Thermal Schedule
Temperature-time trajectory defines the specific heating, soaking, reflow, and cooling stages of an automated printed circuit board soldering process. The reflow solder profile governs the formation of reliable electrical and mechanical solder joints during the assembly of electronic components. It does not apply to manual soldering or wave soldering methods.
Thermal Zones
Temperature control occurs in distinct stages within the conveyor oven. First, the preheat zone gradually raises the assembly temperature to avoid thermal shock to the components. Following this, the soak zone ensures that all parts reach a uniform temperature.
Then, the reflow solder profile reaches its peak temperature where the solder paste melts and wets the terminal pads.
Structural Assembly
Joint reliability depends on the duration of the liquid phase. The time above liquidus must be long enough to allow proper wetting but short enough to prevent excessive growth of brittle intermetallic compounds. This reflow solder profile must be optimized for lead-free alloys like SAC305.
Defect Prevention
Deviations from the specified curve can lead to severe assembly defects. Too rapid a cool-down rate can cause cracks in ceramic chip capacitors, while too slow a ramp can cause solder joint oxidation. Engineers utilize profilometers to measure the actual temperature profile experienced by the board during development.
