used for many years in the military marketplace. Conduction cooling is a method of cooling circuit boards by "conducting" the heat from the circuit card to a mating surface. In most cases the mating surface is the wall of the systems enclosure. Conduction cooling allows the circuit boards to be sealed in the systems enclosure without requiring airflow. Figure 1 is an example of a flight computer that is conduction cooled. The system shown in Figure 1 contains up to six 3U VME cards and is capable of withstanding an ambient temperature of -55 to 85 degrees C as well as a large shock and vibration environment. This article will examine the typical conduction cooled environment as well as some of the design issues that are involved with developing conduction cooled product. VME rules!
Well this is a CompactPCI based article but it needs to be pointed out that VME based systems are used in a lot of applications developed for the military. In fact if a system is not custom, based on a nonstandard backplane architecture, and if a 6U form factor can be used, then typically VME has been used. Why is this? Obviously VME has been available for 20 years and long life is a key factor for military applications. But the driving force has been a standard developed by the IEEE. IEEE-1101.2 is a mechanical specification for developing conduction cooled 6U VME cards. Once this specification was available conduction cooled boards from different manufacturers could be used in the same system with no mechanical fit issues. Systems that could use a 6U form factor now had a standard bus architecture and mechanical standard based on the well understood and widely used VME architecture. CompactPCI on the rise!
With the success of the CompactPCI bus standards in the commercial market there is a lot of interest in providing CompactPCI solutions for military systems. There are several factors driving this interest in CompactPCI:

  • The CompactPCI bus standards are perceived to be the latest in commercial off the shelf (COTS) technology. A COTS solution is a requirement in new military systems requirements.
  • The emerging specifications such as the PICMG 2.16 switched backplane and the PICMG 2.13 redundant system controller are generating a lot of interest
  • More connector space for I/O is a major driving factor.
  • Higher performance and 64 bit bus option provides a faster and wider data bus than is available with VME based solutions.
One of the issues with VME has always been the lack of space on the connectors for I/O signals. Since all conduction cooled systems require rear, or backplane, I/O signals the VMEbus connectors severely limited the number of signals that can be handled. The VME specification was modified to allow for five row DIN connectors and to add a third connector for more I/O but this was only useful for 6U VME cards. If a 3U form factor was required then there was no solution for I/O since the one backplane connector available on a 3U VME board is required to support the VMEbus. Use of CompactPCI resolved these issues. A 3U solution can be provided that allows one connector to be used for a 32 bit CompactPCI bus and the second connector provides almost 100 percent utilization for I/O signals. As we will see later this is critical for some of the system upgrade requirements. The one piece that was missing was a mechanical specification for conduction cooled CompactPCI cards. The missing link was provided by the VME International Trade Association (VITA) standards organization. A standards committee was formed to generate VITA 30.1 - a mechanical standard for conduction cooled Eurocard format boards with 2mm bus connectors. The VITA 30.1 specification provides the conduction cooling standard for both 6U and 3U format boards with CompactPCI electrical bus interface. This standard has been approved and will become an ANSI standard. (A draft copy of VITA 30.1 is available) All of the pieces are in place - PICMG 2.0 CompactPCI standard and VITA 30.1 conduction cooled mechanical standard. All that is left is to build the product and the customers will come! Evolution of a conduction cooled product In order to build a conduction cooled product there are several additional steps required as part of the design process. Along with the electrical design of the product the environmental requirements must be evaluated. The environmental requirements include both thermal and mechanical (vibration and shock) requirements. A typical commercial product has an operating temperature requirement of 0 degrees to 55 degrees C. For a product to be used in most military systems today the operating temperature is -40 degrees to 71 degrees C and for some extreme cases -55 degrees to 85 degrees C with limited temperature excursions to 95 degrees C. In addition environmental concerns such as salt fog, fungus, and chemical interaction need to be considered. SBS Technologies has been providing conduction cooled products for over seven years. Recently we just completed one of several new CompactPCI conduction cooled products that is a good example to use to explain conduction cooled designs. The CCPMCC is a conduction cooled PMC carrier card. The card is a carrier card that provides support for up to two PMC cards. The CCPMCC has a CompactPCI interface as well as routing for all of the I/O signals for the PMC modules. Figure 2 shows the assembled CCPMCC product. In order to show the individual assemblies, Figure 3 shows the expanded mechanicals for the CCPMCC. The wedge locks, cooling plate, and printed circuit board are clearly illustrated in the expanded view. The wedge locks are used to provide the mechanical interface between the circuit card and the chassis. The wedge locks provide mechanical rigidity as well as provide a thermal interface between the cooling plate on the circuit card and the chassis side walls. This cooling interface is where the operating temperature of the card is determined. In all conduction cooled circuit card applications if the operating temperature is specified to be from -40 degrees to 71 degrees C, then this is meant to be at the cooling interface of the wedge lock and chassis mechanical interface. The cooling plate is bolted to the circuit card and is used to transfer heat from the components to the card edge. On the backside of the cooling plate, the plate is machined to allow for different component heights. A thermal transfer material is used to make sure there is good thermal transfer from each component to the cooling plate. The cooling plate also provides mechanical stiffness to the circuit card assembly. This is important for all military applications. From Figure 3 it should be noted that the CCPMCC is a bit unusual. The actual circuit board is 100mm in height and not a full 160mm. Since the CCPMCC does not require a significant number of components, it was decided during the design process to maximize the cooling plate thickness in order to maximize the cooling of the PMC modules. Does it all work? CCPMCC analysis
In order to analyze both the thermal and mechanical performance of the circuit card a finite element analysis was performed. The CCPMCC card was modeled by adding the power consumption of all of the components as well as the mechanical dimensions and layout. Once the model was complete both thermal and mechanical analysis were performed. Figure 4 shows the thermal performance of the card for a card edge temperature of 85 degrees C. The overall temperature rise of the card is about 5 degrees C. Overall the card will provide adequate cooling for the two PMC cards as long as the PMC card power requirements are within the 7 watt PMC specification. The other analysis that was performed was vibration and shock. Using the finite element analysis module it was possible to determine the first and second resonance of the card. This analysis showed that the first resonance was at 800 Hz and the second was at 1400 Hz. In both cases the resonance did not present a problem since even the first resonance is higher than most input vibration spectrums encountered for most applications. Figure 5 shows the deflection of the card for the first resonance. In addition to the vibration analysis an analysis was performed for an operational shock of 15G for 11ms. As expected there was no problem with this level of shock test. One of the advantages of the VITA 30.1 specification is that it builds on over ten years of experience with conduction cooling of VME products. The board form factor is the same. The only major difference is the use of 2mm backplane connectors on CompactPCI instead of the two 96 pin DIN connectors on VME. The 2mm connectors on CompactPCI provide more stiffness to the board than the DIN connectors do on VME. Life is good:
  • CompactPCI provides a superior number of I/O connections over VMEbus solutions.
  • The CompactPCI 3U format provides a usable number of I/O pins on the backplane.
  • The field experience of ruggedized VME 6U products provides positive proof that the mechanical specifications are sufficient for VME or CompactPCI conduction cooled products.
Product available and futures
I would be remiss if I did not point out that there are several products available on the market for conduction cooled CompactPCI applications. Figure 6 and 7 show two PowerPC 750/7410 based solutions available from SBS. The 3U board in Figure 6 is the RL4 product which is an excellent example of what can be achieved on the 3U format. In addition to a full feature set on the base board there is the added advantage of a PMC site for I/O expansion. As an example, by adding a conduction cooled MIL-STD-1553 interface, Fibre Channel, or ARINC 429 the board can be used in several different system applications. Figure 7 shows a 6U version of a PowerPC Single Board Computer with two PMC sites. This card is shown without the wedge locks so that the entire cooling plate can be seen. Some of the exciting future possibilities for CompactPCI in the military market is the PICMG 2.13 specification and 2.16. PICMG 2.13 provides a method of having redundant system controllers in the backplane. For the systems that require a method of providing higher reliability when the main processor fails, PICMG 2.13 is a reasonable solution. With the interest in the military of moving data quickly PICMG 2.16 provides a method of using 1 Gbyte Ethernet links on the backplane for data movement. I believe both 2.13 and 2.16 will be accepted for use on several new military platforms and upgrades. Typical applications
One of the challenges of using VME in the past has been fitting 6U boards into the available space of some the military vehicles. The 3U CompactPCI specification provides a solution to the limited space problem. One of the applications developed at SBS Technologies was a for a fighter aircraft. The original system was totally custom with a processing performance of less than 2 MIPS. The problem was the current system could not be upgraded and the space allowed for the total box was about 4 inches x 6 inches x 9 inches. The only solution was to use a 3U CompactPCI implementation or design a new custom solution. By using a 3U CompactPCI solution we were able to provide a three board set that included a single board computer (RL4) with either MIL-STD-1553 or ARINC 429 and two I/O boards implemented in the same physical space. The advantages were:
  • Lower implementation cost due to the use of commercial off the shelf products
  • Far better performance by using a PowerPC 750 processor
  • The ability to upgrade the system in the future by changing to a different CompactPCI board set
In fact, the ability to change from a MIL-STD-1553 PMC module to an ARINC 429 module allowed the system to be used in two different configurations on the same aircraft. As more military platforms are upgraded and new platforms developed, CompactPCI provides a viable solution to most system applications. A summary of major features that CompactPCI can provide are:
  • 32 bit and 64 bit backplane implementations
  • Air cooled as well as conduction cooled solutions
  • 3U mechanical format for small footprint system solutions
  • 6U mechanical format for larger systems
  • Redundant system support with PICMG 2.13
  • Multiple data channels with PICMG 2.16
  • Multi-sourced COTS solutions