Hmm kind of committed now, the heatsink is marked and I've started drilling. Will change my mind if it gets messy!

Soldering the drains is possible, just not pretty.

  KeepIS said  

I have thread tapped two inverters, then I tried wiseguys idea, as much as Klaus loves basket weaving and tapping holes     wiseguys idea was sooooo much quicker with no chance of breaking a tap, my brain meditates without telling me these days, or is that OLD-timers disease.

OK, it's possible that I too have the old-timers disease since I cannot locate wiseguys incredible tapping method in his long post. Any clue where this info lurks now, it's just possible I missed one of the better ideas wiseguy had .

Yes, I do like manual tapping but have not yet tried basket weaving. Any suggestions how I start with that or do I have to join the basket weaver union first  

I think basket weaving caused my old-timers, but like everything else these days, I don't quite remember what came first

I remember (shocks of disbelief) that the three of us had a discussion about his scandalous suggestion at the time.

It involves drilling a small hole through the aluminum bar, then a slightly larger hole on the other side of the heat bar deep enough for a screw reach and then drive a small nut into the hole. Or with a long screw that fits through the heat bar, pull the nut into the hole - this from fading memory, it my trigger an I remember moment, again use all info with caution, I may have dreamed it all  

I used to call Electric motor rewinding "glorified basket weaving"
Transformer winding is not much different, winding torroids from scratch is not a lot different, but motor winding takes it all to a different level.

Pete

  KeepIS said  

It involves drilling a small hole through the aluminum bar, then a slightly larger hole on the other side of the heat bar deep enough for a screw reach and then drive a small nut into the hole. Or with a long screw that fits through the heat bar, pull the nut into the hole - this from fading memory, it my trigger an I remember moment, again use all info with caution, I may have dreamed it all  

Thanks for the refresher course, it looks like this method was not up to my standards and hence not worth remembering  .

I suppose it might work with short heatsink fins...
But since the hole *has* to be located very accurately to line up precisely with the PCB hole this method looks dodgy to engage the mosfet mounting screw squarely with the nut pressed in at the far side of the heat sink.
I'm surprised you credit wiseguy with that, thought he was working to a better standard .

A refresher of the 'Klaus' method (requires a pedestal drill):

mark and center punch the holes as accurately as possible. Marking could be done through the PCB holes if it is clamped firmly and precisely at the correct location. Tip: the holes should fall into the valleys between the fins - I design my PCB's to do that.
drill 2mm holes *carefully*, with plenty lubrication, at the center punch.

enlarge the holes to 2.5mm

tap the holes M3 using a long taper tap or numbered taps (1-3).

This is for the heat transfer bars some of us use between the FETS and the Heatsink

There are no fins on aluminum heat transfer bars, imagine something flat on both sides  

The hole alignment is easy and the nut is secured perfectly as it bites into the walls of the larger hole and never moves, no mater how many times you remove the PCB.

It's No more dodgy then the pilot holes you drill in the aluminum bars if you tap them.  

Yes we also use a pedestal drill and the method description assumes one knows how to get a nut to go perfectly square down the hole.

Edited 2025-04-23 19:07 by KeepIS

Klaus sometimes you seem to come on here a bit snarky, curiously my daughter also gets like that each month...I know its your idea of sport or fun but we seem to all get on just fine without having to stir the pot resorting to chucking barbs at each other.

My latest method of FET mounting to the heatsink was posted over a year ago, my current method is here:
2024-06-04_174543_FET to Heatsink Mount.pdf

I did also pioneer the method that KeepIS is using and it does work perfectly, when the nut seats at the bottom of the part drilled hole it is perfectly square to the central M3 clearance hole which I usually drill to 3.5mm. The nut can be pushed in using the pedestal drill from the rear or pulled in with a high tensile hex 3mm screw, both methods work fine.  Maybe approaching the task with a positive attitude also helps with the success of the task at hand?

The latest method is even easier especially if you have a proper countersink bit. The major difference is that the nut is the fastener from the topside.

So how tight to do up the screw or nut for compressing the FET/bar/heatsink interface, glad you asked. For ~ 25 years I have used the following method. I keep the spring washer visible and tighten until the split in the spring washer looks like a normal washer ie pulls just flat, this corresponds to the driver becoming stiff, I take it a bit further maybe 10 - 20 degrees rotation, voila job done! It does not squeeze the bejesus out of the silicon pads nor allow the tension to ever become loose.

However I never use a square washer for this job I always use the wider flat spring washer such as these M3 3933411.pdf the obscured thickness dimension is 0.8mm. But note this is an Element14 part and they only seem to stock the stainless steel version - they may be fine - I have only ever used the steel/zinc type.

By the way I have clamped the PCB to the aluminium bar and using a TO220 bush in the PCB's FET hole to avoid PCB damage, I lightly drill each hole about 1mm deep and then remove the PCB and continue drilling the hole starts to full depth which gives excellent registration of PCB hole centres to the bar.
Edited 2025-04-23 21:02 by wiseguy

Apologies to FET Cemetery for polluting his thread with all this stuff.  By the way my colleague (when I was ~17) used to refer to complementary cemetery instead of symmetry.  His rationale was that if something failed with all the direct coupling it resembled a cemetery of dead semiconductors    Not too dissimilar to your handle.

  Quote  

wiseguy
my current method is here:

Even Better

  Murphy's friend said  

"rounded bend if you think it makes a difference"

FYI Whether you are an ex PCB designer or just back shedders there are standards and application notes that describe the correct way to create bends on high current legs to avoid longer term breakdowns and failures, especially with repeated heating and cooling.
AN-1031.pdf