Reflow Soldering: Definition, Equipment, Process

Reflow soldering is an important technique in the circuit board assembly process, accurately soldering the surface mount components to the circuit board. Through solder paste, controlled heating and precise reflow oven temperature, it can create a reliable solder joint between SMDs and the PCB. In this blog, let's explore the definition of reflow soldering, the related equipment and the entire reflow soldering process.

What is Reflow Soldering?

Reflow soldering is a process used for soldering electronic components, mainly in surface mount technology (SMT). The basic process is to put the solder paste coated PCB and surface mount components into the reflow oven, melting the solder paste through heating, forming the solder joint and then cool and solidify, so as to realize the soldering of the components and the circuit board.

Common Reflow Soldering Equipment

Infrared (IR) Reflow Oven: 

It uses infrared radiation to directly heat the surface of soldered components. Infrared reflow oven offers fast heating but because it is sensitive to differences in material absorption rates, it may result in less uniform temperature distribution. Therefore, it is suitable for applications with lower process requirements.

Hot Air Reflow Oven:

This equipment utilizes circulating hot air to transfer heat through convection to the PCB surface and surface mount components. Hot air reflow oven ensures even temperature distribution and stable processes. It is the most widely used reflow soldering equipment, which is ideal for mass production.

Tabletop Reflow Ovens: 

It has a compact design that typically employs infrared or hot air convection heating. It is small, cost-effective and at the same time easy to operate. So it is ideal for small-batch production, R&D labs and educational purposes.

Nitrogen Reflow Oven: 

It introduces nitrogen gas into the chamber to reduce oxygen levels, minimizing oxidation, while using hot air or infrared heating. The nitrogen reflow oven enhances solder joint quality, making it suitable for manufacturing high-reliability products such as medical or aerospace devices. However, it involves higher costs.

Soldering Process: How to Reflow Solder Joints

The reflow soldering is not just a process in the soldering oven, it involves multiple steps, including applying the solder paste to a specific location on the PCB, placing the circuit board components on the solder paste, then heating the component in the reflow oven, and finally testing the soldering effect.

Stencil Printing:

Solder paste is applied to the PCB pads through a stencil. Accurate solder paste deposition is the foundation for good component attachment. The stencil printing process requires ensuring the solder paste is evenly and precisely applied to the pads so that the components can be securely soldered and maintain reliable electrical connections.

Component Placement

The pick-and-place machine precisely places the components onto the pads, ensuring proper alignment with the PCB design. This process requires high placement accuracy, as the precision of component placement directly affects the soldering quality and overall performance of the PCB.

Reflow Soldering

The assembled PCB is passed through the reflow oven, where the temperature profile is divided into the following stages:

Preheating: The temperature gradually increases to activate the flux in the solder paste. This helps remove oxides from the paste and prepares it for the next stage.

l  Soak: The temperature is stabilized, ensuring the PCB is evenly heated. This prevents overheating or uneven heating, which could lead to soldering defects.

l  Reflow: The temperature reaches its peak, melting the solder and allowing it to form strong, reliable solder joints with the pads and component leads. The solder paste melts at this stage, providing the necessary mechanical connection and electrical conductivity.

l  Cooling: The temperature is gradually reduced to room temperature, solidifying the solder joints and ensuring the reliability and stability of the soldering process, preventing cold or insufficient solder joints.

Inspection and Testing

After reflow soldering is completed, the solder joints undergo strict inspection to ensure the soldering quality meets the design specifications. Common inspection methods include AOI (Automated Optical Inspection), X-ray inspection, and flying probe testing. These methods effectively detect soldering defects such as shorts, opens, and cold joints, ensuring the PCB meets the design specifications and can function reliably.

Conclusion

Reflow soldering is one of the cornerstones of electronics manufacturing, enabling accurate and efficient printed circuit board assembly. From stencil printing to the role of reflow ovens, understanding each step of the process is essential to ensure high-quality solder joints and reliable PCBs.