Our project consists of making a reflow soldering device using a normal toaster oven with a graphical LCD display for control and GUI. Soldering is an important and difficult task for custom printed circuit board design especially for integrated circuits that come as chip packages that are impossible to solder by hand. This is particularly true for ball grid arrays (BGA) and small-pitch quad flat packs. If one chooses to design a custom printed circuit board around these chips, then the designer may wish to also purchase a stencil of the designed board that would allow him to squeegee solderpaste precisely on the SMD pads. The designer would then carefully place the components on the board, and heat the solderpaste with a heat gun or a reflow soldering oven. The problem with soldering ovens is that they are expensive and cost thousands of dollars. We have decided to come up with a cheap and working solution to the problem by using a normal toaster oven and controlling it through a microcontroller along with an LCD display that guides the user through the soldering process and constantly provides feedback on the state of the system while reflow soldering. The input to the system would be via a conventional keypad and would consist of target temperature point at specific times that the user would enter based on the solderpaste's recommended tempearture profile. The system would interpolate the temperatures for the in-between time intervals and follow the curve generated by the input. The system would also fulfill the appropriate safety requirements and have the capability of aborting the process in case of a mishap.
High Level DesignRationale and source:
The source of the project was one of the team members Ko Ihara who as part of his M.Eng. project worked on designing a circuit board and came up with the idea of using a toaster oven for reflow soldering. The project idea fully complies with the requirements of the course and the challenging part is coming up with a workable PID feedback control to make the toaster oven follow the temperature curve and heat up or cool down at appropriate times. The Atmel mega32 chip is sufficient for this project as the speed of the chip and the number of I/O pins suffices our needs. Another thing about the project is that it would benefit future students in the course since they would be able to solder boards with less pain and high efficiency and safety.
The project can be divided into three parts namely the LCD display, the oven control and lastly the temperature sensor inside the oven. The following block diagram shows the logical structure of the system.
The microcontroller sends a digital signal to the solid state relay switch which controls the on/off state of the oven for appropriate heating and cooling. The temperature sensor device inside the oven makes a voltage divider circuit in series with a resistor, the voltage signal is connected to the atmel mega32 ADC (analog to digital converter) input. The ADC output is used as a feedback by the program to measure temperature inside the oven and to control the relay state. At the same time, the state of the system is updated in real time on the LCD and the user can see the progress. The keypad is used for input and the LCD provides step by step instructions to the user during the input process. The oven control hardware containing the relay switch is inside a metal box with the fuse circuit in place. It has the three pin plug connections for connecting the oven and the AC voltage supply. It also has harness plugs for connecting the digital input to the microcontroller.