PCBA Tech - How to choose solder paste for PCBA processing?

An important part of the PCBA processing flow is the use of solder paste. Solder paste is one of the important raw materials in PCBA processing. How to choose solder paste for PCBA processing affects the quality of SMT and even PCBA finished products. This article is about PCBA processing. Discussion on how to choose the solder paste.

Solder paste is a slurry or paste that is uniformly mixed with alloy solder powder and paste flux. Because the main metal component of most solder pastes is tin, solder paste is also called solder paste. Solder paste is an indispensable solder material in the SMT process and is widely used in reflow soldering. The solder paste has a certain viscosity at room temperature, which can initially bond the electronic components to a predetermined position. At the soldering temperature, as the solvent and some additives volatilize, the soldered components will be interconnected to form a permanent connection.

At present, most of the coating solder paste adopts the SMT stencil stencil printing method, which has the advantages of simple operation, fast, accurate, and immediately available after production. But at the same time, there are disadvantages such as poor reliability of solder joints, easy to cause false soldering, waste of solder paste, and high cost.

1. The composition of solder paste

The solder paste is mainly composed of alloy solder powder and flux. Among them, the alloy solder powder accounts for 85% to 90% of the total weight, and the flux accounts for 15% to 20%.

Alloy solder powder

Alloy solder powder is the main component of solder paste, and it is also the most important factor to consider when selecting solder paste in PCBA processing. Commonly used alloy solder powders include tin/lead (Sn-pb), tin/lead/silver (Su-Pb-Ag), tin/lead/bismuth (Su-Pb-Bi), etc. The commonly used alloy composition is 63% Sn /37%Pb and 62%Sn/36%Pb/2%Ag. Different alloy ratios have different melting temperatures.

The shape, particle size and surface oxidation degree of alloy solder powder have a great influence on the properties of solder paste. Alloy solder powder is divided into two types: amorphous and spherical according to the shape. The spherical alloy powder has a small surface area and a low degree of oxidation, and the prepared solder paste has good printing performance. The particle size of alloy solder powder is generally 200-400 mesh. The smaller the particle size, the higher the viscosity; the too large the particle size will make the solder paste bonding performance worse; the too fine the particle size will increase the oxygen content on the surface due to the increase of the surface area, which is not suitable for use.

Flux

In the solder paste, the paste flux is the carrier of the alloy powder. Its composition is basically the same as general flux. In order to improve the printing effect and thixotropy, sometimes it is necessary to add thixotropic agents and solvents. Through the action of the active agent in the flux, the oxide film on the surface of the soldered material and the alloy powder itself can be removed, so that the solder can quickly diffuse and adhere to the surface of the soldered metal. The composition of the flux has a great influence on the expansion, wettability, collapse, viscosity change, cleaning properties, solder bead spatter and storage life of the solder paste.

Second, the classification of solder paste

There are many types of solder pastes, which cause certain problems in the selection of PCBA processing. Solder pastes can usually be classified according to the following properties:

1. According to the melting point of alloy solder powder

The most commonly used solder paste has a melting point of 178-183°C. Depending on the type and composition of the metal used, the melting point of the solder paste can be increased to 250°C or higher or lower to 150°C, depending on the temperature required for soldering. Choose solder pastes with different melting points.

2. According to the activity of the flux

According to the classification principle of general liquid flux activity, it can be divided into three levels: no activity (R), towel equivalent activity (RMA) and activity (RA). Choose according to the conditions of the PCB and components and the cleaning process requirements.

3. According to the viscosity of the solder paste

The range of viscosity is very special, usually 100~60OPa·s, and the highest can reach more than 1000Pa·s. Choose according to the different methods of applying cream.

4. According to the cleaning method

According to the cleaning method, it is divided into organic solvent cleaning, water cleaning, semi-aqueous cleaning and no-cleaning. From the perspective of environmental protection, water cleaning, semi-aqueous cleaning and no-cleaning are the development directions of solder paste selection and use in PCBA processing.

Three, surface assembly requirements for solder paste

In the different processes or procedures of surface assembly in PCBA processing, the properties of solder paste are required to be different, and the following requirements should generally be met:

1. The solder paste should be kept for 3 to 6 months before printing.

2. The performance that should be possessed during printing and before reflow and heating

▪ It should have excellent mold release during printing;

▪ The solder paste is not easy to collapse during and after printing;

▪ The paste should have a certain viscosity.

3. The performance that should be possessed during reflow heating

▪ should have good wetting properties;

▪ The minimum amount of solder balls should be formed;

▪ Solder spatter is less.

4. The performance that should be possessed after reflow welding

▪ The solid content in the flux is required to be as low as possible, and it is easy to clean after welding;

▪ High welding strength;

▪ The quality of solder paste.

Fourth, the selection principle of solder paste

The quality of solder paste is related to the quality of SMT processed products. Inferior and invalid solder pastes are likely to cause some soldering defects, mainly due to the problem of PCBA virtual soldering. How to choose solder paste can be based on the performance and use requirements of the solder paste, and refer to the following points:

1. The activity of the solder paste can be determined according to the cleanliness of the printed circuit board surface. Generally, the RMA level is used, and the RA level is used when necessary.

2. Choose solder paste with different viscosity according to different coating methods. Generally, the viscosity for liquid dispenser is 100～20OPa·s, the viscosity for screen printing is 100～30OPa·s, and the viscosity for stencil printing is 200～60OPa·s. .

3. Use spherical and fine-grained solder paste for fine pitch printing.

4. For double-sided soldering, use high melting point solder paste on the first side and low melting point solder back on the second side to ensure that the difference between the two is 30-40°C to prevent the components that have been soldered on the first side from falling off.

5. When soldering heat-sensitive components, a low melting point solder paste containing bismuth should be used.

6. When using the no-clean process, use solder paste that does not contain chloride ions or other strong corrosive compounds.