Fabrication of a flip chip solder bump using a thin mold and ultrasonic filling

A flip chip solder bump is fabricated using a thin dewetting mold and the ultrasonic filling method. The process consists of filling the cavities of a stainless steel mold with lead-free solder using ultrasonic vibration, and transferring the solder bump pattern of the mold to a substrate using reflow soldering. Pressures induced by ultrasonic and mold vibration are predicted analytically, and the ultrasonic pressure is larger than the pressure induced by the surface tension, which traps the vapor at the mold cavity. Contact and non-contact modes are used to fill the through-hole and blind-hole molds, and the solder bump patterns of the mold are transferred to the Cu pads of a polyimide film through reflow soldering.