Jul 11, 2017

Halogen floodlight SMT reflow

This project was waiting on a box fot two years, until I had to solder 100 PCBs. After some research on SMD/SMT reflow oven hacking, I was confident to build my own reflow device with an halogen floodlight.

First test was to check the speed of the temperature rise inside a standard halogen floodlight. Reflow soldering temperature curves are quite demanding, and some adapted ovens can't reach the degrees-per-second speed of the ramp-up stages of these curves.

I bought the spotlight, put an aluminium sheet covering the inside surface of the protective glass (to reduce heat loss), and measured the temperature rise with a multimeter's thermometer.... and wow! More than 5ºC/s... and I better turned the thing off after reaching 300ºC and still rising quickly.
So the floodlight was able to fulfill the needs.
Next step was a temperature controller, that is, the device that keeps the temperature as in a specified reflow curve profile in each moment. Having an AC heater element (in my case the halogen bulb), the two basic components in a AC temperature controller are:
- a temperature sensor
- an AC dimmer (connected to the heater element)

The most common temperature sensors are:

- Thermocoulples
- Thermistors
- RTD's

You can read more about their characteristics in this article.

AC dimmers contain basicly:

- A zero crossing detection circuit
- An AC commutation circuit

You can read more about AC dimmers here.
And more info about detection AC zero cross here.

I decided to adapt 0xPIT's reflow Oven Controller to my needs. This controller uses a Thermocoulples as temperature sensor, a bidirectional optocoupler as zero cross detector, and a Non Zero Cross SSR (Solid State Relay) as AC commutation device (instead of the combination of an Optoisolator Triac and a Triac ).

Then made a new board design, using as dimmer circuit the combination of an Optoisolator Triac and a Triac, wich are cheaper more replaceable and easier to get than SSR's. The board can be made at home on a single side PCB with toner transfer, and in the reflow cicle the triac's temperature stayed well below 60ºC without heatsink, being the load a 500W halogen lamp.

The sources are available in my github: https://github.com/dasaki/nanoReflowController

If your K-type thermocouple comes enclosed in a protective metal cylinder, make sure you remove it to get a quick response to temperature changes.

With the controller working, I just drilled the floodlight enclosure to fit the thermocouple. I used a piece of 6mm OD / 4mm ID teflon pipe to add a bit of thermal insulation between the probe and the case's metal.

0xPIT's original code (which I heavily modified for this project) includes PID to match the required temperature curve. The PID parameters have to be tuned for each hardware setup, changes in the power/size of the floodlight, position of the probe, etc. However, you should be conservative, as the geometry of the heat radiation from the bulb to the board changes a lot with distance, so the center of the board you are cooking will be hotter than the thermocouple reading.

That means you should keep the peak temperatures at the minimum usable for the reflow. To give you some practical numbers, I'm using a peak temperature of 200ºC, so the temperature at the center of the board (shortest distance to the lamp) reached around 220ºC.

The lowest price I've found for the components are: 

500W halogen floodlight       10,00 €
TFT 1.44"                              3,37 €
rotary encoder module           0,90 €
Max 6675+probe                   3,00 €
arduino nano                         2,20 €
blank pcb (15x10cm)             1,68 €
Components on PCB (aprox)   3,00 €
jumper wires (80 units)          1,70 €
teflon pipe (1m)                     0,90 €
TOTAL (aprox.)                    26,75 €




heatsink calculation

pcb trace withs calculator:


  1. looks like a lotz of fun building this

  2. That's Ingenious! I bow to you in respect!

  3. may want to consider posting this on hackaday.io, I'm sure a lot of people would be interested. I've only seen one project which was similar, which used one of those glass cooking appliances which also used a lamp to cook the food.

    i don't really know anything about halogen lamps - is there anything wrong operating them this way? Like would it shorten the lifespan significantly

    1. Already there :)

      It will probably shorten the lifespan of the bulb indeed

  4. Hey your process for applying solder paste looks really neat, can you detail it a little more ?

    1. I made the stencil for the solder paste with the aluminium of a soda can. Had to sand off the coatings/paint of the aluminium before etching the stencil with the same toner transfer technique used to etch PCBs.

    2. Ah cool, thanks. And is it just stock cardboard that holds everything in place ? What are you using to squeegee the paste on ?

    3. Yes, a thin cardboard to keep the board and the stencil aligned.
      My friend Carlos (the one in the video) used a pointy wood stick to apply the paste. However, the most common way to do it is with a "solder paste squeegee".

    4. I made the stencil in a similar way as in this video: