Newbie needs help with reflow.

Questions relating to designing PCBs

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voxmagna
Posts: 18
Joined: Mon Aug 29, 2011 5:00 pm

Re: Newbie needs help with reflow.

Post by voxmagna » Thu Jan 24, 2013 4:14 pm

Thanks, that is helpful. My toaster oven is heavily modded like yours I suspect. I added a second pair of elements of the glass IR type which give a nice cherry red glow. Wired in series they add another 1Kw and that was the only way I could get the rapid heat rise profile to the peak. I can switch the elements to top or bottom only and turn off the recirc fan. I tried that along with using lead and leadfree solders either side on some test board and nothing dropped off.

In UK stencil cutting seems to be mostly done by the pcb shops for more than your $20, then there is shipping on top. I will do some phoning about and see if print shops can do that kind of thing.

macegr
Posts: 380
Joined: Wed Feb 27, 2008 4:44 pm

Re: Newbie needs help with reflow.

Post by macegr » Thu Jan 24, 2013 4:53 pm

You would definitely want to ask about mylar stencils rather than the pro quality steel ones.

Examples:
http://www.pololu.com/catalog/product/446
http://ohararp.com/Stencils.html
http://www.pcbunlimited.com/flexstencil ... p-692.html

My toaster oven was bone stock. I achieved ramping through twiddling the thermostat knob while observing a stopwatch.

MichaelN
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Location: Australia

Re: Newbie needs help with reflow.

Post by MichaelN » Fri Jan 25, 2013 12:15 am

macegr wrote:You would definitely want to ask about mylar stencils rather than the pro quality steel ones.

Examples:
http://www.pololu.com/catalog/product/446
http://ohararp.com/Stencils.html
http://www.pcbunlimited.com/flexstencil ... p-692.html

My toaster oven was bone stock. I achieved ramping through twiddling the thermostat knob while observing a stopwatch.
The O'Hara stencils are Kapton, which is much nicer than Mylar.

voxmagna
Posts: 18
Joined: Mon Aug 29, 2011 5:00 pm

Re: Newbie needs help with reflow.

Post by voxmagna » Fri Jan 25, 2013 3:22 am

Thanks. I am in UK but will do some phoning about. I read a link from a guy that etched his own stencils from thin brass sheet. Since you have all the chemicals available I was interested in having a go, but it seems double work and I do not know if etching gives clean edges. Anybody tried it?

MichaelN
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Joined: Fri May 08, 2009 6:14 pm
Location: Australia

Re: Newbie needs help with reflow.

Post by MichaelN » Fri Jan 25, 2013 5:38 am

voxmagna wrote:I read a link from a guy that etched his own stencils from thin brass sheet. Since you have all the chemicals available I was interested in having a go, but it seems double work and I do not know if etching gives clean edges. Anybody tried it?
Yes, quite a few times. It's a real pain, and the results aren't great (even thin brass is much thicker than the copper on PCBs, and the edges tend to be pretty ragged). I only use it when I can't wait for a laser cut stencil, and I don't have very fine-pitch components.

I use K&S brand 0.005" brass sheet from the hobby store, with Press'n'Peel Blue transfer sheets on one side of the brass. You print the image onto the Press'n'Peel with a laser printer (I just use a piece big enough stuck to a sheet of paper), then iron it onto the brass sheet. The brass needs to be very clean (use steel wool or similar and alcohol to remove any oil), and you need to practice the technique to make sure it adheres to the brass without bubbles. I use the "nose" of the clothes iron to selectively apply a lot of pressure where required. Before I apply the actual pattern, I iron on another piece of Press'n'Peel (printed all black) on the underside of the brass, but you could paint it instead.

I use ammonium persulfate as an etchant, mixed about double the strength you'd use for copper PCBs (otherwise it will take too long to etch).

UNTEngineer
Posts: 274
Joined: Thu Apr 07, 2011 11:08 am

Re: Newbie needs help with reflow.

Post by UNTEngineer » Sun Jan 27, 2013 8:40 am

voxmagna wrote:Thanks, that is helpful. My toaster oven is heavily modded like yours I suspect. I added a second pair of elements of the glass IR type which give a nice cherry red glow. Wired in series they add another 1Kw and that was the only way I could get the rapid heat rise profile to the peak. I can switch the elements to top or bottom only and turn off the recirc fan.
Im curious do you have a website where you posted this build?
Working on a Smart Home Controls System. Visit my blog: http://tae09.blogspot.com/ to find out more, and to keep track with the project.

voxmagna
Posts: 18
Joined: Mon Aug 29, 2011 5:00 pm

Re: Newbie needs help with reflow.

Post by voxmagna » Sun Jan 27, 2013 10:06 am

Sorry no, but I cribbed a lot from several sites. It is not all plain sailing because these things are pretty dangerous to mess around with. I did a few things different. Some mods recommend adding more insulation but they do not really need that because you also have to think of the cool down stage if you are trying to achieve the profiles. If an oven needs insulation then its element power is too low!

I would have liked to build the Sparkfun controller board but it was dificult to get hold of in UK so I bought a SET64rs PID controller off Fleabay. This is a pretty much an all singing dancing PID temperature controller with ramp functions and proportional solid state relay output. However, it is a nightmare to understand and program. I modded my unit internally for RS422 comms and added on a USB converter to talk to it from a PC. I am using a simple and clunky software application to talk to it, but it makes setting it up a lot easier than using the front panel.

The PID controller takes K-Type thermocouples and these are small enough to be glued to a board or stuck with Kapton tape. The PID controller can store up to 16 different values and has 'autotune' That means you activate it then let the oven run to and maintain a temperature and the controller then works out and stores the best numbers for you. For ramps these 'numbers' can be slightly different at different stages so you have to spend a little time calibrating your oven. Once you arrive at a particular profile I can save the settings to a PC file and load it back in for a different solder melt, top element /bottom element running or each of 3 positions inside the oven.

I wired a solid state relay in parallel with the existing thermostat which I replaced because it was rubbish. I also replaced their over temperature bi-metal trip so my oven can get to 260 degrees C. and I can still rely on the cutout as backup. I kept the timer wired in because again, that is a backup to switch off the oven if I forget. It hasn't burned the contacts with the extra 3 amps either.

Mine is a Chinese version branded as 'Andrew James'. A Load of rubbish as far as quality is concerned and I would never allow one in my kitchen having seen how they are made. Mine has a fan and is 1380 watts native plus I added an extra 750 watts. The top and bottom elements are switchable. Many of these toaster ovens have the standard form of steel sheathed pencil element top and bottom like mine. I added the 2 X 1500W quartz glass heaters wired in series. They glow a lovely cherry red and I think put out more Infra red than the steel sheathed types, with a faster warm up time.

You need to remember that if you are trying to emulate the solder profiles you want a fast temperature rise to over 200 deg C and some way to control cooling after the peak. Opening the door works quite well but I am about to try a fan. Before doing anything with the PID controller setting ramps etc or modding the oven, I switched the oven on from cold whilst monitoring some scrap board with a probe fitted. If the temperature rise was not faster than that in the profile, then I would not have gone further. But I can get nearly 3 degrees C per minute without proportional control which is fine. Cooling is too slow even with the door open which is why I will try an exhaust fan, may be connected to one of the controller alarms. I will add these links which I copied from my resource file, but take care with the safety aspects. If I repeated my project I would look for a toaster oven with fan and quartz glass heating elements as I think you get more IR from them, but at the time they were more expensive. I see the cheap Chinese reflow ovens also use quartz glass elements.

http://www.sparkfun.com/tutorials/60
http://www.die4laser.com/toaster/index.html
http://www.temperaturecontrolwiki.com/
http://www.ebay.co.uk/sch/sis.html?_nkw ... 0736766426
http://www.mavromatic.com/2011/04/diy-b ... en-part-1/
http://www.adafruit.com/blog/2011/07/22 ... er-shield/
http://www.recontech.co.uk/index.php/th ... en?start=1
http://www.temperaturecontrolwiki.com/c ... enix_4.pdf

That should be enough to get you interested - But take care!

UNTEngineer
Posts: 274
Joined: Thu Apr 07, 2011 11:08 am

Re: Newbie needs help with reflow.

Post by UNTEngineer » Wed Sep 04, 2013 2:59 am

This sounds intriguing. I'll have to give it a try...but I fear starting an accidental fire with this...is there a safe way to go about modding these ovens?
Working on a Smart Home Controls System. Visit my blog: http://tae09.blogspot.com/ to find out more, and to keep track with the project.

voxmagna
Posts: 18
Joined: Mon Aug 29, 2011 5:00 pm

Re: Newbie needs help with reflow.

Post by voxmagna » Wed Sep 04, 2013 4:55 am

You must regard nothing as safe once you start modifying and exceeding the manufacturer design specifications. Bear in mind these are already (badly) made in China and start out iffy. Now I have seen how cheaply they are made, I would never let my wife have one in her kitchen straight out of the box. You must know what you are doing and you cannot read 'good practice' in an internet post.

If you do not want the hassle, have a look at some of the cheap Chinese IR reflow ovens now selling on Fleabay. I wanted something general purpose for boards and drying/fusing small powder coat painted items, which makes the toaster oven an ideal size.

I now have a gas boiler fan bolted to the back of the toaster oven which can exhaust through the crumb tray I always leave out. It is one thing to PID control the heating, but you also have to think about cooling. You get about the right cooling curve by just opening the door front, but I preferred something more automated - hence a large fan mounted on the rear panel.

It is very important that whatever you do, there is a failsafe over temperature device fitted operating independently from any controls you add or modify and there is an upstream RCD.

UNTEngineer
Posts: 274
Joined: Thu Apr 07, 2011 11:08 am

Re: Newbie needs help with reflow.

Post by UNTEngineer » Sat Sep 21, 2013 10:12 pm

Sorry for the delayed response...

Thanks for the suggestions, I'll look into safe alternatives. Playing with AC power switching and ovens sounds way over my experience level....I will take your warnings to heart.
Working on a Smart Home Controls System. Visit my blog: http://tae09.blogspot.com/ to find out more, and to keep track with the project.

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