Miniature Reflow Oven... New Photo Added 09/19/20: "A Little Perspective" :)
idbruce
Posts: 6,197
With all this thinking and talking that I have been doing about these circuit overlay modules, I want to build a miniature reflow oven to make these boards 
Since making the hot wire foam cutting bows, I have become a lot more knowledgable about resistance wire and aluminum brazing. With this new knowledge and a little PWM of a small fan, I think I can build a miniature convection oven
Since making the hot wire foam cutting bows, I have become a lot more knowledgable about resistance wire and aluminum brazing. With this new knowledge and a little PWM of a small fan, I think I can build a miniature convection oven
Comments
I must say, go for it! But remember to finish it! 😂 I’m glad to know I’m not alone with this!
Here is one project that truly needs finishing, because I have a fair investment of time and money wrapped up in it:
forums.parallax.com/discussion/166498/ldi-pcbs-and-the-propeller-new-photos-uploaded-02-19-2019-8-05-pm
It seems that I have almost everything I need, except for a small four wire fan and a new Propeller Project Board
Temptations, temptations....... Oh those temptations
Sadly - for me - this does not always corelate to the reality, I seem to have more and more unfinished projects, while starting new ones. Sometimes I have good excuses 'It is to hot or windy to finish the clear coat on the Mercedes SL' so lets start a vegetable garden. Shoot the watering needs time so I need to build some watering system leading me back to the Propeller and soon the car is full of dust and not clear coated at all.
It is a vicious thing and one needs discipline to stay on top. I have started to set myself deadlines for various projects, written down. It helps a little bit.
Enjoy!
Mike
I believe the drawing tells a lot of the story, but it does not show any air circulation or temperature monitoring. I have several thermistors that I will be poking through the square tubling, and as for air circulation, I believe I will have to experiment with that a bit.
The drawer frame will be covered with a stainless steel mesh, which will allow the heat to flow freely, and a small PCB will set on this mesh. This small oven should be able to accommodate the circuit overlay boards that I have been discussing elsewhere.
With a 5VDC, 4A, AC adapter, I should be able to heat up the Nichrome C wire to 800 F, which should be more than substantial enough to heat up the inside of a small section of tubing to 423 F, but I could be wrong
EDIT: And if 800 F won't heat it up well enough, I will just have to find a little larger AC adapter to crank up the heat
Maybe an ATX computer supply could be used
I will only have to heat up 9~12 cubic inches of space and the heat within that small space can be controlled with ventilation. Toaster ovens certainly have a lot more space to heat up, therefore they need larger heating elements and thus more wattage.
Jeff Haas
I don't want to convert a toaster oven. I want to build my own design.
EDIT:
Correction, that is 9~27 cubic inches of space
I am aware of this, please refer to the link in Post #2. Like I said, I can always increase the size of the power supply or upgrade the size of the wire. I believe the maximum temp for Nichrome is around 1650 F. If I provide support for the wire, I can achieve that temperature rather quickly with the right power supply. With 1650 F at my disposal, I do not believe I will have any trouble keeping up with the temperature rises, but the cooling period may require a bit of experimentation.
You wouldn't fill a swimming pool with a Solo cup which may be the same type of scenario of your Nichrome wire... The cross sectional surface area of your wire is pretty small compared to a standard reflow heater element. In the same given space the wire might not be adequate for what you want to accomplish.
Yea, maybe not, but I think it will. I think a lot of it will depend on the on how tight the the chamber is sealed, until cooling is necessary.
1/4" X 3/4" X 3"
Now I need to drill (4) four holes through it and hope I do not break it in the process
As previously discussed, I believe most of my troubles will come from the cooling period. Until I can figure out exactly how much venting I will need for cooling, I think I will experiment with an adjustable vent, such as those found on charcoal grills.
I have been putting a lot of thought into this comment. To increase my likelihood of success, I am now changing my plan a bit.
Instead of using a 5VDC, 4A AC adapter as the power supply, I will now be using a 12VDC, 4A AC adapter as the power supply. Additionally, instead of using a formed 11" piece of 23 gauge Nichrome wire, as the heating element, I will now be coiling a 26" piece of 23 gauge Nichrome wire. I may not need it, but it is better to have excess than not enough. I can always fine tune it later, if this arrangement works.
I posted this link, some time ago
There is a whole lot of truth, some of it painful, in that link.
With my hands cupped over the element, I must say that the heat built up rather quickly. However, I must admit that I am glad that I responded to the input and upgraded the element and the power source. Considering that the heating chamber will only be approximately 21.3 Cubic Inches, I think it may just get hot enough and quickly enough, but the cooling still worries me.
However, it may also be just an excercise in futility
During a little research, I found that common toaster ovens have a 0.6 ~0.8 Cubic Foot capacity. If I convert my approximate 21.3 Cubic Inch heating chamber to Cubic Foot, we end up with an approximate ......
21.3 cubic inch = 0.01233 cubic foot
So if we now compare this 0.01233 cubic foot to the 0.6 or 0.8 cubic foot of a common toaster oven, we can clearly see that I have a much, much smaller area to heat and cool
This equates to an oven which is approximately 1/49TH the size of a 0.6 cubic foot toaster oven and approximately 1/65TH the size of a 0.8 cubic foot toaster oven
Why does the cooling worry you? Can't you just open a vent to speed cooling?
Previously, I haven't had anything to add but I've been following along with interest. Thanks for sharing your progress with us.
The cooling worries me because I want to braze the top and bottom ventilation plates to the square tubing, and because I do not have a lot of 1/8" thick plate.
My thoughts are to have indentical, small ventilation holes in both the top and bottom plate, which will always be open, even during the heating process (could be wrong
This is my current thoughts, but may not be the outcome
A tiny oven will not be as effective as a larger one because the idea of a reflow oven is to circulate the air so that it is even all around the interior, and therefore heat the components and pcb consistently. In a reflow oven, depending on the model and construction, there will always be hot spots and cold spots.
In my cheaper commercial oven, I find that I only ever use the central part of the oven. There is heating from above and below the board(s) which sit on a rack filled with slots to allow the air to be circulated around the whole oven. When I do double sided boards, for the second side I place the boards on an old board wrapped in alfoil. This prevents the soldered underside components (solder) from reaching melting point and so the parts remain on the pcb. In commercial assembly houses, the parts on the first side are glued down with dots of glue (an extra process) when placing the components with the pick-n-place machine.
In my oven, there are 40 temperature steps that I have profiled to achieve the required temperature gradients. These steps have variable times that I have preset. Cooling is fan forced to an external vent. My profile is a little slower than the preferred graph, but it doesn't exceed the requirements. Opening the drawer slightly after reaching the down slope aids the cooling process but I don't usually bother.
Just my 2c