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I have a ceramic heat exchange chip(10m x 10m). I know you are suppose to lay this on the chip, and lay a heatsink on top of that.

What is the best medium for connecting the heat exchange to the chip and to the heatsink?

I would guess thermal paste.

further explanation: I need a heatsink on a Motor driver chip. the chip is 5mm x 5mm. The more heat I pull off of it the more current I can drive. so I'm transferring the heat to a 10mm x 10mm heat exchange and then applying a heat sink

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    \$\begingroup\$ A very, very thin layer of thermal paste is key. Thin. Thermal paste is a terrible conductor of heat (its just better than air) \$\endgroup\$ – Bryan Boettcher May 14 '12 at 18:19
  • \$\begingroup\$ Thanks that will help me from putting a huge globe on it. Does thermal paste act like a glue and harden? \$\endgroup\$ – Ashitakalax May 14 '12 at 18:21
  • \$\begingroup\$ A thermal epoxy will. \$\endgroup\$ – Bryan Boettcher May 14 '12 at 21:43
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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.

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You're right to be concerned about applying thermal paste to a chip directly; what if it somehow contaminates the chip. In this paper1 from Hewlett Packard about removing the lid from an integrated circuit to get better heat transfer, they describe using thermal paste between a chip die and heat sink:

The lidless design uses Dow Corning 340 thermal grease as the thermal interface above the die. This is a conservative choice considering that there are thermal greases available that have thermal conductivities more than three times that of Dow Corning 340.5

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  • \$\begingroup\$ sorry I'm not applying the thermal grease directly to a vlsi chip, but onto a QFN. I added an further explanation to my question. thanks. \$\endgroup\$ – Ashitakalax May 14 '12 at 18:39
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    \$\begingroup\$ If there is a lid between the chip and the metal heat sink, why do you think you need an extra piece of ceramic between the component and the heat sink? Ceramics are not good conductors of heat compared to metal; you're just adding a poorly conductive layer between the part and the heat sink. Ceramics are needed when electrical insulation is required. They are better heat conductors than some other kinds of electrical insulators. \$\endgroup\$ – Kaz May 14 '12 at 18:49
  • \$\begingroup\$ really I accidentally ordered the wrong part, so I wanted to double check. Another advantage of using the heat exchange, is it is highly resistive. So since I have a 5mm x 5mm chip and want to pull off more heat, but the QFN clearance is close to other components. I will place the 10mm x 10mm resistive heat exchange, and this should protect against possible shorts. \$\endgroup\$ – Ashitakalax May 15 '12 at 18:24
  • \$\begingroup\$ A 10x10 mm object won't pull more heat from a 5x5 mm object. The contact area is still only 5x5 mm. The larger object has a bigger surface area for radiating heat, that is all. A reasonable solution would be a metal heat sink that makes a 5x5 mm contact patch with the chip, and is built in some kind of mushroom or T shape so that it doesn't touch the other components. \$\endgroup\$ – Kaz May 15 '12 at 18:34
  • \$\begingroup\$ I agree, 10mm x 10mm is the smallest I could find on digikey \$\endgroup\$ – Ashitakalax May 15 '12 at 18:36

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