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Old Aug 19, 2008, 10:03 PM
PLD
Magicsmoke maker
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Australia, QLD, Charters Towers
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Yippee!
My first reflow oven

Since I'm doing a lot of surface mount electronics, I was starting to get rather tired of soldering everything by hand, especially for general production work, so the choice was forced and it was time to look at making a reflow oven.

I was lucky and an online store had some 12" toaster ovens for sale at only $20 -new-, $9 delivery, ordered it last Friday and it arrived on Monday.

Let me first say that it's no surprise that this thing was $20... believe me the build quality was scary including the 240V wiring, a few of the crimps were just waiting to fall out

Anyhow, this oven is a 4 quartz element device (two top, two bottom), I ripped out the timer, thermostat (bimetalic) and proceeded to wire in my own 2-board controller system.

Board #1 is just a fairly generic opto-TRIAC (with zero crossing) driving a BT137 TO220 TRIAC, a snubber cap/resistor was also added though it probably isn't absolutely required with quartz heater elements (might be with wire wound ones). Using this the power can be turned on/off using DC logic levels and the built in zero-crossing detector (MOC6036) means I don't induce ugly noise on the power lines.

Board #2 is the brains, it is driven by an ATMega48 and the sensor is a PT100 type resistor (-50~550'C range). It's also got a 2400bps serial link to the PC for monitoring/logging purposes.

I'm using a 3 phase reflow process, rather than the more typical 4~5 phase. I just go from room temperature to 170'C at about 1K/sec, after which I pause for about 20 seconds and then proceed to push up to 220'C for no more than a total phase duration of 60 seconds, after that power is removed and the unit cools down.

One thing I found is that the solder reacts very quickly to infrared heat, however my temperature sensor is working off air temperature so to keep things better balanced I reflow on a metal tray and also put another metal plate between the tray and the top quartz elements. Using this diffuser the reflows were more controlled as the temperature of the components and boards were now more equalized with the solder.



Anyhow, that's about it... just thought I'd share.
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Old Aug 20, 2008, 01:53 AM
That tree again!!!!
thanhTran's Avatar
Germantown, MD, US
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Looks great! Great work as well!

I've thought about trying this, but the thought of buying & making stencial, buying solder paste, etc. really puts me back. What do you think?

-Thanh
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Old Aug 20, 2008, 02:16 AM
PLD
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I suppose it comes down to how much stuff you're doing.

Solder paste can be acquired rather cheaply. I know you can get it from places like DealExtreme for about $4 per 50g.

As for stencils, I use thin brass sheet which I etch as I would circuit boards.

It's not something I think I'd bother with for one off projects, though as something to help you churn out a dozen or more repeats of the same project at once, then yes, it's worth it.

If anyone's interested, I'm happy to post the schematics / code.

Paul.
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Old Aug 20, 2008, 03:06 AM
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When I helped build one of these (ATMega128 based) the oven did not cool down fast enough on its own and I would have to crack the door open slightly to provide cooling and allow the heating elements to kick in as needed to slow the cooling. Did you have a problem with this? We also did not need any clay insulation, the oven shell was replaced and it heats up just fine without too much radiated heat.

Even if you don't have a stencil, you can do parts like a QFN 56 or some of those parts with ground pads on the underside for cooling. Even with a professional quality stencil, fine pitch parts like a PQ-208 will smear a little between pins so the DIY should not feel like they need perfect application as the solder will tend to flow towards the pins rather than forming a bridge, unless you used too much paste.

A temperature controlled oven with a proper reflow temperature curve is the best to have, but in a pinch I have used a stock oven to do the soldering. Just put a small piece of standard solder on the PCB, when it melts your paste should be melted too (assuming similar melt points). But really, only do this in a pinch.

Circuit Cellar did an article on how to make these that could also be a good reference for those ready to make their own. But no need for a $150 oven like they used, the $20 ones work fine too.
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Old Aug 20, 2008, 04:09 AM
PLD
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I have also found the cooling to be the only part that needs to be manually done. I'll set it up so that a beeper goes off, or better yet, perhaps I should connect a solenoid up to the door and have it push it open.

I've ended up insulating the top and sides with 5mm thick balsa attached using RTV silicone. Since the oven isn't going past 210~220'C it should be perfectly good. I placed my hand on top of the balsa after a test run and it's just 'warm', so the job is being done

Paul.
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Old Aug 20, 2008, 04:13 AM
PLD
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Just took a look at that article you linked - you're right, a bit pricey ($150) considering this one was $20 and has the top and bottom elements.

My total reflow cycle is about 300 seconds, of which up to 240 are preheat, the last 60 are the actual reflow event.

Using the mega48 I've only consumed about 3K of code, which could probably be whittled down with some ingenious thinking but for now I'm more than content with it.

Rather than the LCD 16x2 matrix I'll just use the PC serial link... for now (not that it's even required since you can visually see that things are happening.

Paul.
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Old Aug 20, 2008, 05:37 AM
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Nice Paul!

Regarding the clay, I remember when reading about building a Pizza oven that it can't be heathen (spelling?) up to much the first couple of times. It seems that the clay has to settle first.

This was also with a garden stove we bought a year ago.

So, you might want to try it again, but apply less heat.

CD
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Old Aug 20, 2008, 07:30 AM
PLD
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Crashing Dutchman,

I ended up going with 5mm thick balsa planks bonded to the metal using RTV silicone. Contact temperature of the metal is in the order of 200'C absolute max which is well below the charring/ignition point. I'll monitor it and see how it goes but it should work nicely as an insulator.

Next trick really for me now is to pull the 240V switching electronics out of the unit and place everything into a blackbox that just turns on/off the power via a normal powerpoint (or even just slice an extention lead in half) with only the temperature sensor coming from the oven. Will also pick up a solenoid to see if I can look at popping open that door at the end of the process.

Paul.
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Old Aug 20, 2008, 08:18 PM
PLD
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Has anyone tried doing two reflows, that is so you can do a double-sided board?

A lot of my production boards have components on both sides, namely because in order to get the compact dimensions R/C demands a lot of the time, it's my only choice (also cuts down on weight since you don't use excess FC4 board).

I suppose really, I should just try it and see

Paul.
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Old Aug 20, 2008, 08:54 PM
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I haven't tried it at home, but we do have boards at work built by a local assembly house. At least on our boards they count entirely on surface tension to hold the bottom parts on. You should probably just try it. You need to identify the side of the board where the components are most likely to stay on by surface tension and do that side first. Resistors and ceramic caps hold very well. Some of the chips that I thought would be too heavy stay on fine too because of the large pad area. I know there are glues you can use if the bottom parts won't hold by surface tension but don't have any experience with them.

PS You would have to be very careful when opening the door for the cooldown cycle so you didn't dislodge any components. A solenoid might be too abrupt. Perhaps a large servo would work.
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Old Aug 20, 2008, 09:05 PM
PLD
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Jeff,

Servo for the door... brilliance - now I've got a use for those huge "standard" servos that I never use

Looking at most of my boards I can see that usually the IC/microcontroller is the highest profile component on the board, so it makes things a bit easier in that respect.

Got to love surface tension.

Paul.
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Old Aug 21, 2008, 01:32 AM
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Melbourne, Australia
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Nice work Paul, definitely interested in code/schematics, I have a bunch of ATMega48/88's here (among many others ). I have heard the Black and Decker Infrawave ovens are good, but $20 is sure a bargain.
I have been planning on setting up one of these for a while, just don't have the time. Was planning on doing pretty much the same as you, but with a solid state relay, PID sort of thing.

For the double sided boards, when you do it for the second time, the first lot of solder will melt, but the surface tension should hold the bottom parts in place.
Try it and see
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Old Aug 21, 2008, 02:05 AM
PLD
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Well, I tried to do the 2nd reflow --- but then my oven died on me, we rather the temperature sensor started reading everything WRONG, must have broken one of the very fine wires on the PT100 device

Anyhow, I was smart enough to buy two of them - so I've just finished installing the new one (couldn't remove the old one as it was JB'd in - this one I just use RTV silicone).

Gussy, you can get those ovens from DealsDirect.com.au ---- they're horribly constructed but for $20 it gets you want you need

Originally I had planned on PWM'ing the 50Hz but now that I've done a few runs I've found that simply switching on/off periodically every second or so as required is more than ample, so a relay with a 100,000 contact rating would work fine, that said using the TRIAC method works nicely too and it has the added advantage of the built in zero-crossing detector.
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Old Aug 21, 2008, 02:47 AM
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Just a thought, if surface tension isn't enough to keep the bottom components in place perhaps it's possible to cast them in a jig using automotive high temperature silicone caulking after the first side is made. Naturally after applying plenty of detaching agent. Another option would be to just pot the lot in epoxy, but that would ultimately add weight to the circuit and make it almost impossible to fix any faults. Never mind me, though, I am no expert
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Old Aug 21, 2008, 07:15 AM
PLD
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Replaced the temperature sensor tonight... all fixed in nicer and I've increased the sampling to 64 samples averaged (since they're about 700 max per sample, I can safely fit 64 of them into a single unsigned 16 bit int )

Anyhow, here's some pics of the 'controller box' I've made. All the parts on the controller brain are surface mount so that's why it appears rather blank.

I've got to attach the RS232 D-connector to the panel as well as a socket for the temperature sensor too. Later on if I'm grotesquely bored I'll put a 16x2 LCD matrix on the front.

Oh yes, also have to install a small 9V transformer to power the microcontroller, or I can just plug a jack in the back and power from an external brick/wart.
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