Reflow oven (aka Toaster oven) modification

I like the idea of moving the bottom heaters to the top - duly noted and added to list :slight_smile:

I was wondering about a fan as well
but concerned that blasting the fan with hot air at around 250 degrees might make it a very short-lived fan

so wondered about blowing air in rather than sucking it out
still need to make sure the fixing does get too hot else the fan will still melt and slide off the oven :slight_smile:

perhaps fit the fan in the bottom
with both heaters (now) in the top the fan will be a long way from the heat source

for lighting you might consider white LEDs
again mounted in the base to be as far from heat as possible

either that or cut a window in the back and shine the light through that
mounting the window will be fun

I use a bright torch to watch through the window - its a 12W LED in fact! Intense point source lighting helps bring out
the shiny beads of solder as they melt. Of course an LED isn't compatible with high temperature, but a small 12V halogen bulb
would be, there are holders for them with glass-fibre insulated wires too.

Some pictures of the conversion to top-side elements are in posting #44 here: http://forums.parallax.com/showthread.php/130196-How-I-do-SMT-soldering-in-toaster-oven/page3?highlight=toaster%20oven

I have tried just putting a fan in front of the oven to help the entire structure cool down faster.

Becareful with when you open the door or any other rapid cool-down method. Components, especially ceramic caps, are very sensitive to CTE mismatches. You need to make sure you are not cooling down too quickly.

I've watched as people open an oven door right after reflow occurs. That's a recipe for intermittent issues.

Great ideas y'all. Keep em coming.

Of course an LED isn't compatible with high temperature, but a small 12V halogen bulb
would be, there are holders for them with glass-fibre insulated wires too.

I was just at the hardware store the other day looking at these MR16 lamps... I know they get real hot, so putting them inside the oven is probably no big deal for these bulbs.

I'm thinking mount these MR16 lamps behind the oven (beside the exhaust fan), and then have some L-shaped aluminum in front of the lamp so I'm not staring directly in to the bulb... it will be diffused lighting inside the oven. I can probably run these bulbs using a 5V adapter so the intensity isn't too much (or build an adjustable voltage regulator with a pot.)

oven.jpg

Why not just use fan thats on the oven. With the right timing it should work fine just figure when to turn off the heat but leave the fan on

I wish it were that easy. The built-in fan only turns on when heat is also on. There is no separate switch for it.

It's much easier to modify an oven with a built in fan than to try and add one on.

No need for PID and all that complexity.

You may have concluded this... but in my opinion, the PID would serve you well and could eliminate the need for a fan altogether if you do it right.

here's what I am achieving with simplistic control

algorithm is
if temp < target then
turn on heater
else
turn off heater

the ramp up slopes are less than I would like
the cool down is even slower
I open the door a smidge as soon as reflow is done
the sudden drop at the end is door fully open

profile.jpg

next step is to add a fan in the floor
a) to aid cooldown
b) might improve ramp-up as well!

I notice that th heaters are on full time during ramp up
is my oven under powered?
or is the profile "close enough for government work"?

next step is to add a fan in the floor
a) to aid cooldown
b) might improve ramp-up as well!

My understanding of a convection fan (during heating) is it circulates the hot air more evenly (so there are no hot spots and shadows).
I've read to achieve a fast ramp up (200C in 2 minutes), insulating the glass door, or you need to have more powerful heating element.

(my oven has 4 heaters, and can achieve the 200C in 3 minutes from a totally cold oven, for the first batch)

the cool down is even slower

Yup... this is my problem.
Even opening the door isn't doing it for me....
I have a big oven, 16cu.ft., big enough for a 12" pizza), so more heat trapped inside too.

For others using a small bread toaster oven, opening the door is sufficient for them. I think I really need an exhaust fan, or something.

mine is 9 litre capacity with 1 kW of heating (4 elements 2 above, 2 below)
glass door is double skinned
but the oven body is just sheet metal
I've been running with its "triusers" off while I get things working
maybe putting the outer cover on will help :slight_smile:

I use a 4 element oven, 2 top & 2 bottom, 1500W, observations with k-type thermocouple to digitlal multimeter are that it heats up fast enough.
skyjumper found the same a while ago - 3 elements not quite enough.

CrossRoads:
I use a 4 element oven, 2 top & 2 bottom, 1500W, observations with k-type thermocouple to digitlal multimeter are that it heats up fast enough.
skyjumper found the same a while ago - 3 elements not quite enough.

+1. I agree.

Using 1500 watts here too.

4yi, here is the temperature curve of my reflow oven (850 W with 2 elements, k-type thermocouple; PID )

reflow curve.pdf (38.7 KB)

My cool-down curve is achieved by:

1 switch off

2 open the door

3 carefully slide the wire mesh shelf out as far as I can (don't want to disturb
components while solder still molten)

4 waft a piece of card gently to produce a draught over the board for a while.

Takes about 1 minute for the board to be cool enough to handle (just).

If using a fan it would need to blow otherwise it will melt itself, and will need to be
quite small (to prevent components being blown about. A short steel tube to keep it
mounted remotely from the heat might be wise.

Simply opening the door is probably causing a faster cooldown than 6°C/second, which is a excellent way to damage components. Especially ceramic caps. And the nice thing about cracked ceramics, they rarely show signs of failure right away.

You really need to allow the board to come down from the peak soldering temperature before exposing it to room temperature air.

I doubt it, the oven stays hot for a long time, its walls are radiating heat efficiently and
until the board is outside to some extent you won't get much cooling. The board itself holds a
lot of heat and limits the rate of cooling of the components. Even fanning a board for
a minute leaves it hot enough to burn fingers.

A couple of observations...

I happen to use a heavily-modified toaster oven with a rocket scream PID controller. As best as I can tell, the unit ramps up as desired, i.e. it follows the reflow pattern recommended for my Kester 'green' solder paste (i.e. no Lead). The only part that is likely 'non-compliant' so far is the cooldown phase, i.e. I have to figure out how far to open the door so that the temperature drops as intended, hopefully without cracking a ceramic cap. If you'd like to see some pictures, see this thread.

As for the construction process, I disassembled the oven and removed all controls. A ebay-sourced SCR (2x20A) is perfect for this formerly-'Black and Decker' $30 toaster oven since the four quartz-tubes are OEM-limited by a fat diode. The SCR achieves the same thing, but with control and I used the second channel for the convection fan. On the front face, I mounted a leftover plastic enclosure that I salvaged from a Neuton mower whose motor had gone bad. It holds the Arduino 2009 and the Rocketscream shield perfectly. I stuffed the (formerly empty) voids between the inner and outer oven cavity with 2000*F rated insulation.

Inside, I further reduced the mass 'visible' to the quartz lamps by stuffing the same insulation between the lamps, filling the rear of the cavity where no PCB would ever go, etc. The unit still gets hot during operation, but nowhere as hot as it would get without the benefit of insulation. I even insulated the door window, leaving just a little peep-hole to limit heat loss through the glass as well. I presume this is why the oven has no issues ramping up as needed - there is 'spare' capacity and the quartz elements are not on 100% of the time. At full power, it draws around 1340 Watts, IIRC.

If anyone in the Boston area is interested in looking at the thing or would like some leftover insulation, I'd be happy to share.

... and one more thing... does it necessarily make sense to go through all this effort when you can buy a certified reflow oven on Amazon/eBay for about $285, delivered? Consider that the materials alone (oven, insulation, heat sink, reflow shield, K-thermocouple, etc.) probably came out to about $125.

Sometimes the journey is just as much fun as the destination. :slight_smile: