Transistor mounting w. self-drilling screws - can it be done?

[e96mlo]

Has anyone tried this?

I'm thinking more of fast prototyping than of high end equipment making.

The screws I'm thinking of have a drill-like tip and kan be used without predrilling any holes in at least 5 mm alu. Fast and easy

I'd like to hear your comments, please.

/Marcus

[Petter]

It is the worst idea I have ever seen

Well, not exactly, but if you need a heatsink, you probably want to make sure that you have good contact ((and possibly even isolation)).

Self drilling screws are neat but not optimal for either as you are unlikely to get clean holes without burr.

If you want something set up fast, consider using clip-on heatsinks.

Petter

[e96mlo]

Are there any clip-ons available for bigger heatsinks?

The ones I know of are only for TO220 cases and are not suited for more than a couple of watts.

/Marcus

[joensd]

hi,
very interesting!
i´m just about to make my brain smoke about the same problem.
just don´t wanna use aluminium L-profiles cause auf thermal connection.
self-drilling screws is definitely bad.
but even if you tap? the heatsink (i mean making threads into the heatsink) i doubt this will last forever!?
screwing a little too much or replacing transistors more often and the thread´s gone.
i just think aluminium is too soft for a good thread?
any hints,advices are welcome

Petter: i´ve never seen a big heatsink with clips but it would be a solution just can´t imagine how to attach those

TIA
Jens

[Peter Daniel]

Be serious, are going to remove and install the same device more than 20 times? Because if the hole is tapped properly (with the use of lubricant) I can guarantee that it will last that much.

If you don't want to tap the hole, you can use sheet metal screws. Just drill the hole, countersink the edge and install the screw.

Self drilling screws are for very thin material (like 0.5 mm) . Try to install it on 5mm aluminum, are you gonna do it with a screwdriver? I can't imagine it's possible.

[e96mlo]

It can be done using drilling mashines with torque adjustment. I did that at a job some years ago. But that never included the need for flat surfaces for cooling, only attaching metal parts to each other in a non accurate way.

Maybe it can be used to fasten a metal bar across the transistor? Then the holes are at the sides of the device and the need for accuracy is less.

/Marcus

[Peter Daniel]

I thought you wanted to do it in a fast and simple way. If you have to use drilling machines with torque adjustment for installing self drilling screws (that how I understand your post) it's much easier to make a thread and install machine screw.

[Variac]

I used sheet metal screws to mount the aluminum cased power resistors on my SOZ. It is important to use the correct drill of course. In this case I needed to use a drill one size bigger than recommeded. Forcing the screw into a block is different than into sheetmetal. And forcing too much gets you a broken screw and heartache. As always in these situations, lube and patience win out! ( I should be a relationship columnist )

It happened to me and luckily the head came off and I got it out with vicegrips. That's when I got the bigger drill!!

At the end I had to run a sanding block over the holes to remove the tiny ridge around the holes. All in all, they are amazingly secure, One possible problem is that with the steeper thread pitch, they might loosen from thermal cycling, but I'll bet not-they feel SOLID.

[whitetrash]

I used a form of self tapping screw to mount some transistors to a heatsink. The type I used are called taptite and look like a normal M3 machine screw except for the first couple of mm of screw which is tapered. The idea is to drill an undersize hole in the alu (from memory 2.7mm for an M3 - but check this) and then screw it in. I used a 20mm long screw and on my test site I used max force and couldnt get it to strip so it goes in pretty solid. The surface needs to be countersunk too, as the screw going in causes burrs on the heatsink. Anyway, it works well and is a snap to use.