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Electronic devices are composed of separate components with different functions. By sodering these components to the printed circuit board and forming electrical connections, we can build many kinds of electronics products. Surface-mount technology (SMT) prevails worldwide in the PCB assembly process. Apart from the bare board and components, there are many important raw materials used in SMT assembly, and solder paste is one of them. The quality of the solder paste directly affects the soldering quality of the PCB and even the quality of the entire board, which has a significant impact on the reliability and service life of the final products.

What is solder paste?

Solder paste is used to solder electronic parts on the circuit board. It is one of the important materials indispensable to the progress of modern technology, which contributed to the miniaturization of Today's electronic devices. Solder paste is similar in shape to the toothpaste we use every day. It can stick the electronic parts onto the surface of the circuit board so that these parts will not shift position even with a slight vibration when flowing in the SMT assembly lines. After flowing through the reflow oven, the solder paste melt and re-solidify, during which the components are fixed on a specific position of the circuit board, and the electrical connections will be formed.

Figure 1 PCB with solder paste on the pads

Solder paste is mainly composed of power and flux.

Powder

The powder is a metal alloy that affects the solderability and strength of soldering. It mainly includes Sn (Tin), Ag (Silver), Cu (copper), and Bi (Bismuth). The smaller diameter of the powder, the easier it is to be printed on the circuit board through the stencil as smaller particles are much easier to squeegee into the aperture of the stencil. And it is less likely that the unwanted solder remains on the edge of the aperture, which also improves the printing ability for fine-pitch parts. However, with a larger area contacting with the air, smaller power is easier to oxidize. 

Flux

Flux is actually the pusher that turns the solder into a “paste” state because the solvent in the flux helps dissolve and mix different chemicals. Flux also contains other three substances including Rosin/Resin, activator, and rheology modifier.Flux can remove oxides and dirt on the metal surface. During high-temperature operation, it will form a thin film on the surface of the metal to isolate the air and protect the solder paste from oxidation easily.

How to print solder paste on PCBs in SMT assembly?

The deposition of solder paste to the printed circuit board is considered one of the most important processes of the SMT assembly process. If the solder paste is not printed in the correct position, it may not be able to achieve the functions of soldering parts, and some unexpected quality problems may occur. The amount of solder paste also matters as too much solder paste may cause short circuits while too little may lead to non-wetting. To accurately deposit the correct amount onto each of the pads to be soldered, a stencil should be used. For many PCB manufacturers, this process is operated by a solder paste printer. 

Figure 2 Solder paste deposition

Automatic solder paste printer

Generally, factories with mass production capacity will use fully automatic solder paste printers in SMT lines. As long as the relevant parameters are set, the solder paste printer can automatically load the board, align the stencil, print the solder paste, and then unload the board and transfer the board to the Pick & Placement procedures through the conveyor belt.

Figure 3 Automatic solder paste printer in PCBGOGO

Semi-auto solder paste screen printer

Semi-auto solder paste screen printers are commonly seen in factories with a small number of diverse product lines. The board loading, unloading, and stencil alignment are usually manually operated. Only the solder paste is printed automatically. The operation of this machine required an experienced technician otherwise it is easy to encounter defective boards. 

Manual solder paste printer

Some hobbyists and amateur makers are in favor of this type for its low cost and simple operation. Only a squeegee, a stencil, and solder past are needed. However, it is not recommended to use this printer for small component pitches unless your operating skills are really good.

Conclusion

Printing solder paste on a PCB, and then soldering electronic components through a reflow oven is the most common process in the electronic assembly manufacturing industry today. To ensure the quality of the final assembled board, careful consideration should be given when selecting the solder paste and the printer. PCBGOGO is a professional PCB manufacturer running their SMT factories in China. With high-quality solder paste used and fully automatic production equipment such as GKG Solder paste printer, YAMAHA pick & place machines, reflow oven, PCBGOGO can be your right partner for SMT projects.