More and more semiconductors are produced in packages (the chip's case) which have become impossible to hand solder with a solder iron. The switch from through-hole (THT) to surface mount technology (SMT) has actually made hand soldering much easier (although still many believe the opposite to be true) but now we are faced with BGA and other packaging forms which only have contacts on the bottom of the chip.

In order to be able to work with these chips, a reflow solder process is needed. So we've built our own DIY reflow oven, using a Raspberry Pi as a universal, web enabled, PID Reflow Oven Controller driving a convectional hot air oven originally designated for gastronomy/bakery purposes. Since it's all open-source you may also use it to build the world's most sophisticated pizza oven controller and experiment with temperature curves for fresh/frozen pizza :)

• Raspberry Pi
• Supported SPI driven Cold-Junction K-Type Thermocouple converter:
  • MAX31855
  • MAX6675
• GPIO driven Solid State Relays (230V heating/fan)
• PWM driven MOSFET (12V cooling)
• Python control daemon (running on the Pi)
• HTML5/Websocket OS independent multi user web-client
  • Live Monitoring & control
  • Browser based Profile/Curve Management + Slope Calculator

At this point (although still only a PoC-Demonstrator) the software is a fully functional PID controller and even incorporates a simulator, so you can try, play and extend the software without the need for real hardware. If the daemon doesn't find a thermocouple, it will automatically switch into simulation mode. It's still a little bit hackish and the interface could use a little more care but when we get time we'll throw some of it against progressing this further.

The EKA KF412 is an entry level “professional” hot air oven produced by Teknoeca srl in Italy and was chosen to be used as a lab/reflow oven because the hot air (convectional) temperature transfer seemed preferable to infrared and its maximum temperature rating of 300°C compared to 250°C found on many other ovens in this class.

Attribute	Value	Photo
Size	540 x 450 x 405 mm
Power	2.6 kW @ 230 V
Temp. Max.	300 °C
Weight	21 kg
Capacity	4 Trays (330 x 260 mm)

The original control knobs/switches have all been removed and the heater and fan each have been equipped with a zero-cross detecting solid-state relay. The relays have been mounted on the bottom sheet metal of the oven, to act as heatsink for the SSR (if necessary).

First tests with an ATmega based reflow controller have proven the capabilities of the oven to meet the ramp up and maximum temperature demands of an average reflow profile. However, due to the insulation of the oven chamber which increases its thermal inertia, simply turning off the heat will not result in cooling down. After passing the maximum reflow temperature, a quick but controlled cooldown is necessary in order to bring the solder paste back into a rigid state. Simply opening the door cannot be a solution for three reasons:

• Opening the door will produce vibrations at the point in time when the soldering paste is in the most fluid state which in turn could result in bad joins or even move parts out of placement.
• The oven has no way to control the temperature anymore in order to meet the reflow profile.
• Autonomous operation is impossible.

The software already drives a second GPIO that is for cooling. A radial fan will be mounted on the back and blow fresh cold (room temperature) air into one of the two metal pipes that come out of the oven chamber in the back. This will give the Raspberry full control over both heating and cooling and should provide the quickest and most reliable process with a DIY convectional reflow oven. We still need to figure out some kind of valve so that hot air cannot escape from the oven through the cooling system.

We performed tests with N4148 diodes as temperature sensors (hackish, simple and cheap approach) but it soon became clear that a reliable reflow controller would need reliable temperature measurement. The diodes were changing their characteristics too quickly and each one had a different/unstable compensation curve. More research finally led to K-type thermocouples as a solution.

The Maxim MAX31855 Thermocouple-to-Digital Converter (MAX6675 upgrade) performs cold-junction compensation and digitizes the signal from a K-type thermocouple. The data is output in a signed 14-bit, SPI-compatible, read-only format. It resolves temperatures to 0.25°C and exhibits thermocouple accuracy of ±2°C for temperatures ranging from -200°C to +700°C.

We have used a Adafruit MAX31855 breakout board, which was sponsored by Watterott Electronic to help our research efforts but you can also easily hack your own adapter and try to sample the chips.

MAX31855	RPi PIN	Rev1	Rev2	Schematic
Vin	NC	NC
3Vo	P1-1	3V3
GND	P1-9	GND
DO	P1-11	17
CS	P1-13	21	27
CLK	P1-15	22

The picoReflow software uses the luckily already published, open-source MAX31855 python library to get the temperature from the chip.

The door state (open/closed) is also covered by a microswitch connected to a GPIO (P1-12/GPIO18 & P1-14/GND) and will return the door state back to the webclient.

SSR connected to P1-23

MOSFET connected to P1-19

SSR connected to P1-21

In the current software implementation is a 200°C cut-off for the fan. As soon as the temperature rises above it, the controller will disable the fan in order to minimize vibrations while the solder paste is in fluid state. We will have to A/B test if this is necessary or not but for now it's implemented.

The picoReflow server is based on Python with only a few other dependencies. It was developed with foucs on Reflow purposes but as more and more people stumbled upon this project and are creatively using it for completely different (although related) use-cases, we'd like to push further development of this project into a more generalized next-generation FOSS PID controller which can be used for many more use-cases than just building a Reflow oven. If you're a Python/JavaScript developer and want to join, please contact us, we have all the infrastructure we need to do remote hackathons as well.

Screenshots of new the interface (Profile-Editor)

We've tried to keep external dependencies to a minimum to make it easily deployable on any flavor of open-source operating system. If you deploy it successfully on any other OS, please update this:

• python2.7
• greenlet-0.4.2+
• bottle-0.12.4+
• gevent-1.0+
• gevent-websocket-0.9.3+

$ sudo apt-get install python-pip python-dev libevent-dev
$ sudo pip install ez-setup
$ sudo pip install greenlet bottle gevent gevent-websocket

$ emerge -av dev-libs/libevent dev-python/pip
$ pip install ez-setup
$ pip install greenlet bottle gevent gevent-websocket

Clone the sources from github

$ git clone https://github.com/apollo-ng/picoReflow.git

Create the initial config and start the server

$ cd picoReflow
$ cp config.py.EXAMPLE config.py
$ ./picoreflowd.py

Point your browser to 127.0.0.1:8081

https://github.com/apollo-ng/picoReflow

With the curve editor you can change existing and create new temperature curves. Just drag the points in the graph to the position you want and drop or use the text fields below to enter specific values manually. The resulting slopes are calculated automatically for convenient datasheet comparison.

In order to get more realistic temperature values during software development and testing, without actually having to run or even have an oven, we have implemented a temperature simulation module which is active automatically, when no temperature sensor is found.

The following thermal parameters are the basis to simulate the energy flow from the heating element through the oven and finally into the environment.

Every <x 14>\Delta t</x> the following calculations are performed:

Thermal energy flowing into the heating element:

<x 14> Q_h = P_heat * \Delta t </x>

Temperature change of heating element by heating:

<x 14> \Delta T_h1 = Q_h / C_heat </x>

Temperature change between heating element and oven:

<x 14> P_ho = (T_h - T) / R_ho </x>

<x 14> \Delta T_1 = (P_ho / C_oven) * \Delta t </x>

<x 14> \Delta T_h2 = - (P_ho / C_oven) * \Delta t </x>

Temperature change between oven and environment

<x 14> P_oe = (T - T_env) / R_oe </x>

<x 14> \Delta T_2 = - (P_oe / C_oven) * \Delta t </x>

Temperature of oven and heating element at this timestep:

<x 14> T = T_old + \Delta T_1 + \Delta T_2 </x>

<x 14> T_h = T_h_old + \Delta T_h1 + \Delta T_h2 </x>

This project already has inspired a couple of people who are using this concept and free software stack for their own purposes:

• http://www.cube37.com/projects/reflow_toaster/design
• The domain seems to have gone offline, there is a snapshot on archive.org

http://ararpi.simplesite.com

A custom built oven to bake carbon composite parts.