# Patent

## Title
Method of fabricating board having high density core layer and structure thereof

## Abstract

A circuit board includes a core layer substrate having a plated through hole filled with a dielectric material. The plated through hole has a sidewall coated with an inner electroless copper layer, and an electroplated metal layer plated on the inner electroless copper layer before the plated through hole is filled with the dielectric material. The outer portion of the filled plated through hole is thicker than the center portion and tapered toward the center portion to form a depressed surface on the filled plated through hole. The core layer substrate is covered with a patterned electroless copper layer and a patterned electroplated copper layer that connect with the inner electroless copper layer and electroplated metal layer of the plated through hole. The patterned electroplated copper layer forms a flat copper pad above the plated through hole.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to a method of fabricating a board and its structure, and in particular, to a method of fabricating a board having high-density core layer and the structure thereof.
2. The Prior Arts
With the trend of electronic products becoming smaller, thinner, lighter, and shorter and at the same time having more additional functions, the number of I/Os residing on the chips is rapidly increased. The corresponding electronics packaging technology has also been evolving to accommodate the diverse integrated circuits. Nowadays, most high-level products adopt the flip-chip technology; the package density has line width dropped from 90 nm to 65 nm, and even to 45 nm. Meanwhile, the line width on the IC board has become thinner, going from 50 μm to 20 μm (pitch 40 μm), and even to 15 μm (pitch 30 μm).
Apart from the earlier packaging technologies such as dual in line package (DIP), there are many other packaging technologies such as the chip carrier (CC), flip chip (FC), pin grid array (PGA), tape carrier (TC), hermetic package, ball grid array (BGA), quad flat package (QFP), lead on chip (LOC), chip scale package (CSP), bare die, tape carrier package (TCP), etc.
Of all these packaging boards, the line width and the line width/line spacing (L/S) of VOP (via on plated through hole) made by the traditional FC, CSP technologies cannot reach 40 μm or below, which becomes a bottleneck in the reduction of the overall board density.
SUMMARY OF THE INVENTION
A primary objective of the present invention is to provide a method of fabricating a board having high-density blind via stacked structure. The fabrication method mainly includes the steadily making of multiple-layers of blind vias that are stacked on plated though holes on a core layer. The main fabrication procedure uses core layer with the plated through hole (PTH) for filling resin, encapsulation, grinding, buffing, and polishing. Laser etching, sand blasting, plasma etching, chemical etching, and other methods are used for partially removing the filling material filled inside the PTH of the core layer until the core layer has reached an appropriate flattened depression in the center part of the PTH. A thin copper metal layer is coated on the surface at a thickness of 3 ̃12 μm and image transfer and pattern plating operations are performed to fill and level the depression portion of the filling material up to desired thickness, and to produce the copper pad, and the necessary circuitry.
The semi-additive processing (SAP) technique can be utilized for making the circuits and traces if the high-density thin line structure is needed. Here, the copper layer on the surface can be removed completely, and then the methods such as chemical vapor deposition of copper and the pattern plating can be adopted for making the product. Because of the added flexibility of and reliability of the electroplating process and the structure being adopted, it can substantially improve the design flexibility of the product and the reliability of the electrical circuits. Due to the availability of the SAP technology, the thin line fabrication is able to reach the capability for the line/space (L/S) at 20/20 μm.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be apparent to those skilled in the art by reading the following detailed description of a preferred embodiment thereof, with reference to the attached drawings, in which:
FIGS. 1A ̃1I are a plurality of cross-sectional views showing the fabrication process of a board having high-density core layer in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1A ̃1I are a plurality of cross sectional views showing the fabrication process of a board having high-density core layer according to an embodiment of the present invention, in which FIGS. 1A ̃1D show steps which are similar to the conventional fabrication process of a board having high-density core layer, and FIGS. 1E ̃1I show the remaining fabrication process for the board having high-density core layer according to the embodiment.
The fabrication method for the board having the high-density core layer of the present invention mainly focuses on the core layer substrate having a plated through hole (PTH) 14 (shown in FIG. 1C), the outside of the PTH 14 of the core layer substrate is formed with a depression portion 22 as shown in FIG. 1E; and a surface metal layer 12 is thinned down to 3 ̃12 μm. And then image transfer and pattern plating operations are performed to fill the depression portion 22 (namely, removing the thickness of the filling material 20), thus to form the circuit layer 26 and a copper pad 28 (overplating) that is required, as illustrated in FIG. 1I. The PTH 14 is adapted to provide conducting between the internal wiring and the external wiring of the board.
There are two methods for forming the aforementioned depression portion 22: one is by excavation and the other by an insufficient filling method. Specifically speaking, when the depression portion 22 is formed on the outside of the PTH 14, by using the laser etching, sand blasting, plasma etching, chemical etching, etc. to remove the filling material 20 inside the PTH of the core layer, a portion of the filling material 20 which is filled in the PTH 14 can be removed, and thus the depression portion 22 is formed. Optionally, the other method is by filling the filling material 22 into the PTH 14 steadily, while not filling up the PTH 14 to the extent as shown in FIG. 1D, until the depression portion 22 is formed on the outside of the PTH 14. Of these two methods regarding the formation of the depression portion 22, the excavation method is easier to perform than the insufficient filling method. The filling material 20 is of a high dielectric coefficient insulation material.
After the depression portion 22 is formed, the material on the substrate 10 needs to be removed for fabricating the super-thin circuit and to continue making the circuit layer 26. When the material on the substrate 10 is removed, because both the depression portion and the substrate are made of insulation material, the electroless copper 19 must be formed on these two items before the circuit layer 26 is electroplated (shown in FIG. 1F). Then, by using a patterned photoresistive layer 24 as shown in FIG. 1G, the depression portion 22 is filled and leveled; and the circuit layer 26 is made as shown in FIG. 1H. Finally, the patterned photoresistive layer 24 is removed as shown in FIG. 1I.
Hence, the SAP technique can be utilized to make the circuit if high-density thin line structure is needed. Here, the metal layer 12 on the surface can be removed completely, and then methods such as electroless copper deposition and the pattern plating can be adopted for making the product. Due to the adoption of the electroplating process and structural processing being adopted, the area and the thickness of the copper coated at the corner of the PTH increase significantly and can be flexibly adjusted. Such structure can substantially improve the electrical reliability of the product and the mechanical strength of the circuits. In addition, because of the feasibility of this structure, the conventional stacking limit of the 3 blind vias on the PTH can then be increased to 4 or above. And also, due to the possibility of using the SAP technology, thin line fabrication capability could reach a line width/line spacing of 20/20 μm.
In order to fabricate the core layer substrate as illustrated in FIG. 1D, a substrate 10 of 0.8 mm thickness is first provided (as shown in FIG. 1B), and a metal layer 12 of 1 ̃12 μm thickness is formed on the substrate 10. The substrate 10 is usually made of Bismaleimide Triazine (BT) or other organic material, and ceramic material. Then, using mechanical drilling, the metal layer 12 and the substrate 10 are penetrated through to form a PTH 14 with 100 ̃300 μm diameter (preferably 150 μm) as shown in FIG. 1C. And then the electroplated metal layer 16 is formed with 10 ̃30 μm in diameter on the metal layer 12 and on the side wall of the through hole (already coated with electroless copper 18) on the core substrate as shown in FIG. 1D. Based on the aforementioned method, it can be decided to whether fill the PTH 14 entirely or partially with the filling material 20.
Although the present invention has been described with reference to the preferred embodiment thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.

## Claims

1. A circuit board having a high-density core layer, comprising:
a core layer substrate having a plated though hole filled with a dielectric cylinder, said plated though hole having a sidewall coated with an inner electroless copper layer, and an electroplated metal layer plated on said inner electroless copper layer before being filled with said dielectric cylinder, said dielectric cylinder having an outer portion tapered toward a center portion, said outer portion being thicker than said center portion to form a depressed surface;
a surface electroless copper layer covering said core layer substrate, said plated through hole and said depressed surface for connecting with said inner electroless copper layer and said electroplated metal layer; and
an electroplated copper layer covering said surface electroless copper layer;
wherein said electroplated copper layer and said surface electroless copper layer are patterned to expose portions of said core layer substrate and form a circuit layer on said core layer substrate and a flat copper pad covering said depressed surface of said dielectric cylinder and said plated though hole.