Solder Paste Types and Their Uses

Different solder paste types achieve varying results. Users must, therefore, carefully choose the type of paste for their projects. It ensures high-quality joints and reduces defects. Below, we explore the options and provide helpful selection tips.

What Is Solder Paste?

Solder paste is a thick, sticky material for mounting components on a printed circuit board. It contains solder powder and flux in different amounts and other variations, such as particle sizes.

The variations give rise to different types of the product. Because the paste prepares the pads for soldering, having the right kind for your PCB is the key to a successful project.

So, how many types of solder paste formulations are there today? The following section explains the different options and their categorization criteria.

Solder Paste Types

There are different types of solder paste formulations for PCB assembly, classified based on the following: the side of the solder spheres, flux type, alloy type, and application methods.

By Powder Particle Size

The IPC J-STD-005 standard grades the particles from size 1 up, with 1 being the largest. The table below shows their suitable uses and the powder sizes in microns.

• Size 1: Rarely used in modern electronics. (75-150)
• Size 2: Suitable for specific THT components. (45-75)
• Size 3: General applications. (25-45)
• Size 4: Fine-pitched components. (20-38)
• Size 5: Extra fine pitches. (10-25)
• Size 6: Ultra-fine pitches. (5-15)
• Size 7: Specialized applications. (2-11)

In modern SMT, Type 4 is rapidly becoming the standard over Type 3 due to smaller components (0201s, 01005s). Type 3 is still “common,” but Type 4 is the new “standard” for many assembly houses.

Among Type 3, 4 and 5, Type 3 is the cheapest of the three. It mounts larger components, such as LEDs. Type 4 is suitable for most applications, while grades 5 onward mount IC packages.  

By Flux Type

Flux chemistry also groups the paste. Options include rosin-based, no-clean, and water-soluble fluxes. Let’s explore what each composition means when used in soldering printed circuit boards.

• Rosin-Based: Rosin consists of natural organic acids (extracted from pine trees). The material readily dissolves in a solvent, such as isopropyl alcohol, enhancing the cleaning process.
• No-Clean: This paste requires no cleaning after soldering. It has materials that leave minimal residuals. The residuals of no-clean fluxes are also low-activity and don’t require removal.   
• Water-Soluble: This type has organic materials and solvents. The composition makes it easy to clean using de-ionized water. It suits high-reliability PCBs requiring complete removal of residuals.

By Alloy Composition

Solder is a low-melting alloy that combines tin and lead or other metals, such as copper and silver.  The alloy is typically either lead-based or lead-free. This composition difference classifies the product.

• Leaded: Lead-based paste contains lead in its solder balls. The Sn/Pb alloy produces high-quality joints, making it a preferred choice in high-reliability PCB types and applications.
• Lead-Free: It contains no lead and instead uses tin and other metals (typically SAC305 – Tin/Silver/Copper). The lack of lead makes it an eco-friendly option and RoHS-compliant.

By Application Method

There are different ways of applying the paste when mounting surface-mount components and sometimes through-hole types. These require different types of pastes for efficient transfer.  

• Stencil Printing: This type is formulated to pass through openings on a stencil and under the pressure of a squeegee blade.
• Jet Printing: Some formulations are meant to stream from jet-printing nozzles. They typically have fine particles to ensure precision.
• Dispensing: The types in this category have characteristics that allow them to flow from narrow tips smoothly.
• Pin Dipping: This application method involves dipping component leads or pins in the paste instead of printing or tip dispensing.

Which Solder Paste Type Is Better?

The best type to use depends on your project’s unique needs and constraints. For instance, you must use lead-free soldering if you intend your PCBs to operate in a RoHS region (like the EU).

On the other hand, some applications require lead-based solders for reliability needs. These include critical PCBs for automotive, medical, military, and aerospace industries.

Critical PCBs may also require water-soluble fluxes.  As you can see, each application has factors that influence the type of paste to use. The following section will help you choose the best type for your PCB project.

Choosing a Solder Paste for Your Application

Solder paste selection can mean the difference between clean, robust joints and poor wetting and other soldering process issues. Use these tips to guide your decisions and make the right choice.

Components and PCB type

Fine particles offer better printability and higher transfer efficiency than larger ones. This property makes them the best for miniaturized components. For instance, you need type 4 for 0.5 mm packages.

High-density PCBs also require smaller particle sizes than standard boards. Additionally, it’s crucial to use the same type of solder when repairing or reworking boards.

Reflow Profile Constraints

Different alloys melt at varying temperatures. Ensure the alloy is compatible with the reflow process profiles to ensure high joint quality. That’s because the best wetting occurs at slightly above the melting point.

Cleaning Needs

Here, you have a choice of soluble and no-clean types. No-clean fluxes reduce PCB production costs by eliminating the cleaning step. However, they don’t suit high-reliability PCBs.

Choose water-soluble fluxes if the board has strict cleanliness requirements. It ensures the removal of all residuals, whether conductive or non-conductive.

Environmental Standards

Your choice also depends on the standards your project must meet. One of them is environmental obligations as defined by RoHS directives, which require the use of lead-free alloys.

Mistakes to Avoid When Selecting Solder Paste

Solder paste mistakes can lead to incompatibility with components, weak solder joints, and various other outcomes. Here is what to avoid for proper component mounting and other needs.

1. Choosing the wrong solder particle size. The spheres must easily pass through the stencil apertures or any other application opening to avoid transfer problems. A general rule is that at least five solder spheres should fit across the smallest aperture width.

2. Choosing an inappropriate flux. Different fluxes have varying characteristics. Your choice must be the best for your application.

3. Not considering applicable standards in the electronics industry. It helps to ensure the type you use complies with the regulatory requirements. They include those concerning the environment and joint reliability.

4. Not matching the paste with the application method. Manufacturers formulate the product for specific requirements, such as fine-feature printing. Your choice must take that into account.

5. Not buying from reputable manufacturers. The product is one of the most essential materials for surface-mount technology. Ensure its quality to avoid soldering issues.

Conclusion

Solder paste selection is a fundamental part of the soldering process. This requirement makes understanding the different paste types crucial. We hope the information in this article helps you make those critical decisions when matching the product with your applications.