thermal baggie things

Does anybody know who mades those little flat baggie things full of silicone grease? I want to put something between the bottom of a PC board, and a mounting plate, to transfer some heat. Something like 2" square would be good.

The gap is 0.25 inches, but I could fill half or so of that distance with an aluminum plate if necessary.

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John Larkin         Highland Technology, Inc 

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John Larkin
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Fry's sells it by the tube. Seriously, you don't have any in the shop?? In Silicon Valley, you should be able to knock on your neighbor's door and borrow a dab.

Since you are not in an engineering design location, basically any computer shop has heat sink grease, but you won't like the price. They sell it for CPU mounting, so they can get away with highway robbery since the customer won't be buying much.

Needless to say, less is more when it comes to heat sink grease. The grease is to fill voids. It is not all that great of a thermal conductor. So you should absolutely fill it with the metal of your choice. Consider if you need electrical isolation.

Reply to
miso

I've seen slightly tacky pinkish rubber pads a mm or two thick used in PCs

something like this:

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-1334-ND/2633627 I guess

Reply to
langwadt

I want some thermally conductive pad or a baggie-like thing filled with a thermally conductive gel or putty, not just the grease itself.

I've seen them used on laptop computers, between the CPU and an outer cover. Compressible squishy things.

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John Larkin         Highland Technology, Inc 

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John Larkin

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That's neat; I ordered one to try. Thanks.

It's expensive, but if I hack it into small squares it will be a couple of bucks per.

I'll have to fill most of the gap with an aluminum block, and compress this a bit in the remaining distance. That's best thermally anyhow.

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John Larkin         Highland Technology, Inc 

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John Larkin

Look in the Farnell/Newark catalog. Its just a thick sil-pad.

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Reply to
Nico Coesel

Do you need to transfer much heat or just a little? If just a little you could use a bag of pretty much any liquid.

I think you are assuming what you saw rather than taking a really good look at it. None of the thermal pads or grease products are nearly as good as a hunk of pretty much any sort of metal. The grease or pads are not to take up space, but are there to make a better connection between the heat sink and the thing being cooled (or heated). If you use even

1/8th inch pad without compressing it to a much smaller space, it becomes a pretty good insulator. I used to know some brand names, can't remember one at the moment, but I'm sure if you find the data sheet on one it will tell you it is only effective if squeezed to some small dimension or under some large force.

Even the really fancy heat sinks with a working fluid don't use the fluid as a conductor, it evaporates and condenses at the other end.

The "thermally conductive pad" that is normally used to fill a space is called a metal plate. So like the man said, fill the gap with a metal plate and then squeeze the grease or pad between that and your hot thing.

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Rick
Reply to
rickman

26-1334-ND/2633627

I've just had a atom pc with display disassembled, there was a milled alu block bolted to the chassis with thermal paste, on top of that one of those rubber pads for each chip and the pcb bolted on top ICs facing down

-Lasse

Reply to
langwadt

100% metal would wreck trace impedances and short solder bumps. I need something compliant. The PC board will flex, too, so we wouldn't make a good thermal contact to a flat piece of metal.

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I want to fill the gap between the board and the extrusion, in the area under the FPGA and the ADC, the two big black chips.

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John Larkin         Highland Technology, Inc 

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John Larkin

Lookup Berquist they have silpad as well as that thermal interface material for larger gaps.

0.25 is kinda thick....

Cheers

Reply to
Martin Riddle

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a hole or two so you could make a alu,pcb,alu sandwich would be nice

-Lasse

Reply to
langwadt

I did contact B and they are sending me a bunch of samples, 5 different materials. Most are 0.1" thick but one is 0.25. Their stuff runs from about 1 to 5 w/m-K; air is 0.025.

They were very helpful.

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John Larkin         Highland Technology, Inc 

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John Larkin

John Larkin formulated on Wednesday :

I have used Bergquist products over the years and found them to work just fine. And you are right they are generous with samples. Consider a little thermal grease between the Sil-Pads etc and the frame etc to maximize thermal conductivity.

Reply to
BeeJ

Berquist makes "Sil-Pads" and similar gap fillers.

The thickest Sil-Pad they have is 0.010" thick at 3.5 W/m-K, which is lousy. You would do better with polished aluminum (or copper) and no thermal band aids. They also have pads for larger gaps: I've never used these. They appear to have them up to 0.200 and

0.250" but mostly in "soft" fiberglass pads.

In a former life, I had to move about 70 watts each from a pair of flange mounted RF transistor to an aluminum heat sink. The brilliant mechanical designer left me with a 0.125" gap to fill between the xsistor flange and the heat sink. So, I machined and polished an assortment of copper and aluminum heat spreaders (shims) for the purpose. I then had some fun determining which combination of material, silicon grease, Sil-Pads, and polishing worked best. I don't have the numbers but I do recall the results. The winner was 10 mils of gold plated on a copper shim on the xsistor side, and both the copper and aluminum heat sink side polished to a mirror finish. Adding silicon grease or pads increased the thermal resistance of the sandwich. The shim gold plating cold flowed into the gold plating on the xsistor base, make an air tight, metal to metal bond. The polished ends did much the same thing, although not air tight. Unfortunately, the copper to aluminum connection was galvanically incorrect, so I had to settle for an aluminum spacer. Later on, to squeeze more power out of the xsistors, we went to removing the gold from the transistor bases, and polishing them flat.

For your application, it's unlikely that the PCB is going to be flat. Same with the mounting plate. Silicon grease and Sil-Pads were not made to fill large gaps. They were intended to fill in tiny cracks and crevasses in the surface roughness. Most of what I've seen uses far too much silicon grease. The thicker Sil-Pads are the solid form equivalent of too much silicon grease.

I suggest you center drill the PCB, spacer, and mounting plate with a big fat bolt, and compress the sandwich. If possible, machine the spacer surface to a slightly concave contour to maximize surface contact area. I also suggest you use a tiny thermistor probe to measure the temperature at various points. I think you'll find that it works better by NOT using silicon grease and pads. As a side benefit, we found that not using grease and pads made assembly much cleaner and easier (using a torque wrench), which resulted in more consistent results in production.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

Without hardware to compress the thermal sandwich, I don't think putting anything under the PCB is going to do much. However, you might be able to gain some mechanical compression by compressing some kind of thermal spacer on both the top of the chip to the top cover, and on the bottom of the PCB to bottom of the extrusion. Hopefully, the forces can be balanced sufficiently to prevent the board from bending.

Perhaps it might be easier to just attach with thermal conductive epoxy two aluminum heat sinks to each chip. If that's not good enough, fab an aluminum plate the same size as the PCB, attach two heat spreaders to the chips with glue, and bolt the aluminum plate to the spreaders.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Jeff Liebermann

I put a thermocouple on the underside of the board, under the FPGA, and stuck a pin-fin heat sink on the FPGA. The underside temp only went down by about 1 C, so the heat sink isn't doing much. It's about 60C on the bottom side under the FPGA, and I'd like to pull that down. The FPGA and the ADC are heating one another.

In a thermal image, there's a small hot spot on the top of the FPGA. It could be that the heat sink base cools that off, by spreading the heat laterally away from the hot spot, even if the pin fins don't do much. I think it does, but I need to measure actual die temperature to find out. That could be done using an ESD diode, or a ring oscillator.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

I know they sell all sorts of thermal management for plastic packages, but really you need to have the foo in the package itself. Either a slug or a flip chip. Flip chip today isn't marketed for thermal properties, but back in the day, that was one of the reasons they were used. It puts the die in a better position to radiate, plus the lack of bond wires makes a better thermal connection from die to lead frame. What doesn't radiate from the slug is conducted through the leads. For bonded packages, extra wide bonding wire is used to conduct the heat since the thermal resistance is inversely proportional to the cross sectional area of the bonding wire.

But back to your project, I think I understand better what you want. That is, the pad is between the bottom of the chip and the PCB. I don't see that being very useful since the PCB is just an insulator.

I did go through the Digikey catalog and that graphite sheeting sure is cool stuff.

On your board layout, you can always add more copper to pins that are at a static level. Power and ground of course, but there could be others depending on the chip.

Reply to
miso

I kind of pity the companies selling CPUs to home builders. They just don't understand that too much grease is worse than too little grease. Especially true when so many heat sinks these days are polished like mirrors, i.e. fewer voids to fill.

AMD had a unique scheme where the grease would melt with running the CPU. You didn't spread it. I only used that once, so I guess the scheme was worse than letting Joe computer builder spread the grease. That is the next PC I build didn't use that scheme.

As an aside, AMD stopped selling OEM CPUs to builders for a while. It seems a few people didn't use a heat sink at all. They force you to buy a crappy heat sink so at least you know you need one.

Reply to
miso

I remember seeing a friend of mine (honest) powering-up an early home-built PC, before they had the on-chip cutout. Started booting up, great, works, then a jet of flame from the CPU. Forgot to attach the heatsink.

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John Devereux
Reply to
John Devereux

I have used this one previously, fills the gap nicely:

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They handle up to 20mm if needed, you need to order an oversize, to provide a little static pressure

Regards

Klaus

Reply to
Klaus Kragelund

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