Patent ID: 8361899

Claim:
A method for fabricating a semiconductor assembly, comprising: forming an attachment area and a non-attachment area on a lead finger of a lead frame, the attachment area being more wettable to a solder ball than the non-attachment area during reflow; contacting a solder ball carried by a semiconductor die with the attachment area of the lead finger; reflowing the solder ball while the solder ball is in contact with the attachment area of the lead finger; and controllably collapsing the solder ball to establish an electrical connection between the semiconductor die and the lead finger of the lead frame; and wherein the method further includes attaching the solder ball to a contact pad of the semiconductor die; forming the attachment area and the non-attachment area includes: depositing a photoresist on a surface of the lead finger containing copper (Cu); patterning the photoresist to form an opening corresponding to the attachment area, the opening exposing a first portion of the surface of the lead finger, a second portion of the surface being covered by the photoresist; depositing silver (Ag) on the first portion of the surface of the lead finger through the opening in the photoresist; removing the photoresist from the second portion of the surface of the lead finger having the deposited silver; and oxidizing the second portion of the surface of the lead finger thereby forming copper oxide (CuxO); controllably collapsing includes: preventing the solder ball from migrating away from the silver in the attachment area with the copper oxide in the non-attachment area; and establishing an electrical connection between the contact pad of the semiconductor die and the lead finger of the lead frame via the solder ball; and the method further includes at least partially encapsulating the semiconductor die, the solder ball, and the lead finger with an encapsulant.