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.
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.
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.
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
- 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