ntroduction of SMT or Surface Mount Technology components have made it more difficult for Do-It-Yourself enthusiasts to solder these components using a soldering iron. The switch from through-hole components to SMT types had actually made hand soldering easier initially. However, with the introduction of BGA and similar packages that require blind soldering and extremely small packages that are difficult to handle manually, hand soldering with a soldering iron is now practically impossible.
Such special packages need a reflow soldering process to solder them properly to the PCB. This is easy to make with a single board computer such as the Raspberry Pi or RBPi, and a convectional hot air oven designated originally for a bakery or gastronomy purposes. The RBPi helps to make it an open-source universal reflow oven that is also web enabled and PID controlled. Another advantage of using the RBPi as a controller for this oven is it can be used also as a sophisticated pizza oven. Unfortunately, the two functions are not interchangeable, meaning you must not heat food in an oven that you have once used for reflow soldering purposes.
Apart from the RBPi, you will also need SPI-driven cold-junction thermocouple converters, for which, you can use the MAX31855 or the MAX6675, useful for K-Type thermocouples. The above ICs offer cold-junction compensation and digitize the signal from K-Type thermocouples. The data will be in a signed 12/14-bit, SPI compatible, read-only format.
You will also need Solid State Relays for the heaters and the fan, and if these are of the GPIO driven types, intermediate drivers will not be necessary for the RBPi. The heaters are best made of PWM driven MOSFETs, preferably operating at 12 VDC.
The control software runs as a Python daemon on the RBPi. An OS independent multi-user web-client offers live monitoring and remote control. The profile/curve management and slope calculator is browser-based. The software developed for demonstration is a fully functional PID controller, while incorporating a simulator.
The demonstrators have used an EKA KF412 professional hot air oven produced by Teknoeca srl from Italy. The oven uses hot air temperature transfer (convection) as against infrared, and this was considered preferable. The power consumed by the oven was 2.6 KW at 230 V, producing a maximum temperature of 300°C.
The oven chamber has adequate insulation and this increases its thermal inertia. Therefore, once the oven crosses the maximum reflow temperature, a natural cooling process does not offer quick but controlled cooldown. This is necessary to return the solder paste to its rigid state.
A radial fan mounted on the back of the oven handled the above situation. The RBPi drives this fan via a GPIO pin, and it blows fresh cold air at room temperature into the oven. This gives the RBPi total control over the heating and cooling rates of the oven, and it is possible to define a proper reflow solder profile. It is important to know that the reflow profiles for lead-free solder are different from the profile required by leaded solder.
