This project of transforming a toaster oven into a solder reflow oven is quite common and has been attempted by many people. By doing some research on the internet, I found many webpages that describe the past researches on this project and I picked a couple of them which I think are very helpful and informative:
Resources:
1. http://www.instructables.com/id/Hack-a-Toaster-Oven-for-Reflow-Soldering/?ALLSTEPS
2. https://www.sparkfun.com/tutorials/60
3. https://courses.cit.cornell.edu/ee476/FinalProjects/s2006/ki38/Webpage/index.html
4. http://zavax.wordpress.com/2013/04/16/convert-a-toaster-oven-into-a-pcb-reflow-oven/
5. http://reprap.org/wiki/Toaster_Oven_Reflow_Technique
6. http://hforsten.com/toaster-oven-reflow-controller.html
7. http://hobbybotics.com/projects/hobbybotics-reflow-controller-v8-03/
8. http://danstrother.com/2011/01/15/reflow-oven-controller/
There are many ways and steps that people use to tackle this projects, depending on the types of ovens that they have or the requirements and goals that they want to achieve. But overall there are some basic elements that all the past researchers have to understand in order to work on this project:
1. Reflow soldering basic and temperature profile:
The goal of the reflow process is to melt the solder and heat the adjoining surfaces, without overheating and damaging the electrical components. The reflow soldering process has four stages, or zones, each having a distinct thermal profile: preheat, thermal soak, reflow and cooling.
- The first stage is preheating to the thermal soak stage.
- The second stage is thermal soaking the printed circuit board and all the components. This stage ensures even temperature distribution and activates the flux to clean the surfaces.
- The third stage is raising the temperature to melt the solder paste to perform the actual soldering.
- At the final stage, everything is cooled at a controlled rate.
There is a temperature profile provided from the Kester company for the Sn63Pb37 solder paste that we’ll use for our project:
2. The toaster oven that will be used:
Different types of toaster ovens have been used for this transformation. Our toaster oven, which is of type 120V-60Hz 1500W is also successfully transformed in one of the past research that I found.
3. Temperature controllers:
One of the most commonly used tool for measuring temperature is call the thermocouple, which is basically a device consisting of two dissimilar conductors that contact each other at one or more spots. It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouple is generally inexpensive, interchangeable and can measure a wide range of temperatures. Thus, the thermocouple seems like an ideal option for sticking into our toaster oven.
4. Controller Circuit:
We need to make a PCB controller that helps us keeping track the oven’ temperature as well as controlling the heating rate in order to achieve the reflow profile shown above. The oven’s temperature will be measured by the thermocouple and the data will then be read into a micro-controller which is the main part of our controller circuit. Ideally, all the data and numbers will be shown on a LCD display which helps us to manage and see how well the system is working.
Below is a sample logical structure of a system from a past project:
5. Program and Hardware design:
After our circuit is put together, we need to design a program to make the system works the way we want and allow the LCD screen to display the appropriate information. Below is a sample diagram of such system:
6. Test the resulted product:
After the transformation is finished, we need to test our final product to check if it meets all our initial requirements or not. Also, we want to write up a complete instruction for our toaster solder oven for the later students to be able to use.
Just an addition, I also found a very interesting video online by a famous electrical engineer that describes and instructs how he performed this project: http://element14.tv/ben-heck/episode-36-see-ben-build-a-solder-reflow-oven/
Next step is to define all of the functions you need for the system. So this will include a way to control the oven with a low voltage digital signal, a way to measure the temperature, a way for the user to interact with the system (display and inputs), and the controller itself. Once you have a detailed definition for each function, then you can list all of the possible solutions for each function, and the pros and cons. We will discuss this tomorrow morning.