Quote from: alfabitcoin on May 14, 2013, 05:55:07 PM

Quote from: BkkCoins on May 14, 2013, 03:10:59 PM

an aside, I'd like to know if using silver epoxy solder available at some electronics shops is viable for on the fly upgrades. It may be easy to paint a little bit on the QFN tabs and then carefully place and hold until hardened. My question would be heat transfer - whether silver/epoxy will conduct enough heat? )

Have updated original post now.

http://www.mgchemicals.com/products/adhesives/electrically-conductive/silver-conductive-epoxy-8331/

Hmm yeah I've not found the most up-to-date reference (1992) on the thermal conductivity, but here's a paper: http://infohouse.p2ric.org/ref/27/26227.pdf

Combining this with the info of alfabitcoin, it seems that conductivity of the conductive epoxy is 'high', while it is still very much lower than typical solder (SnPb or including Ag).

Roughly said;
1 Watt/mºK for conductive (silver) epoxy
30-50 Watt/mºK for SnPb solder, Ag solder would presumably be higher.

This seems to be significant to not recommend epoxy for at least the thermal via connection (base of asic chips)?

EDIT:
Quick guestimate:
Thermal difference between heatsink and chip;
Assuming 1W/mºK
Assuming 7x7mm surface
Assuming 2.5W/chip
Assuming 0.2mm epoxy compound between chip and heatsink

Delta T= (2.5/(0.7*0.7) * 0.02 / 0.01 = ~10 degrees Kelvin or Celcius

Roughly losing 10 degrees from chip to heatsink (in PCB here) is not nice... (plz check my calculations, it's late and I'm sleepy Tongue

)

Same formula for solder with 30 Watt/mºK would result in;
Delta T= (2.5/(0.7*0.7) * 0.02 / 0.30 = ~0.34 degrees Kelvin or Celcius