Something I thought might be helpful for builders and other techy/service types. Last week I was working with a student in one of my computer classes and we were involved in fixing a MB that was having a thermal shutdown problem with the CPU. It is a late model device with an Intel P4 3GHz that is mounted on a MCI MB (not really pertinent) The CPU was getting up to around 80C and shuting down, so we decided to lift the cooling fan and CPU to check out the contact area. problem was a broken finger on the plastic grip in one corner had snapped. (You'd think intel could supply slightly better hardware for a $300 CPU). There was a lot of dust, etc so we decided to clean of the heat sink compound and replace with fresh compound. We did a thorough cleaning of the compound, a type which is commonly found on computer CPUs, a silver based compound that has high thermal conductivity. Alas, when we went to the storage cabinet in the classroom, we found the compound we usually have on hand, a similar silver filled compound was all used up. So we took out the alumina thermal grease we use for transistor packages and applied this. We uaed to use this all the time on the older 386s and 486s. We removed the broken finger on the cooling fan grip and replace with a tye-wrap that was small enought to fit. Everything worked well and brought the temperature down to about 38C, which was acceptable. The next day, I decided to grab the heat sink compound I had on my home workbench and take it to class with me. We were going to do an experiment, because I had been thinking about this and bouncing it off of 40 years experience in aerospace electronics and satellite assembly. I knew the silver filled compound had better thermal transfer characteristics than the Alumina filled silicon grease compound. In assembly of most space electronics you cannot use these compounds and rely on the solid film tpes when needed. We decided to repeat the compound replacement on the computer, using the more commonly used silver filled on the CPU. After replacing and reassembly we found that the CPU temp was slightly lower at about 32C aaginst a room ambient of 24C. About what we expected to see. OK where am I headed??? There is a significant difference between the two compounds that has little to do with the thermal properties, but has a little to do with the slight improvement of the metal filled compound. . . it is also ELECTRICALLY conductive. My cautionary note here relates to those of you who might opt to use the silver filled material on a power transistor application. Unlike the computer CPU, most power transistors have power and signal voltages on the collector or source/drain. IF the heatsink is part of the circuit and the entire side of the circuit is grounded, there is probably no reason for concern. But, if it is isolated and is "hot electrically", or there is a mica or other insulator between the Device and the heatsink/mounting surface, you could cause a short circuit by using any heat sink compound that is not an electrical insulator. I hope this is helpful and maybe saves someone from a shocking experience. 73 de WA6VNN, Al
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