What is the reflow soldering process?

The reflow soldering process is a core component of surface mount technology (SMT) in electronics manufacturing. It involves heating and melting solder paste pre-applied to PCB pads to achieve mechanical and electrical connections between electronic component leads and pads.

‌1. Working Principle‌

‌Four-Stage Temperature Control‌

‌Preheating Zone‌: After the PCB enters, the solvent in the solder paste evaporates, the flux is activated and wets the pads/leads, and oxygen is isolated.

‌Tempering Zone‌: The PCB is uniformly preheated (approximately 150–180°C) to prevent deformation caused by subsequent high-temperature shocks.

‌Welding Zone‌: The temperature rapidly rises to the solder paste melting point (approximately 217°C for lead-free solder), and liquid tin wets the pads and pins to form reliable solder joints.

‌Cooling Zone‌: The solder joints rapidly solidify and cure, completing the connection.

‌Heat Transfer Method‌

Heating is achieved through hot air circulation (the mainstream method) or infrared radiation assistance to ensure temperature uniformity.

‌II. Technical Advantages‌

‌High Precision‌: Localised heating avoids damage to heat-sensitive components (e.g., BGA).

‌Low Defect Rate‌: Automated control reduces defects such as bridging and tombstoning.

‌Compatibility‌: Supports lead-free solder and high-density mounting of micro-components.

‌Efficiency and Cost‌: Mass production reduces unit cost, with a yield rate of up to 99.9%.

III. Advanced Technology Evolution‌

‌Nitrogen Reflow Soldering‌: Injecting nitrogen reduces oxygen content, minimises oxidation, and improves solder joint wettability.

‌Vacuum Reflow Soldering‌: Eliminates 97% of bubbles and voids.