If you’re trying to build your own reflow oven there are a couple of key considerations to think about before you put any money down.
The first is the size of the oven itself. Most PCBs do not take up much space so your oven doesn’t need to have a large volume – you’ll only end up expending large amounts of energy to heat up the air inside.
The second is the total power of the heating elements. The greater the power the faster the rise times you can achieve. Reflow soldering works by requiring the PCB, components and solder paste to go through a thermal profile where temperature rates of 1-3°C/s are required. With a greater power density (compared to the volume of the oven) you are more likely to achieve this.
The third consideration is the type of heating elements your oven uses. Both the popular EZ Make Oven and the Controleo3 Reflow Oven use short/medium wave infrared heaters. These are quite popular in US mini ovens but are actually much harder to find here in the UK. They allow for very rapid heating but as they rely on infrared radiation will cause darker components to heat up faster than lighter ones which makes controlling the whole reflow process difficult. In the UK mini ovens are more likely to be fitted with stainless steel heating elements relying on conduction to get the heat into the oven. This is slower but you get an even heat throughout.
Curtains please…
After doing a bit of research including on the EEVblog forums I settled on the Xunda 12L 1200W which I bought from this seller on their eBay site. It was also the same oven used by DJ Legion and featured on Hackaday a while back.
I am trying to avoid making too many modifications to my oven and am following the design philosophy of Adafruit’s EZ Make Oven where a separate controllable outlet is used as a PWM to adjust the temperature of the oven. For comparison the Controleo3 requires you to make significant modifications to your oven which as a first-timer found a little daunting.
The one thing I wasn’t sure about was whether the temperature dial on the oven would kick in and interfere with my outlet’s ability to directly switch the heating elements on and off. To test this I set the temperature to the maximum of 230°C and left the oven to run. I measured the temperature using the Python script I’d created previously and on the video left the reflow profile visible to give an idea of what I need my oven to achieve.
After 100s you can see the rate of temperature increase starts to slow. This coincided with an audible click coming from the temperature dial which then switched in and out for the remaining duration and causes the oscillations in the temperature which become more apparent the closer to 300s you get.
Unfortunately this was not what I wanted and I knew I was going to have to take the oven apart to disable the temperature control.
A dressing down
The Xunda oven is a cheap and cheerful model and taking it apart was as straightforward as undoing a handful of self-tapping screws to release the cover which gives you access to everything inside.
The dial at the top controls the temperature, the one in the middle controls which elements are used – there are two at the top and two at the bottom and the dial at the bottom is a timer – but fortunately there’s a ‘Stay On’ position.
On the other side there a simple spring mechanism for the glass door which has a notch to lock the door in a slightly open position.
Each element is rated at 115V – 300W. The top two and bottom two are connected in series on independent circuits to work with the 230V mains voltage used in the UK and produce a combined power output of 1200W.
The temperature control mechanism seems to operate on a similar principle to that of a bimetallic strip however instead of using two dissimilar metals there is a slightly longer piece that is bent into a spring so that it expands to a greater length when heated and trips the power. The temperature at which this happens is controlled by the dial which presses a plunger into the spring to increase its tension.
Taming the beast
To disable this I used a high-temperature cable tie to keep the contacts closed. A piece of wire with the insulation removed would work equally well.
Putting everything back together and running the oven again I get the temperature profile below. When the oven hit 250°C I pulled the plug to prevent it from overheating.
After a slow start the rate of temperature increase is close to that of the solder profile so the Xunda should perform well as my reflow oven.
