Table of Contents
Solder Paste in PCB Assembly: Selection, Printing, Defects & Expert Tips
This article was originally published in Jan 2023 and last updated on July 19, 2026.
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.
What is Solder Paste and Why Use it?
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:
- Low Melting Point: Given the heat sensitivity of the components, it should have a low melting point. In case the melting point is too high, there could be a possibility of the components being subjected to the maximum possible heat before completely melting.
- High Conductivity: It serves the purpose of not only making physical contact but also providing the electrical connection to enable the free flow of electrical current through the components.
- Compliant to RoHS Directive: The use of solder paste should be done such that it complies with the strict guidelines set by 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.
Solder Paste Type: Particle Size, Flux & Alloy
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.
By Particle Size (Powder Type)
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 |
Pro Tip:
For standard SMT assembly with 0603 components and 0.4mm pitch ICs, Type 4 solder paste (20-38μm) is the most versatile and widely recommended choice. For ultra-fine pitch applications like 01005 components, Type 5 or finer is required.
By Flux Type
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.
By Alloy Composition
Whereas the original passage is concerned about the size of particles, the choice of alloy is just as important. The selection of alloy will determine the melting temperature, mechanical strength, and reliability of the solder joint.
- SAC305 (Sn96.5/Ag3.0/Cu0.5): Industry standard for lead-free soldering applications. Provides the optimal combination of cost, thermal fatigue resistance, and wetting properties, thus suitable for all general-purpose applications such as automobiles and industrial electronics.
- Low Temperature Alloys (for example, Sn-Bi): Used for components that require low temperature or when secondary reflow is necessary, with primary solder joints remaining intact.
How to Select the Right Solder Paste for Your PCBA Project?
The selection of an appropriate type of solder paste is not universal. In order to have a high yield rate and reliability, you need to consider your project based on the following criteria. Please use the decision guide together with the technical information above.
| Project Factor | Recommended Solder Paste Characteristics | Rationale |
| Component Density & Miniaturization | For high-density boards with 0201/01005 components and <0.4mm pitch BGAs, choose Type 4 or Type 5 powder with fine particle size. | Finer powders ensure better print release and prevent clogging in narrow apertures, reducing the risk of insufficient solder. |
| Product Reliability Requirements | For automotive, medical, or aerospace applications, opt for ROL0 (low-activity) no-clean flux combined with SAC305 or high-reliability alloys. | Low-activity flux leaves minimal ionic residues, preventing electrochemical migration. High-reliability alloys offer superior thermal cycling and shock resistance. |
| Production Volume & Mix | For high-mix, low-volume or prototype runs, Jet Printing with a fluid solder paste offers flexibility. For mass production, stencil printing with Type 4 paste is cost-effective. | Jet printing eliminates stencil costs and allows quick changeovers. Stencil printing provides high throughput for repetitive jobs. |
| Regulatory Compliance | If your products are exported to EU or North America, ensure the solder paste is RoHS-compliant (lead-free). For medical implants, also consider biocompatible alloys. | Non-compliance can lead to legal barriers and market rejection. |
| Process Window & Equipment | Match the paste’s viscosity to your printing equipment (squeegee speed, pressure). Also, ensure your reflow oven can handle the recommended temperature profile. | Incompatible viscosity causes smearing or insufficient release. Wrong profile leads to cold joints or component damage. |
Recommendation from FS Technology:
In case of general electronics applications such as consumer and industrial controls, which account to about 80%, we advise the use of SAC305 Alloy in combination with Type 4 and No-clean ROL1 Flux, as it offers the most reliable solution at an affordable price. For specific needs, our engineering team is available for personalized consultation.
Best Practices for Solder Paste Storage and Handling
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.
Detailed Storage Guidelines:
- Shelf Life: 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).
- Avoid Heat: 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).
- Orientation: When feasible, store solder pastes in syringes or cartridges vertically, tip down. Packages should occasionally be turned 180 degrees if they are horizontally stored.
- FIFO System: 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.
Critical Handling Rules
- Do Not Use Cold: 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 (e.g., using a heat gun), as this causes condensation and flux separation.
- Mixing: 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 damage the solder paste structure.
PCB Solder Paste Printing: 3 Solder Paste Application Methods
This is the most critical step in SMT assembly, as over 70% of soldering defects originate from the printing process. There are three primary methods to apply solder paste onto the PCB.
Stencil Printing (Mass Production Standard)
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 per board.
Post-Printing Inspection:
After the solder paste printing is completed, SPI (Solder Paste Inspection) testing is conducted to assess the uniformity and quality of the solder paste, ensuring each pad receives the correct coverage in terms of height, area, and volume. 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 (For Flexibility and Prototyping)
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.
Advantage:
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, making it ideal for prototypes and high-mix low-volume production.
Manual Application (For Rework and Hobbyists)
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.
Common Solder Paste Defects and How to Prevent Them
Even with proper materials and equipment, soldering defects can occur. Recognizing the root causes and implementing preventive measures is key to maintaining high yield. Below are the most frequent defects encountered in SMT assembly, along with practical solutions.
| Defect | Visual Appearance | Primary Causes | Prevention & Corrective Actions |
| Solder Bridging | Solder connects two adjacent pads or pins, creating a short circuit. | Excessive solder paste volume; stencil apertures too large; low paste viscosity; improper squeegee pressure. | Reduce stencil thickness or aperture size; adjust printing pressure; use higher viscosity paste; ensure proper alignment. |
| Tombstoning (Manhattan Effect) | One end of a chip component lifts off the pad, standing upright. | Uneven solder paste deposition on the two pads; different thermal mass on pads; rapid heating during reflow. | Ensure uniform paste volume on both pads; optimize reflow profile (soak zone); use nitrogen atmosphere to balance wetting forces. |
| Solder Beading (Solder Balls) | Small spherical solder balls scattered around the pad or under components. | Paste slump (spreading) before reflow; excessive oxidation; fast ramp-up rate causing spattering. | Use paste with higher viscosity and anti-slump properties; control reflow profile with a gradual ramp; ensure stencil aperture design prevents paste bleeding. |
| Insufficient Solder (Open Joint) | Pad is not fully covered; component lead appears dry or lifted. | Stencil clogged; insufficient paste volume; poor stencil release; worn squeegee. | Clean stencil regularly (under-stencil wiping); increase aperture size or stencil thickness; check squeegee condition; perform SPI to catch volume variations. |
| Poor Wetting / Cold Joint | Solder does not spread properly; appears dull, grainy, or has a non-wetting surface. | Inadequate flux activity; contaminated pads/components (oxidation); reflow temperature too low or time too short. | Choose appropriate flux (ROL1/ROL2); bake PCBs prior to assembly to remove moisture; verify reflow profile with thermocouple. |
| Voids (in BGA/CSP) | X-ray reveals gas bubbles within the solder ball. | Outgassing from flux or PCB/component surface; insufficient preheat; too rapid cooling. | Optimize reflow profile with longer soak; use low-voiding flux paste; ensure components and boards are dry. |
FS Technology’s Quality Assurance:
We employ SPI inspection after printing and AOI (Automated Optical Inspection) plus X-ray after reflow to detect and eliminate these defects at every stage, ensuring your PCBA meets IPC Class 2 or Class 3 standards.
Conclusion
Solder paste is the lifeblood of modern SMT assembly. From selecting the correct alloy and particle size (Type 3, 4, or 5) to rigorous storage protocols and precise printing processes, every step impacts the final quality of your PCBA.
At FS Technology, we combine state-of-the-art equipment (including advanced SPI and jet printing systems) with strict process control to ensure your boards are assembled with the highest reliability. Whether you need high-volume production or rapid prototyping, our engineering team is ready to assist you in selecting the optimal solder paste for your specific application.
FAQs about Solder Paste
The difference lies in the powder particle size. Type 3 ranges from 25-45μm and is suitable for standard components and larger pitch ICs. Type 4 ranges from 20-38μm and is required for finer pitch components like 0201 and 0.4mm pitch BGAs, providing better print release and reduced solder balling.
Yes, but the temperature must be strictly controlled between 0°C and 12°C (32°F-55°F). Do not freeze the paste. Also, ensure the container is sealed tightly to prevent moisture absorption.
This is called “conditioning” . It allows the entire container of paste to reach room temperature evenly. If opened cold, moisture from the air will condense on the cold paste, leading to solder spatter and voids during reflow.
Typically, this is due to low humidity in the environment, prolonged exposure to air (draft), or a stencil temperature that is too high. It is recommended to keep the production environment at 22-28°C and 40-60% relative humidity, and to return unused paste to the jar if printing pauses for more than 30 minutes.
Use a solder paste formulated with low-voiding flux, optimize the reflow profile with a longer soak zone to allow outgassing, and ensure both the PCB and BGA components are properly baked to remove moisture prior to assembly.
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