What is Solder Paste?


Introduction to Solder Paste Composition and Usage

In recent years, electronic devices such as automobiles and household appliances have become increasingly multifunctional. As a result, the electronic components used on circuit boards have significantly reduced in size and increased in number, making the process of soldering crucial for joining the circuit board and the components. To ensure reliable soldering of these miniaturized electronic components, the Surface Mount Technology (SMT) process using "solder paste" is essential.This article explains the composition and usage of solder paste, which is expected to continue as the main choice in electronics manufacturing.

1.   What is Solder Paste?
1-2.Types of Solder Powder in Solder Paste (Grade)
2.   Storage and Shelf Life
3.   Usage Instructions
3-1.Applications of Solder Paste
4.   Summary

1. What is Solder Paste?

Solder paste is a mixture that combines solder alloy in powder form with a paste-like resin primarily composed of rosin, which is known as flux. It is used in Surface Mount Technology (SMT). The solder paste is printed on the printed circuit board and then electronic components are placed on boards followed by reflow soldering to create a soldering bond between the board and the components.

Since the miniaturization of components requires higher printing precision of the solder paste, the print performance of solder paste has a significant impact on the quality of the attachment of the components to the board. To avoid printing and component placement defects, it is necessary to select the appropriate solder paste for fine-pitch component assembly. Here-on, we will summarize the following two areas for solder paste:

●Solder powder type (grade) of the solder paste

You should use the solder powder type as a reference for selecting solder paste that suits your intended application.

1-1. Composition

The composition of solder paste can broadly be divided into "solder powder" and "flux." Generally, lead-free solder powder is an alloy mixture of tin, silver, and copper metals. The solder powder particles are spherical and come in various sizes, with mainstream powder sizes ranging from 20 to 45μm in diameter.

The remaining component which is flux, consists primarily of resin such as rosin and synthetic resin, as well as activators and thixotropic agents as additives, and solvents for viscosity adjustment of the solder paste. The flux performs the following functions. When there is an oxide film on the solder alloy, it eliminates the oxide film on the metal surface of the solder joint area, which otherwise hinders the bonding process between the component and the solder, facilitating proper bonding. Furthermore, even if the oxide film is removed, it can quickly regenerate during the soldering process. Therefore, the flux prevents re-oxidation of the metal surface by coating it. Additionally, the flux plays a crucial role in reducing the high surface tension of the solder, allowing it to effectively wet and spread on the surfaces to be soldered. This makes the flux an essential component in the soldering process.

1-2. Solder powder type (grade) of the solder paste

Table 1 shows the classification of solder powder types for solder paste based on IPC standard J-STD 005,
along with their respective particle sizes in microns.

Type Particle Size (Microns)
(80% Minimum between)
1 75–150
2 45–75
3 25–45
4 20–38
5 15–25
6 5–15
7 2–11
8 2-8
Table 1: List of Solder Powder Types in Solder Paste

Choosing the appropriate solder powder type is crucial, considering the size of the components and the printing conditions of the solder paste. Typically, Type 3 or 4 is used in consumer electronic devices and general electronic products, while Type 4 or 5 is commonly used in smaller electronic devices like smartphones.
Smaller particle sizes from Type 5 to Type 8 tend to be more costly and cause more frequent failures such as non-wetting and poor melting occuring, which would need more development of the flux materials.

2. Storage and Shelf Life

The following is an explanation of storage and shelf life of solder paste. Generally, solder paste should be refrigerated for storage. This is because higher temperatures can cause solder metal oxidation and unwanted reactions between the solder powder and flux. However, there are solder pastes available that can be stored at room temperature.
The shelf life of solder paste varies depending on the storage method and the product, but it typically ranges from 6 to 12 months stored unopened and refrigerated. Using solder paste beyond its expiration date can result in increased viscosity and solder paste hardening, leading to potential printing issues. It is recommended to use the solder paste within the designated shelf life period.

3. Usage Instructions

Solder paste is used during printing, which is then followed by component placement onto the deposited paste on the board and then the solder paste on the board is reflowed. The first stage involves printing the solder paste onto the PCB (Printed Circuit Board). Due to the increasing number and decreasing size of electronic components placed on a single PCB, automated printing equipment is used to perform the printing process swiftly and accurately.
Next, various types and sizes of electronic components, including chip resistors and capacitors, are placed on the PCB where the solder paste has been printed. This process is called component placement. After placing the components onto the PCB, the assembly is placed in a reflow oven and heated to melt the solder paste. By subjecting all components on the PCB to the same heating conditions, the variation in solder joint quality can be minimized.

3-1. Applications of Solder Paste

During the printing process, solder paste can be applied using three different methods, depending on the desired application technique.

The first method is manual application, typically employed during research and experimental stages when a large quantity of boards are not being produced. In this method, solder paste is meticulously applied onto the board pads using a syringe or through manual printing, accommodating varying conditions.

The second method involves using a printing machine with a stencil. To apply the solder paste, a stencil with aperture openings corresponding to the desired paste application areas is positioned on top of the PCB. The solder paste is then printed on the stencil through the stencil apertures using a squeegee blade of the printing machine on to the board pads. After removing the stencil from the PCB, the solder paste remains only in the board pad areas with the stencil aperture openings. This method enables faster and more accurate application compared to manual methods.

4. Summary

So far, we have introduced the type and use of solder paste, which is essential in the manufacturing of electronic component PCB assemblies. As PCB assembly continues to advance with increased functionality and higher density, the performance requirements for solder paste will also increase.
We hope this article assists and helps to inform you when selecting the solder paste to align with your specific needs.

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