Why is it called reflow soldering?

Reflow soldering is one of the three main SMT mounting processes. Reflow soldering is mainly used for soldering circuit boards for mounting components. Melt the solder paste by heating, allowing the installed components to fuse with the circuit board pads, and then cool the solder paste with reflow soldering to solidify the components and pads together. However, most people understand reflow soldering machines, which are machines that solder components onto PCB boards through reflow soldering, and are currently widely used in most electronic factories. To understand reflow soldering, the first step is to understand the process of SMT.

Why is it called reflow soldering

Originally, solder paste was a mixture of metal tin powder, flux, and other chemicals, but the tin in it can be said to exist independently in the form of small solder beads. After passing through a reflow soldering furnace and at different temperatures in several different temperature zones, when the temperature exceeds 217 degrees Celsius, those small solder beads will melt, and countless small particles will fuse under the catalysis of flux and other materials. In other words, those small particles will flow again. This process is usually referred to as reflux, which refers to the return of tin powder from the previous solid state to the liquid state, and then from the cooling zone back to the solid state.

Introduction to reflow soldering methods:

The advantages of different reflow soldering are different, and the process flow is naturally different.

Infrared reflow soldering: With high radiation heat transfer efficiency, high temperature gradient, and easy control of temperature curve, the upper and lower temperatures of PCB are easy to control during double-sided soldering. There are shadow effects, uneven temperature, which can easily cause localized burning of components or PCBs.

Hot air reflow soldering: The convective conduction temperature is uniform, and the welding quality is good. The temperature gradient is difficult to control.

Forced hot air reflow soldering: The infrared hot air mixed heating combines the advantages of infrared and hot air stove, and can achieve excellent welding results during product welding. Forced hot air reflow soldering can be divided into two types based on its production capacity:

1. Temperature zone equipment: suitable for mass production. The PCB board is placed on the walking belt and needs to pass through several fixed temperature zones in sequence. If there are too few temperature zones, temperature jumps may occur, which is not suitable for welding high-density assembly boards. And it has a large volume and consumes a lot of electricity.

2. Small desktop equipment in warm regions: Small and medium-sized batch production is rapidly developing in fixed spaces, with temperature changing over time according to set conditions and simple operation. It can repair defective surface mount components (especially large components) and is not suitable for large-scale production.

Due to the characteristics of "reflow" and "self positioning effect" in the reflow soldering process, the requirements for mounting accuracy are relatively loose, making it easy to achieve high automation and speed in welding. Meanwhile, due to the characteristics of reflow and self positioning effects, the reflow soldering process has stricter requirements for pad design, component standardization, component end and PCB quality, solder quality, and process parameter settings.

Cleaning is the process of removing pollutants and impurities from the surface of the cleaned object through physical and chemical reactions. Whether cleaning with solvents or water, it involves processes such as surface wetting, dissolution, emulsification, and saponification., By applying different mechanical forces, dirt is peeled off from the surface of the surface assembly board, then rinsed or rinsed clean, and then dried, dried or naturally dried.

Reflow soldering is a key process in SMT production, and a reasonable temperature curve setting is the key to ensuring the quality of reflow soldering. Inappropriate temperature curves can lead to welding defects such as incomplete soldering, virtual soldering, vertical components, and excessive solder balls in PCBs, affecting product quality.