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PCB Solder Paste: Types, Application & Troubleshooting

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In electronics manufacturing, it is essential to bond every circuit board component to ensure the device works smoothly. Solder paste can make a strong bond between the components without hampering the electrical connection. Solder paste is like glue that holds each electronic component together, providing solid connections between boards and components during soldering.
In this article, ELEPCB will discuss solder paste, its use in circuit boards, the proper method of using a paste, selection advice, and a comparison between solder paste and solder flux.

What is Solder Paste?

Solder paste is a mixture of paste-like flux and solder powder, which is used in electronics to join the components in PCB. It is viscous in state and is gray. This paste is applied to the areas where components need to be soldered onto the PCB. It is typically applied using a stencil or a dispenser machine before components are placed onto the PCB and then heated in a reflow oven to complete the soldering process.
When heated during the soldering process, the flux helps clean the metal surfaces, and the solder particles melt, creating a strong electrical and mechanical connection between the components and the board.
solder paste for smd

What is the Use of PCB Solder Paste?

Below are some key reasons why PCB paste is used in electronics manufacturing:

Bonding Components

It is a bonding agent that attaches the electronics components like capacitors, resistors, connectors, and integrated circuits, creating a secure bond between the leads of components and the PCB pads. PCB paste is widely used in surface mount technology (SMT) soldering, which is the process of creating a PCB from SMD components or mounting the components in the pads of the PCB.

Conductivity

It has excellent electrical conductivity. It efficiently transmits electrical signals between the components and ensures electronic devices’ proper functioning.

Thermal Conductivity

It helps to dissipate the heat generated by electronic components during the operations.

Mechanical Strength

Solder joints created by solder paste provide strong mechanical support to electronic components. This mechanical strength is crucial for withstanding mechanical shocks, vibrations, and other environmental stresses.

Miniaturization

With the trend toward miniaturization in electronics design, solder paste offers a precise and efficient method for attaching small and delicate components to densely populated PCBs. It allows for assembling complex electronic circuits in compact and space-constrained devices.

Types of Solder Paste

Solder pastes can be mainly classified into the following types according to different composition contents and applications:

Lead-Based Paste

The lead-based PCB paste contains lead as a major component in the solder alloy. However, its use is declining because of environmental worries about lead toxicity. It was once widely utilized but is now being phased out in many applications in preference for lead-free substitutes.

No-clean Paste

After the soldering procedure, this kind of solder paste leaves very little residue. Electronics can operate normally with this residue because it is not conductive or acidic, requiring no special cleaning procedures. No-clean solder paste is useful in situations where thorough cleaning could be challenging or not feasible.

Wate-Soluble Paste

After soldering, water can be used to remove water-soluble solder paste readily. However, it is critical to ensure that the solder joints are thoroughly dried thereafter to avoid corrosion. While water-soluble fluxes make cleaning more accessible than other types, adequate drying techniques are required to ensure the integrity of the electronic assembly.

Rosin Based Paste

This solder paste contains rosin flux made from natural pine resin. Rosin-based fluxes are preferred for general-purpose soldering because of their high wetting qualities and ability to remove surface oxides from metal surfaces. However, they leave a sticky residue after soldering that may need to be cleaned to avoid long-term dependability difficulties.

what is solder paste

How is Solder Paste Applied to PCB?

Applying solder paste to PCB is typically done in a few steps:

Step 1:
Preparation

Step 2:
Stencil Printing

Step 3:
Solder Paste Application

First, the PCB is prepared by cleaning its surface to remove any contaminants or oxidation that could interfere with the soldering process.
A PCB stencil, usually made of stainless steel or polyester, is placed over the PCB. This stencil has openings, called “apertures,” that align with the locations where components will be soldered onto the board.
It is then applied onto the stencil. It is done manually using a squeegee to spread the paste over the stencil openings, or it can be done automatically using a solder paste dispenser machine.

Step 4:
Squeegeeing

Step 5:
Component Placement

Step 6:
Reflow Soldering

If applied manually, a squeegee is used to press the paste through the apertures in the stencil, ensuring that the paste is evenly distributed and fills the openings properly.
After applying PCB paste, electrical components such as resistors, capacitors, integrated circuits, and connectors are carefully put on the PCB and aligned with the matching PCB paste deposits.
The PCB, complete with components and PCB paste, is then placed in a reflow oven. The temperature in the oven is carefully managed to slowly bring the PCB and paste to a specific temperature, usually around the solder alloy’s melting point.

Step 7:
Soldering

Step 8:
Cooling and Inspection

As the PCB paste reaches its melting point, the solder particles liquefy and form metallurgical bonds between the component leads and the PCB pads. The flux helps to remove oxides and contaminants from the metal surfaces, ensuring strong and reliable solder joints.
After soldering, the PCB is cooled to solidify the solder joints. It then undergoes inspection to verify the quality of the solder connections. Automated optical inspection (AOI) and X-ray inspection are commonly used techniques to check for defects such as insufficient solder, solder bridges, or misalignment.

How is Solder Flux Different from Solder Paste?

FeatureSolder FluxSolder Paste
CompositionTypically, liquid or gel substance is used to clean and prepare metal surfaces for soldering.A mixture of flux and solder particles, providing both fluxing and bonding properties.
ApplicationApplied separately onto the metal surface before soldering.Pre-mixed and applied directly onto the components to be soldered.
PurposeCleans metal surfaces, removes oxides and promotes solder flow.Facilitates soldering by holding components in place and enhancing conductivity.
FormUsually liquid, gel, or solid form.Semi-solid consistency, similar to toothpaste.

Conclusion

In the precision-driven world of electronics manufacturing, solder paste is more than just a material; it is the critical foundation for reliable PCB performance. The longevity of products in automotive, medical and IoT applications is directly determined by your choice of solder paste.

At ELEPCB, we combine material science with SMT expertise to solve your toughest assembly challenges. Contact ELEPCB today to make your PCB strong by using solder paste.

FAQs

A1: The strength of PCB paste varies based on how it’s made and applied. But usually, people don’t pick solder paste for its strength in holding things together. Its main job is to make sure electricity flows smoothly in electronic devices. The connections it forms are usually strong enough to keep circuits stable. If extra strength is needed, people might use things like screws or glue along with soldering.
A2: To ensure the reliability of solder joints with solder paste, consider the following steps:
  • First, try to follow the recommended soldering profiles and guidelines provided by manufacturers.
  • Then, perform a visual inspection or use automated inspection techniques to verify solder joint quality.
  • Conduct reliability testing, like thermal cycling or vibration testing. This ensures the board’s long-term performance.
A3:
  • Ensure the PCB surface is clean and free of contaminants.
  • Use a high-quality stencil or dispenser machine for precise application.
  • Ensure even distribution of solder paste on PCB pads and proper alignment of components.

A4: Lead-based paste includes lead in the solder mixture, while lead-free versions align with environmental standards by omitting lead.

The shift towards lead-free options is driven by environmental considerations, offering dependable solder connections without using lead.

A5:

  • No-clean solder paste leaves minimal residue after soldering. It doesn’t affect electronics operation and doesn’t require cleaning, too.
  • Water-soluble solder paste can be easily removed with water after soldering. Thus, thorough drying is necessary to prevent corrosion.
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Rashila
I am an Electrical and Electronics Engineer, and I have 5 years of work experience with electronics and communication jobs. I'm the full time content creator of ELE company.
About Benjamin

Benjamin is the general manager of ELE PCB, a leading PCB design and manufacturing company based in China. He has over 10 years of experience in the PCB industry, and has been involved in various projects.

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