Micro-machined Conformal silicon Molds for wafer bumping and probing

Bolder-bumped flip chips are an enabling technology for high density portable electronics such as mobile phones and personal digital assistants (PDAs). One of the critical fabrication processes for flip chips is wafer bumping. In the present study, a wafer bumping process with solder sphere transfer using a conformal silicon mold is developed. MEMS processes are employed to fabricate through holes on the silicon mold with a pattern conforming to the pads on the wafer under bumping. In addition, V-grooves are made on the surface of the silicon mold to serve as guiding tracks. The conformal silicon mold is placed on the top of wafer. The solder spheres are poured at one side of the mold and then pushed by a blade to run along the guiding tracks. Eventually a number of solder spheres fall in all through holes to complete the transfer process. Furthermore, another conformal silicon mold is designed and fabricated for wafer probing. This silicon mold has arrays of probing beams matching with the solder bumps to be inspected. Again MEMS processes are employed to make such suspended structures of which elastic deflection may compensate the variation in solder bump height. The design concept and fabrication processes of aforementioned devices are illustrated in details.