VPX HAS A HEAT DISSIPATION PROBLEM
Instead of using the board edges to conduct heat, why not use the entire board?
In a recent post, Embedded Board Standards Are No Longer “Standard” – So Why Aren’t You Looking for Better Alternatives? we talked about the problems related to VPX as a “standard.” It’s clear that it’s become more accurately an “open specification,” which means it’s lost any advantage in terms of interoperability or modularity.
But VPX as a design choice has more issues than that. Let’s look specifically at its heat problem.
VPX Wedge Locks Are A Heat Bottleneck
The majority of the VPX boards that are sold go into conduction-cooled systems. In this scenario, you have to take the heat from the entire board, a 6U card. For the sake of argument, let’s say that it’s 150W that needs to be dissipated. It may be higher than that, but it’ll serve well for this discussion.
You have to take that 150W of heat across the board’s surface to the ejectors (the fasteners that lock the board in place), also known as the wedge lock. The wedge lock then transfers the heat to the chassis to cool it. This is where the real problem is. The surface area of the wedge lock that can actually conduct that heat is very limited. Consequently, it can’t transfer enough surface-area heat from the cold plate out to the wedge lock and from the wedge lock to the ATR box.
This clearly wasn’t a problem back at the origin of VPX, because the higher-power boards were only dissipating 30 or 40W. Now, because everything is so highly integrated and processors consume so much power, 150W solutions are common. Heck, the Xeon processor alone could consume 85 to 120W. Also, bear in mind that older systems placed each function on a different card. Today, it’s the norm for a motherboard to hold the CPU, memory, graphics, communications, I/O, and so on.
We came up with a way to integrate that board into a box that sits on the base of the chassis, as shown in Figure 3. As a result, the board’s entire surface gets conducted directly through its bottom. Think of it as a 100% heat pipe going directly from the processor to the box’s aluminum frame, and NOT through a thin little wedge lock.
Clearly, the advantage of this technique is that there’s more surface area to use for cooling, and when you do cool it, you cool the frame. You transfer all the heat directly to the frame of the vehicle or aircraft (and not heat/cool the cabin).
ZIF Rod Displacement Curve Torque Specification
ZIF III Advantages over Wedge Style Board Retainers
Introduction
The advantages that ZIF III board retainers provide over wedge style retainers provide system integrators
the opportunities for better heat management and simplified board loading.
Description
The ZIF III retainer is designed to mount to the PC board with the board in contact along the full length of
the retainer. The board and retainer assembly is securely locked into a cold-wall with a simple quarter turn. There are no torque wrenches or settings needed with the ZIF III.
The unique locking design of the ZIF III produces a uniform pressure distribution along the PCB edge and
once the assembly is mounted into the cold plate, provides improved heat transfer and resistance to
extreme shock and vibration. Both braze and screw mounting of the ZIF III is available and both provide
uniform mounting pressure to the PCB with no board warping.
Figure 1. ZIF III retainer both mounted and un-mounted on PCB.
The quarter-turn lock and unlock allow quick loading and unloading of boards. The slot in the hex-head
rod is colored to provide visual indication of the lock-status of the retainer.
Figure 2. Typical ZIF III assembly and cold plate mounting arrangement.
ETS Electronic Components Page 1 9/28/2006 ZIF III Advantages over Wedge Style Board Retainers AN1017
Figure 3 demonstrates how force is evenly distributed along the length of the ZIF III retainer and PCB
rather than concentrated into narrow bands, as shown in the wedge retainer. The even pressure and
uniform contact maximize force and heat transfer on both sides of the cold wall, as shown by the arrows.
By design, the wedge style retainers have the potential to create air gaps and hot spots.
ETS Electronic Components Page 2 9/28/2006
ZIF III Advantages over Wedge Style Board Retainers AN1017
Conclusion
ZIFIII retainers provide a simplistic quarter-turn lock and uniform clamping pressure. These and other features should be considered for all mission-critical applications.
Referenced Documents
ZIF III retainers are tested per:
MIL-STD-810
MIL-E-16400
MIL-S-901
Complete Details on ZIF III and other ZIF products can be located at:
Written by: Damon Niswonger
Date: 9/28/200
ETS Electronic Components Page 3 9/28/2006
ZIF III Advantages over Wedge Style Board Retainers AN1017