Patent ID: 7432188

Claim:
A method for forming bumps on an under bump metallurgy layer, the method comprising the following steps of: (a) providing a wafer, having a plurality of solder pads and a protection layer, the protection layer covering a surface of the wafer and exposing parts of the solder pads; (b) forming a first under bump metallurgy layer (UBM layer) on the protection layer and the solder pads; (c) forming a first photo resist on the first under bump metallurgy layer; (d) patterning the first photo resist to form a plurality of openings, and the openings formed on the first under bump metallurgy layer corresponding to the solder pads; (e) forming a second under bump metallurgy layer in the opening of the first photo resist; (f) plating a metal bump in the opening of the first photo resist; (g) removing the first photo resist; (h) removing part of the first and second under bump metallurgy layers so as to form an undercut structure under and around the first and second under bump metallurgy layers; (i) forming a second photo resist on the protection layer and filling the undercut structure with the second photo resist; (j) removing the part of the second photo resist to form residual of the second photo resist only in the undercut structure before reflowing the metal bump processing; and (k) reflowing the metal bump.