A typical printed circuit board contains a large number of electronic components that are interconnected through thin copper traces. These components are bonded to the board through a process called soldering. Soldering not only holds a component in place but also provides electrical connectivity between the component's pins and the circuit board pad. There are different techniques that can be utilized to solder components on a circuit board.
Soldering With Soldering Iron
A soldering iron is a common and an easy-to-use tool to solder components on circuit boards and is used by professionals as well as hobbyists. This tool has a tip that is heated electrically. The tip is used to heat the solder flux to a maximum temperature of about 400 degrees Celsius. The heat melts the flux, which then flows on the circuit board and covers the component's pin. When the tip is retracted, the flux solidifies within a few seconds, creating a bond between the pin and the pad.
Reflow Soldering
Reflow soldering is generally used when a large number of surface mount devices are to be soldered at the same time. This method is more suitable for a production environment, where individual soldering with an iron is impractical and time consuming. Reflow soldering requires a specially designed oven to heat the solder flux. First, the surface mount devices are placed on their respective circuit board pads with a solder flux paste on all the device terminals. The paste is sticky and keeps the components in position during soldering. The board is then placed in the oven. Most reflow ovens have four operational zones or stages. In the first zone the temperature of the oven is raised slowly to about 200 degrees Celsius. The rate of temperature rise is kept at about 2 degrees Celsius per minute during this stage. This rate is reduced significantly during the next stage, which lasts for about two minutes. In the third stage the temperature is raised to about 220 degrees Celsius, which allows the flux to melt and start bonding the components' terminals with their corresponding pads. In the last stage the temperature is rapidly decreased to solidify the solder flux, which completes the bonding process.
Soldering Surface Mount Components
Surface mount device components can be soldered by a soldering iron or through the process of reflow soldering. If there are only a few components to be soldered, then a soldering iron is preferred since it is less energy intensive and does not require a reflow oven.
Through-Hole Component Soldering
Through-hole components are almost always soldered using a soldering iron, either manually or through an automated soldering robot. Using reflow technique for these components is very tricky and is therefore not recommended. The problem is that the through-hole components are soldered on the opposite side of the board. This would require the board to be placed in the oven with the components facing down. As a result, these components may fall down when the flux melts during the soldering process.
