For surface mount soldering you can use a variety of tools. Anything from a heat gun to an oven will work. Many people invest in their own reflow ovens (yes, you CAN buy an already set up reflow oven!) or a new toaster oven. What did we do? We bought a $5 used toaster oven online. To “set it up” for reflow purposes we cleaned it and then stuck a thermocouple that was attached to a multimeter in temperature mode into a hole on the bottom of the toaster oven. And voila! We were good to go.
So, here are the reflow basics we learned:
I.What is a Reflow Oven?
A reflow oven is a basically a temperature controlled heating device used for soldering.
II. Reflow Profiles
If you buy solder paste, you can check the product’s data sheet to figure out both melting temperature and reflow profile for the material. Because reflow has more than one stage, reflow profiles are important in helping figure out the proper temperature to heat your PCB without bridging or warping.
III. The 5 Stages of Reflow
There are 4 (or 5 if you buy solder paste with flux in it) stages to a good reflow job.
1. Preheating. This is the longest stage. The board must be brought up to temp. slowly and evenly or the board could warp. Limit the rate of heating to 1-3°C/sec.
2. Thermal Soak. Once you hit the desired preheating temp (150°C +/- 20°), keep the temperature steady and allow it to sit for 1-2 minutes. This activates the chemicals in the solder paste.
3. Reflow. After the solder paste is activated, bump the temperature up again to your melting point at a rate of 1-3°C/s. This is where the magic happens. For lead-free soldering paste, at any temp. above 217°C it’s considered reflow. Stay in this phase long enough so that the solder turns shiny and liquidy and the parts move into place, but not long enough that the solder begins to evaporate. This should take 30-75 seconds.
4. Cool down. The quicker your PCB cools down the better, as it will help strengthen the joints. Usually this happens form 2-4°C/sec. I tend to just turn the oven off and open the door.
5. Washing. (only do this if you don’t have a no-clean solder paste). Wipe the board down with rubbing alcohol, deionized water and detergent, or other mild solvents to get rid of any chemical residue.
IV. Toaster Oven Experiment to Figure Out Rough Reflow Profile
Before we did any actual soldering, we ran some experiments on our oven to figure some things out. We wanted to see how everything would play out a couple times before we wrecked an expensive PCB. So, with our $5 toaster oven, thermocouple multimeter setup, stopwatch, and a pen, we started. We simulated the generic reflow profile for our lead-free solder paste to the best of our abilities. Timing how long it took to get to each phase, and practicing how the oven worked and how accurate the dials were. How we could keep the PCB in thermal soak if the temperature kept rising? We looked at all possible outcomes.
After multiple trials, we found that the best technique was just to watch the temperature and stopwatch and adjust accordingly. The oven didn’t heat up at the same speed everytime, and didn’t even go to the same temperature every time. Also, every board would reflow at a different rate.