Author Topic: Soldering MOSFETS directly to Aluminium Plate  (Read 10038 times)

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Offline capt bullshot

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #25 on: December 15, 2017, 10:57:12 am »
Some time ago, a colleague tried something similar:

using a aluminium core PCB (as they are quite popular with LED lighting, see zeqing's post), single side and single layer copper. He reflowed a total of 6 or 8 TO220 Mosfets on this PCB, worked fine. There's one major issue: you need to fix the MOSFETs while reflow in some way, but still allow them to reflow, so not too fixed, just kept in position by some special mechanical guidance. Otherwise their pins won't fit anymore into your other PCB because they float a bit around while reflowing.
Using this kind of PCB it doesn't matter if the tabs are all connected together or electrically separated, since you can etch the PCB before assembly.
Otherise, IMO using a sheet of copper instead of aluminium looks like a good idea for your problem.

Edit: if this is for volume production, check with the MOSFETs manufacturer for the reflow soldering of TO220 or similar packages. Usually these packages are not specified for reflowing the tab and may cause issues.
« Last Edit: December 15, 2017, 11:01:44 am by capt bullshot »
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Offline DerekGTopic starter

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #26 on: December 15, 2017, 02:06:40 pm »
Is this on an aluminium substrate pcb?

Can you please share some more information about this process?

Thanks in advance.
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Offline capt bullshot

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #27 on: December 15, 2017, 02:30:20 pm »
Yes, it's called IMS:
https://www.pcb-pool.com/ppuk/info_pcbpool_alupanel.html

Don't know much about the process itself. What my colleague did was some kind of pre-production testing, and we came to the result "yes, it's a genius idea" but no one wanted to actually mass produce the assembly - reflow solderability of the packages and final pin alignment were the issues.
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Offline CM800

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #28 on: December 16, 2017, 09:18:28 am »
Yes, it's called IMS:
https://www.pcb-pool.com/ppuk/info_pcbpool_alupanel.html

Don't know much about the process itself. What my colleague did was some kind of pre-production testing, and we came to the result "yes, it's a genius idea" but no one wanted to actually mass produce the assembly - reflow solderability of the packages and final pin alignment were the issues.


I'm aware of a company that uses the same tech for their power electronics, they ended up doing the whole assembly from P&P to re-flow in-house. Wouldn't surprise me if that was the reason... they probibly use jigs for alignment.
 

Offline mswhin63

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #29 on: December 17, 2017, 06:09:45 am »
Just a quick thought, What about spot welding. Low voltage so no problem with damaging the MOSFET and the current is isolated around the pins of the spot welder. More so maybe a spot welder with multiple 2 pin terminal that can weld multiple point on the case metal.


Only an idea, never tried it and would be interested in another project I have for myself.
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Offline T3sl4co1l

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #30 on: December 17, 2017, 06:41:57 am »
Unlikely to benefit, given the warping and thermal shock associated with spot welding, and the copper or other high conductivity substrates involved.

You could perhaps add a filler layer: a foil of solder, or anything lower melting than the base material, to get a larger bond area for less stress.

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Offline radar_macgyver

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #31 on: December 17, 2017, 06:49:57 am »
Spot welding, by definition, welds a single spot - not ideal for heat transfer purposes.
 

Offline DerekGTopic starter

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #32 on: December 17, 2017, 07:14:02 am »
Thanks again to everyone who has contributed to this thread.

After the excellent suggestion to use copper plate instead of aluminium, we have decided this is the best way forward.

We will make up a jig to clamp the 4 x mosfets into the correct position on the copper plate after applying a layer of "thick" solder paste (approx 0.5mm) via a specially made stencil. We will then pass them through a vapour phase oven & then xray the finished plate.

If everything looks good, this is the production path we will follow.
« Last Edit: December 17, 2017, 12:22:00 pm by DerekG »
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Offline SeanB

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #33 on: December 17, 2017, 10:54:46 am »
I would second the jig and an aluminium core PCB, as this can easily and cheaply be done to precisely fit the device pinout and even have slots and other things, like mounting tabs, milled in for essentially free during production, plus you have quite good thermal resistance from the copper to the base, and can even integrate other parts in there simply, like a heatsink thermal sensor, power devices like resistors that need heatsinks, or even just test points. Then you only need to have a few holes to mount to the main heatsink with a thermal pad to lessen the tolerances needed ( thin pad is a lot cheaper than tight control of surface finish on 2 parts) and spring clips to hold each device down to the heatsink. That way rework is possible, and sevicing or repair is a lot easier as you can remove the individual parts easier. just use a thick copper on the aluminium core PCB, and make the pads larger with a solder mask to keep the solder under the TO packages, and in later versions you can use a SMT version ( lower cost immediately there) and simply have some surface mounting pins, to keep the footprint for the leads to the main board the same, to save a respin.
 

Offline SVFeingold

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #34 on: January 10, 2018, 06:05:44 am »
One thing I've not seen mentioned - phosphoric acid. Regular old (acid strength, not beer strength) phosphoric acid. It makes soldering to steel, stainless, and aluminum no different from soldering to a regular PCB. Much less hassle than oil and scrubbing and such.

There are probably even more effective chemicals, I'd look into aluminum etchants at that point, but phosphoric acid has worked great for me.
 
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