Thin film RDL for IC package

1. A thin film redistribution layer (RDL) structure for an integrated circuit (IC) package, the thin film RDL structure comprising:

a redistribution layer having

a plurality of top metal pads, and

a plurality of bottom metal pads,

wherein a density of the top metal pads is higher than a density of the bottom metal pads;

a molding compound having

opposite uppermost and lowermost surfaces, and

a uniform thickness between the uppermost and lowermost surfaces; and

a plurality of metal pillars, each configured on top of a corresponding top metal pad among the plurality of top metal pads,

wherein

the redistribution layer further comprises a topmost dielectric layer,

the plurality of top metal pads and a bottom end of each metal pillar among the plurality of metal pillars are embedded in the topmost dielectric layer, and

each metal pillar among the plurality of metal pillars has

a middle portion embedded in the molding compound,

a top end protruding upwardly above the uppermost surface of the molding compound, and

the bottom end protruding downwardly below the lowermost surface of the molding compound.

2. A thin film RDL structure as claimed in claim 1 , further comprising:

a plurality of solder balls, each configured on bottom of a corresponding bottom pad among the plurality of bottom metal pads.

3. A thin film RDL structure as claimed in claim 1 , further comprising:

a bottom protection layer configured on bottom of a corresponding bottom metal pad among the plurality of bottom metal pads.

4. A thin film RDL structure as claimed in claim 3 , wherein the bottom protection layer is selected from the group consisting of Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) and Organic Solderability Preservatives (OSP).

5. A thin film RDL structure as claimed in claim 3 , further comprising:

a plurality of solder balls, each configured on bottom of the bottom protection layer of a corresponding bottom metal pad among the plurality of bottom metal pads.

6. A thin film RDL structure as claimed in claim 5 , wherein the bottom protection layer is selected from the group consisting of ENEPIG and OSP.

7. A thin film RDL structure as claimed in claim 1 , wherein the bottom end of each metal pillar among the plurality of metal pillars is electrically coupled to a corresponding top metal pad among the plurality of top metal pads.

8. A thin film RDL structure as claimed in claim 7 , further comprising:

a bottom protection layer configured on bottom of a corresponding bottom metal pad among the plurality of bottom metal pads; and

a plurality of solder balls, each configured on bottom of the bottom protection layer of a corresponding bottom metal pad among the plurality of bottom metal pads.

9. A thin film RDL structure as claimed in claim 8 , wherein the bottom protection layer is selected from the group consisting of ENEPIG and OSP.

10. A thin film RDL structure as claimed in claim 9 , further comprising:

an under-filling material between the chip and the uppermost surface of the molding compound,

wherein the top end of each metal pillar among the plurality of metal pillars is embedded in the under-filling material.

11. A thin film RDL structure as claimed in claim 1 , further comprising:

solder paste directly on the top ends of the plurality of metal pillars; and

a chip directly on the solder paste.

12. A thin film RDL structure as claimed in claim 11 , further comprising:

an under-filling material between the chip and the uppermost surface of the molding compound,

wherein the top end of each metal pillar among the plurality of metal pillars is embedded in the under-filling material.

13. A fabricating process for a thin film redistribution layer (RDL) structure for an integrated circuit (IC) package, the fabricating process comprising:

applying a release layer on top of a temporary carrier;

applying a bottom seed layer on top of the release layer;

applying a first patterned photoresist on top of the bottom seed layer;

forming a plurality of patterned bottom metal pads;

stripping the first patterned photoresist;

forming an RDL using the plurality of bottom metal pads as a starting point, the RDL having a plurality of top metal pads, wherein a density of the top metal pads is higher than a density of the bottom metal pads;

applying a top seed layer on top of the RDL;

forming a second patterned photoresist on top of the top seed layer;

forming, in the second patterned photoresist, a plurality of metal pillars, each on top of a corresponding top metal pad among the plurality of top metal pads;

stripping the second patterned photoresist and stripping the top seed layer between the plurality of metal pillars;

applying a molding compound to encapsulate the plurality of metal pillars;

thinning the molding compound from top to expose top surfaces of the plurality of metal pillars, so that the molding compound has

opposite uppermost and lowermost surfaces, and

a uniform thickness between the uppermost and lowermost surfaces;

after said thinning, removing a portion of the molding compound from top to expose top ends of the plurality of metal pillars, wherein each metal pillar among the plurality of metal pillars has

a middle portion embedded in the molding compound,

the top end protruded upwardly above the uppermost surface of the molding compound and having the exposed top surface, and

a bottom end protruded downwardly below the lowermost surface of the molding compound; and

mounting a chip on the top ends of the plurality of metal pillars.

14. A fabricating process for a thin film RDL structure as claimed in claim 13 , further comprising:

before said mounting the chip, applying solder paste on the exposed top ends of the plurality of metal pillars.

15. A fabricating process for a thin film RDL structure as claimed in claim 14 , further comprising:

underfilling a space between the chip and the uppermost surface of the molding compound.

16. A fabricating process for a thin film RDL structure as claimed in claim 15 , further comprising:

removing the temporary carrier; and

removing the bottom seed layer.

17. A fabricating process for a thin film RDL structure as claimed in claim 16 , further comprising:

forming a protection layer on bottom of each bottom metal pad among the plurality of bottom metal pads; and

planting a plurality of solder balls, each on bottom of the protection layer of a corresponding bottom metal pad among the plurality of bottom metal pads.