The Impact of Reflow Soldering Time on Soldering Quality

Reflow soldering time is a critical parameter determining the reliability of solder joints, directly affecting the degree of solder melting, metallurgical bonding quality, and defect occurrence rate. The following analysis examines its specific mechanisms based on the core temperature zone process (preheating → holding → reflow → cooling):

1. Effects of insufficient time

‌Incomplete solder melting: Insufficient time leads to inadequate liquidisation of the solder paste, resulting in cold solder joints (rough surface, loose interior), with solder joint strength decreasing by over 60%.

‌Poor wetting: Solder fails to adequately wet the pad (wetting angle >90°), causing cold solder joints or solder detachment, resulting in unreliable electrical connections.

‌Residual gases not fully evacuated: Incomplete evaporation of solvents and moisture leads to the formation of bubbles or voids inside the solder joint, reducing mechanical strength.

2. Effects of excessive time

‌Solder joint oxidation and discolouration: Prolonged exposure to high temperatures accelerates surface oxidation of the solder, causing the solder joint to lose its lustre, become brittle, and even peel off.

‌Excessively thick intermetallic compounds (IMC): Once the IMC layer thickness exceeds 5μm, it tends to stabilise, but excessive thickness increases joint brittleness and significantly reduces shear strength.

‌Component thermal damage: Sensitive components (such as ceramic capacitors) may crack due to thermal stress or be corroded by residual caustic substances from excessive flux evaporation.

Core principle: The time must match the solder paste type (lead-free peak ≥217°C) and PCB layer count, with dynamic adjustment of temperature zone ratios to reduce thermal stress.

3. Comprehensive optimisation strategy

‌Temperature zone coordination: The preheating zone heating rate should be ≤3°C/second to ensure uniform solvent evaporation; the holding zone should maintain a constant temperature for 60–120 seconds to remove oxides.

‌Defect prevention: Avoid multiple reflow passes (maximum 2 times); when repairing, the hot air gun temperature should be <300°C and the time should be