Interconnect substrate and method of making interconnect substrate

1. An interconnect substrate, comprising:

an insulating resin layer having a first surface and a second surface opposite the first surface;

a redistribution layer provided on the first surface of the insulating resin layer;

a first connection terminal exposed at the second surface of the insulating resin layer; and

a conductive via provided in the insulating resin layer to electrically connect the redistribution layer and the first connection terminal,

wherein the insulating resin layer includes:

a first resin layer constituting the second surface and containing a first filler;

a second resin layer provided on the first resin layer on a same side as the first surface; and

a third resin layer provided on the second resin layer on a same side as the first surface, the third resin layer containing a second filler and constituting the first surface,

wherein an average particle diameter of the first filler is greater than an average particle diameter of the second filler, and

wherein in a cross-section of the insulating resin layer perpendicular to the first surface, a percentage of the first filler in the first resin layer in terms of area is 40% to 70%, and a percentage of the second filler in the third resin layer in terms of area is 50% to 80%, the percentage of the first filler in the first resin layer in terms of area being lower than the percentage of the second filler in the third resin layer in terms of area.

2. The interconnect substrate as claimed in claim 1 , wherein the average particle diameter of the first filler is 0.05 μm to 1.00 μm, and the average particle diameter of the second filler is 0.01 μm to 0.50 μm.

3. The interconnect substrate as claimed in claim 1 , wherein the average particle diameter of the first filler is 3 to 100 times as great as the average particle diameter of the second filler.

4. The interconnect substrate as claimed in claim 1 , wherein in a cross-section of the insulating resin layer perpendicular to the first surface, a number density of the first filler in the first resin layer is 2 particles/μm 2 to 10 particles/μm 2 , and a number density of the second filler in the third resin layer is 20 particles/μm 2 to 40 particles/μm 2 , the number density of the first filler in the first resin layer being lower than number density of the second filler in the third resin layer.

5. The interconnect substrate as claimed in claim 1 , wherein the second resin layer contains glass fibers.

6. The interconnect substrate as claimed in claim 1 , wherein the third resin layer is thinner than the first resin layer and the second resin layer.

7. The interconnect substrate as claimed in claim 1 , further comprising a build-up substrate having a second connection terminal bonded to the first connection terminal.

8. An interconnect substrate, comprising:

an insulating resin layer having a first surface and a second surface opposite the first surface;

a redistribution layer provided on the first surface of the insulating resin layer;

a first connection terminal exposed at the second surface of the insulating resin layer; and

a conductive via provided in the insulating resin layer to electrically connect the redistribution layer and the first connection terminal,

wherein the insulating resin layer includes:

a first resin layer constituting the second surface and containing a first filler;

a second resin layer provided on the first resin layer on a same side as the first surface; and

a third resin layer provided on the second resin layer on a same side as the first surface, the third resin layer containing a second filler and constituting the first surface,

wherein an average particle diameter of the first filler is greater than an average particle diameter of the second filler, and

wherein a thickness of the first resin layer is equal to a thickness of the second resin layer, and each of the first resin layer and the second resin layer is thicker than the third resin layer.

9. The interconnect substrate as claimed in claim 8 , wherein the average particle diameter of the first filler is 0.05 μm to 1.00 μm, and the average particle diameter of the second filler is 0.01 μm to 0.50 μm.

10. The interconnect substrate as claimed in claim 8 , wherein the average particle diameter of the first filler is 3 to 100 times as great as the average particle diameter of the second filler.

11. The interconnect substrate as claimed in claim 8 , wherein in a cross-section of the insulating resin layer perpendicular to the first surface, a percentage of the first filler in the first resin layer in terms of area is 40% to 70%, and a percentage of the second filler in the third resin layer in terms of area is 50% to 80%.

12. The interconnect substrate as claimed in claim 8 , wherein in a cross-section of the insulating resin layer perpendicular to the first surface, a number density of the first filler in the first resin layer is 2 particles/μm 2 to 10 particles/μm 2 , and a number density of the second filler in the third resin layer is 20 particles/μm 2 to 40 particles/μm 2 .

13. The interconnect substrate as claimed in claim 8 , wherein the second resin layer contains glass fibers.

14. The interconnect substrate as claimed in claim 8 , further comprising a build-up substrate having a second connection terminal bonded to the first connection terminal.