Reflow soldering: the profile explained

Once paste is printed and parts are placed, the board goes through reflow: a controlled temperature curve that melts the paste and forms every joint at once. The shape of that curve — the profile — separates shiny joints from cold ones and lifted parts.

The four stages

• ·Preheat — ramp up steadily (about 1–2 °C/s) to avoid thermal shock
• ·Soak — hold around 150–180 °C to activate flux and even out temperatures
• ·Reflow — peak above the paste's melting point (about 245 °C for lead-free) briefly
• ·Cooling — come down at a controlled rate for strong grain structure

Lead-free vs leaded

Lead-free (SAC305) paste melts near 217 °C and peaks around 245 °C; leaded melts lower (about 183 °C). Follow the paste datasheet — the profile belongs to the paste, not the board.

Getting it right

Big thermal masses — ground pours, tabbed power packages, QFN thermal pads — lag the rest of the board, so soak long enough for them to catch up. A thermocouple on the board beats trusting the oven's setpoint.

Reflow is the last step after a clean stencil print and placement.

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