Ti has recommended Arctic Silver Céramique as a thermal grease but it is not electrically conductive, so the required electrical contact to the topside thermal tab is incidental. Are there electrically conductive thermal greases you can recommend? We anticipate using Chemtronics CW7100 (http://www.chemtronics.com/products/americas/TDS/Cw7100tds.pdf). If there is a preferred choice, I would like to know about it.
Steve is out of the office right now, but he'll be back next week to help you with this.
Audio Applications Engineering Manager
Dallas, TX USA
Russell, proper application of Ceramique actually produces reasonable resistance and capacitance between a heatsink and a PowerPAD, in spite of the fact that the Ceramique data sheet says (as you accurately mention) "It is a pure electrical insulator, neither electrically conductive nor capacitive." In application as TI recommends Ceramique is compressed to a thickness of 1 to 2 mils, and we expect resistance in the range of several ohms to several kilohms and capacitance of 150pF or more in that condition. (Ceramique actually has some conductivity and dielectric constant, though both are small, and that plus the thinness of the layer and the likelihood of some slight metal-to-metal contact at surface irregularities are behind the resistance and capacitance results I noted above.)
Please DO NOT use thermal compounds with high electrical conductivity or capacitance, UNLESS you can be certain the compound cannot migrate to the pins of the ICs to which it is applied. Compounds with either of those characteristics can cause partial or near shorts, resistive or capacitive. You do not want to have to troubleshoot that... Also, please DO NOT use thermal compounds that are not mechanically stable under prolonged pressure and heat. These can migrate out of the target contact area. We have found that Ceramique eliminates these problems.
I should add that we get fairly frequent inquiries from customers complaining of unstable operation and poor audio performance, when they have used truly non-conductive thermal pads (with low capacitance) or have even insulated the PowerPAD or have not grounded the heatsink. This problem is almost always eliminated with the use of Ceramique or an equivalent between the PowerPAD and the heatsink, with a solid heatsink ground. We have worked with a variety of thermal compounds to arrive at our conclusions. Part of our experience certainly was the pain of discovering the problems and issues I described above. We do not want you to suffer through any of that.
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