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Building a reflow oven. Have questions.

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MarkS:
Doing research on this topic, it seems fairly simple. However, several questions have arisen that I cannot find answers to, so I hope someone here can help.

1.) While I understand what "zero crossing" is, I'm not sure why switching on zero crossing is important. One project I've seen had its own zero crossing detector circuit, but I can buy an SSR with built in zero crossing detection. Why?

2.) Is there a recommended location within the oven to place the board? My naive inclination is the center of the oven.

3.) Same as #2, but for the thermocouple. Should it be installed from the top or back? Centered or to the side? Does it matter?

Thanks in advance,
Mark

mindcrime:

--- Quote from: MarkS on June 15, 2021, 03:29:29 pm ---Doing research on this topic, it seems fairly simple. However, several questions have arisen that I cannot find answers to, so I hope someone here can help.

1.) While I understand what "zero crossing" is, I'm not sure why switching on zero crossing is important. One project I've seen had its own zero crossing detector circuit, but I can buy an SSR with built in zero crossing detection. Why?


--- End quote ---

Maybe consult this? http://www.crydom.com/en/tech/newsletters/solid%20statements%20-%20ssrs%20switching%20types.pdf

In particular, note this bit:

Zero-crossing relays are ideally suited for most commercial and industrial loads, such as resistive heating elements, lamps and ballasts, and any other load with low initial impedance or capacitive
characteristics.



--- Quote ---2.) Is there a recommended location within the oven to place the board? My naive inclination is the center of the oven.

--- End quote ---


To some extent the answer is "it depends". Ideally there would be no thermal gradients inside the cooking chamber, and the answer would be "it doesn't matter". In practice, there may be "hot" and "cool" spots in your oven. If so, you may need to tweak the board position to be in an ideal spot. If that comes up, once you identify the "sweet spot" you'd probably want to be consistent with your positioning. Thoroughly insulating the inside the oven with reflective insulation that reflects IR radiation may help smooth out any temperature gradients.


--- Quote ---3.) Same as #2, but for the thermocouple. Should it be installed from the top or back? Centered or to the side? Does it matter?

--- End quote ---

One thing somebody pointed out to me, is that you really want to track "board temperature" not just "air temperature in the chamber". They will differ, although I'm not sure by how much. Anyway, to get a simulation of your "board temperature" without actually taping the thermocouple to your board, you can take a small piece of scrap PCB material, tape the thermocouple to it with kapton tape, and place it as near as possible to the board you're actually soldering. This is the approach I've been using and so far it seems to work reasonably well.

Also, FWIW, there's been a lot of recent discussion around building reflow ovens in this thread. If you haven't seen it yet, you might find some useful thoughts by browsing through there.

MarkS:
You have been EXTREMELY helpful! Thank you!

mindcrime:

--- Quote from: MarkS on June 15, 2021, 04:52:38 pm ---You have been EXTREMELY helpful! Thank you!

--- End quote ---

No problem. To be fair though, to the extent that I know anything about this stuff, it's mostly due to the generous help I've received from other EEVBlog users, in that other thread. Folks like @fourfathom, @coppercone2, @beanflying, @wizard69 and many others have been critical in helping me figure out how to get something useful put together.

jmelson:

--- Quote from: MarkS on June 15, 2021, 03:29:29 pm ---
2.) Is there a recommended location within the oven to place the board? My naive inclination is the center of the oven.

3.) Same as #2, but for the thermocouple. Should it be installed from the top or back? Centered or to the side? Does it matter?

--- End quote ---
Well, in general center the board between multiple heating elements.  Closer to the element gets hotter.

I have never gotten a thermocouple in air to work.  I've burned several boards.  What I did was get some micro-sized
thermocouple extension wire, stripped and twisted the ends and poked it into a plated through hole in one of the boards.  That way, the thermcouple reads the actual board temperature.

I also use a ramp and soak temperature controller I got on eBay.  These allow you to program linear temperature ramps and temperature holds.  I ramp to 180 C for a couple minutes, hold at 180 for one minute, then ramp to 230 C in a couple minutes, hold for one minute and cool to room temp.

I use this system to reflow from one big board up to 8 small ones at a time, and have done over 2000 boards with it.
I got a large GE toaster oven with 4 heating elements.  Two are above the rack, running left to right, toward the front and the back.  Two more, same arrangement, below the rack.

Jon

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