{"id":472,"date":"2012-12-02T21:16:03","date_gmt":"2012-12-03T03:16:03","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=472"},"modified":"2014-04-13T20:24:03","modified_gmt":"2014-04-14T01:24:03","slug":"everything-you-know-is-wrong-april-2003","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=472","title":{"rendered":"Everything You Know Is Wrong April 2003"},"content":{"rendered":"

Answers to those Doggone Thermal Design Questions<\/strong><\/p>\n

By Tony Kordyban<\/strong><\/p>\n

Copyright by Tony Kordyban 2003<\/p>\n

Dear Everything, etc.,<\/em><\/p>\n

Last year, when our company still had a thermal engineer, she designed a heat sink for the microprocessor on my board.\u00a0 Without the heat sink, it was about 25 degrees C over its operating limit.\u00a0 The heat sink brought down the temperature about 20 degrees.\u00a0 That wasn’t quite good enough, so she added a special thermal grease between the microprocessor and the heat sink, and the temperature went down another 10 degrees.\u00a0 It was a great success.<\/em><\/p>\n

I am now doing a cost reduction on that product.\u00a0 One way of saving money is to use a processor with less horsepower — and less heat, too.\u00a0 Without a heat sink it is only about 8 degrees C over its operating limit.\u00a0 Can I save even more money by eliminating the heat sink and just use the thermal grease to reduce the temperature by 10 degrees?<\/em><\/p>\n

Penny-pincher fromPalmyra<\/em><\/p>\n

 <\/p>\n

Dear Penny,<\/p>\n

Following is the answer that I am sorely tempted to give, but EVERYBODY ELSE PLEASE UNDERSTAND THAT THE FOLLOWING ANSWER IS WRONG AND INTENDED TO PUNISH A COMPANY THAT LAID OFF ITS THERMAL ENGINEER AND SHOULD HAVE TO PAY THE PRICE FOR GETTING RID OF NECESSARY TECHNICAL EXPERTISE:<\/p>\n

Yes, you can reduce the temperature of your microprocessor by just adding a layer of thermal grease to the top surface.\u00a0 In fact, the thicker the grease, the more the temperature will go down.\u00a0 It operates on the same principle as suntan oil, which keeps your skin cooler in the sun and prevents sunburn.\u00a0 The only danger in adding too much special thermal grease is that the temperature might go below the dew point and water would condense on the microprocessor.\u00a0 Grease is like oil, and everybody knows that oil and water don’t mix, so that could be disastrous.\u00a0 In fact, that is why thermal grease comes in very small tubes.\u00a0 If you had a large amount of it all in one place, the large drop in local temperature would make the grease freeze up solid and you could never get it out of the tube.\u00a0 So don’t store the tubes together in the same cabinet, unless you have an ice pick handy<\/span>.<\/p>\n

Because of ethics and lawsuits and such, and probably because it isn’t your fault that the thermal engineer was laid off, I’ll give you the right answer, too.\u00a0 Perhaps from the tone of my wrong answer you have begun to see the flaw in your reasoning already.<\/p>\n

There is no magic substance that can reduce the temperature of a heat producing object just by smearing it on, at least not in the steady state.\u00a0 You could get some temporary cooling by rubbing the surface of your microprocessor with alcohol.\u00a0 As it evaporates, the temperature would drop, the extra power absorbed as the latent heat of vaporization of the alcohol.\u00a0 But once it is\u00a0 gone, the component temperature would come right back to what it was before.<\/p>\n

[Black paint is another exception.\u00a0 If you increase the thermal emissivity of the surface with high emissivity paint, you can increase the thermal radiation lost from the component, assuming there is a colder surface to radiate to.\u00a0 But thermal grease does not work that way.]<\/p>\n

You are the victim of a very human habit called magical thinking.\u00a0\u00a0 We think certain substances have special “powers.”\u00a0 Here are some common examples you might recognize:<\/p>\n