Methods of forming lead free solder bumps

1. A method of forming an electronic device, the method comprising:

forming an under bump seed metallurgy layer on an electronic substrate;

forming a nickel layer having a thickness of at least 1 μm (micrometer) on the under bump seed metallurgy layer so that the under bump seed metallurgy layer is between the nickel layer and the electronic substrate, wherein portions of the under bump seed metallurgy layer are free of the nickel layer; and

forming a lead free solder layer on the nickel layer so that the nickel layer is between the solder layer and under bump seed metallurgy layer.

2. A method according to claim 1 further comprising:

before forming the nickel layer, forming a plating mask on the under bump seed metallurgy layer wherein portions of the under bump seed metallurgy layer are exposed through the plating mask, wherein forming the nickel layer comprises plating the nickel layer on the portions of the under bump seed metallurgy layer exposed through the plating mask, and wherein forming the solder layer comprises plating the solder layer on the nickel layer.

3. A method according to claim 2 further comprising:

before forming the nickel layer, plating a copper layer having a thickness of at least 5 μm (micrometer) on the portions of the under bump seed metallurgy layer exposed through the plating mask so that the copper layer is between the nickel layer and the under bump seed metallurgy layer.

4. A method according to claim 2 further comprising:

after forming the solder layer, removing the plating mask; and

after removing the plating mask, removing portions of the under bump seed metallurgy layer free of the nickel layer.

5. A method according to claim 1 wherein the nickel layer has a thickness in the range of about 1.5 μm (micrometer) to about 5 μm (micrometer).

6. A method according to claim 1 wherein the nickel layer has a thickness in the range of about 1 μm (micrometer) to about 5 μm (micrometer).

7. A method according to claim 1 further comprising:

before forming the nickel layer, forming a copper layer on the under bump seed metallurgy layer so that the under bump seed metallurgy layer is between the copper layer and the electronic substrate, wherein portions of the under bump seed metallurgy layer are free of the copper layer, and wherein the copper layer is between the nickel layer and the under bump seed metallurgy layer.

8. A method according to claim 7 wherein the copper layer has a thickness greater than about 5 μm (micrometer).

9. A method according to claim 7 wherein the nickel layer has a thickness in the range of about 1 μm (micrometer) to about 5 μm (micrometer).

10. A method according to claim 1 wherein the solder layer comprises tin and at least one of silver, bismuth, copper, indium, antimony, gold, and/or zinc.

11. A method according to claim 10 wherein a weight percent of tin in the solder layer is greater than about 95 weight percent.

12. A method according to claim 1 wherein forming the solder layer comprises forming a first layer of tin and forming a second layer of silver, bismuth, copper, indium, antimony, gold, and/or zinc.

13. A method according to claim 12 wherein the first layer is between the second layer and the nickel layer.

14. A method according to claim 12 wherein the first layer of tin is greater than about 95 weight percent of the solder layer.

15. A method according to claim 12 further comprising:

after forming the solder layer, heating the first and second layers to provide an alloy thereof.

16. A method according to claim 15 wherein heating the first and second layers comprises heating the first and second layers to a temperature that is less than a melting temperature of at least one of the first and/or second layers.

17. A method according to claim 15 wherein heating the first and second layers comprises heating the first and second layers to a temperature that is less than a melting temperature of one of the first and second layers and greater than a melting temperature of the other of the first and second layers.

18. A method according to claim 15 wherein heating the first and second layers comprises heating the first and second layers to a temperature that is less than a melting temperature of either of the first and second layers.

19. A method according to claim 18 wherein heating the first and second layers comprises heating the first and second layers to a temperature above a melting temperature of the alloy thereof.

20. A method according to claim 1 wherein forming the under bump seed metallurgy layer comprises:

forming an adhesion layer on the electronic substrate; and

forming a conduction layer on the adhesion layer, wherein the adhesion layer is between the conduction layer and the electronic substrate, and wherein the adhesion layer and the conduction layer comprise different materials.

21. A method according to claim 20 wherein the adhesion layer comprises Ti (titanium), TiW (titanium-tungsten), TiN (titanium nitride), and/or Cr (chromium).

22. A method according to claim 20 wherein the conduction layer comprises Cu (copper), Ag (silver), and/or Au (gold).

23. A method according to claim 1 wherein forming the solder layer comprises:

plating a first portion of the solder layer in a plating bath including first and second plating metals using a first plating voltage and/or current so that the first portion of the solder layer has a first concentration of the first plating metal; and

plating a second portion of the solder layer in the plating bath including the first and second plating metals using a second plating voltage and/or current so that the second portion of the solder layer has a second concentration of the first plating metal different than the first concentration.

24. A method of forming an electronic device, the method comprising:

forming an under bump seed metallurgy layer on an electronic substrate;

forming a nickel layer having a thickness of at least 1 μm on the under bump seed metallurgy layer so that the under bump seed metallurgy layer is between the nickel layer and the electronic substrate, wherein portions of the under bump seed metallurgy layer are free of the nickel layer;

forming a lead free solder layer on the nickel layer so that the nickel layer is between the solder layer and under bump seed metallurgy layer; and

after forming the lead free solder layer, removing portions of the under bump seed metallurgy layer free of the nickel layer.

25. A method according to claim 24 wherein the under bump seed metallurgy layer includes an adhesion layer comprising a first metal and a conduction layer comprising a second metal, wherein the adhesion layer is between the conduction layer and the electronic substrate, and wherein the first and second metals are different.

26. A method according to claim 25 wherein the first metal comprises Ti (titanium), TiW (titanium-tungsten), TiN (titanium nitride), and/or Cr (chromium), and wherein the second metal comprises Cu (copper), Ag (silver), and/or Au (gold).

27. A method according to claim 24 further comprising:

before forming the nickel layer, forming a copper layer having a thickness of at least 5μm on the under bump seed metallurgy layer so that the copper layer is between the nickel layer and the under bump seed metallurgy layer, wherein portions of the under bump seed metallurgy layer are free of the copper layer.

28. A method according to claim 24 further comprising:

after removing portions of the under bump seed metallurgy layer, reflowing the lead free solder layer.