Author Topic: Soldering MOSFETS directly to Aluminium Plate  (Read 10039 times)

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Offline DerekGTopic starter

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Soldering MOSFETS directly to Aluminium Plate
« on: November 30, 2017, 02:10:55 am »
I'm hoping that someone may have some experience with soldering the heat sink of a MOSFET directly to a sheet of aluminium in an oven (either infrared or vapour phase)?

Can it be done? Is it reliable? What to watch out for? What flux to use? What solder paste to use?

I'm wanting to solder TO-220 / TO-3P / TO-247 through hole MOSFETs onto a 2 mm (or 3mm) thick aluminium plate. The MOSFETs would need to be clamped in place before soldering & the legs would hang over one edge of the plate so they can be soldered onto a regular PCB adjacent to the plate.

We require 4 MOSFETS to be soldered to the plate, so there would be 12 component legs to give some strength between the boards. Both boards would be mounted with 4 screws into our plastic enclosure.

The MOSFET heat sinks are all at the same potential, so we can safely connect them all together on the one aluminium plate.

This would be a production process with several hundred boards being run through the oven over the morning.

I presume the aluminium plate will require some preheating. The aluminium plate to be used is a standard commercial grade plate which is why I ask about special fluxes etc to clean it first (& get the solder to adhere to it).

This process would alleviate us having to manually screw all the MOSFETs to the aluminium plate & to do away with the manual process of applying heat transfer paste.

We will have the aluminium plate laser cut to shape to begin with & later on, punched as the volume grows.

Any thoughts appreciated. Especially from anyone who has actually tried it.
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Offline Falcon69

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #1 on: November 30, 2017, 02:15:22 am »
i'm no expert, but, wouldn't you'd have to heat the heatsink up so much, that by the time you got it hot enough to take the solder, it would fry the mosfet?

I guess you can heat the heatsink first, get it tinned with solder, wait for it to cool down, then quickly hit it with solder iron and see if you can get the mosfert soldered on without damaging it.
 

Offline KL27x

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #2 on: November 30, 2017, 02:30:58 am »
I have experience in trying. It is some kind of hard getting solder to stick to aluminum, at all.

I have heard it is possible by putting oil on the surface, scraping/grinding the surface away, then soldering it. All the while under a layer of oil to keep out any oxygen.

I have tried it. I didn't get it to work.

Seriously, try getting solder to stick to aluminum, at all. If you can do that, then you can move on to figuring out how to get the FET on there. Until then, I think you're dreaming that this is going to be cheaper than screws and paste. Take the cheapest products out of China and show me where they save money by soldering anything to aluminum! Their labor might be cheap, but they also have reflow ovens and solder and aluminum heatsinks.  :-//

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« Last Edit: November 30, 2017, 02:39:37 am by KL27x »
 

Offline Rerouter

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #3 on: November 30, 2017, 02:48:51 am »
To solder to aluminium, coat the area in gel flux. Scrape the base material through the flux. Then begin soldering. With the thick oxide layer broken and the flux to further rip of the very thin layer that would have just formed you can get solid connection to the base metal.

I've done it this way for a number of old gauges with Ali housings. You get the solder to bite to the metal first, then come back and solder what you need by melting the blob you have attached.
 

Offline nanofrog

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #4 on: November 30, 2017, 03:33:32 am »
Tips for Soldering Aluminum might be of some use. That said, even if you manage it, I expect you'd have reliability issues.

You'd be better off using copper plate/sheet/bar IMHO, and 0.125"/3.175mm thick would work and is easy enough to source. You can even get it on eBay (example).
 
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Online Ian.M

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #5 on: November 30, 2017, 03:40:15 am »
Getting Aluminum to tin for a one-off is doable by abrading through a puddle of flux, solder or high temperature oil that excludes air. The surface must be freshly abraded then abraded through the puddle as even the thin layer of oxide that forms while heating will totally prevent success.   It is in no way a production-ready process.

I'd also be concerned about the difficulty of cooling the heatsink fast enough not to be *way* outside the MOSFET's permissible soldering thermal profile, without thermally shocking it with a too rapid quench.

Consider using a specialist thermal adhesive.  To get a reliable bond with thermal epoxies, the heatsink surface and device mounting surface will need to be freshly abraded and cleaned.   Alternatively look at self-ashesive silpads, and mounting clips rather than screws.
« Last Edit: November 30, 2017, 03:42:19 am by Ian.M »
 

Offline thermistor-guy

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #6 on: November 30, 2017, 04:22:29 am »
...
Consider using a specialist thermal adhesive.  To get a reliable bond with thermal epoxies, the heatsink surface and device mounting surface will need to be freshly abraded and cleaned.   Alternatively look at self-ashesive silpads, and mounting clips rather than screws.

Aremco make some printable high-temperature adhesives:
http://graphitestore.com/Graphite/High-temp-coatings-and-adhesives/custitem_gs_applications/Screen-Printable

e.g. Aremco-Bond 556-HT-SP for chip bonding.

The OP may be able to print the adhesive onto the heatsinks in a semi-automated way. I would ask board assemblers about this: what methods and materials would best suit their processes.
 

Offline floobydust

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #7 on: November 30, 2017, 05:27:04 am »
I've never successfully soldered to aluminum. Be prepared for a lot of baked semi's.

Instead, I would use D2PAK MOSFETS on aluminum-core PC board. These are common PCBs in the automotive industry and for LED lighting.
Then mount that to your final heatsink.
 

Offline HalFET

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #8 on: November 30, 2017, 06:31:53 am »
This is definitely a vapour phase job (since it's an option for you). The fluorocarbon liquid makes an inert atmosphere that should protect the aluminium from oxidation during soldering.
 

Offline T3sl4co1l

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #9 on: November 30, 2017, 06:37:32 am »
This is definitely a vapour phase job (since it's an option for you). The fluorocarbon liquid makes an inert atmosphere that should protect the aluminium from oxidation during soldering.

Ooh, that's actually a good idea.  It's probably not quite good enough, though: aluminum will getter any remaining oxygen, so the vapor needs to be very pure indeed, not diluted with air, say.  Anyway, you still need the skull-and-crossbones flux to get through the initial oxide layer.  Would be cool if the surface could be prepared inside an inert gas glovebox, then soldered.  Hm.

Anyway, guessing that's a lot more setup than OP is looking to use here. :-\

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Online Ian.M

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #10 on: November 30, 2017, 06:42:18 am »
It may be worth looking at getting the aluminum sheet copper or tin plated so its solderable.
 

Offline T3sl4co1l

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #11 on: November 30, 2017, 07:03:27 am »
I'm hoping that someone may have some experience with soldering the heat sink of a MOSFET directly to a sheet of aluminium in an oven (either infrared or vapour phase)?

Can it be done? Is it reliable? What to watch out for? What flux to use? What solder paste to use?

To echo what others have said...

No. Not even no, Hell No... :-DD

Quote
I'm wanting to solder TO-220 / TO-3P / TO-247 through hole MOSFETs onto a 2 mm (or 3mm) thick aluminium plate. The MOSFETs would need to be clamped in place before soldering & the legs would hang over one edge of the plate so they can be soldered onto a regular PCB adjacent to the plate.

0. Those aren't SMT packages, for a reason.  (There are TO-263 and TO-268's for that!)  Many of those come with tin plating which is just fine and solderable, but many also come with nickel plating that's a nightmare to bother with.  I've SMT'd TO-220's here and there for little projects, I know.  :horse:

1. Use copper.  Copper is solderable, also more conductive so you need less of it, perhaps even to the extent that you need one or two fewer transistors.

2. Use a PCB.  Heavy copper PCBs are available most anywhere.  There's even the kind with machined (I guess) inner "layers" of considerable thickness.  Though you may end up paying as much for the PCBs as for trying to solder aluminum...

3. Use more transistors.  You're looking at five or ten bucks of transistors, versus sky's-the-limit inert gas, breathe-the-fumes-and-you-die soldering.  Transistors are fucking cheap.  I haven't seen a single design where the transistors were anywhere near a significant fraction of the total product cost.  The labor of inserting them and screwing them down easily overwhelms their cost (except maybe in China, but then, locally sourced, classic type, transistors are quite cheap, too).

They're not vacuum tubes.  Pile them on! ;D

3a. While you're at it, give up on all that annoying mounting hardware!  Spring clips are fantastic: they don't bend the device, they spread the load evenly, and you can use much bigger devices (MAX247 and such) that aren't hampered by pesky trivia like screw holes.

There are even heatsink-and-clip duos that are cam-lock or clip-in-place.  No screwdriver at all! ;D

Quote
We require 4 MOSFETS to be soldered to the plate, so there would be 12 component legs to give some strength between the boards. Both boards would be mounted with 4 screws into our plastic enclosure.

Plastic?

So, no heatsink?

God man, just stretch the PCB a bit and use D2PAKs on 2oz copper!  You won't notice a difference!

Tim
« Last Edit: November 30, 2017, 07:06:03 am by T3sl4co1l »
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Offline mairo

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #12 on: November 30, 2017, 08:27:53 am »
I used to work for a company (well known one) involved in 'power electronics' and they developed a method of soldering the THD type packages directly to the heatsink of one particular product, which gave them good advantage in terms of the product's performance..

From memory it was around 3-5 mosfets per side (so 6 to 10 in total) on a long rectangular heat sink - the process was done on a modified for this purpose solder pot, all fets were soldered at the same time. Once the process was mastered there were no issues with damaging the mosfets while soldering. Having said this, if a mosfet was to fail in this module the whole assembly had to be thrown away as you could not rework it most of the time safely, or within reasonable time.
« Last Edit: November 30, 2017, 08:32:15 am by mairo »
 

Offline mairo

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #13 on: November 30, 2017, 08:30:24 am »
Altho, I am not 100% sure if the heatsink was from Aluminium (it looked like from memory)
 

Offline HalFET

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #14 on: November 30, 2017, 08:42:24 am »
This is definitely a vapour phase job (since it's an option for you). The fluorocarbon liquid makes an inert atmosphere that should protect the aluminium from oxidation during soldering.

Ooh, that's actually a good idea.  It's probably not quite good enough, though: aluminum will getter any remaining oxygen, so the vapor needs to be very pure indeed, not diluted with air, say.  Anyway, you still need the skull-and-crossbones flux to get through the initial oxide layer.  Would be cool if the surface could be prepared inside an inert gas glovebox, then soldered.  Hm.

Anyway, guessing that's a lot more setup than OP is looking to use here. :-\

Tim

Vacuum vapour phase is actually a thing.  >:D

 

Offline mikeselectricstuff

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #15 on: November 30, 2017, 09:36:47 am »

This process would alleviate us having to manually screw all the MOSFETs to the aluminium plate & to do away with the manual process of applying heat transfer paste.

Rivets instead of screws would be quicker and more secure.
Or SMD MOSFETS on an aluminium PCB.
Or clips.
Why mess about inventing a new process - the time taken will easily cover many thousands of rivet/screw operations.
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Offline sstepane

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #16 on: November 30, 2017, 09:54:36 am »
I'd solder MOSFETs on copper (stainless steel with flux) pads with holes for screws and afterwards screw these onto aluminium heatsink. May be adding thermal paste between pads and heatsink. It will be much easier and more confident than trying to stick it to aluminium, which is solderable only in oxygen-free environment.
 

Offline DerekGTopic starter

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #17 on: November 30, 2017, 11:37:20 am »

Thank you to everyone who has contributed to this thread.

Using copper plate instead of aluminium is of great interest. Something that I had not thought about before.

We are using the expensive Infineon 10.7 milliohm 200V mosfet IPP110N20NAAKSA1. Two push the power in one direction & the other 2 push the power to a second control board once the first board no longer requires it.

We currently use 2oz copper with a large heat sinking area around the mosfets (on both sides with solder filled plate through holes).

The design originally started out pushing 1.5KW & is currently pushing 3.4KW (bolting the mosfets to an aluminium plate heatsink using heat transfer paste).

We are wishing to automate the process as production numbers rise which is where my interest comes in.

We are also looking to push more power using the newly released expensive IXYS 250V 12 milliohm mosfet IXFQ120N25X3.

Production in Australia is not cheap, hence everything that can be automated needs to be automated.

We also want repeatability. We strive to ensure that every pcb is identical in every way.

Please keep the great suggestions coming. I'm taking them on board & will discuss them all with our other R&D & production engineers to pick the eyes out of it.

We don't want any internal fans as these usually fail at some time in the future & we don't wish to go above 2oz copper on the existing board as there is a lot of SMD with finer tracks already on it.
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Offline mikeselectricstuff

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #18 on: November 30, 2017, 11:59:12 am »
To ensure a uniform contact ( especially with temperature cycles) I wonder if some sort of spring-clamp detail would be best, to ensure consistent pressure.
Instead of paste, maybe a soft metal deformable shim ?
Have a look at what high-end PC coolers are using.
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Offline HHaase

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #19 on: December 02, 2017, 12:52:49 pm »

This process would alleviate us having to manually screw all the MOSFETs to the aluminium plate & to do away with the manual process of applying heat transfer paste.

Rivets instead of screws would be quicker and more secure.
Or SMD MOSFETS on an aluminium PCB.
Or clips.
Why mess about inventing a new process - the time taken will easily cover many thousands of rivet/screw operations.

I will agree that rivets are a potential option here.   At my regular employer we rivet TO-220 packages to heat sinks,  hundreds to thousands of boards each day.
Combined with self-adhesive sil-pads and you have a fairly quick process with consistent thermal contact.
Depending on volume you could potentially look at automatic rivet feeders. 
Though for the wattages you're looking at, and the cost of the part, I'd of course say to do extensive testing on any potential new process.

My concern with soldering directly to the plate is that you're looking at different expansion rates between a PCB and an aluminum plate.  Over time you risk fracturing solder joints due to the flex.
And, like mentioned, you need to either chemically or mechanically remove that outer oxide layer on the aluminum.   

If you need to keep with the thermal paste instead of pads, you'd be surprised what an automated system dispensing system is capable of handling.
Depending on the volume that you're using,  sometimes just the ability to use larger containers of liquids can be enough of a cost savings to justify the equipment purchase.

Another thought is to automate the existing screw process.   Depending on how far you want to go,  there's equipment to handle it.    Even just going to a hand operated driver with an integrated screw feeder can be a huge time saver.   I'd guess you don't do the volume for a full robotic system, but they are out there if you ARE doing that kind of volume. 

 

Offline thermistor-guy

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #20 on: December 04, 2017, 02:00:11 am »
...
Another thought is to automate the existing screw process.   Depending on how far you want to go,  there's equipment to handle it.    Even just going to a hand operated driver with an integrated screw feeder can be a huge time saver...

Or make the existing screw process more efficient. One company I worked for used duo-taptite screws to fasten motherboard housings. These screws roll a thread instead of cutting it, so there is no debris when the thread gets formed.

The metalwork came with formed/extruded holes of the correct diameter. Factory staff would drive the taptite screws in with an air-gun. One short burst and the screw would be driven fully home, forming the thead as it went, and fastening two of the pieces. The process of assembling the metalwork pieces into a chassis was very quick.
 

Online Ian.M

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #21 on: December 04, 2017, 03:07:30 am »
Not a good idea.  A taptite screw will deform the heatsink surface round the hole leading to poor thermal contact.  One can alleviate it to some extent in a thick enough heatsink by countersinking the hole, but the countersink depth (and thus diameter) has to be chosen extremely carefully as if it is too small the deformation due to the taptite will leave an annular raised ring round the hole, lifting most of the device mounting surface off the heatsink and if it is too large the device tab will dish, tilting the device, also resulting in poor thermal contact. 

The use of pop or impact rivets is also strongly dis-encouraged - the shock of the disposable mandrel separating in pop rivets, or of impact riveting can distort the tab or even crack the die.

See OnSemi's AN1040/D Mounting Considerations For Power Semiconductors

As the O.P's using thin plate, countersinking other than just to break the edge of any burr left by drilling and tapping is NOT an option.  As  the heatsink is to be laser cut and later punched, the extra cost of rectangular cutouts for mounting clips will be small or even negligible.
« Last Edit: December 04, 2017, 03:12:17 am by Ian.M »
 

Offline TheDane

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #22 on: December 11, 2017, 12:42:14 pm »
While not technically being soldering, another form of joining metals is welding: Spot welding
https://en.wikipedia.org/wiki/Spot_welding

I do not know if it will fry your semiconductors during the process. I know it will easily join aluminum and other materials like copper just fine.
In any case, it seems like your construction is a one-time deal only. The devices are mounted, and never leave for replacement/service.

Best of luck!
 

Offline RoGeorge

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #23 on: December 11, 2017, 01:09:51 pm »
The Al and Sn can form alloys, but the problem with soldering on Al is that an oxide Al layer is formed almost instantly when heating the solder joint. The oxide layer keeps the Sn and Al apart, so no melting together, no soldering. But, any oxide layer requires oxygen in order to be formed, so if we prevent the Al to oxidase, then we might solder on Al.

One trick to keep the Oxygen away from the solder point is to solder under oil.
You can try it for yourself by
- putting a big drop of oil on the solder point (even the cooking oil will do it),
- scratch the oxide layer (just a few scratches to break the already existing Al oxide layer, but do the scratch under oil so it won't immediately oxidize back),
- then solder under the oil drop.

I tried that by soldering normal wires on food grade Aluminum wrapping foil, and it worked unexpectedly well. The solder drop spread and wet on the Aluminum foil like it would have been on a Copper foil.
For soldering transistors on a big radiator, I guess you will need to preheat the whole radiator before soldering, not a simple thing to do, but totally doable.

NOTE
If you decide to try soldering under oil, use a sacrificial soldering tip.
My solder tip was very hard to wet with solder again after using it under cooking oil, so you may want to cover the soldering tip itself in protective Al foil to avoid oil contamination.
« Last Edit: December 11, 2017, 01:18:13 pm by RoGeorge »
 

Offline zeqing

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Re: Soldering MOSFETS directly to Aluminium Plate
« Reply #24 on: December 15, 2017, 09:17:58 am »
maybe not so hard, see my pictures :-DD IC controller directly on the aluminium board, assembled by makerfabs.
« Last Edit: December 15, 2017, 09:20:21 am by zeqing »
 


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