heatsinksurface-mount
There's a SoC I want to throw a heatsink on. What's the best way to do this for maximum heat transfer and longevity?
I know of thermal pads, thermal adhesive, and thermal paste — but when it comes to the standard black plastic packages, I have no clue which of the options would be best.
Yes, thermal paste is what you wish. The type of heat sink/exchange on top of it doesn't remove the need for paste unless it has some or thermal tape already.
With the much reduced use of DIP ICs, few component retailers offer DIP heat sinks any more (and possibly few of the younger electronic circuit designers know about the stuff!). However, if you are looking at personal / DIY use, glue-on DIP heat sinks in anodized aluminium are available, often as "new old stock", on sites like eBay, and surplus vendors.
For instance, I found this on eBay for under $1:
While it is designed for 14 to 16 pin DIPs, it is trivial to chop it down the middle and use half for an 8-pin DIP.
Thermal paste between the heat-sink and the IC would be useful, or one could use peel-and-stick thermal paste pads for convenience:
Some form of clamp would also be needed to hold down the heat sink, if glue-on isn't good enough.
Alternatively, 2-part and slide-on heat sinks for DIP used to be quite common, and can still be found as "new old stock". These eliminate the need for a clamp:
However, these heat sinks are not as convenient to use for a smaller DIP, such as an 8-pin package.
Finally, sometimes one would sand the face of a metal coin flat, then clamp it onto a DIP IC, as an interesting-looking heat sink. End of history lesson :-)
Best Answer
I don't know about maximum efficiency but there are little stick-on heatsinks that can reduce the junction temperature of BGA and LQFP packages.
I got like 90 of them (2 sizes) from Aliexpress for about $7 US shipped, but you can also find them at distributors. Chances are good the adhesive might be better in terms of longevity and thermal conductivity from the disties, but $$.
There are other options such as using a high tech thermal pad between the chip and a finned case, but there isn't so much advantage in that for the relatively low power levels and home/office environment. Chances are your SoC will be obsolete before it fails if you keep the junction temperature reasonable.
As others have said, most of the heat is typically conducted out through a thermal pad (with copious thermal vias on your board) to planes on the board. Keeping the board cool can be done with a fan, or perhaps something attached to the opposite side of the board (insulation is really important or something could be damaged).