Toaster oven for reflow

We use an old Cole-Parmer chem lab hotplate with a piece of 1/2-inch aluminum jig plate on top of it, and a thermocouple held down with a screw.

Anecdotally it's pretty easy to scorch boards in a toaster oven.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Right. When we moved, our big reflow oven was down

so we got a Chinese benchtop oven for small runs. It has glow-bars in the top, which IR-scorched rows of parts on the board. On one first article board that I got, the main power input was a short, which mystified me. Turns out that a row of potted dc/dc converters lined up with one of the overhead heating elements.

The bricks stick above the board and have minimal thermal conduction downward, and have solder joints and toroids and such inside.

At home, a convection oven is probably better, maybe with a heat shield above the board to reduce radiation heating.

When I bake pies, I usually put a baking sheet on a shelf above the pie in our electric oven, so the crust doesn't scorch; same idea.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

yeh easy to get the reflow-to-incinerate ratio wrong with IR

and don't forget to remove the magnet from the hot plate ..

Lasse Langwadt Christensen wrote in news: snipped-for-privacy@googlegroups.com:

One can easily perform it with great control and accuracy as well.

Use the oven as a hot chamber AFTER it has come up to and risen above reflow temp a couple of tens of degrees. As it is settling *sans* power, you place the board in. If at any time you intend to apply power again, it had better be for a short spurt.

Prior to this final reflow cycle, you should also have used a 'cooler' oven to pre-heat the entire assembly to a mere hundred degrees or less differential from the final process chamber. (yes I know they are toaster ovens). The final transfer to the main reflow 'oven' should never be done with a room temp board. The ramp up is too fast and the difference will soak heat out of the small chamber.

IR thermometry can be used to examine the chambers and the inserted PCB.

Otherwise, it is a good idea to epoxy down a small thermistor node onto the board and monitor it live.

Whatever that is about.

I use mine for repair, not to reflow solder new PCBs. The problem is where to measure the temperature. Obviously somewhere near the PCB being soldered, but when I reflow several boards, the rather wide variation in temperature has caused problems. So, I'm down to one or two boards at a time.

Another problem is resisting the temptation to open the door. Don't open the door until the oven had cooled down to about 70C. I suspect I destroyed a few large value MLCC caps with the thermal shock.

Controlling the oven is currently a manual operation. The oven is plugged into a power strip. I have a crude temperature profile graph scribbled onto a piece of paper. I try to follow the curve as best I can.

I also put a blob of solder paste and some SMD component on a blank PCB nearby. When the solder melts, the SMD part will move. I do this to predict what the solder might be doing under hard to see components, such as BGA packages. For repair using reflow, liquid or paste flux is a must.

I also have a hot air desoldering station, but find that the toaster oven works better for boards with large BGA chips and on boards where there's a risk of blowing away small parts with the hot air blast.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

mandag den 12. november 2018 kl. 19.00.55 UTC+1 skrev snipped-for-privacy@decadence.org:

then it isn't really IR anymore, it is more convection

if you use a laboratory hot plate the magnet under the plate used to stir is strong enough to move components

I just grabbed an old toaster oven from a garage sale ($5 or so). It has nichrome-strip elements rather than quartz, and no fan. My hunch is that the metal-strip elements are probably better than quartz for heat-up/cool-down speed, as there's less thermal mass involved. No fan, unfortunately.

It heats up fast enough to do a good job of following the temperature profile recommended by the manufacturers of the solder paste I've used. In fact, it would overshoot significantly if the controller weren't PWMing the power during much of the cycle.

For control, I eBay'ed an older Omega temperature controller with thermocouple. It's one which has a PID algorithm with a self-training feature, and a four-stage temperature-profile capability... this is enough to implement the "ramp up to preheat temperature, soak for a while, ramp to peak temperature, ramp down" profile well enough. I put this in a box, with a hefty solid-state relay to drive the toaster oven.

I probably need to fiddle a bit with the settings, as it does overshoot the "preheat/soak" temperature a bit, but it's not bad.

The biggest shortcoming of this approach is that the toaster oven isn't designed for a rapid cool-down, and without manual intervention the cooling part of the process would take far too long. I've found that I have to open the front door of the oven, and aim a small desk fan at it, to drop the temperature at something like the proper rate. If necessary the controller will activate the heating elements during the cooldown, "fighting" the cooling effect of the fan to achieve the programmed rate of cooling.

I have a feeling that achieving the proper temperature profile (the ramping and soaking) with a purely manual control could be tricky, especially if the toaster oven's heating elements have only a simple on/off control. A commercial controller (or an Arduino equivalent) can pulse the heat on and off fairly rapidly to achieve the right rate of heat delivery. This will be harder to do, consistently, with manual switching.

Some ideas...

A piece of 1/4 inch aluminum plate heat shield / spreader between a toaster's elements and intended victim might prevent toasting.

Sparkfun Electronics started off using a hot plate (though they tried toasters, too.)

A colleague has one of these for assembling protos, and loves it:

Cheers, James Arthur

It's way simpler to use the hotplate + jig plate as though you were making pancakes. Works great. Plus it's out in the open air, so you can watch the highest thermal-mass components to make sure they reflow correctly.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Thanks for everyone's comments.

My understanding is that the main problem with the quartz heating elements is that they directly heat the board. That may cause black parts to heat more rapidly than white, or parts that are located off-center to heat more slowly, and you could even have some shadowing of smaller parts by taller ones.

One solution several mentioned is shielding the board from direct radiation by the heating elements. But it seems to me that that changes the board heating from radiant to convection, which would inevitably slow down temperature changes because now the air has to change temperature. I guess that wouldn't matter so much up through the soak phase, but it seem that taking it on up to the reflow stage, and back down, needs to happen fairly rapidly, and I'm not sure that would happen with shielding.

Another option is a four-quartz-element oven, in which the board would not be directly above or below an element. But I'm not sure that completely solves the radiant problem.

Walmart also sells an Oster oven for $40, model TSSTTVDFL2, which has four elements and a fan. I'm not sure what the elements are, but they are smaller than the quartz elements in the other ovens, and dark grey. If this is basically an air-heated oven, then it might be the most easily controllable, with none of the disadvantages of quartz. But the question would remain as to how fast the temperature can change.

Jeff, I'd love to know how your oven is configured - the number of elements, what type, and whether there is a fan.

I've watched a number of additional videos on hotplates, and I guess it's something I should consider. But there still doesn't appear to be much control over temperature. In any case, more people like hotplates than I had realized. I assume it's the even conduction heating that's the main appeal.

I've had zero success with the toaster oven approach - tried it's own controller (utterly useless) and a decent self tuning temperature controller (which had to re-tune for every change of board size and gave completely different results if you didn't).

For a long while I've been happily using a hotplate - set it to 215C (for leaded solder paste), place the board on it, wait until solder paste melts, give it an extra 30 seconds and it's done. Can only do one side.

I've just bought a French (CIF) draw style reflow oven which was a LOT

two shielded quartz elements. The temperature control is (almost) good but it's badly let down by having no forced air cooling so although you can get a decent profile heating up, it stays hot far too long. I'll modify it by adding a cooling fan when I get time but for the money it should have had one already. If I hadn't been in a tearing hurry to do a job I would have been better buying a more up market Chinese belt job or a second hand one.

For $100 I don't think you can beat a hotplate. If your time is free I think you would do better to start with sheet metal than a toaster oven. I spent ages insulating mine and the insulation cost nearly as much as the oven - and it was still awful.

MK

Mine was maybe $50, mostly for the jig plate that I got from McMaster-Carr. Half inch is the ticket.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I have a small one of these:

It is not $50, but when you compare the price of a mid-range FPGA or a GHz ADC in a BGA, then a lab-size vapor phase seems to be a good thing.

Or the time lost in chasing part time contacts or semi-burnt chips.

cheers, Gerhard

Agreed about minimizing debugging time. Round here we usually send PCBs out for stuffing, and build protos by hand. If we needed an FPGA we'd probably use a demo board and some hand wiring. (We don't do a lot of high speed A/D and D/A--our fast stuff is mostly wideband RF coming from heterodyne or Doppler interferometers.)

My fastest Doppler interferometer went DC-8 GHz--it was for measuring hypersonic tin debris in an EUV light source. (The funding got pulled before it was finished, but the design worked great.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

11 years ago I got a P&P machine, and did up my own oven. I got a ramp and soak thermocouple controller on eBay. I got a roll of some micro thermocouple extension wire and made my own thermocouples. After some fried boards, I figured out to poke the thermocouple into a plated- through hole on one of the boards. I got the largest toaster oven they had at WalMart, and put a SSR in series with the thermostat. I fed the thermocouple through a gap in the oven, so it is convenient to poke it into the board.

This all works very well, I can program the ramp and hold profile into the controller and get pretty good results, both with leaded and lead- free solder. (I have to set different temperatures for the different solder.) I have done close to 2000 boards in it.

The only issue is that the heating is not uniform, but it does pretty well, maybe 10 - 15 C variation across all but the edge inch of the oven interior space. Poking the thermocouple into the board is a slight annoyance, but reading the actual PCB substrate temp makes it all work a lot better.

My oven has a convection fan, but I have not found that it makes any difference. I think the fan is WAY too small for the volume of the oven. My oven has 4 straight elements that run left to right, two on top and two below the rack.

Jon

Turns out that a row of potted dc/dc converters lined up

We use two different Murata DC/DC converters, their datasheet says "NO REFLOW" period.

Jon

Does that imply hand soldering only? May as well go thru-hole.

We hadn't had any problems until we used the little substitute ovens. Our big one is back online now.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

You could design temperature sensors into your boards. Thermistors or even copper traces.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Thanks for the info. Are these quartz elements? Is there any covering on them to block direct radiation?

It's interesting that the fan doesn't make any difference. If you still have temperature variations across the rack with the fan running, then you must be right about it not moving enough air to make any difference. Either that or direct radiation is so fierce that air temperature just doesn't matter.

Putting the thermocouple into a plated-through hole makes a lot of sense. It sounds like you consider this to be important to your success with your oven.

Do you think you would need the controller if you were standing there the whole time reading the thermocouple? Could you control it manually with a power switch?

Any sense for how many times the controller toggles power in a typical run?