How to Use Solder Paste in PCB Assembly

A crucial component in electronic devices is the PCBA board, consisting of two main parts: PCB and Components. The PCB supports and connects the components, which, in turn, provide the actual functionality. Initially, these two parts are separate, but through the assembly and soldering process, they are eventually combined into a unified whole.

In this assembly process, the use of certain key materials is necessary, and one of these is solder paste. This article will provide a detailed overview of the role of solder paste and a guide on how to correctly use solder paste in the PCB assembly process.

Solder paste printing during PCB assembly

What is Solder Paste and Its Purpose

One of the most reliable methods to connect two metals without melting them is soldering, and solder paste is essential for achieving this. Solder paste is a special metal alloy composed of metal solder particles and a unique combination of flux. During the soldering process, the distinctive properties of solder paste allow it to melt and form a permanent bond, thus establishing a reliable connection between metals.

To ensure good performance during the soldering process, solder paste must meet several important standards. Firstly, it must have a relatively low melting point, considering the sensitivity of components to temperature. If the melting point of the solder paste is too high, there is a risk of reaching the maximum temperature the components can withstand before it fully melts. Secondly, the conductivity of solder paste is crucial as it is used not only for mechanical connections but also as an electrical connection, ensuring uninterrupted flow of electrical current to the components. Lastly, to ensure safety for both humans and the environment, the raw materials of solder paste must comply with the strict requirements of the RoHS directive.

In the past, approximately 40% of solder pastes in electronic manufacturing consisted of a combination of lead and tin. However, due to the environmental hazards associated with lead, and with advancements in technology and increased environmental awareness, the electronic manufacturing industry has gradually transitioned to lead-free processes. Consequently, lead-free solder paste has become the mainstream choice, reflecting a positive response to environmental concerns and industry focus on health and sustainability.

Choosing the Right Solder Paste Type

Using high-quality and the correct type of solder paste is a crucial step in ensuring the quality of PCBA boards. In assembly projects, FS Technology offers various grades and types of solder pastes to meet different requirements. You can choose based on the materials planned for soldering and the needs, or select according to the size of solder balls in the solder paste.

To meet the requirements of different projects, solder pastes are usually divided into different grades and categories. These grades are not solely based on the accurate size of solder balls but are evaluated based on around 80% of the solder balls falling within a specific size range. Here are the various grades and their corresponding solder ball sizes.

Grade of Solder Paste

Size of Particle

Type 1

75mm to 150mm

Type 2

45mm to 75mm

Type 3

25mm to 45mm

Type 4

20mm to 38mm

Type 5

10mm to 25mm

Type 6

5mm to 15mm

Type 7

2mm to 11mm

Type 8

2mm to 8mm

In addition to considering the particle size, choosing the appropriate solder paste based on the type of flux is also a viable option. Different types of PCB flux can better match specific solder paste formulations to achieve better soldering results or meet specific process requirements. However, not all environments allow for the use of this type of flux, and it must be ensured that the environment is clean enough.

  • Water-Soluble Flux Paste: Composed of organic components and ethylene glycol base. Residues of water-soluble flux can be easily removed by using readily available cleaning products on the market.
  • No-Clean Solder Paste: This type of solder paste is typically used in the manufacture of no-clean flux. It has relatively low solid residues and resin content. The main advantage of using a no-clean flux is the potential cost savings on cleaning supplies.
  • Rosin-Based Solder Paste: Solder pastes of this kind are made from rosin and can be cleaned using solvents after soldering.

Storing PCB Solder Paste

Depending on the temperature and humidity of the PCBA factory, store these ready-to-use solder pastes in specially designed packaging that keeps the product’s temperature between 0° and 30 °C (86 °F and 84 °F) for at least 48 hours.

Solder paste should be delivered overnight or through chilled transportation. In summer, transport at night as much as possible to reduce time and avoid high temperatures. Paste packaging may include cold packs that arrive melting. If the 48-hour travel time is not exceeded, this is typical and not a sign of damage or improper handling. Upon arrival, solder paste must be moved right away to controlled storage. The use of refrigeration is advised but not always necessary.

At normal temperatures (22 °C/72 °F), solder pastes usually have a storage life of between three and six months. Unrefrigerated: 25 °C (77 °F) / Refrigerated: 0°-12 °C (32 °F-55 °F). The performance of solder paste may be impacted by exposure to temperatures above 30 °C (85 °F). Keep solder paste away from heat sources that are hotter than 32 °C (90 °F). When feasible, store solder pastes in syringes or cartridges vertically, tip down. Packages should occasionally be turned 180 degrees if they are horizontally stored.

Paste must be used before the date indicated on the product-specific TDS, which is the Date of Manufacturing (DOM) on the label. On a FIFO system with the date and time of removal from storage recorded, the oldest solder paste lots should be utilized first. Cold usage of solder paste is not permitted. Before opening the paste package, filler material should be allowed to attain room temperature, which is 68°–77 °F (20°–25 °C). Normally, this takes between four and six hours. DO NOT HEAT THE PASTE FORCEFULLY.

The solder paste in jars ought to be lightly and thoroughly mixed with a plastic tool for at least one minute after it has reached room temperature. Syringes and cartridges for solder paste don’t need to be mixed. It is not advised to use automated solder paste mixing equipment since it could harm the solder paste.

PCB Solder Paste Printing Process

Stencil Printing

PCB Stencil Printing

The most common practice is stencil printing, which is a special metal grid structure typically made of stainless steel. Manufacturers first create a PCB stencil with holes or apertures of the same size and shape as the surface mount pads in the design. To ensure accuracy, FS Technology employs laser cutting technology to process the stencil and applies a coating material on the surface to smooth the apertures and facilitate even distribution of solder paste.

The stencil is then placed in an SMT device and aligned with the PCB below. Solder paste is uniformly applied to the stencil, and a squeegee blade is used to scrape off excess paste, allowing it to drop onto the PCB surface through the apertures. The entire process typically takes about 15 to 45 seconds.

After the solder paste printing is completed, SPI testing is conducted to assess the uniformity and quality of the solder paste, ensuring each pad receives the correct coverage. Finally, the stencil is cleaned after completing a batch of PCB solder paste printing to ensure that residue from the previous printing does not impact the distribution of solder paste on the next board.

Jet Printing

While stencil printing is advantageous for large-scale PCB assembly, constructing the stencil takes a considerable amount of time and precise control over the solder paste quantity for each component is challenging. Therefore, Jet Printing is becoming increasingly popular.

Jet Printing utilizes a modified formula to create a more liquid-like solder paste. Inkjet printers, using the same PCB design data as stencil printing, apply a precise amount of solder paste to each interface pad on the board. Depending on the solder material required for the application, the printer can easily switch and alternate between solder paste cartridges, from which tiny droplets of solder paste are jetted out.

Jet Printing technology allows PCB assemblers to precisely control the amount of solder paste for each interface, providing them with significant flexibility. This means that solder paste quantities for each solder point can be adjusted based on specific requirements.


In repair or hobbyist projects, the printing method is usually not employed to apply solder paste to PCBs. Instead, there are two common manual methods:

  • Using a syringe, precisely apply solder paste to the pins and then use a soldering iron to bond it with the pins.
  • Place solder wire between the two pins of the component and then use a soldering iron to heat it.

Regardless of the method chosen, the ultimate goal is to establish a reliable connection between the PCB and the component. After manual soldering is completed, it is advisable to perform a quality check and conduct functional tests to ensure the reliability of the connections.

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