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mrwireless 13th September 2009 10:10 PM

Screwing into Heatsink - advice needed
Can anyone suggest how best to attach circuit boards/transistors to an aluminium heatsink? Obviously some sort of screw is needed but i don't want to use self-tappers because i know it's going to get cross threaded at some point. Should i be using machine screws? Is it best to tap the hole first? I'd like a reliable fix that can be done/undone without degrading. Any advice much appreciated, thanks.

kannan_s 13th September 2009 11:19 PM

Aluminium is a soft material - best is to use screws with nut and washers to tighten the Transistor. also try using a torque adjustable screw driver would be better to apply correct force

Geek 14th September 2009 12:27 AM

If you're mounting cases that tolerate redrilling to a larger hole (TO-3, TO-220 (non-IsoTab), TO-247) and the heatsink is thick enough, you can drill and tap the sink for 6-32 and the case to clear (5/32" or 9/64" is better).

If it needs to be electrically isolated or your skills aren't up to par, just drill the sink to clear a #4 screw (1/8" hole does nice) and use a proper mounting kit as kannan_s suggests.


tinitus 14th September 2009 12:30 AM

Dip the tap in spirit/alcohol when cutting the threads
Unbraco or torx screws are nice
Unfortunately not available in brass, which is my preferred

When drilling holes and cutting threads there will be a very small raise in the aluminium around the hole
Its important to remove that carefully
The easy way is to countersink the hole before cutting the threads
But countersinking may also result in a small raise of material around the hole, depending on your tools and skills

mrwireless 20th September 2009 01:58 PM

thanks all for the helpful advice

lgreen 9th October 2009 04:10 PM

Ah, the most pain-in-the-a$$ part of diy audio, drilling and tapping your heat sinks! I HATE doing this but you cannot avoid it.

Most of the time I drill a hole and tap it for a 4-40 thread; then use a machine screw, lockwasher and washer. A 4-40 will fit most semiconductors and gives you a margin of error in placement if you are not so accurate. I use normal household oil on the drill and tap by hand (using regular oil). Then when done wash with normal dishwashing liquid and the garden hose. Do it slowly and you minimize the chances of snapping the tap inside the hole. When this happens -- and it will -- you are in trouble!

If there is a slight raise in the metal around the tap I just leave it (most times) because the hole is usually a wider diameter than the raised part.

If you happen to drill into the spaces between fins, no problem, the hole can be cleared out and any length screw will fit in there. If you happen to drill into a fin the long way, you will have trouble with tapping all the way through, so I just make sure the hole is deep and use shorter screws. A 4-40will hold pretty good even at a short length.

So yes- drill and tap first then clean up the heatsink. It makes an awful mess.

mrwireless 12th October 2009 10:50 PM

thanks lgreen, great advice....i still haven't done the job yet so this will come in handy

richie00boy 12th October 2009 11:44 PM

You won't find any of the screws mentioned here in England :) Use an M3 machine screw and yes you should tap the hole. Be aware though that the base material is aluminium and will not take much undoing/doing cycles before the threads get too worn.

MJL21193 12th October 2009 11:59 PM

1 Attachment(s)
I use self tapping sheetmetal screws, some lube and driven into the correct size hole:

Attachment 143439

Lots of the old in-out and no stripping. The key to not stripping the holes is not being so damn brutal and make sure the screw is going in correctly in the first place. A tip: before re-driving the screw, back it off until you feel it "click" - that means it's on the correct thread and will go in easily.

Trying to remove a broken tap from a heatsink can be very frustrating...:)

Conrad Hoffman 13th October 2009 12:22 AM

There are many methods that will work but be aware that heat sinks are made out what some machinists refer to as "pink eraser #3". The stuff is gummy and the inexperienced often break taps in it. Whatever size tap you use, be sure it's sharp; the junk from the local hardware store might not be up to the task. Next, use the correct size tap drill. If you need a #43, that's what you need, not some fractional size that appears close. Best bet is to order from a machine shop supply company and get good name brands of exactly what you need. You can also order spiral flute taps that send the chips out the back, rather than the front- my choice if tapping gummy aluminum, especially blind holes.

Now, if you really want to do it like a pro, it's going to cost you, but not that much. Order a Balax thread forming tap (no substitutes) and the correct size tap drill for it- this will *not* be the same size as for a cutting tap and once you drill the hole you *must* use the Balax. Drill the hole and run the thread forming tap in with some oil. It will go in quite easily, leaves no chips to worry about, and produces work-hardened threads that are tougher and longer lasting than cut threads. Not having grooves cut in the sides, the tap is way stronger. Not having cutting edges, they don't get dull. The downside? They cost about $15 each. OTOH, you'll likely never replace it. File or chamfer the hole to remove the burr and you're good to go. For any ductile material, these are all I use.


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