What is Solder Paste?

Solder paste is a material used in electronics manufacturing to attach surface mount components to printed circuit boards (PCBs). It is a mixture of tiny solder particles suspended in flux. The solder particles are typically made of a tin-lead alloy, although lead-free solder paste is becoming increasingly common due to environmental and health concerns.

Solder paste is applied to the PCB using a stencil or syringe, and then the components are placed on top of the paste. When the board is heated in a reflow oven, the solder melts and forms a strong electrical and mechanical connection between the component leads and the PCB pads.

Solder Paste Essentials

To ensure high-quality solder joints and reliable circuit performance, it is essential to understand the key properties and characteristics of solder paste. Here are some of the most important factors to consider:

Alloy Composition

The alloy composition of the solder particles in the paste determines its melting point, wetting properties, and mechanical strength. The most common alloys used in solder paste are:

Alloy	Composition (by weight)	Melting Point (°C)
Sn63/Pb37	63% tin, 37% lead	183
Sn62/Pb36/Ag2	62% tin, 36% lead, 2% silver	179
SAC305	96.5% tin, 3% silver, 0.5% copper	217-220
Sn42/Bi57/Ag1	42% tin, 57% bismuth, 1% silver	138

The choice of alloy depends on the specific application requirements, such as operating temperature, environmental conditions, and regulatory compliance.

Particle Size

The size of the solder particles in the paste is another critical factor that affects its performance. Smaller particles allow for finer pitch components and more precise control over the amount of solder deposited. However, smaller particles also have a higher surface area to volume ratio, which can make them more susceptible to oxidation and require more flux to achieve proper wetting.

Solder paste is classified by its particle size using a mesh number, which indicates the number of openings per linear inch in the sieve used to sort the particles. Common mesh sizes for solder paste include:

Mesh Size	Particle Size Range (µm)
Type 3	25-45
Type 4	20-38
Type 5	15-25
Type 6	5-15

Flux

The flux in solder paste serves several important functions:

1. It removes oxides and other contaminants from the surfaces of the solder particles, component leads, and PCB pads, allowing for better wetting and a stronger solder joint.
2. It protects the surfaces from re-oxidation during the reflow process.
3. It helps to evenly distribute the solder particles and control the flow of molten solder.

Solder paste flux is classified by its activity level and halide content:

Flux Type	Activity Level	Halide Content	Cleaning Required
ROL0	Low	<0.05%	No
ROL1	Low	<0.5%	Yes
ROH0	High	<0.05%	No
ROH1	High	<0.5%	Yes

The choice of flux depends on the specific application requirements, such as the type of surfaces being soldered, the reflow profile, and the cleaning process (if any).

Rheology

Rheology refers to the flow properties of solder paste, which determine how easily it can be dispensed and how well it maintains its shape after being applied to the PCB. The two main rheological properties of solder paste are viscosity and thixotropy.

Viscosity is a measure of the paste’s resistance to flow under stress. A paste with high viscosity will be thicker and more difficult to dispense, while a paste with low viscosity will be thinner and more prone to slumping or spreading.

Thixotropy is a measure of the paste’s ability to change its viscosity depending on the amount of shear stress applied. A highly thixotropic paste will have a high viscosity when at rest, but will become less viscous when stirred or dispensed. This property allows the paste to be easily applied to the PCB while still maintaining its shape and preventing slumping.

The rheology of solder paste can be affected by various factors, such as the particle size distribution, flux composition, and storage conditions. It is important to select a paste with the appropriate rheological properties for the specific application and to follow the manufacturer’s guidelines for storage and handling to ensure consistent performance.

Storage and Handling

Proper storage and handling of solder paste are critical to maintaining its quality and performance. Here are some key guidelines to follow:

1. Store solder paste in a refrigerator at a temperature between 0-10°C (32-50°F) to prevent premature flux activation and minimize oxidation.
2. Allow the paste to warm up to room temperature before opening the container to prevent condensation from forming on the surface of the paste.
3. Gently mix the paste before use to ensure even distribution of the solder particles and flux.
4. Use clean, dry tools and equipment when handling solder paste to avoid contamination.
5. Follow the manufacturer’s recommendations for the maximum storage time and shelf life of the paste.

Solder Paste Application Methods

There are two main methods for applying solder paste to a PCB: stencil printing and syringe dispensing.

Stencil Printing

Stencil printing is the most common method for applying solder paste in high-volume production. It involves using a metal stencil with apertures that match the size and location of the pads on the PCB. The stencil is placed over the PCB, and solder paste is spread across the surface of the stencil using a squeegee blade. The paste is forced through the apertures and onto the pads, creating a precise and consistent deposit of solder paste.

Stencil printing requires careful control of various parameters, such as:

1. Stencil thickness and aperture size
2. Squeegee pressure and speed
3. Snap-off distance (the gap between the stencil and PCB)
4. Print direction and pattern
5. Paste viscosity and rheology

Proper optimization of these parameters is essential for achieving high-quality solder paste deposits and minimizing defects such as bridging, solder beads, or insufficient solder volume.

Syringe Dispensing

Syringe dispensing is a method for applying solder paste in low-volume production or for rework and repair. It involves using a syringe filled with solder paste and a pneumatic or manual dispenser to apply the paste directly onto the PCB pads.

Syringe dispensing offers several advantages over stencil printing:

1. Flexibility: Syringe dispensing can be used for a wide range of pad sizes and shapes, and can easily accommodate changes in the PCB design.
2. Precision: Syringe dispensing can achieve very precise control over the amount and location of solder paste deposited.
3. Cost: Syringe dispensing requires less equipment and tooling compared to stencil printing, making it more cost-effective for low-volume production.

However, syringe dispensing also has some limitations:

1. Speed: Syringe dispensing is slower than stencil printing and may not be suitable for high-volume production.
2. Consistency: Syringe dispensing is more dependent on operator skill and may result in less consistent solder paste deposits compared to stencil printing.

Reflow Profile

After the solder paste has been applied to the PCB and the components have been placed, the board undergoes a reflow process to melt the solder and form a permanent connection. The reflow profile is a critical factor in achieving high-quality solder joints and preventing defects such as tombstoning, bridging, or insufficient wetting.

A typical reflow profile consists of four stages:

1. Preheat: The board is gradually heated to a temperature below the melting point of the solder to activate the flux and remove any moisture or volatiles.
2. Soak: The board is held at a constant temperature to allow the flux to fully activate and the components to reach a uniform temperature.
3. Reflow: The board is rapidly heated to a peak temperature above the melting point of the solder, causing the solder to melt and wet the surfaces of the components and pads.
4. Cooling: The board is cooled at a controlled rate to allow the solder to solidify and form a strong, reliable joint.

The specific temperatures and durations of each stage depend on various factors, such as the type of solder paste, the size and thermal mass of the components, and the PCB material and thickness. It is important to follow the paste manufacturer’s recommended reflow profile and to use proper equipment and process controls to ensure consistent results.

Solder Paste Defects and Troubleshooting

Despite careful process control and optimization, solder paste defects can still occur due to various factors such as contamination, improper handling, or equipment malfunction. Here are some common solder paste defects and their possible causes and solutions:

Defect	Possible Causes	Solutions
Bridging	Excessive solder paste volume, slumping, or poor stencil design	Reduce paste volume, adjust stencil aperture size or shape, increase stencil thickness
Solder Beads	Excessive paste volume, poor paste release from stencil, or contaminated stencil	Reduce paste volume, clean or replace stencil, adjust print parameters
Insufficient Solder	Insufficient paste volume, poor paste transfer efficiency, or oxidized paste	Increase paste volume, optimize print parameters, use fresh paste
Voiding	Entrapped flux or volatiles, poor wetting, or rapid cooling	Optimize reflow profile, use paste with lower voiding potential, improve PCB and component design
Tombstoning	Uneven heating or cooling, poor paste or component placement, or insufficient paste volume	Optimize reflow profile, improve component placement accuracy, increase paste volume

To troubleshoot solder paste defects, it is important to systematically analyze the process and identify the root cause of the problem. This may involve:

1. Inspecting the solder paste deposits using visual or automated inspection methods
2. Monitoring the reflow profile using thermocouples or profiling systems
3. Analyzing the stencil design and print parameters
4. Checking the solder paste quality and storage conditions
5. Reviewing the component placement and board design

By understanding the key factors that affect solder paste performance and following best practices for storage, handling, and processing, manufacturers can minimize defects and achieve high-quality, reliable solder joints.

Frequently Asked Questions (FAQ)

1. What is the shelf life of solder paste?

The shelf life of solder paste depends on the specific formulation and storage conditions, but is typically around 6-12 months when stored at the recommended temperature (0-10°C or 32-50°F). However, it is important to follow the manufacturer’s guidelines and to use the paste before the expiration date to ensure optimal performance.

2. Can solder paste be reused?

Solder paste should not be reused after it has been dispensed or printed onto a PCB. Once the paste has been exposed to air and heat, it can start to degrade and lose its flux activity, leading to poor solder joint quality. Any unused paste should be properly stored in a sealed container and used within the recommended shelf life.

3. How do I choose the right solder paste for my application?

Choosing the right solder paste depends on various factors, such as the type of components being soldered, the PCB material and finish, the reflow profile, and the environmental and reliability requirements. Some key considerations include:

• Alloy composition (e.g., lead-free vs. leaded, melting point)
• Particle size (e.g., Type 3, 4, 5, or 6)
• Flux activity and halide content (e.g., ROL0, ROH1)
• Rheology and printing properties (e.g., viscosity, slump resistance)
• Voiding potential and wetting ability
• Compatibility with cleaning processes (if applicable)

It is important to consult with the solder paste manufacturer or supplier and to conduct thorough testing and evaluation to select the best paste for your specific application.

4. How do I store and handle solder paste?

To ensure optimal performance and shelf life, solder paste should be stored and handled according to the following guidelines:

• Store paste in a refrigerator at 0-10°C (32-50°F) in a sealed container
• Allow paste to reach room temperature before opening the container to prevent condensation
• Gently mix the paste before use to ensure even distribution of solder particles and flux
• Use clean, dry tools and equipment when handling paste to avoid contamination
• Follow the manufacturer’s recommendations for maximum storage time and shelf life
• Dispose of expired or contaminated paste according to local regulations

5. What are some common causes of solder paste defects?

Some common causes of solder paste defects include:

• Incorrect paste volume or stencil aperture design
• Poor paste rheology or printing properties
• Contamination or oxidation of paste or PCB surfaces
• Incorrect reflow profile or temperature control
• Poor component placement or board design
• Insufficient or excessive cleaning of PCBs or stencils
• Improper storage or handling of paste or materials

To prevent solder paste defects, it is important to carefully control and monitor all aspects of the solder paste printing and reflow process, from material selection and storage to process parameter optimization and quality inspection. Regular maintenance and calibration of equipment, as well as employee training and adherence to best practices, can also help minimize defects and ensure consistent, high-quality results.