Hi,
I'm screwing TO-220 and TO-264 transistors onto a heatsink. Any tips on type of screw, washer etc. to make a good thermal solution that is also durable with years of thermal cycling?
In my case, it's OK that the transistor package, heatsink (probably Cu) and screw are electrically connected.
It could be so easy that what I'm looking for is M3 screws, a drop of Loctite and a springy washer. Of these, locating the right kind of washer is probably the hardest part since it must prevent overtorquing and thermal expansion / contraction from breaking the transistor while maintaining thermal conduction.
Thanks,
Børge
I'm screwing TO-220 and TO-264 transistors onto a heatsink. Any tips on type of screw, washer etc. to make a good thermal solution that is also durable with years of thermal cycling?
In my case, it's OK that the transistor package, heatsink (probably Cu) and screw are electrically connected.
It could be so easy that what I'm looking for is M3 screws, a drop of Loctite and a springy washer. Of these, locating the right kind of washer is probably the hardest part since it must prevent overtorquing and thermal expansion / contraction from breaking the transistor while maintaining thermal conduction.
Thanks,
Børge
One phrase I remember from many many years ago (might even have been @DouglasSelf in one of his WW articles) and that was to use Belleville washers. In practice I've never had issues with screws coming loose etc but would just say that I find it is best to screw the packages down before soldering them and never ever over tighten. Gauging how much torque is not easy but you can get a good idea on a test package and seeing just how (un)tight it needs to be to resist sideways movement. Data sheets often give torque settings.
And yes, a dab of Loctite tamper proof stuff (or nail varnish) does no harm and is something I do. Beware some general 'threadlock' liquids used in engineering as they can rot plastic pretty quickly.
And yes, a dab of Loctite tamper proof stuff (or nail varnish) does no harm and is something I do. Beware some general 'threadlock' liquids used in engineering as they can rot plastic pretty quickly.
I would just add, apply small amount of loctite (or any kind of super glue thats heat resistant) on the nut and screw joint part, no need to worry about it eating plastic. Works like a charm.
https://www.mcmaster.com/belleville-spring-washers/. Type in Belleville in the search box!
Carry everything, almost. I suspect shipping is way more than the cost of the parts.
Normal practice is to use a machine screw, a coned (Belleville) washer, a slightly larger flat washer to protect the plastic package, and then on the other side of the heatsink a lock washer and nut. If you use a thread locking compound it must be a weak one. Full strength thread locking compound will require breaking the bolt before it releases. The compound is applied to the bit of screw thread that sticks out from the nut. Normal practice is to have at least 1 1/2 threads sticking out of the nut. No more than 1 1/2 plus the diameter of the bolt.
PM me a shipping address if you can’t find the parts you need.
Carry everything, almost. I suspect shipping is way more than the cost of the parts.
Normal practice is to use a machine screw, a coned (Belleville) washer, a slightly larger flat washer to protect the plastic package, and then on the other side of the heatsink a lock washer and nut. If you use a thread locking compound it must be a weak one. Full strength thread locking compound will require breaking the bolt before it releases. The compound is applied to the bit of screw thread that sticks out from the nut. Normal practice is to have at least 1 1/2 threads sticking out of the nut. No more than 1 1/2 plus the diameter of the bolt.
PM me a shipping address if you can’t find the parts you need.
Thank you for the tip on the Belleville washer. That looks like a great fit for the TO-264 where there is a flat surface to engage. On the TO-220 there is a flange sized 0.235"/5.97mm with the screw hole in the middle.
An M3 "spring washer" would fit snugly just there, see link below.
In my case I will likely have a copper cooling element that is custom made with grooved M3 holes. It will be inconvenient to fit a washer and nut behind the transistor. If I use Loctite I'll have to be very careful not to fill the hole in the cooling element and have gunk come out of it onto the transistor I'm cooling. Or I can use through-bore holes and remove excess glue from the rear side after it solidifies.
Børge
I get most of my mechanical parts from Accu in the UK. Highly recommended. https://www.accu.co.uk/search-uk?Ex...3+(3mm)&cs_ids=703&query=spring+washer&page=1
An M3 "spring washer" would fit snugly just there, see link below.
In my case I will likely have a copper cooling element that is custom made with grooved M3 holes. It will be inconvenient to fit a washer and nut behind the transistor. If I use Loctite I'll have to be very careful not to fill the hole in the cooling element and have gunk come out of it onto the transistor I'm cooling. Or I can use through-bore holes and remove excess glue from the rear side after it solidifies.
Børge
I get most of my mechanical parts from Accu in the UK. Highly recommended. https://www.accu.co.uk/search-uk?Ex...3+(3mm)&cs_ids=703&query=spring+washer&page=1
The "Belleville" springs are also known as DIN 2093.
A larger diameter flat washer like DIN 9012 may be a good idea to distribute the force onto a larger area.
A larger diameter flat washer like DIN 9012 may be a good idea to distribute the force onto a larger area.
For the insulated bushings / "shoulder washers" which prevent the mounting screw from making electrical contact with the transistor package, I find that the Big Bagz O Partz sold on eBay are quite good. Buy brown (high temperature) when you can, but white (normal temperature) is okay too.
For TO-220: link to eBay
For TO-264 / TO-247 / TO-3P: second link to eBay
They're dirt cheap and in my experience, just as good as the $0.50 per piece shoulder washers at Mouser
For TO-220: link to eBay
For TO-264 / TO-247 / TO-3P: second link to eBay
They're dirt cheap and in my experience, just as good as the $0.50 per piece shoulder washers at Mouser
So does anyone use self tapping screws? They should be self locking because they're tight in the threads, also you wouldn't have the problem of
trying to feed a nut or bolt through the fins of a heatsink. I haven't had to use a heatsink in any project yet, so just curious if it's practical.
trying to feed a nut or bolt through the fins of a heatsink. I haven't had to use a heatsink in any project yet, so just curious if it's practical.
No.
Not enough threads, their pitch is coarser than machine (regular) screws. So less grip.
Here we get kits, heat sink, screw, nut, washer, and nylon bush, about 5 cents each in 10 quantity.
Not enough threads, their pitch is coarser than machine (regular) screws. So less grip.
Here we get kits, heat sink, screw, nut, washer, and nylon bush, about 5 cents each in 10 quantity.
True on normal self tapping screws intended for sheet metal, trying to grip aluminum heatsinks, aluminum being a very pool mechanical qualities metal.
That said, I have repaired Peavey MI amplifiers, which use a much finer thread.
Never measured it but is comparable to regular "nut and washer" intended screws, about twice as fine as usual.
Also some old TO3 transistor sockets, made out of punched phenolic paper and plated brass contacts,used regular sheet metal screws but springiness and quality surface came from the threaded contact itself.
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That screw will not be easy to find, tapered and threads on the taper part.
Similar shaped screws are used for orthopedic surgeries, to set bones, and hold implants / support plates in place, some have a relieving slot to remove debris, like in taps.
That TO-3 socket has a shape for the screw which is common in cars, at least used to be, to hold say grilles in place against metal, the clip (spring steel?) was shaped like this on one side, and fitted over a hole, and a sheet metal screw was used.
This item may be used in different areas as well.
I have an old Philips tuner / cassette / amp, the chip amps are held in place using spring clips instead of screws.
Similar shaped screws are used for orthopedic surgeries, to set bones, and hold implants / support plates in place, some have a relieving slot to remove debris, like in taps.
That TO-3 socket has a shape for the screw which is common in cars, at least used to be, to hold say grilles in place against metal, the clip (spring steel?) was shaped like this on one side, and fitted over a hole, and a sheet metal screw was used.
This item may be used in different areas as well.
I have an old Philips tuner / cassette / amp, the chip amps are held in place using spring clips instead of screws.
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I’ve always used regular #6 sheet metal screws with those TO-3 sockets. They don’t work themselves loose.
If the heat sink is thick enough for at least 1.5 to 2 threads, it will hold, which may not be the case for the TO-220 and TO-264 heat sinks.
TO-3 generally are fitted onto thicker meat sinks, so some choice is possible, like yours.
Even my Kenwood amp has tapped holes in the heat sink to hold the STK module in place, it is thick enough for several threads.
Putting the nuts in place sometimes needs a watch maker's tweezers....tedious.
TO-3 generally are fitted onto thicker meat sinks, so some choice is possible, like yours.
Even my Kenwood amp has tapped holes in the heat sink to hold the STK module in place, it is thick enough for several threads.
Putting the nuts in place sometimes needs a watch maker's tweezers....tedious.
Have a look at the DIN 7500 tri-lobular thread-forming screws which produce ISO metric treads.
All you need is to adjust the bore diameter accordingly. No tapping required.
In case you remove the thread forming screw, you could replace it by a normal ISO metric screw.
A huge benefit of thread forming is that the material in the thread region becomes more dense, thus more robust.
I don’t have practical experience with such screws, but with thread forming in aluminum, which is superior to thread cutting. The crucial part is the correct bore diameter.
All you need is to adjust the bore diameter accordingly. No tapping required.
In case you remove the thread forming screw, you could replace it by a normal ISO metric screw.
A huge benefit of thread forming is that the material in the thread region becomes more dense, thus more robust.
I don’t have practical experience with such screws, but with thread forming in aluminum, which is superior to thread cutting. The crucial part is the correct bore diameter.
I just placed an order for washers with Accu.
My rough design plan is to have electrical and thermal connection to a first cooling element and then only thermal to the second one. It looks like M3 screws and non-flat metallic washers are a good choice at the transistors. Between the cooling elements I will probably use M3 screws with insulating washers that go some way into the holes.
I'll use my 3D printer to draft the mechanical shape of things and probably use it to print the distance pieces and plastic washers needed. The 85C softening temperature of PETG is probably a bit on the lower side, but I can use it for the initial design
My rough design plan is to have electrical and thermal connection to a first cooling element and then only thermal to the second one. It looks like M3 screws and non-flat metallic washers are a good choice at the transistors. Between the cooling elements I will probably use M3 screws with insulating washers that go some way into the holes.
I'll use my 3D printer to draft the mechanical shape of things and probably use it to print the distance pieces and plastic washers needed. The 85C softening temperature of PETG is probably a bit on the lower side, but I can use it for the initial design
Another possibility for electrically insulated screw connections is suitable plastic screws... For instance you can get PPS (polyphenylene sulfide) bolts which have a good performance at high temperatures (where nylon might start to creep and slacken).
For TO-220 I saw a clamp - a flat spring with a platform for a screw and a ledge. It additionally presses the plastic case of the transistor from above.
THFM 1
Fischer Elektronik GmbHAttachments
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This is similar to the one in my Philips sets, it allows some play, easy to use on an assembly line.
Finding M3 screws molded in PPS in small quantities may be a problem.
Most people would not need the MOQ on offer.
Practical solution is a flanged round tube nut, or a slightly under size hole, so self tapping screws can be used.
On a repair job, the constraint is using what is already in place, modifying the heat sink may not be easy.
Finding M3 screws molded in PPS in small quantities may be a problem.
Most people would not need the MOQ on offer.
Practical solution is a flanged round tube nut, or a slightly under size hole, so self tapping screws can be used.
On a repair job, the constraint is using what is already in place, modifying the heat sink may not be easy.