Soldering is an important step in circuit board assembly, which connects various components to the circuit board. In the process, we usually use a variety of soldering materials to ensure the efficiency of joining, such as soldering paste, solder wire and flux. Among them, soldering paste plays an important role in surface mounting technology (SMT).

So, what exactly is soldering paste? How is it used? How does it differ from solder wire and flux? Let's explore these issues in this blog to help you better understand the importance of soldering paste in modern electronics.

What is soldering paste?

Soldering paste, also called solder paste, is a gray, sticky substance. It is a mixture of metal solder powder and flux, mainly used in surface mount assembly as an adhesive and solder that can connect electronic components to the PCB. During reflow soldering, the solder paste melts and forms solid electrical connections between the component leads and the board.

At the same time, solder paste is critical in large-scale electronics manufacturing because it automates component placement and soldering while increasing the accuracy and efficiency.

What are solder paste types?

Types of solder paste can be classified in a variety of ways. We can divide it into different types according to the composition of solder paste, application and soldering process:

By solder alloy

According to solder alloys, solder paste is mainly divided into two categories: lead-based solder paste and lead-free solder paste.

Traditional solder pastes usually contain tin and lead, which have a low melting point and are often used in electronics manufacturing where environmental standards are not required.

Common lead-free solder paste alloys include tin, silver, copper, etc., with a slightly higher melting point than lead-based solder paste. Compared to traditional solder paste, lead-free solder paste is more environmentally friendly and suitable for RoHS-compliant products.

By flux type

According to different flux, soldering paste can be classified in four types: RMA (Rosin Mildly Activated) Solder Paste, RA (Rosin Activated) Solder Paste, Water-soluble Solder Paste and No-Clean Solder Paste.

By melting point

According to the melting point of the solder paste, we can divide it into three categories: low, medium and high-temperature solder paste.

Low-temperature solder paste has a melting point of about 138°C. It is suitable for heat-sensitive components or materials and low-temperature soldering.

Medium-temperature solder paste is the most common type. The melting point of lead-based solder paste is about 183°C, and lead-free solder paste is between 217 and 221°C, which is widely used in standard electronic assembly.

High-temperature solder paste has a higher melting point and is suitable for use in high-temperature environments, usually for special applications that require high-temperature resistance.

By particle size

Solder particles in solder paste can be divided into different grades according to size, such as Type 3, Type 4, Type 5 and so on. The smaller the particle, the better for fine-pitch application, such as the soldering of microchips or high-density circuit boards.

By application

According to solder paste’s different applications, it can also be divided into general-purpose solder paste, precision solder paste and special process solder paste.

What is soldering paste used for?

Solder paste connects surface-mounted components to the printed circuit board and plays a crucial role in the assembly of surface-mount technology.

Component Mounting

Applying the solder paste to the pads of the PCB where the electronic components will be placed. The solder paste can temporarily hold the components in place before soldering, preventing them from moving.

Reflow soldering

After the components are placed on the PCB, the circuit board is moved into the reflow oven. The solder in the paste melts under high-temperature heating in the oven, connecting pins of components to PCB pads. After the solder solidifies, a solid solder bond is formed.

Repairing or reworking PCBs

Solder paste can also be used to repair or rework existing circuit boards. When the existing circuit board needs to be replaced or added more components, we can remelt the solder paste to complete the new soldering.

Improve soldering quality

The flux in the solder paste can remove oxides from the metal surface when heated, helping the solder flow and adhere better, ensuring solder joint quality.

Mass production of electronics

Solder paste is the key material to achieve automated patch soldering, which can complete multiple components soldering at the same time in a single heating process. This greatly improves production efficiency, it’s very suitable for mass production.

Overall, solder paste is an indispensable material in the manufacturing process of modern electronic devices, helping to form efficient and reliable electrical connections. From consumer electronics to industrial equipment, solder paste plays an important role.

How to use solder paste?

Following the steps below, with precise operation and proper tools, we can successfully use solder paste to solder the SMT components to PCBs.

1. Prepare tools and materials

Before soldering, prepare the required materials and tools, including solder paste, stencil, printing equipment or manual coating tools and so on.

2. Clean the PCB

In order to ensure soldering accuracy and efficiency, we need to clean the circuit board before applying the solder paste, ensuring that the pad is free of oxides and impurities

3. Apply solder paste

Apply the solder paste to the pad of the circuit board through the stencil or screen press to ensure that the solder paste on each pad is appropriate and even.

4. Mount components

After applying the solder paste, the electronic components are precisely placed on the PCB using a pick-and-place machine or manually.

5. Reflow soldering

After the component is mounted, the circuit board will be placed in the reflow oven. The solder paste will be melted by heating, and the pins of the components are solidly soldered together with the pad. After cooling, the solder solidifies to form solid solder joints.

6. Inspection

After the soldering, inspect the quality of the solder joints to ensure that they are intact and secure.

Through the steps above, solder paste can help achieve efficient and accurate soldering.

Solder paste vs solder wire vs flux

They are all essential materials in the soldering process, but their uses and applications are different:

Solder paste is an important material in modern electronic manufacturing, especially in SMT. It simplifies and speeds up the soldering process by combining solder particles with flux, making it ideal for automated assembly and precision rework. With a comprehensive understanding of solder paste, we can greatly improve the quality and efficiency of electronic product assembly.