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标题: My first reflow oven [打印本页]

作者: liyf 时间: 2012-3-31 09:33
标题: My first reflow oven
（http://www.rcgroups.com/forums/showthread.php?t=911539）
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.

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.

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.

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.

                                                Okay,  obviously some minor tweaking with timer settings etc yet to be done - but everything's now packed in a box and sealed up... except I forgot to put the AVR programming port on the back *sigh*.

Here's a bunch of pics.

Going for gold

I think I'm about home now

I made my vacuum pump, to make it easier to control I used a prefabricated T-piece in the line. The line is a bit stiff as it's a good 5 years old, I'll go get some fresh stuff tomorrow if I can.

Also, here's a shot of the board after the reflow - the board was assembled with the vacuum tool, I can see it's going to take some practice but there does seem to be the "improved productivity" aspect to it.

Oh yes, notice that I moved the caps around on the board and now the surface tension doesn't yank things out of place

Paul.

Things are now going 'well'

Seems like the problems were mostly related to the reflow peak being too low and too short. I put the boards that were not functioning back in for the following reflow profile and they came out working 100%;

* 300 seconds @ 80'C
* Ramp to 100'C
* 100 seconds at 150'C
* 90 seconds to 220'C (the 220'C bit only lasts for about 30 seconds)
* cool-down.

An important change I made in the software also was to compensate for heater element overshoot (that is, the oven keeps getting hotter even after you've turned off the element, so your 220'C could go as high as 235~240'C)

Fortunately the adjustment was a very simple one; for every second that the element is on, you add 1'C to the actual oven temperature, up to 16 seconds (eg 16'C overshoot), then for each subsequent second the element is -off-, you deduct 1'C from the predicted oven temperature. Obviously those values have to be tweaked to each oven but it works pretty good on my oven now and the max I saw was about 222'C.

Just redrawing the controller and 240V board... then I'll post them up (in actuality, I botched the Eagle CAD file the other night so I had to start again

).
First... a disclaimer.

WARNING

These schematics and circuit-boards are provided in good faith and come with absolutely no warranty and/or implied liability on my behalf.

If you are not experienced with mains power equipment do not attempt this project.  A mistake in construction or handling could be lethal

Now, find attached the various schems and circuit boards, though I suspect people will make up their own circuit boards based on what gear they have available (eg pin-through etc).

If you want the source code, send me a PM, I won't post it here.

Paul

                                             Here's some more pictures.

Starting to ramp up the number of boards I do in each batch as my confidence in the setup grows.

Okay, so I got tired of keeping the PC on and I wanted to test a new creation anyhow

Added:

*Arming button

*Serial LCD panel (uses a cheap/easy Tiny2313 @ 8MHz, powered directly from the 4 wire serial lead (+, -, RX, TX). I could just make it a 3 wire by dropping the TX line (relative to the device).

Anyhow, chatter aside, here's the photos.

                                    Closeup of the screen.  Obviously over time I'll probably tweak the display data. Nice thing is now that I don't have to do anything more to the serial-LCD module.             146.4 KB · Views: 170

                                             Just a bit of a status update...

Reflow oven now is going in 'production' and things are behaving very nicely overall. There's still some fine tuning to be done to minimize the out-balling of the solder but other than that I'm very happy.  Double-sided (two reflows) is now par for the course.  The only trick with doing double sided is that I have to put the boards onto a flat-plate rather than the open-mesh wire that I can use for the first side in order to keep the boards resting horizontally.

I've also moved my oven up to the top shelf right next to the 250mm exhaust fan, really keeps the fumes down

  ( one minor artifact I've found though is that the fan can induce a small vibration onto the oven if I'm not careful - haven't seen any troubles yet from it but it's something I'll watch ).

Also, today I received my order of 100 and 75 micron film (.004 and .003 respectively).  For larger SOIC I'm happy to keep using 100 micron as it's quite a bit stronger than the 75.  However I suspect as I move to MLFs I'll have to switch to 75 to prevent excess paste being put down.

作者: robter 时间: 2015-10-30 22:50
这个很好，感谢楼主

作者: gl542400 时间: 2017-2-11 15:12
又路过一个地方！

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