Yet another toaster reflow success story

[nsayer]

When I was young, I was into Heathkits in a big way. But as you all know, in the 80s and 90s the industry transitioned to SMD and, well, through-hole kit assembly stopped being even remotely cost effective (if it ever was) and I said "goodbye" to hardware hacking and became a software weenie.

I bought an electric car almost exactly a year ago, and stumbled across OpenEVSE. Long story short(er), that introduced me to Arduino, Eagle and OSHPark. Very soon I was back to designing my own stuff again - this time with proper tools and fabrication support. But I was still doing through-hole stuff.

Just to see what it would take, I decided to try a SMD version of one of my simpler things - an EV Simulator board (a very, very useful diagnostic tool if you're into designing and building EVSEs). I can only imagine what the DigiKey picker must have thought of someone buying single quantity 0805 resistors!

So I got the board and the parts and went to the hardware lab where I work with a microscope and it took me two hours to hand solder what I could have done in 20 minutes through-hole. I swore it off as a big waste of time, but happened to post about my experience on the OpenEVSE mailing list. In no uncertain terms, Chris (the main OpenEVSE developer guy) told me I was doing it wrong and that I should use reflow instead.

I quickly got a google education on the subject and saw that tons of folks had done their own reflow ovens. I looked at them all and... I dunno... I sort of found something different with each one that I didn't really like so much. Lots of folks used SSRs, but those are awfully pricey. An opto-isolated triac circuit can be done a lot cheaper. I also wanted to separate out the AC power control systems from the PID and thermocouple stuff, and actually build the former into the oven itself. It would wind up being a lot safer that way.

Skipping forward a bit, I call my result "Toast-R-Reflow." I went to Fry's (those not in the western U.S. may not be familiar with Fry's. It's a quite unique store. They sell components, computer parts accessories, appliances, office supplies... a quite eclectic mix) and semi-randomly picked a toaster oven. It turns out I made a not-too bad choice. It was easy to nondestructively disassemble it, while being "sporty" enough to keep up with at least a SnPb profile (which is good enough for me).

My power board is a pair of opto-isolated (MOC3020) BTA-20 triacs. By splitting the problem in two, I was able to simplify the thermal management. Each channel is thermally designed to switch up to 8 amps, which means the two combined (here in the U.S.) will do just under 2 kW. Plenty. Since there are two channels, this opens up the possibility of setting the oven to "broil" if you wanted to do double-sided reflow, for example. I haven't contemplated that because this particular oven struggles to follow the profile sometimes even with both elements in play.

The power board is through-hole - that solves the chicken-and-egg problem of building your first reflow oven. My first controller was an uno + breadboard with an AD595 thermocouple amplifier. At the time, AD8495 breakout boards weren't available. I now sell them and Adafruit sort of stole my thunder. :/ That controller's last job was to reflow its own replacement.

My latest controller is an ATTIny84 with a 2x16 LCD, an AD8495 thermocouple amp, a 1.8v LDO fed into AREF (to improve the resolution of the A/D), and a button. It has just enough firmware space for a dedicated-profile system. I have a design in mind for an ATMega + MAX31855, which conceivably could allow for a profile editor UI so that altering the set points wouldn't require an ISP.

Anyway, if there was a next step for me, it would be to try and either boost the capabilities of my current oven (perhaps by stuffing the inner walls with fiberglass) or replacing it with a beefier one so that I could use RoHS solder paste, which would open up Europe as a market for my stuff.

I've also wondered if there might not be a market for a real hobbyist reflow oven as its own product. At its core, it's a double-walled sheet metal box with a pair (or four) heating elements and the control system I've already designed. I'd be surprised if I couldn't arrive at a retail price of $199.

[IanB]

Nice.

I seem to recall reading from other sources too that 120 V toaster ovens struggle to cope with the desired temperature profile. If the supply is upped to 240 V then 3 kW becomes easily obtainable and I wonder if that might work out better? Unfortunately, I think a 240 V toaster oven would have to be sourced from Europe since I don't believe it would be easy to find one in North America.

[IanB]

But having said that, a quick scan of toaster ovens in the UK suggests they have a typical power rating of 1500 W. If there was a 3000 W oven it would likely be a big and expensive one.

[nsayer]

I'm not sure 3 kW would be necessary with a small interior space and a blanket of fiberglass around the chamber. And in North America, that would be problematic from a supply perspective, since most ordinary household outlets are on 20A breakers, which really implies a maximum truly usable continuous current of more like 16A. That's just shy of 2 kW, and that implies you basically have the whole circuit to yourself. If we're talking about a hobbyist reflow oven, I don't think it can require more power than a consumer toaster oven, or else it won't really be a hobbyist tool.

Where I think real gains in efficiency can be made are in the interior geometry. Toaster ovens are quite tall. That's not at all necessary for a reflow oven. I can envision a space more like 8"x10"x6" - very flat by comparison. With two elements running across both the top and bottom, that should heat evenly enough to work, and would potentially accommodate boards with parts up to, say, 3" tall, which I think for most things that need to be reflowed would be fine.

A reflow oven also doesn't need a glass door, like most toaster ovens have. Yes, we all want to peek inside, but a reflow cycle is so short that I think we can be patient.

[Fred27]

I've been working on my own reflow oven too. I think a physically small oven is a real help. I found a 1kW 240v oven (UK) that's only 9L and it can manage 2C per second.

I've not finished the controller yet, but I successfully used it to reflow a PCB under manual control.

[Coder96]

Has anyone tried using a pizza oven as a reflow oven?

That maybe closer to the dimensions you describe.

The cheapest one I found on Amazon for 55 usd didn't have any temp selection. Who knows if it can get up to temp.

[nsayer]

I've not finished the controller yet, but I successfully used it to reflow a PCB under manual control.

Wanna buy one of mine? 

[nsayer]

Has anyone tried using a pizza oven as a reflow oven?

That maybe closer to the dimensions you describe.

The cheapest one I found on Amazon for 55 usd didn't have any temp selection. Who knows if it can get up to temp.

That's a thought. Pizzas are supposed to be baked at 260°C, so far as I know. But the real question for reflow is how quickly it can get there.

[hamster_nz]

I've been working on my own reflow oven too. I think a physically small oven is a real help. I found a 1kW 240v oven (UK) that's only 9L and it can manage 2C per second.

I've not finished the controller yet, but I successfully used it to reflow a PCB under manual control.

I think smallest, lightest, cheapest is a pretty good mantra for a hobby reflow oven. If it is big and heavy it will heat up slowly (and clutter up the bench)!

I used this one: http://www.thewarehouse.co.nz/red/catalog/product/Necessities-Brand-Toaster-Oven-White-9L?SKU=1689799

Adding themal blankets might be a waste of time unless it is to protect your controller... given how slow they cool down when the door is closed there isn't that many watt/seconds leaking out. I might try heating mine up to 220C, leaving the door closed and monitoring the temperature - the rate of cooling would be the max difference that perfect insulation could make to the rate of heating.

[mjkuwp]

...
I quickly got a google education on the subject and saw that tons of folks had done their own reflow ovens. I looked at them all and... I dunno... I sort of found something different with each one that I didn't really like so much. ...

you said it exactly.  Every oven is the same, yet every oven is different because nobody is totally happy with what the other person did.

There is a Kickstarter  - Reflowster so from that you can judge the market a little bit.  You can sell one for $200?  so... you can build one for $80? including labor?  Check out Dave's recent post about pricing products.  Considering the possible safety issues involved and judge that against the profit you could make.  I believe it is not worth it but it is only my opinion  : )

You can shorten the heating elements of your oven to get more power.  I changed mine from 1200W to 1700W.

[nsayer]

  You can sell one for $200?  so... you can build one for $80? including labor?

I did take a 2x  rule into account when I said that. 2.5x would be $249.

With my current design, no.

But if I was going to make reflow ovens hundreds at a time, I'd optimize the design - and then? $80?  Eh... not sure. I'm guessing closer to $100.

I certainly believe the electronics can be done for around $50 in quantity. I'm basing that on $10 for a thermocouple, $20 for a power supply and $8 or so for an LCD module. A small board and the rest of the components should be doable for $10.

Can one get a double-wall sheet metal box with a door and two heating elements for $50 at scale? I'm guessing so, but I honestly don't know.

[mark03]

I'm not sure 3 kW would be necessary with a small interior space and a blanket of fiberglass around the chamber. And in North America, that would be problematic from a supply perspective, since most ordinary household outlets are on 20A breakers, which really implies a maximum truly usable continuous current of more like 16A. That's just shy of 2 kW, and that implies you basically have the whole circuit to yourself. If we're talking about a hobbyist reflow oven, I don't think it can require more power than a consumer toaster oven, or else it won't really be a hobbyist tool.

In North America, you could always run it off of the electric-dryer 240V circuit.  That's what I did, with a 5-kW duct heater.  I'm not sure I could "recommend" it unless you have a four-prong grounded outlet.

http://www.keteu.org/posts/reflow_oven.html

tl;dr:  The problem with toaster ovens, if you can call it a problem, is that the heat-transfer mechanism is almost entirely radiative (infrared).  While this seems to work ok, convection is better.  And for that, you need [a lot] more than 1-2 kW.  I would be leery of shrinking the interior space in an infrared oven.  At least make sure you leave room for a fan in there.

[nsayer]

In North America, you could always run it off of the electric-dryer 240V circuit.  That's what I did, with a 5-kW duct heater.  I'm not sure I could "recommend" it unless you have a four-prong grounded outlet.

Our dryer is gas. 

I do take your point, though, and we do have a 240 outlet in the garage (for an EVSE - that's what got me into being a Maker in the first place, of course), but if there's a way at all to do it at a max of 2 kW @ 120V, then I think it's important to go that route to make the market as large as possible.