A lead frame having thereon a plurality of semiconductor devices is prepared by immersing it in a flux solution to activate the lead frame surface and to remove oxide film. The lead frame is then transferred to a coating block unit, and the lead frame is moved along a lateral passage which extend through the coating block unit. During movement through the passage, the lead sections of the lead frame are coated on both upper and lower surfaces thereof with liquid solder which ascends by capillary action along a slit which projects upwardly from a liquid solder bath and communicates with the passage. The solder in the slit, where it communicates with the passage, is continually replenish by capillary action from the bath as the solder coats the lead sections as the lead frame moves through the passage.