A flexible unitary mask has a plurality of through holes. A substrate has a plurality of wettable pads in recessed regions defining volumes. The through holes are aligned with the wettable pads. Molten solder is directly injected through the through holes of the flexible unitary mask into the volumes with the wettable pads, such that the through holes and the volumes with the wettable pads are filled with solder. The solder is allowed to solidify, forming a plurality of solder structures adhered to the wettable pads. The flexible unitary mask is peeled from the substrate after the solder has solidified.