There are two main steps involved in the reflow soldering process of PCB. The first is to apply the solder paste and the second is to assemble the components.
1. Solder Paste
Before applying the solder paste, you have to use a solder mask to ensure the solder is applied only to the areas where the copper pads will be installed. If you do not apply the solder mask properly, it will lead to the formation of solder bridges.
By using the solder paste, you can apply the solder paste only on the areas in PCB where soldering is needed.
2. Pick and Place
Once you have applied the solder, then you can assemble the components on the board. If the number of components is less, you can manually assemble the components.
But if the components are more or if you are soldering on a commercial level, then it is best to use a pick and place machine. With the pick and place machine, you can place the components on the board automatically. The components will be held onto the board by the surface tension of the solder paste.
But in some cases, you have to apply glue to ensure the components do not fall off from the board. If you apply glue, it will be difficult to rework the PCB. Once you have placed the components on the board, then you can move them to the reflow soldering machine.
Reflow Soldering Stage
The reflow soldering process consists of several stages to ensure the joints are strong. The temperature of the board is gradually increased before the soldering process to prevent thermal shocks.
The four different stages of the reflow soldering process are as follows,
You have to heat the board slowly to bring it up to the required temperature. If you subject the board to rapid heating, it will lead to thermal shock and break the board.
Besides, if the heating is not stable, it will cause hot and cool spots on the board. Some parts of the board will have a higher temperature. And, some parts of the board will have a lower temperature. For infra-red reflow soldering, you have to raise the temperature of the board by 2 to 30C. In some cases, you can also increase the temperature by 10C.
Once you have brought the board to the required temperature, you have to move it to the thermal soak area. In this, you have to maintain the board at a steady temperature. This will ensure that all areas of the board are brought up to the required temperature. Additionally, it will also remove the solder paste and activate the flux.