Hi,
Anybody has experience with heatsinks on FPGAs?
In the V5 documentation, Xilinx says the heatsink can be glued to the FPGA but that it is safer to screw it to the board to avoid mechanical contraints to the FPGA ball when under vibrations.
But then the screws take some space on the board that we can't really afford... This would be at the expense of signal integrity (longer PCB tracks...).
Any feedback welcome :o)
Many thanks, Jean-Baptiste.
I've been putting heatsinks on FPGAs for quite a while, including heatsinks with fans on them. Gluing is fine for in the lab, but if it is going into the field, you'll want a better mechanical connection to the FPGA if the heat sink has any mass to it. That can be done with screws to the board, or with some arrangement of springs on the chassis if your mechanical arrangement allows it. You could also fabricate a hold-down bracket that screws to the board, thereby only taking a little bit of board real-estate at two points (one we did had a bracket that went diagonally across the heatsink and held to the board with two screws).
Another small heatsing fastening method I have seen is a simple torsion bar hooked into a pair of through-hole eyelets placed on opposite sides. PCB real-estate for a pair of these is something like 3-4 square milimeters. No screws, easy to service, negligible PCB area and the eyelet tabs can be however long as necessary to clear nearby SMT components.
We have done before on Virtex-2. It's very good handle to remove the top case of the FPGA if you have a desire to look inside so if you can do have mechanical mounts to the pcb.
John Adair Enterpo> Hi,
This is a non-trivial problem. What is the environment? Will it be subject to shock and vibration? What is the board orientation?
In our experience the only thing that works reliably is to screw it down. Our applications are ground-mobile military and passenger rail vehicles.
The forces have to be carefully controlled so that you don't deform the BGA solder balls. You will have to check with Xilinx to see what compression forces are acceptable for each package. To control this we use spacers between the PCB and the heat sink, and a Bergquist ultra-soft thermal gap pad between the heat sink and the BGA package. You have to adjust the spacer height to get the right amount of compression of the pad.
We have found that the PCB is not stiff enough and will bow under the pressure. So, on the bottom of the PCB we add a thin FR4 insulating spacer and then a stainless-steel stiffener plate. The plate has countersunk holes, and we use flat-head screws from the bottom and lock nuts at the top (heat sink).
We have been using Radian 'Icicle' series half-brick DC-DC converter heat sinks.
We have not yet had to do this with Xilinx devices, but we have been doing this with Freescale PowerPC processors (up to 480 pin BGAs).
================================
Greg Neff VP Engineering
*Microsym* Computers Inc. snipped-for-privacy@guesswhichwordgoeshere.com-- Posted via a free Usenet account from http://www.teranews.com
Hello Greg,
thank you for your feedback.
We do have to consider vibrations. I think the standard says something like [50Hz-500Hz]/3G. But it is a telecom test equipment that will be horizontal and static. So not so bad.
Reading you, screws is the right option. Makes sense. PCB will have to be reinforced from underneath.
To control the force that is applied by the scew, Xilinx mentions some spring based mechanism that prevents the screw to deform the solder balls.
Thank you for taking the time to send me this.
Jean-Baptiste
Thanks Daniel!
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Last time I looked at this, a few years ago, the best solution was something designed for the PC market. That market is big enough to support engineering design work on screws, springs, backplates, and so on. And the end product is often amazingly inexpensive, though that wasn't a consideration for my project.
You're welcome.
I personally like the torsion bar and I have seen it on a few PC motherboards. The bar regulates pressure and if you use a soft, sticky(-ish) thermal pad between the heatsink and FPGA, it will dampen vibrations while maintain contact and heatsink position. Of course, this method is suitable only for small thermal loads (>>> afford...
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A note on glue. We've been using glue to attach heatsinks for years and have thousands of boards in the field without detachment problems, BUT...
1) We have found that the heatsinks that come self-sticking do not hold. 2) We use only DeltaBond 156-K, which requires a long cure time. 3) We don't use fan-equipped heatsinks which may create their own vibration or supply boards for use in high-vibration environments.That being said, if you're short on board space and your board is going into a typical P.C. or similar environment, glue can work for you.
Here's a picture of one of our high-volume boards:
Regards, Gabor
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