Polymer layers embedded with metal pads for heat dissipation

1. A method comprising:

forming a metal pad over an interconnect structure;

forming a passivation layer over the interconnect structure, wherein the passivation layer comprises a portion over the metal pad;

coating a first polymer layer over the passivation layer;

plating a dummy metal pad and a first via in the first polymer layer, wherein at a time the dummy metal pad is plated, the dummy metal pad is a discrete metal pad fully encircled by the first polymer layer, and an entirety of the dummy metal pad is disconnected from other conductive features;

forming a second polymer layer over the first polymer layer;

forming a second via and a third via in the second polymer layer, wherein the second via is electrically coupled to the first via, and the third via has a bottom surface contacting a top surface of the dummy metal pad;

forming a redistribution line over and contacting the second polymer layer, wherein the redistribution line electrically couples the second via to the third via;

coating a third polymer layer over the second polymer layer; and

forming an Under-Bump-Metallurgy (UBM) extending into the third polymer layer, wherein the UBM is electrically coupled to the dummy metal pad.

2. The method of claim 1 , wherein the third via and the dummy metal pad are configured to have no current flowing through when a current flows through the second via.

3. The method of claim 1 , wherein the plating the dummy metal pad comprises:

forming an opening in the first polymer layer, wherein no conductive material is on sidewalls and bottom surfaces of the opening; and

plating a metallic material in the opening to form the dummy metal pad.

4. The method of claim 1 , wherein the UBM and the dummy metal pad are electrically coupled to an electrical ground or a power supply node.

5. The method of claim 1 , wherein the UBM is in a first package component, wherein the method further comprises bonding a second package component to the first package component.

6. The method of claim 1 , wherein the dummy metal pad has a top surface and a bottom surface coplanar with a top surface and a bottom surface, respectively, of the first polymer layer.

7. A method comprising:

forming a first package component comprising:

forming a metal pillar in a first polymer layer, wherein a top surface of the metal pillar is substantially coplanar with a top surface of the first polymer layer;

forming a second polymer layer over and contacting the top surface of the metal pillar and the top surface of the first polymer layer;

forming a first dummy metal pad in the second polymer layer, wherein a top surface and a bottom surface of the first dummy metal pad are coplanar with a top surface and a bottom surface, respectively, of the second polymer layer; and

forming a first Under-Bump-Metallurgy (UBM) electrically coupled to the first dummy metal pad;

bonding the first UBM to a first metal pad in a second package component through a solder region, wherein after the bonding, an electrically inter-coupled structure comprising the first dummy metal pad, the UBM, and the first metal pad is electrically floating;

forming a second dummy metal pad in the second polymer layer;

forming a first metal via and a second metal via over the second polymer layer and at a same level, wherein the first metal via is electrically coupled to the second dummy metal pad through the second metal via, and the second metal via has a bottom surface in contact with a top surface of the second dummy metal pad;

forming a second UBM electrically coupled to the second dummy metal pad; and

bonding the second UBM to a second metal pad in the second package component, wherein after the bonding, the first metal via is configured to have currents flowing through, and the second dummy metal pad and the second metal via are configured not to have currents flowing through.

8. The method of claim 7 , wherein the first dummy metal pad and the second dummy metal pad are formed in a same process step.

9. The method of claim 7 , wherein the first metal pad is a signal pad configured to conduct electrical signals.

10. The method of claim 7 , wherein the metal pillar is a signal path configured to conduct electrical signals.

11. The method of claim 7 , wherein the forming the first dummy metal pad comprises:

forming an opening in the second polymer layer, wherein no conductive material is exposed through sidewalls and a bottom surface of the opening; and

plating a metallic material in the opening to form the first dummy metal pad.

12. The method of claim 7 , wherein the first UBM is in physical contact with the solder region.

13. The method of claim 7 , wherein all bottom surfaces of the first dummy metal pad are in contact with dielectric materials.

14. A method comprising:

forming a metal pillar in a dielectric layer;

after the metal pillar is formed, forming a first polymer layer over and contacting the dielectric layer and the metal pillar;

forming an opening in the first polymer layer to expose a top surface of the dielectric layer, wherein all sidewall surfaces and a bottom surface of the opening are surfaces of dielectric materials;

filling a metallic material into the opening to form a dummy metal pad;

forming a second polymer layer over the first polymer layer;

forming a Post-Passivation Interconnect (PPI) over and electrically coupled to the dummy metal pad, wherein the PPI comprises:

a first metal via and a second metal via in the second polymer layer, with the first metal via electrically connected to the metal pillar, and the second metal via having a bottom surface over and contacting the dummy metal pad, wherein the first metal via is configured to have currents flowing through, and the dummy metal pad and the second metal via are configured not to have currents flowing through; and

a redistribution line over and contacting the second polymer layer, wherein the redistribution line interconnects the first metal via and the second metal via;

disposing a solder region over and electrically coupling to the PPI; and

bonding a metal pad in a package component to the solder region, and the package component is over the solder region.

15. The method of claim 14 , wherein the solder region is in contact with an electrically conductive feature in the package component, wherein after the bonding, the dummy metal pad and the electrically conductive feature in combination are electrically floating.

16. The method of claim 14 , wherein after the bonding, no conductive feature is electrically coupled to the dummy metal pad from sidewalls and a bottom of the dummy metal pad.

17. The method of claim 14 , wherein the dielectric layer comprises an additional polymer.

18. The method of claim 14 , wherein the first polymer layer is in physical contact with both the dielectric layer and the metal pillar.

19. The method of claim 14 , wherein the package component is a package substrate, an interposer, or a printed circuit board.

20. The method of claim 14 further comprising forming an Under-Bump-Metallurgy (UBM) electrically coupling to the dummy metal pad.