Electronic – How to attach a heat-sink to legacy ICs

Yes, thermal resistance/conductance is a two-way street, for passive conductive cooling devices, with equal speed limits posted for both directions.

OK, now for the caveats: If your heat sink is rated for forced air flow, and you have no forced air flow, then the heat flow through the heat sink will be different. Your heat sink is rated for certain conditions at both ends. If you meet those conditions (at both ends), except for reversal of temperature conditions, then you can expect equal but opposite heat flow.

Say that your 10C/W heat sink is rated 10C/W for conduction of heat from a 1W source to still air, with the contact area with air being fins. Now, you put those fins INSIDE your enclosure, in contact with still air, and you place the outside end of your heat sink in contact with a device (say, a cold plate) that will keep that end of the heat sink 10C cooler than the inside air. In that case you will get 1W of energy flow from the fins of the heat sink to the cooling device (cold plate).

You would want to pay attention to such things as: Warm air rises and cooler air falls. Air fins are most effective when hot air can rise from them and allow cooler air to come in contact with the fins. Cooling fins, on the other hand, would be more efficient when placed such that cooled air can fall away from them.

Consider clamping or screwing a heatsink in place and using standard and relatively low cost thermal grease.

If this is for a on off and/or does not have to look pretty, consider whether retaining with a cable tie would be acceptable. This may need holes to be made in PCB, but may not. You MAY even be able to slide a thin tie lengthwise under the DIP package.

You can make your own with a goodish chance of success.
Use epoxy of choice and fill (mix) with about 50% of substance liable to make it suitable conductive thermally.
This*might* be graphite, carbon black, metal powder (copper probably good, brass probably good, Aluminum may be excellent).

You can get powdered Aluminum from paint suppliers (or could). As this was the main fuel of the space shuttle solid boosters it may be harder to buy now than it was just over 10 years ago. Filing the handle of an old saucepan may do almost as well :-).The finer the Al powder the greater the oxide coating % so super fine may not be good.

NOW I Gargoyle it - should have done that 1st.

This man claims excellent results making his own from about 3 parts epoxy and 1 part thermal paste - makes sense.

Thermal adhesive:

ebay UK has a few offerings

This is GPB2.25 for a small tube with free economy international postage from Hong Kong BUT it doesn't sound like it makes a very strong bond.

ebay also has lower cost adhesive thermal tape. How good it is is tbd.

GBP1.65 free postage one square inch from Manchester - sounds possibly OK

2 GBP from Gloucester - hopefully their dimensions are wrong.

About GBP5 from GB may be OK but getting up

GBP2.88 from Wimborne 35mm x 35mm