Abstract: In a flow soldering apparatus, a substrate is clamped by a pair of nails of a chuck mechanism and transferred over a molten solder barrel while being held in contact with a molten solder. The nails are held resiliently to each other, so that the substrate is allowed to expand thermally between the nails without bowing. The chuck mechanism has a scraper nail which extends transversely over the entire lateral length of the substrate at the front side of the substrate in a substrate transfer direction to push away an oxide film formed on the molten solder. The chuck mechanism is constructed to allow the molten solder to enter underneath the substrate in the lateral direction with respect to the substrate transfer direction. After completing one soldering operation, the nails change the set of the opposing two sides to another set of two opposing sides of the substrate, so that the substrate is subjected to the soldering operation from a direction different from the preceding soldering operation.