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Old 27th October 2007, 09:46 PM   #1
loovet is offline loovet  Sweden
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Default Mounting power transistors

I’m now at the stage where I will mount my transistors to the heat sink and have some questions regarding this. I will be using metric M3 screws to mount the transistor and the “base” of my heat sinks are 8 mm thick, so the depth of the hole it self will probably be some 7 mm.

When mounting a power transistor to the heat sink, how much power should one use when tightening a screw?

Are you all using a bottom tap when making the threads? Ore are you using a normal throughout tap?

Any tips to make it as good as possible?

Thanks in advance

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Old 27th October 2007, 11:19 PM   #2
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Lacking a torque wrench, I just make mine snug--don't reaf on the screwdriver or wrench (which ever you use). You should see some heatsink compound squeeze out from the mica.

A small-handle screwdriver is safer in that it's more difficult to over tighten the screw.

Haven't lost a transistor yet due to thermal management.
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Old 28th October 2007, 01:17 AM   #3
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You can get away with either a bottom tap or a through hole tap with 7mm deep holes. 8mm is a righteously thick backplate! 5 mm of thread should be plenty for those screws.

I'm not sure if an M3 is closer to a #6 or a #4. But, a number 4 tap is wee little thing -- very easy to break.

Tip of the day -- I like to use a hand tightening drill chuck (without the drill attached) to tap. If you don't tighten it up too much, the chuck will slip when you hit a rock, rather that breaking the tap. Also, it makes it easier to turn the tap rather than pushing it -- which tends to break small taps.

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Old 29th October 2007, 01:02 AM   #4
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It sounds like you don't want a threaded through-hole, so the type of tap depends also on the length of screws to intend to use. I always try to use a bottoming tap for blind holes, but I'm anal like that. (Although I start off with a tapered tap.)

Is there any way for you to make a clamp (or use a big washer) to distribute the pressure on the transistor? If you're dissipating a lot of heat, you might need the extra clamping pressure and need to use a larger screw. Just remember, aluminium is soft and strips easily, so use the exact tap drill (and a pilot drill first) and a tapping guide block to keep the tap straight.
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Old 29th October 2007, 03:03 AM   #5
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I enjoyed reading your posts, jupiterjune and Tosh! Words of experience, thanks for sharing.
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Old 29th October 2007, 09:45 PM   #6
flg is offline flg  United States
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1 More little refrence on the topic...

And there is a few more there somewhere
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Old 29th October 2007, 09:53 PM   #7
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Originally posted by Nelson Pass
I recommend a nice wide washer on top.

The TO247 case has good support at the hole, and you
just want to avoid cranking down too hard. We use
torque wrenches set at 8 inch-pounds.

For tapping blind in aluminum I would also start with a tapered plug tap and finish up with an bottoming tap to increase usable thread depth and durability as Tosh suggests. A wax lube on the tap helps to draw the cuttings out with the tap and cleans up nicely. IMHO, cutting M3x.05* or M3.5 threads requires a soft touch as the tap will only cut down about 3mm per pass before it clogs up; if you power through this initial resistance there is a fair to good chance the tap will snap off flush in the hole.You will never forget the first time this happens to you (e.g: Cadillac 429ci cylinder head.)

*Fine threaded hex bolts + large washers are swell.


I used
IRWIN #80213 6 - 32 NC Tap + 7/64" Drill Bit
Lenox H5405 Lube Tube (bandsaw cutting wax and for general machining)
Aavid 4180G ALUMINUM OXIDE PAD TO-218/TO-247/TO-3P (Mouser)
#6 –32 x .625” hex bolts
Homedepot Washers, 1” dia. .125” hole .06” thickness
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