Over the past few decades, surface mount technology (SMT) has revolutionized the electronics manufacturing industry. One of the key materials used in SMT assembly is solder paste. Solder paste is a homogenous mixture of tiny solder alloy particles suspended in a flux. When heated during the soldering process, the flux removes oxides and promotes alloy melting and bonding to form solder joints. Selecting the right solder paste is crucial for producing high-quality and reliable solder joints in SMT assembly. This article outlines the main types of solder paste used in SMT, their properties, applications, and selection criteria.

　　 Solder Paste Types

　　There are three main types of solder paste used in SMT assembly:

　　Lead-containing solder paste

　　Lead-based solder paste contains lead (Pb) as the primary alloying element. A commonly used lead-based solder alloy is eutectic 63Sn/37Pb, which has a melting point of 183°C. Lead-based solder paste offers excellent solderability, providing strong, reliable solder joints. However, due to environmental legislation, lead-free solder paste is now more commonly used.

　　 Lead-free solder paste

　　Lead-free solder paste contains alloys based on tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), zinc (Zn), indium (In), and other elements. Common lead-free solder alloys include:

　　SAC alloys: Sn/Ag/Cu alloys, melting point approximately 217–220°C

　　SAC305: 96.5Sn/3Ag/0.5Cu alloy, melting point 217°C

　　SAC387: 95.5Sn/3.8Ag/0.7Cu alloy, melting point 217°C

　　Sn/Ag: 99Sn/1Ag alloy, melting point 221°C

　　Sn/Cu: 99.3Sn/0.7Cu alloy, melting point 227°C

　　Sn/Zn: 91Sn/9Zn alloy, melting point 198°C

　　Sn/Bi: 97Sn/3Bi alloy, melting point 139°C

　　Lead-free solder paste complies with environmental regulations and offers high solder joint reliability. However, lead-free solder paste often requires higher soldering temperatures than lead-containing solder paste.

　　 No-clean solder paste

　　No-clean solder paste contains fluxes that leave residues that do not need to be cleaned after soldering. Most modern solder pastes used in SMT are no-clean formulations. No-clean flux chemistries include:

　　Rosin-based: A common no-clean flux, leaving a harmless tacky resin residue.

　　Water-soluble: Residues can be removed with water after soldering.

　　Organic acids: Similar performance to rosin but more active, suitable for difficult applications.

　　No-clean solder paste improves production efficiency as a separate cleaning step is not required after soldering.

　　 Solder Paste Properties

　　Key properties to consider when selecting solder paste include:

　　Metal content: The proportion of solder alloy particles in the paste, typically 88–92% for standard pastes.

　　Alloy composition: The type of solder alloy (lead-based, SAC, Sn/Ag, etc.). Determines the melting point.

　　Particle size: The diameter of the suspended solder particles, typically 25–45 µm. Smaller particles result in better solderability of fine features.

　　Flux type and activity level: Determines oxide removal capability and residue.

　　Viscosity: Thixotropic flow behavior. Affects stencil printing performance.

　　Sag resistance: Ability to maintain shape after printing. Important for small deposits.

　　Tackiness: The solder paste's adhesion/stickiness to the surface. Affects solder ball formation.

　　Shelf life: The storage time of the paste before use. Typically 6–9 months under refrigeration.

　　 Solder Paste Applications

　　Different types of solder paste are suitable for different SMT assembly applications:

　　 Solder Paste Selection Criteria

　　Key factors to consider when selecting solder paste include:

　　Soldering process temperature: Match the solder paste melting point with equipment parameters.

　　Alloy compatibility: Avoid tin whiskers by using pure tin finishes.

　　Component type: Select particle size and tackiness for the components used.

　　PCB surface finish: Match the solder paste with the circuit board surface finish (OSP, immersion silver, etc.).

　　Product reliability requirements: Select appropriate solder paste (e.g., high-reliability alloys).

　　Environmental considerations: RoHS requires the use of lead-free solder paste.

　　Residue cleaning: Use no-clean paste if cleaning is not possible.

　　Printing equipment: Match rheological properties with the printer.

　　Cost: Balance performance and cost.

　　Correct solder paste selection, screening through prototyping, evaluating printing and reflow performance, and inspecting solder joint quality are key to successful SMT processes. Solder paste manufacturers provide extensive technical resources to guide solder paste selection.

　　 Typical Solder Paste Formulations

　　Solder paste products consist of specific solder alloy powders with specific flux formulations. The following are typical solder paste compositions from leading manufacturers:

　　Product datasheets provide detailed specifications on metal load, particle size distribution, viscosity, sag resistance, tackiness, and other parameters.

　　 Solder Paste Handling

　　Proper solder paste handling and storage extends shelf life and prevents defects:

　　Store the paste in sealed cans/cartridges at 2–10°C, away from sunlight. Allow it to reach room temperature before use.

　　Do not remove processed paste from the stencil and mix it with unused paste to avoid contaminating the fresh paste.

　　Avoid introducing contaminants such as oils, solvents, metals, paper/plastic fibers, etc., that affect the properties of the paste.

　　Use the paste within the recommended shelf life according to the manufacturer's data. First In, First Out (FIFO) inventory turnover.

　　Ensure the seal of the can/cartridge during storage to prevent the paste from drying out or oxidizing.

　　Regularly measure and record the paste viscosity, slump resistance, and tackiness to identify changes over time.

　　Do not refreeze the paste after thawing, as this will disrupt the stability of the suspension.

　　Proper handling and process control are key to maximizing solder paste life and minimizing defects.