What is Solder Paste?

Solder paste is a gray, paste-like material that consists of tiny solder particles (typically lead-free alloys such as tin-silver-copper or SAC305) suspended in a flux medium. The flux helps to remove oxides and other contaminants from the metal surfaces, allowing the solder to wet and form a strong bond with the PCB pads and component leads.

Solder paste is applied to the PCB using a stencil printing process, where a thin metal stencil with apertures corresponding to the PCB pads is placed over the board. The solder paste is then spread across the stencil using a squeegee, filling the apertures and depositing the paste onto the pads. After the components are placed on the PCB, the assembly is heated in a reflow oven, melting the solder particles and forming permanent solder joints.

Choosing the Right Solder Paste

When selecting solder paste for your assembly process, there are several key factors to consider:

Alloy Composition

The most common lead-free solder alloys used in electronics manufacturing are:

• SAC305 (96.5% tin, 3% silver, 0.5% copper)
• SAC387 (95.5% tin, 3.8% silver, 0.7% copper)
• SN100C (99.3% tin, 0.7% copper, with a small amount of nickel and germanium)

Choose an alloy that is compatible with your PCB Surface Finish and components, and meets the required mechanical and thermal properties for your application.

Particle Size

Solder paste is classified by its particle size, which is typically expressed as a mesh size or type. Common sizes include:

• Type 3 (25-45 microns)
• Type 4 (20-38 microns)
• Type 5 (15-25 microns)
• Type 6 (5-15 microns)

Smaller particle sizes allow for finer pitch components and more precise printing, but may be more prone to clogging stencil apertures. Choose a particle size that is appropriate for your component pitches and stencil thickness.

Flux Type

Solder paste flux is classified by its activity level and residue characteristics:

• No-clean (NC): Leaves a minimal, non-conductive residue that does not require cleaning
• Water-soluble (WS): Requires cleaning with water or an aqueous solution to remove the residue
• Rosin-based (RMA): Leaves a non-conductive, rosin-based residue that may require cleaning for some applications

Select a flux type that is compatible with your cleaning process and meets the reliability requirements for your end product.

Viscosity and Rheology

Solder paste viscosity and rheology (flow characteristics) are important for achieving consistent print quality and preventing defects such as slumping or bridging. The paste should have a stable viscosity over time and be able to recover quickly after shear stress (such as during printing). Consult with your solder paste supplier for recommendations based on your specific application and printing equipment.

Storing Solder Paste

Proper storage of solder paste is essential for maintaining its quality and performance. Follow these guidelines to ensure optimal shelf life and print ability:

Refrigeration

Store solder paste in a refrigerator at a temperature between 0-10°C (32-50°F) when not in use. This slows down the flux activation and prevents the solder particles from settling or agglomerating.

Thawing

Before using refrigerated solder paste, allow it to thaw at room temperature for at least 4 hours. Do not open the container until the paste has fully thawed to prevent condensation from forming on the surface.

Stirring

After thawing, stir the solder paste gently with a spatula to ensure a homogeneous mixture. Avoid introducing air bubbles, which can cause printing defects.

Shelf Life

Most solder pastes have a shelf life of 6-12 months when stored at refrigerated temperatures. Check the manufacturer’s recommendations and use the paste before the expiration date.

Using Solder Paste

To achieve reliable and high-quality solder joints, follow these steps when using solder paste:

Stencil Printing

1. Align the stencil with the PCB and secure it in place
2. Apply solder paste to one end of the stencil and spread it across the apertures using a squeegee
3. Lift the stencil away from the PCB, leaving the solder paste deposits on the pads
4. Inspect the print quality and adjust the process parameters (squeegee pressure, speed, etc.) as needed

Component Placement

1. Place the components onto the PCB, aligning the leads or terminals with the solder paste deposits
2. Ensure that the components are seated properly and not shifted or tilted
3. Use a pick-and-place machine for high-volume production or precise placement of small components

Reflow Soldering

1. Preheat the PCB Assembly to activate the flux and evaporate the solvents
2. Ramp up the temperature to the solder liquidus point, allowing the solder particles to melt and form a homogeneous liquid
3. Hold the peak temperature for a specified time to allow the solder to wet the surfaces and form intermetallic bonds
4. Cool the assembly to solidify the solder joints
5. Inspect the solder joints for defects such as bridging, voiding, or poor wetting

Cleaning (if required)

1. If using a water-soluble or rosin-based flux, clean the PCB assembly with the appropriate solution and method (e.g., spray, immersion, or ultrasonic cleaning)
2. Rinse the assembly with deionized water and dry thoroughly
3. Inspect the assembly for cleanliness and residue removal

FAQ

1. Can I use expired solder paste?

No, it is not recommended to use expired solder paste as its performance and print ability may be compromised. The flux activity may be reduced, leading to poor wetting and solder joint defects.

2. How do I know if my solder paste has gone bad?

Signs of deteriorated solder paste include:
– Separated or settled solder particles
– Hardened or dried out paste
– Foul odor indicating flux degradation
– Poor print ability or inconsistent deposits

If you notice any of these signs, discard the paste and use a fresh batch.

3. Can I mix different brands or types of solder paste?

No, mixing different brands or types of solder paste is not recommended as they may have incompatible alloys, flux chemistries, or rheological properties. Stick to a single brand and type for consistent results.

4. How do I dispose of unused solder paste?

Consult your local regulations for proper disposal of solder paste, as it may be considered hazardous waste due to the presence of lead or other heavy metals. Do not dispose of solder paste down the drain or in regular trash.

5. Can I reuse solder paste that has been left out at room temperature?

No, solder paste that has been left out at room temperature for an extended period (more than 8 hours) should not be reused. The flux may have degraded and the solder particles may have settled or oxidized, leading to poor print ability and solder joint defects.

Conclusion

Choosing the right solder paste and using it correctly is essential for achieving reliable and high-quality solder joints in electronic assembly. Consider factors such as alloy composition, particle size, flux type, and rheology when selecting a solder paste for your application. Store the paste in a refrigerator when not in use and allow it to thaw completely before printing. Follow best practices for stencil printing, component placement, reflow soldering, and cleaning to ensure optimal results. By understanding and managing the critical aspects of solder paste, you can improve your assembly process and product quality.