Posted by: crash! ® 08/28/2008, 17:39:59 Author Profile eMail author Edit

I need to attach a high power mosfet to a metal heat sink. I'm planning to use an electrically insulating heat transfer pad between the mosfet & heat sink with an M3 screw holding it together. The mosfet data sheet calls out a torque of 1.13 Nm (10 in-lb) which equates to a force of approx 400lb. I'm concerned about maintaining this force over time and thermal cycles. I haven't been able to find a belleville washer in an M3 size that can put out 400lb of force when it's partially compressed. Has anyone worked on this problem before? Any suggestions are greatly appreciated!

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: Attaching a heat sink to a mosfet
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Posted by: randykimball ® 08/30/2008, 00:59:49 Author Profile eMail author Edit

It does not help with heat sinking, but "we" do have nylon screws an nuts these days, ya know. Plus, nylon fasteners cut down on the parts count. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

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: Attaching a heat sink to a mosfet
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Posted by: zekeman ® 08/29/2008, 09:42:38 Author Profile eMail author Edit

Agree with all of the above, but what is the purpose of the electrical insulator, since it will inhibit heat conduction as well?

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: : Attaching a heat sink to a mosfet
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Posted by: Kelly Bramble ® 08/29/2008, 10:30:35 Author Profile eMail author Edit

Yes, your right Zekeman though I think the OP is using both the correct and wrong words to describe the film. The "insulating heat transfer pad" does act as a thermal resistor or insulator in the heat transfer circuit. However the practice of placing these strips or thermal transfer grease between electrical components and heat sinks is to actually improve the heat transfer between the component and the heat sink. Manufacturers tend to not build the bottom and top surfaces of electrical components very flat. Additionally, the heat sink probably has some flatness issues as well. The reality is that when you place the heat sink against the MOSFET, the imperfect (non-flat) surfaces will result in only three areas or points of contact between the components. These areas are probably the furthest or outer most areas of the MOSFET and heat sink. So, technically most of the usable contact area of both devices will do very little to transfer heat energy as there will be a gap or air between them. The "insulating heat transfer pad" is relatively compressive compared to the MOSFET and heat sink. So the "insulating heat transfer pad" will close the gaps between the components and give more surface area for heat transfer, resulting in improved thermal transfer between the MOSFET and the heat sink. In a nut shell, the "insulating heat transfer pad" heat transfer is better than air. A method to eliminate thermal grease or "insulating heat transfer pad" is to ensure both components have very flat surfaces, thus increasing the heat transfer contact area. Other techniques involve more thermally conductive materials between the MOSFET and heat sink. Modified by Kelly Bramble at Fri, Aug 29, 2008, 10:32:36

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Posted by: zekeman ® 08/29/2008, 19:53:46 Author Profile eMail author Edit

Kelly, Thank you. It's been a while, but I always used thermal grease for the job and if the MOSFET is sufficiently flat as well as the contact heat sink surface, it is still a serviceable solution. But I suppose, the heat transfer gods have decreed a superior method, maybe owing to the potential outgassing problem with grease.

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I Just Love Posts With Pictures!
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Posted by: Kelly Bramble ® 08/29/2008, 09:15:19 Author Profile eMail author Edit

Clearly stated question with a picture! Woo Hoo! Nice picture BTW!

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Posted by: randykimball ® 08/29/2008, 08:58:17 Author Profile eMail author Edit

One more small note, remember: Belleville springs washers can be stacked, like cups to increase forces, and each in reverse to stack distance. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

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Posted by: Kelly Bramble ® 08/28/2008, 22:18:13 Author Profile eMail author Edit

First, an M3 is roughly equivalent to a ANSI 1/8 (.125) fastener. So, you could substitute and ANSI sized Belleville spring washer. Second, see DIN 2093 Spring Disks standard for another solution. Third, are you sure your going to thermal cycle enough to have a problem? You could secure the fastener with a thread locker or have a locking thread installed in the heat sink. Another possibility is to turn that MOSFET around put a bunch of thermal vias in the board and have a fastener go through the MOSFET, board and heat sink on the far side. Or, just use a heat sink that can acomodate a locking washer and locking screw.

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Posted by: Bruised ® Bruiser 08/29/2008, 02:31:33 Author Profile eMail author Edit

Make sure you use heatsink grease for this - gets rid of air and therefore improves thermal conduction. It sounds like you are already aware that the central pin will conduct with the metal tab on the transistor. A nylon bush will prevent any problems with your fastener conducting if the circuit warrants this. Also, a more 'traditional' arrangement would see the heatsink at right angles to what you have shown - this sort of transistor is typically mounted at the edge of the pcb for this purpose. I have never encountered thermal cycling creating loosening of the fastener, a simple spring washer being the only countermeasure.

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