Reflow Oven

A reflow oven is a multi-zone heating chamber used in surface-mount assembly and board-level repair to melt solder paste and form every joint on a board simultaneously. The board travels through (or sits inside) a sequence of thermally controlled zones that raise it through a carefully shaped time-temperature profile rather than heating one joint at a time with an iron. On ASIC mining hardware, reflow is how factory hashboards are populated, and it is occasionally used in shops doing whole-board recovery.

The four standard zones

A conventional profile moves through four stages: preheat, which ramps the board up (typically 1-3°C per second) to drive off solvents and activate flux; soak, which holds a plateau so the whole assembly reaches a uniform temperature and oxides are reduced; reflow (the spike), which pushes above the solder's liquidus so the paste fully melts and wets; and cooling, which solidifies the joints. For lead-free pastes such as SAC305, peak temperatures generally sit between 235°C and 250°C, with time above liquidus held to roughly 30-90 seconds.

Why ramp control matters

The ramp rates on either side of the peak are as important as the peak itself. Heating too fast thermally shocks ceramic components and large BGAs; cooling faster than about 4°C per second can crack joints or warp the substrate. A well-tuned oven profile minimizes voiding and produces consistent intermetallic growth across hundreds of joints at once, which is difficult to match by hand.

For finer points on melt behavior and metallurgy, see our entries on eutectic solder and intermetallic compound.