Use of electrolytic plating to control solder wetting

1. A method comprising:

forming a copper pillar;

electroplating a metal layer on a top surface and a sidewall of the copper pillar;

electroplating a metal cap above the top surface of the copper pillar in direct contact with the metal layer, wherein the metal cap is less than 1 μm; and

forming an intermetallic by heating the metal layer and the copper pillar in a non-reducing environment, the intermetallic comprising elements of both the copper pillar and the metal layer, wherein the intermetallic comprises one of: only Cu 3 Sn; only Cu 3 Sn and SnO 2 ; and only tin, Cu 3 Sn, and SnO 2 ,

wherein molten solder will wet to the metal cap and will not wet to the intermetallic.

2. The method of claim 1 , wherein forming the copper pillar comprises:

patterning an opening in a photoresist layer above a quad layer plating structure comprising a first layer, a second layer, a third layer, and a fourth layer, each layer is conductive and is formed successively one on top of another; and

electroplating the copper pillar structure in the opening by applying an electrical potential to the second layer, copper from a plating solution plates out on the fourth layer exposed at a bottom of the opening.

3. The method of claim 1 , wherein forming the copper pillar comprises:

patterning an opening in a photoresist layer;

electroplating the copper pillar structure in the opening; and

removing a portion of the photoresist layer immediately adjacent to the copper pillar to form a space, the space is formed by exposing the photoresist layer to a high temperature high pH plating bath.

4. The method of claim 1 , wherein the metal layer comprises tin and the metal cap comprises gold.

5. The method of claim 1 , further comprising:

forming a solder bump directly on top of the metal cap.

6. The method of claim 1 , further comprising:

electroplating a solder cap directly on top of the metal cap; and

heating the solder cap to a molten state after completely removing the photoresist layer.

7. A method comprising:

forming a copper pillar;

forming a solder bump directly on a top surface of the copper pillar by electroplating a solder cap and reflowing the solder cap to form the solder bump;

electroplating a metal layer on a top surface of the solder bump and on a sidewall of the copper pillar, wherein the metal layer comprises tin and is less than 1 μm; and

forming an intermetallic from a first portion of the metal layer in contact with the sidewall of the copper pillar by heating the metal layer and the copper pillar in a non-reducing environment, the intermetallic comprising elements of both the copper pillar and the metal layer, a second portion of the metal layer in contact with the solder bump remains unreacted, wherein forming the intermetallic includes:

converting a portion of the metal layer in direct contact with the copper pillar to Cu 3 Sn; and

oxidizing an outermost portion of the metal layer to forming SnO 2 ,

wherein the intermetallic comprises one of: only Cu 3 Sn; only Cu 3 Sn and SnO 2 ; and only tin, Cu 3 Sn, and SnO 2 .

8. The method of claim 7 , wherein forming the copper pillar comprises:

patterning an opening in a photoresist layer above a quad layer plating structure comprising a first layer, a second layer, a third layer, and a fourth layer, each layer is conductive and is formed successively one on top of another; and

electroplating the copper pillar structure in the opening by applying an electrical potential to the second layer, copper from a plating solution plates out on the fourth layer exposed at a bottom of the opening.

9. The method of claim 7 , wherein forming the copper pillar comprises:

patterning an opening in a photoresist layer;

electroplating the copper pillar structure in the opening; and

removing a portion of the photoresist layer immediately adjacent to the copper pillar to form a space, the space is formed by exposing the photoresist layer to a high temperature high pH plating bath.

10. The method of claim 7 , wherein forming the solder bump directly on top of the copper pillar comprises:

electroplating a solder cap directly on top of the copper pillar and within the opening;

removing the photoresist layer; and

reflowing the solder cap to form a hemispherical solder bump in which the surface tension limits molten solder from extending beyond the top surface of the copper pillar during reflow.

11. The method of claim 7 , further comprising:

heating the solder bump to a molten state after formation of the intermetallic,

wherein molten solder will wet to the top surface of the copper pillar and will not wet to the intermetallic.

12. A method comprising:

forming a copper pillar;

electroplating a first metal layer on a top surface and a sidewall of the copper pillar;

electroplating a second metal layer on top of the first metal layer, wherein the second metal layer is less than 1 μm; and

forming an intermetallic by heating the first metal layer and the copper pillar in a non-reducing environment, the intermetallic comprising elements of both the copper pillar and the metal layer, wherein the intermetallic comprises one of: only Cu 3 Sn; only Cu 3 Sn and SnO 2 ; and only tin, Cu 3 Sn, and SnO 2

wherein molten solder will wet to the second metal layer.

13. The method of claim 12 , wherein forming the copper pillar comprises:

patterning an opening in a photoresist layer above a quad layer plating structure comprising a first layer, a second layer, a third layer, and a fourth layer, each layer is conductive and is formed successively one on top of another;

electroplating the copper pillar structure in the opening by applying an electrical potential to the second layer, copper from a plating solution plates out on the fourth layer exposed at a bottom of the opening;

removing an exposed portion of the quad layer plating structure such that a remaining portion of the quad layer plating structure remains beneath the copper pillar; and

removing remaining exposed layers of the quad layer plating structure.

14. The method of claim 12 , wherein forming the copper pillar comprises:

patterning an opening in a photoresist layer;

electroplating the copper pillar structure in the opening; and

removing a portion of the photoresist layer immediately adjacent to the copper pillar to form a space, the space is formed by exposing the photoresist layer to a high temperature high pH plating bath.

15. The method of claim 12 , wherein the first metal layer comprises tin, nickel, or some combination thereof.

16. The method of claim 12 , wherein the second metal layer comprises gold, palladium, or some combination thereof.