Author Topic: Reflow oven - free unsolicited advice!  (Read 4081 times)

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Offline mjkuwp

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Reflow oven - free unsolicited advice!
« on: October 22, 2014, 11:30:03 am »
From the show notes of Amp Hour #221, Chris mentions he is starting a reflow oven project.

some notes for Chris and all on what I made a year back or more.  Uses electric imp to make a head-less design and gui interface super easy.

 - Convection:  I assume it helps.  I never tried it an other way and others have stated it is necessary for even heat
 - Feedback:  I place all boards on a copper sheet that I blackened.  One spare board has a thermocouple soldered to it.
 - Power: I boosted mine by shortening the heater wires.  It's a bit of labor but it worked well.  Mine went from 1200W to 1700W
 - GUI.  I use a web page and so I did not have to design and build any buttons on the hardware. What a relief.
 - Safety:  Wire the mechanical timer in series with everything.  Regardless of any software, this mechanical timer will shut it down after n minutes.
 - Control:  I control slope of the temperature in the PID loop directly instead of Temperature.  I think this results in much more stable control.

 - state  1 preheating at super low duty cycle until you reach 40C (perform error checking at the end of this)
 - state  2 Drive upward at 1.5C/second
 - state  3 At ~100C change your target slope to one that will get you to 150C after 70 seconds (example)
      - Continue re-computing the target slope in the loop and you will smoothly turn the corner into the soak

 - state 4 After appropriate time expires for the soak, drive upward at 1.5C per second to flow.
 - remove from oven at temp you like: for example 215C
         
By constantly computing the slope and using that as the loop instead of temperature, the web page can extrapolate and continue to update the screen at a higher refresh rate than the data that comes in.  This gives you precise real-time display of the temperature with no lag by virtue of how much inertia the oven has.

I shared everything I could about my project but...

of course I realize everyone makes their own oven, no two are alike and that is the fun of it!



One possible oven:

on Amazon, this Black & Decker model - TRO4075B - is only $54 and is 1500Watts.  about as high as it goes these days.
I made mine from a similar $40 model that seems to be discontinued now.

Soldering thermocouple to the sense pcb with extra high-temp solder

http://mjkuwp94.tumblr.com/Soldering

Tarnishing the copper
http://mjkuwp94.tumblr.com/tarnishing

PID Constants
http://mjkuwp94.tumblr.com/PIDSlope

Main page containing documentation of the whole project
http://themzlab.tumblr.com/reflowoven
 

Offline Falcon69

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Re: Reflow oven - free unsolicited advice!
« Reply #1 on: October 22, 2014, 06:17:53 pm »
hey! that's the same model oven I converted too.  That black and decker one works great.

Except with mine, I used an Auburn Instruments PID Controller, SSR, and a data logger from http://www.vernier.com/products/interfaces/go-link/

works pretty good, although, kinda a pain to program the PID controller, but it works.

I redid the face on mine. Got rid of all the knobs and mounted switches and the PID controller. 

 

Offline marshallh

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Re: Reflow oven - free unsolicited advice!
« Reply #2 on: October 23, 2014, 12:22:52 am »
Good idea to cover the heat shields (which are very leaky) with some metal covers and metal flake IR-absorbing paint.
This way you only get indirect heat, which is important. Otherwise you will see various parts of the pcb (like packages) heat up at much different rates. This is Bad News.
Verilog tips
BGA soldering intro

11:37 <@ktemkin> c4757p: marshall has transcended communications media
11:37 <@ktemkin> He speaks protocols directly.
 

Offline Nate

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Re: Reflow oven - free unsolicited advice!
« Reply #3 on: October 23, 2014, 08:50:31 pm »
Hey mjkuwp,
Thanks for sharing and love your implementation! The web app is an awesome idea since its verrry cross platform. Mine uses USB which would require a bit of effort to access it via a web app(certainly still could be done). If only there was a legitimate, certified, and very cheap wifi module. I'll keep dreaming  ;D .

Looks like ya did some work to get some powerful ramp rates. Very cool although my 2 cents would be to caution you on spending excess effort trying to implement the perfect datasheet profile. Most 10 stage industry convection ovens do not have profiles that look close to it and often can take up to 8-15 minutes per board for a profile to finish. Trying to rush it can be a bit risky to the joints. Heres a good reference on whats acceptable for a majority of components: http://www.jedec.org/sites/default/files/docs/jstd020d-01.pdf , (table 5.2).
 

 

Offline mjkuwp

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Re: Reflow oven - free unsolicited advice!
« Reply #4 on: October 24, 2014, 12:42:53 am »
Falcon69, really nice!  I like the build quality.

how do you program the PID controller?  Can it hold a sequence?   Do you input a bunch of temperature and time points? 


Nate, thanks for your comments.  one thing I like about the wifi is that I don't have to have an electrical connection between the oven and the user interface.  There are other ways to do this such as 433MHz radios or optical of course.

 I definitely am not sweating over small details of datasheets.  I programmed one profile and I've stuck with the same one for all the builds.    If I could get some extra time I would make a flowchart or graphic to explain it.  It is not complicated but still...

I did put effort into increasing the ramp rate but if I built a 2nd oven I don't think I would do that.  There is too much labor and the benefit is...uncertain.  Shortening the heating element to boost the power is relatively easy so I probably would do that, but I would not do the insulation work again.

I just finished listening to this Amp Hour podcast and have to say I don't quite agree with what Dave said about the learning feature of an oven.  If the system is properly tuned and has good feedback this should not be necessary.  The function of a controller and PID loop is to compensate for different thermal masses to a certain extent and other variables as well. 

I have been using a spare PCB with a TC next to the target board(s) all sitting on the same copper sheet.  This gives feedback levels accurate to +/- 10C which is not awesome but I can easily compensate by watching the actual temperature at which the solder melts.  I've just decided I may have done better to connect the TC directly to the copper sheet.  This puts it one thermal boundary closer to the target board.

I've done a lot of boards in my oven from about 2 square inches up to around 8 square inches, 2 boards at the same time and have had good results each time so far as I can tell...knock on wood this continues ;)
 

Offline Falcon69

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Re: Reflow oven - free unsolicited advice!
« Reply #5 on: October 24, 2014, 11:40:41 pm »
mjkuwp,

I have to manual program temperature and ON state at that temp for how many seconds.  I closely monitor it with the data logger.  It holds the setting, thank god. If it reset every time I unplugged the machine, I would cry. LOL

The site I got the info from for programming the PID controller was wrong for my oven.  I did not realize it until I started melting Headphone jacks. LOL.  Lesson learned.  That's when I bought the Data Logger and programmed it that way to get the correct temp settings.  You have to have some type of an external temperature monitor, because just programming the PID for the set points will not work. Each oven is different and heats up differently.

So far, I too have had good results.  The grill in the center oven doesn't seem to be causing any cold spots, but I might start hanging the PCB so that it 'floats' in the middle of the oven instead of sitting on the rack.  See if it is any better.  I can always use the mounting holes of the PCB to do this.

Thanks for the comment on the build. Your is pretty nice to, much cheaper built then mine I think, since I bought the data Logger and PID. I had to use a jig saw with a metal cutting blade to cut away the steel so that I could fit the new front plate, PID, and switches.  That, with the SSR, is a very tight fit inside.  Almost too tight. But it all fit together nicely.  I even managed to keep the stock indication light.
 

Offline mjkuwp

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Re: Reflow oven - free unsolicited advice!
« Reply #6 on: October 25, 2014, 04:25:54 pm »
.... The grill in the center oven doesn't seem to be causing any cold spots, but I might start hanging the PCB so that it 'floats' in the middle of the oven instead of sitting on the rack.  See if it is any better.  I can always use the mounting holes of the PCB to do this.
...

I agree.

I did a bunch of boards just sitting on the metal rack and also did not notice any problems.  You can raise your boards up and this is better than sitting on the rack.   The blackened copper sheet that I added was more about increasing the potential heating rate and more important to provide better temperature feedback and allow multiple boards to run at the same time.  btw, I cannot run thru-hole parts or two-sided.  Everything has to be surface mount.

I originally was soldering the thermocouple to the target board but that is a huge pain.  It worked perfectly of course because the feedback was directly from the board itself.  The feedback and actual solder melt correlated perfectly.

With the copper sheet I can throw several boards in at the same time and count on the copper to distribute the heat.  The feedback comes from an extra bare board that sits on the copper.  As I mentioned previously I think I will change that thermocouple to be directly on the copper.  I should be able to just throw boards on the copper then and hit go!

Another goal i will have is to plot the P, I and D (when it is used) terms on my plot and share those.  I think it will be good for education to see if the oven command is a certain level - why is it there.  When I tuned the oven I watched these terms in the log pane of electric imp and also plotted them in Excel.  Later I added the highcharts plot but didn't think to plot these terms because I was past that part of the project.

I think the ability to plot the relatively contribution of the P,I and D terms would be a nice feature for the  Zallus (Kickstarter) controller if it is not already part of it.
 

Offline engineertype

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Re: Reflow oven - free unsolicited advice!
« Reply #7 on: November 05, 2014, 07:02:29 am »
Copper is a good idea , as is aluminum.  The worst is actually steel which has very poor thermal transfer qualities.

Here's a build guide using a small Black and Decker toaster oven:
http://whizoo.com/reflowoven
 


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