Solder paste is the behind-the-scenes champion of electronics manufacturing today. The unsung material is the essential glue – quite literally – that holds Surface Mount Technology (SMT) feasible, enabling the creation of complex circuit boards that power everything from smartphones to spacecraft. Solder paste is something that anybody involved with PCB design, assembly, or procurement must know about.
The blog will discuss the basic concepts, working principles, composition of structure, types of solder paste and how to store it. If you have no idea how to choose the right one for your PCB, this blog will give you a guidance.
What is Solder Paste?
Imagine a homogenous mixture with the consistency of thick toothpaste. That's solder paste. It's a suspension of microscopic metal alloy solder spheres (typically tin-based) in a special flux medium. The mixture serves a two-fold purpose:
- Temporary Adhesive: It holds SMT components in place firmly against their pads before soldering.
- Permanent Bond: When heated (reflowed), the solder particles melt, coalesce and together with the leads and PCB pads of the devices and make good electrical and mechanical connections.
Solder paste brings the high-speed, automated placement of tiny components like 0201 resistors or fine-pitch BGAs from being an engineering fantasy to a reality. Easy.
How Does Solder Paste Work?
Solder paste plays a role in different stages during the reflow soldering process:
- Normal temperature adhesion: At room temperature, the solder paste has a certain viscosity, which can initially adhere the electronic components to the pads of the PCB, so that the components will not be easily displaced in subsequent processes such as patching, and prepare for welding.
- Heating/melting: During the reflow soldering process, the solder paste is heated to a certain temperature, and the solder alloy powder melts into liquid solder after reaching the melting point (typically 183°C for Sn63Pb37, 217-227°C for common lead-free alloys like SAC305). At the same time, the solvent and some additives in the paste begin to volatilize, reducing the viscosity and volume of the paste, so that the liquid solder can flow and diffuse better.
- Wetting connection: Liquid solder relies on surface tension and wetting to spread evenly on the solder end of the component and the surface of the PCB pad, fill the weld gap, and reliably connect the component to the pad. During the wetting process, the activator of the flux continues to work to ensure that there are no impurities such as oxides on the welding surface, ensuring the quality of welding.
- Cooling/solidification: After welding is completed, the solder cools and solidifies to form a permanent solder joint, realizing the electrical and mechanical connection between the electronic components and the PCB, and completing the welding process.
Solder Paste Compositions
The major components of solder paste are solder alloy powder, flux, and additives. What each component does and its characteristics will be explained below.
Solder Alloy Particles (85-92% by weight)
These may be made of tin (Sn), lead (Pb), silver (Ag), and copper (Cu), among others. The components melt and fill voids in solder joints to form tiny metal balls (powders) that form actual solder joints, enabling mechanical and electrical interconnections between components and circuit boards. Common alloys are:
- Sn63Pb37: Vintage eutectic tin-lead. Replaced for RoHS but remains in use in exempting applications.
- SAC305 (Sn96.5Ag3.0Cu0.5): Most prevalent lead-free alloy.
- Other Lead-Free: SAC variants (e.g., SAC387, SAC405), SnCuNi, SnBi (low temp), SnAg (higher temp).
Generally, the particles should be as spherical as possible to reduce the surface area, decrease the oxidation level, and improve the printing and soldering properties. Meanwhile, the particle size should be evenly distributed, usually in a mesh size of 200-400. Different mesh sizes are required in different application scenarios; fine pitch soldering usually uses fine mesh solder powder.
Flux (8-15% by weight)
Stores activators, resins, solvents, and thixotropic agents. It is a vehicle for the solder alloy powder that gives the solder paste the desired viscosity and fluidity to accurately print on the pads and has cleaning, fluxing, and protective roles during soldering.
Additives
Minor components (<1%). Additives which may include improving corrosion resistance of solder paste, brightness of solder joints, flame retardation, etc., are added according to specific application requirements, in an attempt to meet the special requirements of diverse products for welding quality and environmental compatibility.
Solder Paste Types
There are many types of solder pastes. The following will classify and introduce them according to the alloy solder, flux activity, cleaning method, viscosity and particle size. By reading this part, you will have a better idea of how to choose the right solder paste for PCB.
By Alloy Composition
Lead Based
The most widely used is Eutectic Sn63Pb37. Low melt point, excellent wetting, proven process. Suitable for products that do not have stringent environmental requirements and are not difficult to weld, such as some old-fashioned electronic toys, plain household appliances, etc.
Lead Free
Primarily SAC305 or its derivatives. Propounded by RoHS/WEEE environmental regulations. Enhanced melting point, changed wetting behavior. Applicable to most modern-day electronics.
Lead-free pastes (e.g., SAC305) are now everywhere due to RoHS regulations, aside from exempt uses (military, aerospace, some medical devices).
Specialty Alloys: Low-temperature (e.g., SnBi, SnBiAg), high-temperature (e.g., SnSb, SnAu), high-reliability (e.g., doped SAC alloys).
|
Feature |
Leaded (Sn63Pb37) |
Lead-Free (SAC305, etc.) |
|---|---|---|
|
Melting Point |
183°C (eutectic) |
217-227°C (higher reflow temp) |
|
Wetting Performance |
Excellent |
Requires stronger flux |
|
Environmental Impact |
Contains toxic lead (RoHS-restricted) |
|
|
Cost |
Lower |
Higher (due to silver content) |
|
Reliability |
Proven in legacy electronics |
Better for high-temp applications |
By Flux Type
No-Clean
The most prevalent today. Contains milder activators and resins that leave a harmless, non-corrosive residue after reflow. There is no need for post-welding cleaning process, which can minimize the production cycle and be cost-effective. Suitable for products with not very high cleanliness requirement and no special electrical performance requirement after welding, such as simple assembly of some consumer electronic products.
Water-Soluble
Excessive activity flux with excellent cleaning on hard surfaces. Water for post-weld cleaning is needed to prevent corrosion and improve electrical insulation performance. It is used on products with high cleanliness requirements, such as high-reliability products, aerospace and military products, high-precision instruments and medical devices.
Rosin-Based
Rosin-based is a soldering paste that uses rosin (a natural resin of pine trees) as the core of its flux. It is commonly used in electronics assembly, especially for sensitive devices, because it is less corrosive and more environment-friendly than some synthetic alternatives. Rosin-based pastes are available in different levels of activity (e.g., RMA, RA) to suit various soldering applications.
|
Flux Type |
Pros |
Cons |
|---|---|---|
|
No-Clean |
No post-reflow cleaning needed; benign residue |
Residue may look "dirty"; not for high-reliability |
|
Water-Soluble |
Excellent cleaning power; high activity |
Must be cleaned (residue is corrosive) |
|
Rosin (RMA/RA) |
Reliable, proven performance |
Often requires cleaning (sticky residue) |
By Viscosity
Low Viscosity
Viscosity is generally 100-300Pa·s, has quick falling speed, which is in favor of processes needing high fluidity such as injection and dispensing, and is also easy to print quickly and patch. However, viscosity retention time is relatively short, and the requirements of PCB handling and patching time after printing are relatively rigorous.
Medium Viscosity
Overall viscosity is usually 300-600Pa·s, with good overall performance, suitable for general processes such as screen printing, and can provide good printing accuracy and solder joint quality.
High Viscosity
Viscosity can reach 600-1400Pa·s, with good accumulation effect for solder joint, suitable for fine pitch printing or where high shape requirement of solder joint is needed, and can be able to reduce solder paste collapse and bridging. However, greater pressure and finer steel mesh may be required in printing.
By Particle Size
The particle sizes are divided based on various types of particles. Each type corresponds to a specific range of particle diameters.
|
Type |
Particle Size (Microns) |
Application |
|---|---|---|
|
Type 1 |
75–150 µm |
Coarse soldering tasks. |
|
Type 2 |
45–75 µm |
General-purpose soldering. |
|
Type 3 |
25–45 µm |
Standard SMT assembly; suitable for most stencil printing. |
|
Type 4 |
20–38 µm |
Fine-pitch components; requires smaller apertures. |
|
Type 5 |
15–25 µm |
Ultra-fine pitch components; micro-BGA. |
|
Type 6 |
5–15 µm |
Advanced packaging and miniature components. |
|
Type 7 |
2–11 µm |
Nano-scale applications. |
|
Type 8 |
2–8 µm |
High-density interconnects and cutting-edge technologies. |
How to Store Solder Paste?
Proper storage is critical to maintain paste performance and prolong the PCB shelf life:
- Storage temperature range:
- Generally, solder paste should be stored at 2-10℃ (35.6℉~50℉). Within this temperature range, the activity of the flux in the paste is relatively stable, which can effectively avoid the chemical reaction between the alloy powder and the flux in the paste, leading to a decrease in viscosity and activity, and can also avoid the crystallization of the resin in the flux at low temperature.
- Constant temperature requirements: The storage environment should be kept at a constant temperature as much as possible to avoid frequent and large temperature changes in the paste; otherwise, the performance of the flux in the paste will change, affecting the welding quality.
- Sealed storage Original packaging seal:
- Solder paste is generally stored in wide-mouthed jars and other containers. Storage involves keeping the packaging tightly sealed to prevent paste from oxidizing when exposed to the air, and for preventing the flux ingredients from volatilizing or taking up atmospheric humidity and causing proportion imbalance.
- Sealed after use: After some solder paste has been taken out while in use, the rest of the paste should be sealed and stored back in the refrigerator in time to prevent the volatile components of the flux from continuing to degrade and thus creating greater viscosity as well as other performance changes.
- Room temperature balance:
- Do not open and use the solder paste immediately after taking it out of the refrigerator. Let it sit at room temperature for 2-4 hours to equilibrate it with the room temperature. Do not warm it artificially. This will prevent the paste from absorbing moisture due to sudden temperature changes, and solder balls or solder splashes during welding.
- Other precautions
- Store separately: Never mix the remaining solder paste that is scraped back from the silk screen and stencil with unused new paste. Keep it separate in another container. If it is not expired, it can be used in the production of products with relatively low welding requirements after adding an appropriate amount of solvent.
- Environmental requirements: The environment in which the solder paste is stored should also be as clean as possible to prevent dust and other impurities or contamination from entering the paste.
Apart from following the above proper procedure for storing solder paste, we must conduct solder paste inspection (SPI) regularly, mainly complying with industry standards such as IPC J-STD-005, which explicitly define the composition, physical properties, metal particles, chemical properties, etc. of paste. The results obtained through the test must be compared with the standard to determine whether the solder paste is qualified.
Conclusion
PCB solder paste is more than "conductive glue." It is a complex, multi-function material whose selection, handling, and application have a significant impact on the quality, reliability, and manufacturability of electronic assemblies. By understanding its chemistry, bouldering, application subtleties, and selection considerations, engineers and technicians can optimize their SMT process, minimize defects, and make their products perform reliably in the field. Coming to grips with the sticky chemistry of solder paste is a foundation for electronic manufacturing excellence.
By embracing advancement in solder paste technology, manufacturers like FC PCBA can take innovation to new heights toward a sustainable and connected future.
Frequently Asked Questions
What is solder paste, and how is it used in PCB assembly?
Solder paste is a mixture of tiny solder alloy particles (e.g., tin, silver, copper) suspended in flux. It is used in Surface Mount Technology (SMT) to temporarily hold components in place before reflow soldering, where heat melts the solder, forming permanent electrical and mechanical connections.
What is the difference between lead-based and lead-free solder paste?
- Lead-based: Contains tin and lead (e.g., Sn63/Pb37). Although it is being phased out because of environmental laws (RoHS), it has a lower melting point (183°C) and outstanding wetting characteristics.
- Lead-free: Uses alloys like SAC305 (tin-silver-copper) and has a higher melting point (217-220°C). Though it requires more strict process control, it is RoHS-compliant and ecologically beneficial.
How is usability life linked with solder paste?
The shelf life of solder paste is typically 6 months when stored properly in a refrigerator. After opening, it should be used within 24 hours if left at room temperature or within a few days if resealed and refrigerated.
What indicators are there that my solder paste is degraded?
Indicators of poor quality solder paste are:
- Dry or crusty texture.
- The demarcation resulting from metal particles.
- Poor printing performance or solder balling during reflow.
- If in doubt, perform a slump test or check the expiration date.
Can I reuse solder paste?
Reuse of solder paste is not advisable. Solder paste can dry, oxidize, or be contaminated depending on the humidity in the air, which finally results in poor-quality soldering. Therefore, a new solder paste is essential in each application.
Can I mix different types of solder paste?
No, mixing many kinds of solder paste—e.g., lead-based and lead-free—can result in variable performance, bad solder junctions, and contamination. Always use one type of solder paste for a single assembly.
