Time Sequence
Thermal graph that defines the heating cycle for soldering electronic components without lead. A lead-free reflow profile typically requires higher temperatures than traditional tin-lead processes. It must be managed to ensure the solder melts completely without damaging the circuit board.
Process Zone
Heating occurs in distinct stages known as preheat, soak, reflow and cooling. During the lead-free reflow profile, the ramp rate is controlled to prevent thermal shock to the ceramic capacitors. The soak zone allows the flux to activate and the temperatures across the board to equalize.
This preparation is essential for forming consistent solder joints.
Peak Temperature
Maximum heat reached during the cycle must exceed the liquidus point of the solder alloy. For common alloys like SAC305, the lead-free reflow profile often peaks between two hundred thirty five and two hundred fifty degrees Celsius. Exceeding this range can cause the plastic packages to crack or the board to warp.
Monitoring is performed using thermocouples attached to a sample board.
Joint Formation
Wetting of the metallic surfaces occurs during the few seconds the solder is in a liquid state. The lead-free reflow profile defines the time above liquidus which must be long enough for a bond but short enough to limit intermetallic growth. Rapid cooling after this stage helps to create a fine grain structure in the solder.
This results in a mechanically strong electrical connection.
