Lead-free SMT solder paste is a key soldering material in Surface Mount Technology (SMT), used to achieve alloying connections between electronic components (such as resistors, capacitors, LEDs, ICs, etc.) and printed circuit board (PCB) pads. Its core feature is lead-free and environmentally friendly (complying with international environmental directives such as ROHS, REACH), replacing traditional lead-containing solder paste (such as Sn63/Pb37), avoiding the harm of lead to humans and the environment.

I. Basic Composition

Lead-free SMT solder paste is composed of tin powder (metal alloy) and flux mixed in a certain proportion:

• Tin Powder: Accounts for about 90% (by weight) and is the core component for conductivity and bonding. Common lead-free alloys include:
  • Sn-Ag-Cu Series (e.g., Sn96.5/Ag3.0/Cu0.5): Medium-to-high temperature solder paste, suitable for conventional SMT processes.
  • Sn-Bi Series (e.g., Sn42/Bi58): Low-temperature solder paste, suitable for heat-sensitive components.
  • Sn-Ag-Bi Series (e.g., SnAg0.3Bi35): Medium-to-low temperature solder paste, balancing melting point and mechanical properties.
• Flux: Accounts for about 10% (by weight). Its functions include dissolving oxides on the soldering surface (activator), adjusting solder paste viscosity (solvent, thixotropic agent), preventing oxidation during soldering (antioxidant), and maintaining the shape of the solder paste after printing (anti-slump agent).

II. Classification (by Reflow Temperature)

Lead-free SMT solder paste can be divided into four categories according to reflow soldering temperature:

III. Detailed Introduction to Low-Temperature Lead-Free Solder Paste

Low-temperature lead-free solder paste is an important branch of lead-free SMT solder paste, with its core advantage being a low melting point (usually ≤138℃). It is specifically used for electronic products with lower heat resistance, avoiding damage to components or PCBs from high-temperature reflow.

Core Features

• Low Melting Point: The melting point of common alloys (e.g., Sn42/Bi58) is about 138℃ (much lower than the 217℃ of high-temperature solder paste), effectively reducing thermal stress damage to heat-sensitive components (e.g., LED lamp beads, plastic encapsulated components).
• Environmental Compliance: Lead-free, halogen-free (for some products), complies with ROHS, REACH and other directives, meeting contemporary electronic manufacturing's environmental requirements.
• Soldering Performance:
  • Good Wettability: Can quickly spread and wet pads and component leads, forming full solder joints.
  • Low Void Rate: Special flux formulations reduce gas generation during soldering, lowering rework rates.
  • Anti-Bridging: Not prone to slumping after printing, rarely short circuits between pins after reflow.
• Process Adaptability:
  • Loose Reflow Process Window: Allows for larger temperature fluctuation ranges, suitable for various grades of reflow soldering equipment.
  • Suitable for Tube Printing/Syringe Dispensing: Some models (e.g., X5) are specifically designed for high-precision dispensing, suitable for small components.
• Storage and Use:
  • Requires refrigeration at 5-10℃, shelf life approximately 6 months.
  • Must be warmed to room temperature for 2-4 hours before use to avoid moisture causing solder beads.
  • Stir evenly before use, unused solder paste should not be returned to the original container to prevent contamination.

Typical Alloys and Applications

The core alloy of low-temperature lead-free solder paste is the Sn-Bi series (e.g., Sn42/Bi58); some products add a small amount of silver to enhance mechanical strength. Its main application scenarios include:

• High-End LED Products: Such as LED lamp beads, LED display screens, avoiding light decay or package deformation due to high temperatures.
• Heat-Sensitive Components: Such as sensors, battery protection boards, plastic-cased components, preventing high-temperature damage to internal structures.
• SMT Substrates with Poor Heat Resistance: Such as flexible PCBs (FPC), thin PCBs, reducing the risk of thermal deformation.