An improved apparatus for filling patterned mold cavities formed on a surface of a mold structure with solder includes an injector head assembly comprising a solder reservoir having a bottom surface with an elongated slot for injecting solder into the mold cavities and a carriage assembly configured to carry and scan the mold structure under the injector head assembly. The mold structure is brought into contact with the solder reservoir bottom surface and a seal is formed between an area of the solder reservoir bottom surface surrounding the elongated slot and the mold structure surface and then the mold structure is scanned under the injector head assembly in a first direction from a starting position to a finish position for filling the mold cavities and in a second direction opposite to the first direction for returning back from the finish position to the starting position.