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IntroductionA while ago I decided to start designing my circuits using Surface Mount Devices (SMD). There are many advantages to using SMD components over through-hole components. Some of the major advantages are size and cost. The smaller size means I can make the circuit boards smaller. In addition, smaller components and smaller circuit boards means lower costs. One of the disadvantages to SMD components is they are more difficult to solder than through-hole components. The solution is to use what is called a reflow oven. Within the scope of this project, we will cover a little bit of theory behind reflow soldering. Next, we will discuss the basics of Proportional, Integral and Derivative (PID) control necessary (IMHO) for the repeatable success of a reflow system. Then, we will provide the meat and potatoes of the project so that others may construct their own. Finally, we will comb through key parts of the firmware and round out this project with a summary of the windows interface application. For those of you that are already familiar with reflow and PID concepts, you may elect to skip the theory and jump to the build section. For others that are looking for a brief explanation, we will begin with a closer look at the reflow soldering process.
The Reflow ProcessA reflow oven is used to reflow solder surface mount components onto a Printed Circuit Board (PCB). The basic process entails applying either a leaded or unleaded solder paste to the pads on the PCB. Next, the SMD components are placed on the board and the board put into the reflow oven. The reflow oven applies/adjusts the amount of heat in stages in order to bond (solder) the components to the board. Each stage must be performed using a strict procedure. There are 5 stages for a typical reflow system (Preheat, Soak, Reflow, Dwell and Cooling).
Stage 1 – PreheatThe temperature inside the reflow oven is raised to approximately 125°C at a rate of approximately 2°C per second. This heating rate must be gradual in order to not cause the solder to bubble and splatter. The flux becomes liquid at this temperature and the excess flux will flow away from the pads leaving behind grains of solder.
Stage 2 – SoakThe temperature is slowly raised to approximately 170°C – 175°C and maintained for approximately . At this stage, the temperature of the circuit board and components are nearly the same. Equalizing the temperature prevents cracking or warping of the PCB and/or components during soldering. In addition, the solder flux liquefies and coats the pads.
Stage 3 – ReflowThe temperature inside the oven is ramped up to the soldering temperature of approximately approximately 220°C - 240°C as quickly as possible. At this stage, the grains of solder begin to melt and bond the metal contacts of the components to the associated solder pad.
Stage 4 – DwellThe soldering temperature is maintained for approximately 10-30secs. The solder is now melted and drawn together by surface tension. The flux is also force outward by the surface tension leaving behind a bond between the component and PCB pad.
Stage 5 – Cool DownThe temperature inside the oven is slowly decreased to room temperature. The cool down must take place slowly to prevent potential warping or cracking of the components and/or PCB because of thermal shock.
温度曲线
pcb
原理图
控制板连线
制作资料，固件源代码，上位机软件源代码（c#），文件比较大，分包了
brooksware2000-Hobbybotics-Reflow-Controller-1cb7351.part1.rar
(3.81 MB, 下载次数: 2)
brooksware2000-Hobbybotics-Reflow-Controller-1cb7351.part2.rar
(3.81 MB, 下载次数: 1)
brooksware2000-Hobbybotics-Reflow-Controller-1cb7351.part3.rar
(3.38 MB, 下载次数: 9)
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发表于 2012-10-6 20:46:33
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发表于 2012-10-13 14:51:33
