Nickel-tin microbump structures and method of making same

A method of fabricating an integrated circuit (IC) package substrate, the method comprising:

• forming a dielectric layer above a sub-surface-level metal layer, the sub-surface-level metal layer including copper contacts;
• forming via holes in the dielectric layer, the via holes exposing the copper contacts;
• forming a seed layer in the via holes and above the copper contacts, the seed layer including copper;
• and
• forming microbumps on the seed layer, the microbumps including tin and nickel, wherein a bottom ten percent of a volume of each of the microbumps has a first tin mass fraction, wherein a top ten percent of the volume of each of the microbumps has a second tin mass, and wherein the second tin mass fraction differs from the first tin mass fraction by at least five percent of the first tin mass fraction.

A method of fabricating an integrated circuit (IC) package substrate, the method comprising:

• forming a dielectric layer above a sub-surface-level metal layer, the sub-surface-level metal layer including copper contacts;
• forming via holes in the dielectric layer, the via holes exposing the copper contacts;
• forming a seed layer in the via holes and above the copper contacts, the seed layer including copper;
• and
• forming microbumps on the seed layer, the microbumps including tin and nickel, wherein a total volume of tin of each of the microbumps is equal to at least 75% of a total volume of nickel of each of the microbumps.