July 2004, Issue 168

Easy Reflow
Build an SMT Reflow Oven Controller

OVEN IN USE

Now that you know why you need a reflow oven and how to build one, how will you use it? First, you need to get some additional equipment. You’ll need a good magnifier (a video magnifier would be best) and a syringe of solder paste. It’s easier if you have a pneumatic fluid dispenser (which is easy to find on the ’Net, but you will also need shop air) because using the syringe can be difficult. You will also need a flux cleaning spray, a small classic iron (15 W or less), and, ideally, a hot-air iron for repairs. Don’t worry, the Internet is a good source for low-cost equipment. Even if you don’t have a video magnifier, you can be successful if you have a good optical lens and good eyes!

A clean blank PCB is extremely important. I know from experience that it’s nearly impossible to achieve good reflow results with home-etched PCBs. Solder masks are really helpful in keeping the solder paste on the pads. I don’t know how to get solder masks on home-etched PCBs, so find a good PCB manufacturer and order your PCB with solder masks.

You have to put solder paste on the pads with the syringe either manually or with the fluid dispenser. Then, you must place each component on the PCB on the solder paste spots. These two steps are time-consuming, but things speed up with experience! Remember that it is far quicker to use solder paste than to try to solder by hand. After all of components are placed, you can put the board in your reflow oven and press the Start button. After 10 min. or so, your PCB will be cooked. Beware, it can still be quite hot. Use flux removal fluid and a toothbrush to clean the board (see Photo 4). Refer to the Circuit Cellar ftp site for a detailed illustration of the process.

By the way, you can use this process for boards with components on both sides if you put light components on one side. Reflow only that side first, and then flip the board and repeat the process: apply solder paste, place the components, and then reflow. The components on the bottom side will usually stay soldered thanks to the surface tension of the solder.

Finally, you should check the electrical connections and make the necessary repairs. If you follow the process closely, you shouldn’t have too much trouble. My last 160 mm × 100 mm board had more than 300 fine-pitch components on both sides, but I had only a couple of bad joints and one solder bridge on a 0.65-mm pitch IC.