[steiner]

Is the thermal expansion of the mosfet legs something to be concerned with?

I made a few assumptions. If the legs were simply bent into a 90 degree angle and soldered, I assumed the length of the legs between the mosfet and the board was .200 inches. I measured a small mosfet I had laying around and the width and thickness dimensions of the legs were .035" x .017". I then assumed the legs would be heated from 25C to 100C. This meant the legs would grow .0002484 inches in length. As some of you have said.....this doesn't sound like much. However, the question becomes....how much vertical force does this put on the body of the mosfet that is attached to the heat spreader with an estimated 15 lb force. The answer is each leg applies 11.8 lbs of force due to thermal expansion. Three legs equals 35.4 lbs of force. I don't know what the coefficient of friction is between the mosfet body and the heat spreader. I am assuming you will be using thermal grease between them. Certainly the coefficient of friction will be less than one. Probably closer to 1/2.

If we assume a coefficient of friction of .5 and a 15 lb clamping force, this will require a 7.5 lb load to move the mosfet body. If we assume two of the three legs are experiencing most of the thermal expansion (third leg is not conducting many amps), then each leg applies a 3.75 lb load. Doesn't sound like much. However, this works out to a stress level of 6300 psi in each leg. That is quite a bit for copper.

So here is the dilema. If you apply a great deal of clamping force between the mosfet and the heat spreader....we will get good heat transfer. However, this will allow the stress levels to get much higher in the legs due to thermal expansion as it will be harder for the mosfet body to move relative to the heat spreader. The bend in the legs will reduce the stress level greatly as now they will buckle slightly due to the thermal expansion. The only down side I can see to this is fatigue (bending the legs back and forth every heat cycle).

My two cents.....put the bend in the legs but try to do it as gently as possible. Meaning....don't put sharp bends in them. A nice smooth radius is always better (lower stress riser). I know this is easier said than done.

If you could just put a smooth radius in the legs with them starting out straight down from the mosfet body and gently curving them outward leaving the last portion straight such that it will be parallel with the board. This would give you the clearance you need between adjacent mosfets.

Rick