a vapor generating chamber configured to accommodate a heat transfer fluid and to be filled with saturated vapor generated by the heat transfer fluid;

a heater configured to heat the heat transfer fluid in the vapor generating chamber;

a substrate stage configured to be movable upward or downward in the vapor generating chamber and to support a substrate on which an electronic device is mounted via a solder; and

a temperature gradient regulator comprising at least one mesh plate extending in a horizontal direction in the vapor generating chamber, the at least one mesh plate comprising a plurality of openings through which the vapor moves,

wherein an inner space of the vapor generating chamber is divided, via the at least one mesh plate, into an upper zone and a lower zone,

wherein the temperature gradient regulator is configured to maintain a first temperature at the upper zone and a second temperature at the lower zone,

wherein the second temperature is higher than the first temperature, and

wherein the substrate on the substrate stage moves through the upper zone and the lower, to thereby obtain a desired solder temperature profile.