Author Topic: Use an induction cooking element to solder a TV backlight LED bead on a strip?  (Read 728 times)

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Offline t1d

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I see lots of DIY induction forge circuits, but I am wondering if I can use a cabinet top single element induction cooker and solder paste to reflow on a single new LED bead. Seems like that would get around massively heating the aluminum heat sink strip. I am thinking that the only thing that would heat would be the solder.

I have lots of other equipment, skills and techniques, but I am wondering about this one, particularly. So, there's no need to cover other methods. I also understand the hazards of having leaded solder paste around cooking equipment.

Thoughts and suggestions, please and thank you.
 

Offline Berni

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It is a bad idea because it is difficult to control the amount of heating and the heating will not be evenly distributed over the board.

Induction heaters work by well... induction. So since you need a loop area to have induced current this means that the most current flows close to the edge of the object you are inductively heating since that is where the loop area is the largest. You can see this for yourself if you put a small thin metal plate on a induction cooker, the rim of the plate will get smoking hot and perhaps even glow, while the middle does nothing.

If an induction cooker is all you have then i would recommend placing a thick metal plate on top of the cooker and then placing your board on top of that. This turns it into a good ol electric hot plate that does work well for reflowing boards.
 

Offline perieanuo

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short answer, no
 

Offline Caliaxy

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I am thinking that the only thing that would heat would be the solder.
Is your solder magnetic?
 

Offline Jwillis

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Induction only heats magnetic metals . Typically ferrous metals like iron , steel , and nickel steel alloys . Tin , Silver and Lead are not magnetic .
 

Offline T3sl4co1l

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Induction only heats magnetic metals . Typically ferrous metals like iron , steel , and nickel steel alloys . Tin , Silver and Lead are not magnetic .

Any metal can be heated, and presumably they've designed it to be compatible with aluminum and copper-bottom pans.

There are two key factors:
1. The metal has to be big (wide) enough to intercept a usable amount of magnetic field.  That is, be sized similar to or larger than the coil.  Otherwise most of the field slips past and little heat is delivered.
2. This has exactly the same solution, but a different reason: the cooker will only detect a pot, and remain energized, if the load is large enough.

The easiest solution is to grab a nice aluminum plate, about pan-bottom-sized, and slap it on there.  Use it like an electric griddle.

The plate also blocks magnetic fields from being induced in the circuit being soldered, which can deliver quite high currents and could damage components.

IIRC, IBM or someone was considering something like magnetic-loaded solder so that PCBAs can be induction heated directly, though I don't think I've ever read anything about it so I'm guessing they weren't able to solve the problems with PCB planes shielding the field (and heating up themselves), induced currents frying parts, and simply finding some sort of susceptor material that can be added to solder paste without worsening the soldering process itself, while generating enough heat from magnetic fields (high hysteresis loss).

Tim
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Online Cerebus

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Induction only heats magnetic metals . Typically ferrous metals like iron , steel , and nickel steel alloys . Tin , Silver and Lead are not magnetic .

I don't know why you would think that. Last time I looked Faraday's Law hadn't been modified to add "only for ferromagnetic materials" to it. Now, domestic induction hotplates are only designed to work for ferrous materials, but that does not mean that only ferromagnetic materials experience heating by electromagnetic induction. People regularly melt non-ferrous metals in induction furnaces; at university a metallurgist friend spend a 6 month work placement melting gold and platinum in induction furnaces. Induction furnaces heat things by inducing a current according to Faraday's Law and I2R losses in the material do the actual heating.

Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Jwillis

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Those induction hot plates will not heat copper , aluminum or the solder he is using . They are far to low power for any application other than cooking  in ferrous materials .  READ the opening statement before you jump to conclusions.
 

Offline BrokenYugo

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You could use it to heat a thick steel plate, and use that as a hot plate to reflow or preheat.

 

Offline Berni

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The reason why induction cookers have trouble with non ferrus metals is that there is a ferrite core around the coils that is not designed to work with large air gaps. It relies on the metal pan itself to close the magnetic circuit around the ferrite core and close down the airgap. When you use an aluminium pan most of the field will just close down on itself under the pan rather than go trough it.

However you do get a slight bonus heating effect when using ferrus metals, they have magnetization losses that help make more heat. This is in addition to the eddy current losses that all conductive objects have. At the same time the conductivity on aluminium and copper is really high so you need much stronger induced eddy currents to get the same heating effect, but steel is a terrible conductor among metals, so less current for the same heating effect.

Typically the kind of induction heaters that heat up or even melt aluminium or copper or gold or whatever use a air core coil (allows for stronger fields) and place the work piece right in the middle of the coil rather than on top (solves those magnetic coupling issues)
 

Offline T3sl4co1l

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In the process, some magnetic field goes into the metal, inducing a current (which is how the field reflects off it, in the first place) and thus causing it to heat up.

As seen by the circuit, the effect is a lower work coil inductance and modest Q factor, whereas steel has similar inductance and low Q factor.

Generally speaking, for similar setup, magnetic steel gives a lower Q than stainless, than aluminum, than copper.

In any case, it's just an impedance matching problem.  Which is why I've presumed "they've designed it to be compatible with aluminum and copper-bottom pans".  Simply check if it's rated for that or not, and use whatever metal is appropriate.

Like I said (or hinted at), induction is perfectly reasonable on PCB material as well.  In fact I've used this to tin large areas of copper clad before, just set up an open-face ferrite core on the breadboard, with a half-bridge driver running from a function generator.  Direct drive or series resonant is very reasonable, and as it happens, fixed frequency is pretty easy to control.

Typical Q for very conductive metals, with close coupling, is 10-20.  Meanwhile the inductance drops in proportion to the amount of field being shielded (which may be quite considerable for "close coupling", anywhere from -10 to -80% say?).  Best Q for something like copper foil (like PCBs) is around 6 or 7, from what I've measured.  This is at frequencies where skin depth is about equal to foil thickness, so the shielding is relatively poor and the foil looks somewhat more resistive than reflective.  For similar conditions on magnetic steel, it can be 2-5, quite a lot of resistance indeed -- direct drive (no resonant cap needed) is even feasible.


Typically the kind of induction heaters that heat up or even melt aluminium or copper or gold or whatever use a air core coil (allows for stronger fields) and place the work piece right in the middle of the coil rather than on top (solves those magnetic coupling issues)

Mainly it's that the frequency is higher (which is neither here nor there, really), the high temperatures, and requirement for insulation, preclude use of most any core material, which would get very hot from core loss anyway (requiring water cooling just as the coil does).  Field strength is also secondary; it's simply about how much power you can put into the coil, and its efficiency.  Though the pole pieces used with cooktops do help greatly to improve the field strength at the surface; this helps reduce Q, and also shields the circuitry underneath, an important feature.

That said, there are some cored applications in industry.  Channel induction furnaces use a loop of molten metal, essentially as a shorted turn, around a laminated steel core, and operate at mains frequency (plain old phase control is feasible, though hopefully they're doing something a bit smoother for sake of power factor).  These are typically in the low MW power range.

Overall equivalent figures like L and Q are more important than particulars like field strength and skin depth.  Things are a bit more particular if you're doing stuff like case hardening (the skin depth and thus frequency does matter, albeit not very sharply because skin depth depends on sqrt(f)), or melt stirring (lower frequencies create more MHD and convective action).

(For those unaware, I've been designing induction heaters for many years; this comes from direct experience.)

Tim
« Last Edit: October 14, 2021, 07:30:31 pm by T3sl4co1l »
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Offline t1d

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@ Everyone: Wow, what great responses! Thank you, all, for your efforts to help me!

I did think of a thing, or two, to add...
- My element is only rated for ferromagnetic cookware. It is, specifically, not rated for aluminum and copper. I knew that I would have to trick the unit to turn on, by leaving a ferromagnetic pot on the edge of the eye.
- YouTube has lots of videos of induction heaters being used to solder copper pipe. That type of induction unit either wholly encases the pipe in a circular coil, or has a coil that is "U" shaped. So, I knew that induction could be used, but not how it worked. You gracious folks have filled in that education gap.

So, the pondering of my original question was twofold...
1) Is the solder ferromagnetic, i.e. will it reflow, without the aluminum strip becoming, or needing to become, hot. The answer is no, solder, itself, is not ferromagnetic and, if you did have a forge with enough energy to reflow the solder, applying that much energy might melt the aluminum, as well.

2) The second part of the thinking process was what I thought to be more of the issue... If solder was ferromagnetic, would the flat coil of a residential induction cook-top unit do the same trick as the coiled forges. I think the answer to that is that it depends on the power rating of the individual cooking element. But, a coil would clearly be much more efficient, for the job.

Conclusion
You have educated me well. I understand that the issues at hand. For my use, using the induction element would not be a panacea... that is, a perfect solution, having no complicating factors.

As said, I have lots of other soldering equipment. I even have an old metal coil single burner, in the garage. It should make an adequate amplitude and reserve quantity of heat to deal with the losses of the aluminum heat sink strip. So, I am going to give it a try.

@ Tim
Like I said (or hinted at), induction is perfectly reasonable on PCB material as well.  In fact I've used this to tin large areas of copper clad before, just set up an open-face ferrite core on the breadboard, with a half-bridge driver running from a function generator.  Direct drive or series resonant is very reasonable, and as it happens, fixed frequency is pretty easy to control. Tim
I would l-o-v-e to see a schematic - even just a rough sketch - of this circuit.

Thanks, again, to each of you. I think that we can call this one "Solved."
 
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Offline T3sl4co1l

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Offline t1d

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No problem: https://www.seventransistorlabs.com/Images/Breadboarded_Induction_Test.pdf
Tim
Thanks, Tim! How very interesting! I did not know the IR2101/2 Driver. Nice part. I am sending you a PM.
 

Offline T3sl4co1l

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Offline t1d

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Wow, thanks!
 


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