After designing a PCB board, we need to find a factory to produce it. Once the board is manufactured, we need to solder the components onto it. For small batch testing, technical staff usually does the soldering manually. Once the board’s performance is confirmed to be problem-free, it’s typically mass-produced, at which point we need to find an assembly factory for surface mount production. There are two soldering methods for surface mount assembly: wave soldering and reflow soldering. In the following, we will introduce these two soldering techniques.
Wave soldering is a process where the soldering surface directly contacts the high-temperature molten solder, forming a wave peak to achieve the soldering purpose.
This is typically done by using an electric pump to spurt the molten solder, creating a wave peak for soldering. During the wave soldering process, the soldering surface of the circuit board does not come into contact with the solder simultaneously.
The circuit board is generally placed on a conveyor belt, passing through the solder wave peak at a certain speed. The solder usually does not remain on the soldering surface for an extended period.
Common wave soldering methods
Manual through-hole assembly → Wave soldering
Single-sided surface-mount assembly → Single-sided through-hole assembly → Wave soldering
Double-sided surface-mount assembly → Single-sided through-hole assembly → Wave soldering
Dispensing adhesive → Surface-mount assembly → Through-hole assembly → Wave soldering
Wave Soldering Process
What is reflow soldering In PCB Manufacturing?
Reflow soldering refers to the process of heating and melting solder paste (which is typically a mixture of tin powder and flux) pre-applied to the solder pads, causing it to return to a liquid state (this process is called “reflow”).
This allows the components pre-positioned on the solder pads to come into full contact with the molten solder, achieving a solid solder connection.
What are the advantages of reflow soldering?
When using reflow soldering technology, there is no need to immerse the printed circuit board in molten solder. Instead, the soldering task is completed through localized heating. As a result, the components being soldered experience less thermal shock, preventing damage due to overheating.
Since soldering only requires applying solder to the soldering points and using localized heating to complete the connection, soldering defects like bridging can be avoided.
In reflow soldering technology, solder is used only once, with no reuse. This ensures that the solder is pure and free from impurities, guaranteeing the quality of the solder joints.
What is Reflow soldering process
Reflow soldering is used for processing surface mount boards, and its process can be more complex, divided into two types: single-sided assembly and double-sided assembly.
A. Single-sided assembly: Pre-apply solder paste → Mount components (either manual or automatic machine assembly) → Reflow soldering → Inspection and electrical testing.
B. Double-sided assembly: Apply solder paste to side A → Mount components (either manual or automatic machine assembly) → Reflow soldering → Apply solder paste to side B → Mount components (either manual or automatic machine assembly) → Reflow soldering → Inspection and electrical testing.
The simplest reflow soldering process consists of “screen printing solder paste – mounting components – reflow soldering.” The core of this process is the accuracy of screen printing, the yield of component placement is determined by the machine’s PPM, and reflow soldering involves controlling the temperature rise, peak temperature, and temperature drop curve.
The process described above is for reflow soldering on one side of a PCB. However, in many cases, PCBs have components on both the top and bottom layers. In this situation, the process must be repeated for the second side. First, apply solder paste to the first side, place the components, and then go through the soldering and cooling process in the machine. After that, apply solder paste to the second side, place the components, and follow the same soldering and cooling steps.
When there are components on both sides of a PCB, it’s essential to be mindful of the component placement. Try to place heavier components on one side to ensure soldering reliability.
Reflow Soldering vs Wave Soldering: What's the Difference?
From the introduction above, we know that wave soldering is typically used for through-hole components, while reflow soldering is mainly used for surface-mount components. Nowadays, PCBs are quite complex and generally contain both surface-mount and through-hole components. Therefore, during the assembly process in a manufacturing facility, reflow soldering is performed first to solder surface-mount components, followed by wave soldering to solder the through-hole components.