Reflow Theory

In order to properly reflow your PCB, the temperature must follow a certain profile which depends on if you are using leaded or lead-free solder paste. The profile will also depend on the exact composition of your solder paste, so please refer to the manufacturer's spec sheet to determine the thermal profile set points yourself. Typically there are a few categories of solder paste:

• Leaded vs lead-free - Lead-free is becoming very popular with RoHS certification and this is what I use personally, and I'll be focusing on it in this tutorial
• No clean vs water wash - Please see this article for more info about the differences
• Normal vs low temperature - Low temperature solder pastes typically have a melting point of 138°C or lower and are advantageous if there are thermally-sensitive components on the PCB

Normally you should always store your solder paste in refrigerator unless you buy "no fridge" solder paste. This extends the lifetime of the paste and also allows the components to "stick" to the PCB better when you place the components.

In the Reflowduino Demo code there is a set of temperatures for generic lead-free solder paste and another set of values for low-temperature lead-free solder which has a much lower melting point. Since this is becoming the norm nowadays with all the red tape and RoHS, I’ll be using lead-free solder and it has worked great from my experience.

Generic lead-free solder paste reflow curve (provided by Chip Quik®):

Low temperature lead-free solder paste reflow curve:

The reflow process is typically summarized into four phases:

• Preheat - from room temperature to a soaking point (the first horizontal axis label)
• Soak - "soaking" at the preheat temperature but increasing slowly to another temperature (the third label)
• Reflow - a short period in which the temperature reaches the reflow temperature (the fourth label) and here the magic happens!
• Cooldown - the period after reflow where it cools back down to a comfortable temperature

If you want to take a look at different types of solder paste, Chip Quik® has a variety of solder pastes with datasheets for each one, including reflow curves for each type.

The Reflowduino controls the temperature (measured by the thermocouple) to match the thermal profile by switching the toaster oven on or off with varying duration calculated by a PID algorithm. The temperature is controlled such that it reaches the set point temperature values by increasing or decreasing at the predefined rates of increase/decrease in temperature for the phases of the reflow process.