# How can I attach a heat-sink to legacy ICs?

I work (play) with some legacy chips like the mighty TMS9918. Which, if you didn't know, doesn't require a heat-sink but the thing does get really hot. Amazing they don't fail more often.

Anyway, I actually have a heat-sink for it that is the perfect size (came out of an old Colecovision) but I don't know how to attach it.

Modern CPU and heat-sinks have the mounting holes in the motherboard that clamp it on top of the CPU. This heat-sink or motherboard has nothing like that. It's literally a piece of metal that sits on top of the chip.

I have thermal paste but that isn't glue. In fact, the reason I have this is because it was glued (I'm guessing) to the chip but over time fell off.

So my question is, is there a thermal paste that can also act as glue to hold this on? I want to protect these legacy chips as much as possible.

Thanks.

• Could you post an image of the heatsink you're hoping to use? If you don't absolutely need to use that particular heatsink, you can buy clip-on ones for 40-pin DIPs for a few dollars – DerStrom8 Jun 17 '15 at 14:46

From what you describe, you don't want the heatsink to be removable. For that reason I would suggest using Arctic Silver binary thermal adhesive:

Note this is the Thermal Adhesive, not the Thermal Compound.

It's good for more than 150*C, and is one of the most popular thermal adhesives I've come across. You can buy it on Amazon for a relatively low cost:

Good luck!

• It seems to have a much higher thermal conductivity than any other thermally conductive epoxy on the market. I wonder how accurate the specification is? (They no longer mention it on their website; it is only listed in the review links.) – Oleksandr R. Jun 18 '15 at 0:39

I'm going to throw in a couple of cents, since I don't think Arctic will be very cheap bought as a one-off. But I may be wrong. You definitely will not need that level of heat conductance on the top of a ceramic chip package, let alone a plastic one.

You can do two other things: As Plasma suggest google for "Thermal Adhesive", many cheaper types exist, as an example of what I use, bought in bulk, for not-so-heat-critical things:

Farnell Listing for Fischer Thermal Adhesive

May also be obtainable through easier mediums. This does have the disadvantage of needing a precise measurement tool for the stuff. If you need a small drop mixed, you're looking at miligrams of hardening liquid.

But, what you can also do is much simpler and household-budgetty:

Take a drop (or small stripe) of thermal compound on the heatsink, press it onto something made of glass (bit of alcohol and an old cloth will clean it off, no worries) and see how much it spreads. On glass with high pressure (don't break your glass!) it will spread more than on a chip with a rough surface in comparison, but not by 50%.

Now find the drop that will cover about 80% of your heatsink.

Once you have found the drop-size you need (and over time you will get better and better at it in one go if you need to do it often):

Apply the drop-size you need, put some high-strength glue to the small bits that you expect won't get covered. Such as brand name super-glue, or usually better with plastic dips that get hot: "heat proof" (upto 100degrees C is fine) epoxy or silicone.

Press on and hold in place with a decent clip until the glue is fully hardened.

Done, preso-magnifico and you will likely be using the glue elsewhere often enough to validate its purchase.

You should just use regular epoxy glue.

I can't do better than quote my old answer:

Almost any adhesive will be suitable. Thermal adhesives are better, but that's missing the point. Some numbers from wikipedia: Air has a thermal conductivity of 0.025 (W.m-1.K-1). Aluminium is about 200, so that's a ratio of 10,000 times. If you can fill the gap with thermal grease (0.9), or a polymer like epoxy (0.3), or even water (0.5), the thermal resistance of the gap will decrease by about 100 times.

3M claims a thermal conductive epoxy with 0.72, but this is not much better than average plastic around 0.3 to 0.5. The important thing is that these are all 100 times more conductive than air, and they fill the gap eliminating the air.

So select an epoxy that meets your mechanical needs, something with the right temperature range and flexibility. You can probably use it somewhat above it's high temperature limit, as you don't need full strength. Prepare the surfaces properly and fill the air gap.

• Regrettably the 3M link seems to be stateful. It may be better to link to the datasheet, which is here. – Oleksandr R. Jun 18 '15 at 0:39
• @oleksandrr whoops, thank you for the datasheet link. I would expect 3M to have accurate specifications. – tomnexus Jun 18 '15 at 5:19

As the others said, ultimate performance is unnecessary for the interface material, since the chip's dissipation is likely pretty low to begin with.

I would use a less permanent solution than Epoxy: good old thermally conductive adhesive tape. It is also much less messy to apply than epoxy goop, since it behaves just like double sided tape. It is also removable, if needed.

Also, I did not check if thermal expansion of the ceramic package is similar enough with Aluminium's so they can be bonded without risk.

Gluing the heatsink with the surface of SMD will work to some extent. A few cases it will with stand for dissipation but in several cases it failed by days. I would suggest a continuous air flow in that area will give good result.

Calculate the space near and around the SMD IC, fix a mini fan that is more efficient and low voltage operated(5v or 12v). In addition to the heatsink. (I think it is easily available and you can fix it near the PCB.)