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Another reflow oven build. What quartz heating element locations are best.

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clytle374:
So I just threw out my Goodwill toaster oven as it wasn't suitable.  I got 2 $20 ones from walmart, quartz elements and 1050W.  I was going to install all 4 elements in one oven, ovens are cheaper than just buying the elements.  :-//

I have 20A (I live in a 120V part of the world)outlets on my bench, so I can probably run all 4 of the elements at once(technically, not properly, but PWM can solve that).  At least for a short time.  Being that the elements are both centered from to back, one on the top and one on the bottom, I am going to have to move them to add more.  I cracked the very end off one of the quartz tubes and then realized I can cut them down with a diamond wheel.  So I decided to build a insert for the oven with thin sheet metal.  This will allow much better insulation, reduced volume, and raw metal without holes for the element mounting.

Being that I keep reading about direct infrared heat causing issues I'm wondering if having one element on the top and 3 on the bottom is better?  Maybe all 4 on the bottom and move the rack to the top as heat rises?  Also can't find any good references as to if I want the PCB closer to the top elements or the bottom ones?  Another reason I would like to put all the elements in the bottom is that aluminum will spread the heat faster with less thermal inertia than steel,  but I think the quarts elements might melt the aluminum so if I put all the elements on the bottom I can make the bottom out of steel and the rest out of aluminum. 

Any advice?  Thanks

RoGeorge:
Most PCBs these days are very small.  Unless you are working with very big ones, maybe use something smaller:
https://hackaday.com/2017/07/13/a-bright-idea-for-reflow-soldering/

I've tried a kitchen oven once, mine was underpowered.  Had to wrap its inside with Al film, and stop its fan.  It was about 2kW, 2 elements up, 2 down, yet it was not able to do steep enough temperature slopes.  Being rather big (the size of a plate) it was also cooling too slow, so had to juggle with hand opening the door.

Overall, didn't work very well, and the plan was to use something smaller next time, which I never did since.

Few random example that popped in the search:
https://youtu.be/6bxpy1Ret6Y
https://youtu.be/C7blZigaaaA

After all, it's a hot plate.  Pretty much everything works, even a pan with sand heated on a stove.  :)
https://hackaday.com/2019/06/05/solder-smds-with-a-pan-o-sand/

clytle374:

--- Quote from: RoGeorge on February 22, 2024, 08:24:31 am ---Most PCBs these days are very small.  Unless you are working with very big ones, maybe use something smaller:
https://hackaday.com/2017/07/13/a-bright-idea-for-reflow-soldering/

I've tried a kitchen oven once, mine was underpowered.  Had to wrap its inside with Al film, and stop its fan.  It was about 2kW, 2 elements up, 2 down, yet it was not able to do steep enough temperature slopes.  Being rather big (the size of a plate) it was also cooling too slow, so had to juggle with hand opening the door.

Overall, didn't work very well, and the plan was to use something smaller next time, which I never did since.

Few random example that popped in the search:
https://youtu.be/6bxpy1Ret6Y
https://youtu.be/C7blZigaaaA

After all, it's a hot plate.  Pretty much everything works, even a pan with sand heated on a stove.  :)
https://hackaday.com/2019/06/05/solder-smds-with-a-pan-o-sand/

--- End quote ---

Thanks for the advice.  I almost went with the hotplate method.  The reason I asked about the elements on top is due to reports of top heating damaging some components, and removing them would allow the total volume to be smaller. I'm wondering if the lamp method works because it doesn't put out as much infrared. It is possible that I have made a mistake in choosing this design, but I'm too far into it to scrap the idea without giving it a shot.  Heating up shouldn't be an issue with 2Kw of quartz heaters.  Cooling will be just opening the door with a servo. 

RoGeorge:
Not saying your choice is wrong.  There are plenty of good working reflow ovens like the one you want to build.  Only saying 2kW was not enough for mine.

Try a test first (without taking the time to modify anything), turn the oven full on, with nothing but a thermocouple and an empty PCB inside.  Use a DMM set to measure the temperature with its own thermocouple.  See if the oven can heat fast enough.

Another thing, there's no need to PWM a 2kW AC load, because thermal inertia is very big.  ON/OFF control will be enough.

I've used a triac + optodiac (optotriac) with zero crossing detection.  Don't skip on the optical isolation.  I've used MOC3063-M and BT139-600E connected like in the typical application shown in the MOC3063 datasheet (Fig.10).

Attached are the temperature charts measured inside my oven, interior was about 20x30cm (~8x12inch or so).  These temperatures were measured with the oven's door closed, 2kW total, all 4 heaters full on, with and without Aluminium foil inside.  The green trace (after its first peak) shows cooling with the door closed, then with the door open (then, after some cooling, it was powered again a few minutes later - the last part is irrelevant).

MarkT:
I use only two elements on top, as I don't want bottom-side components falling off (not sure if this is really an issue), but I only do small boards.  Two elements makes for a fairly flat spatial distribution of temperature as they are spaced about the same as the element-to-board spacing.

With a mix of large and small components I use a low temperature pre-soak first to bring the large one's up to similar temperature.

I've noticed flat-flex connector locking tabs are most sensitive to radiant heat issues.

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