What are wave soldering and reflow soldering in PCB manufacturing?

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.

wave soldering

Common wave soldering methods

  1. Manual through-hole assembly → Wave soldering

  2. Single-sided surface-mount assembly → Single-sided through-hole assembly → Wave soldering

  3. Double-sided surface-mount assembly → Single-sided through-hole assembly → Wave soldering

  4. Dispensing adhesive → Surface-mount assembly → Through-hole assembly → Wave soldering

Wave Soldering Process

wave soldring process
wave soldring process
  1. Spraying flux

The circuit board with assembled components is embedded in a fixture and sent into the wave soldering machine at a certain angle and conveying speed by the connecting device at the entrance. The board is then clamped by continuously rotating claws, sensed by sensors, and the spray head moves back and forth evenly along the starting position of the fixture, evenly coating the exposed solder pad surface, solder pad vias, and component lead surfaces with a thin layer of flux.

  1. PCB board preheating

The board enters the preheating area, where the soldering part of the PCB is heated to the wetting temperature. At the same time, due to the increase in component temperature, the board avoids significant thermal shock when immersed in molten solder. During the preheating stage, the PCB surface temperature should ideally be between 75 and 110°C.

Functions of preheating:

① The solvent in the flux evaporates, reducing gas generation during soldering;

② The rosin and activators in the flux begin to decompose and activate, removing the oxide film and other contaminants from the printed circuit board solder pads, component ends, and pin surfaces, while also protecting the metal surface from high-temperature re-oxidation;

③ The PCB board and components are sufficiently preheated to avoid damage to the PCB and components due to rapid temperature rise during soldering.

  1. Thermal compensation

During the thermal compensation stage, the compensated PCB board enters wave soldering with reduced thermal shock.

  1. First and second wave peaks

First wave peak (turbulent slope)

The first wave peak is a “turbulent” wave peak sprayed from a narrow nozzle with a fast flow rate, providing good penetration for the shadowed soldering area of the fixture. Simultaneously, the upward jet force of the turbulent wave allows the flux gas to be expelled smoothly, significantly reducing soldering defects such as missed soldering and insufficient vertical filling.

  1. Cooling stage

The cooling system rapidly lowers the temperature of the PCB board, significantly improving voids and pad delamination issues that occur during the production of lead-free solder eutectics.

Throughout the soldering process, the addition of nitrogen gas protection during the preheating stage and the soldering area effectively prevents the oxidation of bare copper and eutectic solder, greatly improving wettability and fluidity, ensuring the reliability of solder joints.

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.

reflow soldering 2

What are the advantages of reflow soldering?

  1. 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.

  2. 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.

  3. 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.

reflow-soldering-prcess
reflow soldering prcess

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.

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