I. Definition

Leaded SMT solder paste is a common soldering material used in SMT (Surface Mount Technology) processing. It is a paste-like mixture of flux components (rosin, diluent, stabilizer, etc.) and alloy components (mainly tin and lead). Because tin (Sn) and lead (Pb) are the core elements in its alloy composition, it is called "leaded solder paste".

II. Core Components

The components of leaded solder paste can be divided into two parts: alloy powder and flux:

• Alloy Powder:
  • Main components: Tin (Sn) and lead (Pb), common ratio is Sn63:Pb37 (eutectic alloy, melting point 183℃).
  • Low-temperature leaded solder paste: Adds elements such as bismuth (Bi) to lower the melting point (e.g., Sn-Bi-Pb alloy).
  • Trace impurities: Contains iron, zinc, copper, aluminum, etc., whose content must be strictly controlled to avoid affecting soldering performance.
• Flux:
  • Function: Makes alloy powder into a paste, controls fluidity, removes oxides from the soldering surface, provides adhesion for components, etc.
  • Components: Rosin (main film-forming substance), diluent (adjusts viscosity), stabilizer (prevents oxidation), etc.

III. Main Characteristics

Advantages

• Excellent Printing Performance: Good rolling and release properties, capable of fine printing for 0.3mm pitch pads; small change in viscosity during continuous printing, steel stencil operable life exceeds 12 hours.
• High Soldering Stability: No slumping after printing, appropriate wettability, adaptable to different grades of soldering equipment, no nitrogen environment required, wide process window.
• Good Post-Soldering Performance: Low residue and light color, high insulation resistance, can meet no-clean requirements, excellent ICT (In-Circuit Test) performance.
• High Cost Performance: Lower price than lead-free solder paste, high soldering strength, good conductivity, wide application range.

Disadvantages

• Environmental Issues: Contains harmful substances such as lead (Pb), which does not comply with RoHS and other environmental standards, having negative impacts on human health and the environment.
• Appearance and Oxidation: Rapid surface oxidation after soldering, significant color difference (gray-black) from the substrate, affecting product appearance.
• Usage Restrictions: International exports (e.g., to Europe and America) must comply with environmental requirements, and it is gradually being replaced by lead-free solder paste.

IV. Typical Applications

Leaded solder paste is mainly used in electronic manufacturing scenarios without strict environmental requirements, including:

• Consumer Electronics: Soldering of phone connectors, high-density resistors, Type-C charging ports, computer video ports, router PCBs, etc.
• LED and Lighting: LED chip mounting, heat sink soldering (due to low soldering temperature, less likely to damage components).
• Automotive Industry: Mounting and soldering of automotive electronic components (e.g., diodes, sensors).
• SMT General Process: Mounting of electronic components (resistors, capacitors, ICs), reflow soldering, etc.

V. Precautions for Use

• Warm-up and Stirring: Must be warmed to 25℃ before use (heating is forbidden), stir manually for 3-5 minutes or with a machine for 1-3 minutes.
• Addition and Storage: Add solder paste in small amounts multiple times; use within 24 hours after opening, do not return poured out solder paste to the original container.
• Printing and Mounting: Complete component mounting and reflow soldering within 1-2 hours after printing to avoid solder paste oxidation.
• Environmental Requirements: Operating environment temperature 22-28℃, humidity 40%-60%.

VI. Classification and Key Differences

Classification of Leaded Solder Paste

Key Differences from Lead-Free Solder Paste