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By bdring
#4606
...a little off topic...

What do you guys use to apply the paste? Are you doing it manually or via stencil?
User avatar
By sparky
#4624
We use a stencil exclusively. I tried it by hand once - I couldn't do squat.

We are working on a stencil ordering system as well. Cheap. Less than $50 a stencil. But with what we've seen from the PCB service, we don't know how to handle the problem orders. I'll post another topic about this specifically.

Phil and Jmoyer - great feedback! I dunno if a control loop will work. I don't think these resistive coils can react fast enough. The coils in our 'real' oven are IR coils that burn out over time - basically really expensive high wattage bulbs.

Yes - the AC could kill you. The original board just had 12VDC control though a screw terminal that allowed the end customer to attach to whatever relay or control unit they wanted to use. This can also be done with the latest revision PCB - I'm thinking the user could just not mount the relay we provide, and solder into the connection holes.

If you read through some of the toaster tutorials, most gloss over the relay connection. It's the hardest and most screwy part of the project. This is not for the faint of heart. The control board is more for the data aq. and the decision making. How you physically turn on/off the oven is still sorta in the air.

The im-precision of the voltage regulator carries through to the thermocouple amplifier and finally to the ADC on the PIC. As long as all line are steady (lots of decoupling caps) we are ok. If the VDD line is 4.95V or 5.12V, doesn't matter as long as it doesn't vary +/-0.5V (I've seen +/-2V on some wire wrap boards...).

Time for a silly question - How is a triac better than a relay? Solid state relays would be fine, but for one that size, it could easily be $30 a pop.

-Nathan
By jmoyer
#4627
sparky wrote: The im-precision of the voltage regulator carries through to the thermocouple amplifier and finally to the ADC on the PIC. As long as all line are steady (lots of decoupling caps) we are ok. If the VDD line is 4.95V or 5.12V, doesn't matter...
Are you sure?
The AD595 supply can be as high as 30V, so it's output accuracy does not depend on the accuracy of the supply. The A/D reference voltage on the other hand directly affects the way the 10mv/degree output of the 595 is interpreted as a temperature. If it was 2 V low as you have seen, then a 3 volt input (300 degrees C) would be interpreted as 500 degrees!

The Maxim 6675 eliminates this problem. You get a digital temperature value directly, but it uses 3 I/O pins instead of 1, and you are rather low on I/O pins. A common technique is to use a shift register to reduce the number of pins needed to drive the LCD to just 3.
By moorejl
#4629
I do my solder paste in small batches and so the stencil clean up would be a large portion of my time for the tiny boards I make. I broke down and ordered a DG10 DispenseGun from efd inc. for about $100 shipped. It takes a 10cc paste syringe and makes the application much more reliable and easy. I have built around 150 boards to date with ~15 smt components.

If I start to make larger boards, then the stencil is by far the way to go. pcb-pool also has a stencil pool that may be worth looking at.
By rglissmann
#4722
How is a triac better than a relay?
No sparks or mechanical fatigue. 1500Watts can be handled by a Q4025L5 triac driven by a MOC3011 triac driver - all for less than $4 from Jameco. No snubber is needed because the load is not inductive. See http://www.fairchildsemi.com/an/AN/AN-3003.pdf

-Randy
By Oznog
#4808
rglissmann wrote:
How is a triac better than a relay?
No sparks or mechanical fatigue. 1500Watts can be handled by a Q4025L5 triac driven by a MOC3011 triac driver - all for less than $4 from Jameco. No snubber is needed because the load is not inductive. See http://www.fairchildsemi.com/an/AN/AN-3003.pdf

-Randy
Keep in mind that a triac driving a 1500W 120V load would itself dissipate about 15 watts. This would require a decent heatsink, whereas a mechanical or solid state relay runs fairly cool. That level of dissipation may be hard to get out of an insulated tab pkg too, so the heatsink would either need an insulator pad or the heatsink would be electrically hot.

The main benefit behind a triac might be its ability to reduce the power without resorting to harsh on/off cycles. Has anyone seen a need for this?
By Philba
#4826
Oznog wrote:The main benefit behind a triac might be its ability to reduce the power without resorting to harsh on/off cycles. Has anyone seen a need for this?
I believe that using a PID control algorithm, you can maintain a much tighter control on the temperature. Harder to do this with a relay and I suspect a much shorter operating life (chattering contacts don't last as long). By the way, phase control is not really needed for this application when using a triac.

Its best to line up the possible solutions and their pros/cons plus pricing and go from there.

relay - need to handle at least 20A and I'd go for a 25 or 30A spec:
+ cheap, easy to drive, good isolation
- mechanical, subject to wear (spec at least 1M rated cycles), noisy (audible and electrical)

triac. same current needs as a relay.
+ cheap, easy to drive, good isolation (via opto driver), faster switching, phase control
- heat dissipation, electrically noisy (unless using zero crossing logic)

SSR
+ very easy to drive
- expensive

one should also add IGBTs for completeness sake but I don't know enough about them.

From surfing digikey, the relay and triac solutions could be had for under $5. SSR, is way out there in prioce. Based on the above, I'd opt for the triac approach. I'd also, as I said before, move all the AC stuff off the controller board.
By victorf
#4876
This project is of great interest to me. What is the current status of the reflow oven controller project?

What is the current status of the possibility of getting stencils from SFE?

Any enlightenment will be appreciated
By Pete-O
#4896
Check the new tutorial Nate's posted- it's a hoot. The final outcome is that hotplates are cooler (read better) than toaster ovens. It was truly impressive to watch the hotplate do its thing.

He's also been talking about offering a stencil service for a while now. You might see it in the near future.

Pete
By dpaton
#5149
I've been lurking here for a while, but looking over the existing reflow controller, and reading the tutorials, I wonder if it might be better to drive the upper elements at a different drive level than the lower elements in the reflow oven.

Given the propensity of the upper elements to heat so much faster, I would think that it would be amazingly slick to do both topside and bottom side sensing, and adjust so that the board heated evenly on both sides. SSRs are available for cheap on the surplus market (I have a pair of them I grabbed for $10 each, 20A/250V, 5V trugger), and I think a slow PWM signal could keep things under control. It might be as simple as characterizing the heating rate of the upper and lower elements, and adding a calibration constant, but I think there's real power to be had with a dual-element control. Thoughts?

And yes, I know, the first version was only just released today. I figure if I'm going to do this, I'm going to do it right, and only once ;)

-dave
By turbo2ltr
#5163
I have been using a toaster oven for SMD reflow quite successfully for about 4 years now. I've reflowed thousands (literally) of small dual sided boards. Toaster oven was mom's old one (read: free) and is much smaller than the one in the tutorial. Rememer that the bigger the oven, the more thermal inertia it will have, making it harder to control.

The first time I used it, I burned a batch of boards. I found that since the two bottom elements were placed one above the other in the center, there was a hot spot. This was easily fixed by putting a 4x6" piece of aluminum above them. Since aluminum transfers heat very well, it tends to diffuse the heat acoss it. It also acts like a buffer to keep the oven from changing temperature too fast.

I made a little stand for my boards. They are double sided (and I do both sides at the same time) so they need to be suspended. I placed a thermocouple directly under the board. BTW, all my boards are hand pasted using an electronic dispensor, a syringe and a lot of patience. But after seeing the cheap stencills here, I might be converting!

Using the 'toast' mode, I manually modulate the the toast switch to follow the reflow curve. Initially, the oven will heat up at just about the right 'preheat' rate. Once I get close to 140, I turn it off. The temp will continue to rise but will start to level off just like the profile. Once 160 is reached, I will again turn it on and spike it up to about 200. I'll turn the oven off, but the temp will continue to rise. I'll leave the door closed until 215-220 and then open it enough to allow the temp to start to drop at a gradual rate. Waite 5 minutes before trying to move the board. (I dropped a board that was still 'wet', thinking it was cool enough and all the parts came flying off the board.)

After a few runs, you learn how not to overshoot your target temp and using my scopemeter trend-plot feature, I can match the reflow profile exactly. I have thought of making something to do this automatically, but its so easy to do it myself it wasn't worth it.

Image

A couple of comments to the admin. From what I read on your site, you were having problems with the USB connectors not reflowing or burning other connectors.. If you are ramping the temp up like the profile you have here

Image

this could be your problem. The 'soak' period is important to get all the components up to just under reflow temp. You profile doesnt have a soak period. Try to let them soak at 160 for 30-60 seconds and see if it helps.

I also think that solid state would be the way to go here.

-Mike
By wiml
#5192
Not saying that t wouldn't be easier to use a solid-state relay or triac or whatever, but I think a relay would work perfectly well here. You could PWM the heaters with a period of, say, 5 seconds and the contacts would last forever. Electrical noise won't be significant. (It'd certainly be less than from a phase-controlled triac!) It'll make a click but I guess I don't see that as being a problem.

Anyway, just wanted to defend the old electromechanical technology a little. :)
User avatar
By sparky
#5199
Hi,

Oh no - the curve was just from a dry run from On to Off. No control, just data. And we've never used the toaster to reflow boards. Instead, the $2300 industrial oven has all sorts of problems - all of which preset themselves as poorly reflowed and melted connectors. The hot-skillet doesn't have any of those problems.

Congrats getting your toaster going! My brother recommended a ceramic brick (similar to your aluminum plate) but I was afraid of adding anything the would retain heat as it would dampen/smooth the rate at which the oven could be ramped/cooled.

BTW - the toaster controller kit has been posted : http://www.sparkfun.com/shop/index.php? ... itemid=428

We are waiting for the new PCBs to come in with the corrected relay footprint and a ground on the thermocouple (I gotta read the datasheet closer next time).

-Nathan
By dpaton
#5201
Nathan-

My limited experience in oven control tells me that the issue with the toaster over may be the top elements. They not only create a wicked thermal gradient inside the chamber, but they will tend to cook things like your prescious connectors cajun style because of the direct heat. Welcome to the joy of IR ovens. :D

The thing I noticed most "real" reflow ovens (read: convection ovens with inert gas atmosspheres) is that the heat is indirect. I think Mike's aluminium plate is a good start. I think that top plate with sufficient airflow above it (to make sure the upper elements don't melt down) would go a long way toward solving your cooked connector issue.

I also think that recirculating the air in the oven would help. It looks to me like the fan on the side of the toaster oven (mislabeled as an AC transformer in the photo takeapart) only pushes air through the oven, which would lead to the rather tepid slope of the DTemp/Dt curve you posted (looks like 0.8dC/sec?). Four 350W elements should be able to heat the oven cavity at something close to 1.5dC/sec without much trouble.

The final thing that comes to mind is something reflective for the door of the oven, like tinfoil, shiny side out. The oven works on IR, and 5 sides of the box are reflective, but the 6th lets it out, mica window or not (likely not, I think the windows are all pyrex derivatives these days). You'd need to keep a close eye on the top and bottom side temperatures, but with dual thermocouples that wouldn't be hard. I'm debating building a pair of controllers to run the top and bottom elements independently with the same curve. In theory, if I make them dependent on each other, they'll lock together nicely, and all will go swimmingly.


-dave
By BigRedBee
#5203
All this fancy talk is nice (and Great Fun), but I've been reflowing in a $19 Target toaster oven for months. 225 for 4 minutes, 350 for 2 and 450 for 1. Granted, no plastic connectors, but it works. Not a single 'toasted' board.

I'd like to have a computer controlled oven, but is it really necessary? I doubt it. Do I want one? YOU BET! But let's keep things in perspective. If your starting with a toaster oven......well.......there you have it :roll:
Last edited by BigRedBee on Tue May 24, 2005 7:58 am, edited 1 time in total.
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