21 June 2010

Not Flat Enough - or, Oiling a Transceiver

During an operation to install a new ceramic IF filter in a Kenwood TM-D710, I had to remove the rig's PCB from the diecast casing (heat sink), to gain access to the filter's pins (more on the filter issue to come soon in the "Mysterious Case of the Withering Filters"). The rig hadn't ever been serviced before. After unscrewing all of the screws, including those of the hybrid power modules, I lifted the PCB. I immediately noticed that the heat transfer compound spread indicated that the contact area of the heat sink (or that of the hybrid?) wasn't exactly flat, and a comparatively large spot hadn't been making contact at all (clicking on a photo enlarges it). The corresponding surface of the hybrid had been covered with the right thickness of thermal transfer compound, but it never made contact with the heat sink at all. This can be a serious situation, endangering the expensive hybrid amplifier module because of reduced heat transfer and the unequal thermal and mechanical stresses that develop as a consequence.
Using more heat transfer compound in such cases doesn't help much, because the compound itself is not a spectacular heat conductor, it just helps by filling up microscopic surface irregularities at the contact interface and replacing air, that would be an even worse heat conductor. The contact surfaces must be clean and flat and the heat transfer compound layer thickness must be exceedingly small for the compound to serve its purpose effectively, and this fact is emphasized in every power semiconductor manufacturer's application notes.
I thought of another heat transfer agent that is frequently used in other cases (for example, in oil-filled dummy loads). The right viscosity (heavy) oil would fill the gap nicely, stay there due to the forces of affinity and help transfer the heat to the heat sink at the problem spot. During reassembly, I used a couple of drops of SAE 80W-90 gear oil on the trouble area and gently pushed down the hybrids with a sliding motion, so all of the air trapped between the contact surfaces came out. The screws affixing the hybrids must be tight enough but not overtightened, as this could cause the ceramic substrate inside the module to crack. Just apply enough torque so that the split-ring on the screws closes, and then about an eighth of a turn more - but not much more!
This isn't a new trick for this dog - during my misspent youth, I used vaseline jelly in place of the (then) expensive and hard-to-obtain silicon grease. A couple of linear power supplies I built then (early eighties) are still in daily operation, with the same old 2N3055 and 2N3772 pass transistors "greased" this way.