Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-03-07
2003-02-25
Wilson, Gregory (Department: 3749)
Metal working
Method of mechanical manufacture
Electrical device making
C029S825000, C029S846000
Reexamination Certificate
active
06523258
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of manufacturing a printed circuit board having inner via-holes.
BACKGROUND OF THE INVENTION
In recent years, due to the miniaturization and multiple functions of electronic equipment, printed circuit boards have been increasingly becoming multi-layered and higher in component density.
Generally, a method of manufacturing a printed circuit board comprises a step of alternately laminating a plurality of boards with conductor circuits and adhesive sheets (usually called “prepreg”), a step of press-fitting under heat, followed by a step of making a through-hole therein, and a step of making electrical connections between surface and inner layers by means of copper plating or the like in the through-hole.
However, with increase in demand for video movie cameras and mobile communication equipment or the like, printed circuit boards used for these cameras and equipment have been required to be smaller in size and higher in density. To meet such requirement, there is a proposal of a method for manufacturing a printed circuit board disclosed in Japanese Laid-open Patent No. 6-268345. The method of manufacturing a printed circuit board in Japanese Laid-open Patent No.6-268345 comprises a step of making a through-hole in a non-compressible porous substrate having release film on either side thereof, a step of filling conductive paste into the through-hole, a step of electrically connecting both sides of the board to each other by sticking metallic foil on either side of the porous substrate and pressurizing same under heat after peeling the release film, and a step of patterning by itching the metallic foil to form a circuit.
A conventional method of manufacturing a printed circuit board will be described in the following with reference to the drawings.
FIG. 3
to
FIG. 5
show cross-sectional views of manufacturing steps for a conventional printed circuit board. First, as shown in FIG.
3
(
a
), release film
12
is disposed on either side of porous substrate (hereinafter referred to as “prepreg sheet”)
11
. The porous substrate
11
has a square plane, which is 500 mm in side length and t1 (for example, about 150 &mgr;m) in thickness. The release film
12
includes polymer film such as PET (polyethylene terephthalate) of 19 &mgr;m thick, having a silicone-based release layer disposed on one side thereof. Prepreg sheet
11
used is a composite material consisting of non-woven cloth of aromatic polyamide fiber and thermosetting epoxy resin with which the non-woven cloth is impregnated.
Next, as shown in FIG.
3
(
b
), through-hole
13
is formed in a predetermined portion of the prepreg sheet
11
by laser operation.
Subsequently, the prepreg sheet
11
is placed on a table of a printing machine (not shown), and conductive paste
14
is printed on the release film
12
. Then, as shown in FIG.(
c
), the conductive paste
14
is filled into the through-hole
13
. In this case, the release film
12
serves to prevent staining of a printing mask and prepreg sheet
11
.
Next, as shown in FIG.
3
(
d
), the release film
12
on either side of the prepreg sheet
11
is removed at the room temperature. And, as shown in FIG.
3
(
e
), metallic foil
15
such as copper foil is stuck on either side of the prepreg sheet
11
and is pressurized under heat. In this way, as shown in FIG.
3
(
f
), the prepreg sheet
11
and the metallic foil
15
are adhered to each other, and the prepreg sheet
11
is compressed to t2 (for example, about 100 &mgr;m) in thickness (t2>t2), then the metallic foils
15
on either side of same are electrically connected to each other by conductive paste
14
. At that time, epoxy resin, that is one of the components of the prepreg sheet
11
, and the conductive paste
14
become hardened.
And, as shown in FIG.
3
(
g
), the metallic foil
5
is subjected to patterning by photolithography, followed by etching, and thereby, circuit pattern
16
is formed on either side of the prepreg sheet
11
.
However, in the conventional configuration described above, through-holes in a prepreg sheet are reduced in diameter as a result of increasing in density of the circuit pattern, and after filling conductive paste, when the release film is removed from the prepreg sheet
11
, the conductive paste sticking to the release film is removed from the prepreg sheet along with the release film. A possible cause of this problem is that reducing only the through-hole diameter by using conventional release film mainly to meet the requirement for reducing the through-hole diameter causes the through-hole diameter to be excessively reduced as compared with the thickness of the release film, that is, the aspect ratio (the ratio of the diameter of the hole formed in the release film to the thickness of the release film) becomes greater. Accordingly, it is not possible to make the conductive paste projected in convexity from the surface of the prepreg sheet after removal of the release film. If the worse comes to the worst, as shown in FIG.
4
(
d
), the conductive paste will be concavely curved from the uppermost surface of the prepreg sheet, thereby forming a depletion layer
18
. Consequently, it becomes difficult to make electrical connections between the conductive paste and the metallic foil in the following steps.
Also, as shown in
FIG. 5
, for preventing removal of the conductive paste during release film peeling operation, it is possible to lower the aspect ratio by using thinner release film. However, when thinner release film
22
is used, the amount of conductive paste projected from the prepreg sheet will become less. As a result, the absolute amount of conductive paste that should exist in the through-hole after compression by pressure application becomes insufficient. Accordingly, the connection resistance between the metallic foil and the conductive paste will increase.
SUMMARY OF THE INVENTION
A method of manufacturing a printed circuit board of the present invention comprises:
(a) a step of disposing a first release film on a surface of a substrate, and disposing a second release film on a back surface of the substrate;
(b) a step of forming a through-hole through the first release film, the second release film and the substrate;
(c) a step of filling conductive paste into the through-hole;
(d) a step of peeling the first release film and the second release film from the substrate with the through-hole filled with the conductive paste;
(e) a step of placing a first metallic member on the surface of the substrate with the release films removed, and placing a second metallic member on the back surface of the substrate;
(f) a step of compressing under heat the substrate with the first metallic member and the second metallic member disposed on either side thereof; and
(g) a step of forming a desired circuit pattern on the first metallic member and the second metallic member,
wherein the thickness of the first release film is different from the thickness of the second release film;
when the first release film is peeled, a first projected paste portion is formed, which is projected from the surface of the substrate having the through-hole, and a second projected paste portion is formed, which is projected from the back surface of the substrate having the through-hole; and
the first metallic member is electrically connected to the first projected paste portion, and the second metallic member is electrically connected to the second projected paste portion.
Preferably, the thickness of the first release film is greater than the thickness of the second release film, the thickness of the first projected paste portion depends upon the thickness of the first release film, the thickness of the second projected paste portion depends upon the thickness of the second release film, and the thickness of the first projected paste portion is greater than the thickness of the second projected paste portion.
Preferably, a step of forming the through-hole is performed by application of a laser beam, and the laser beam is applied from t
Kawamoto Eiji
Takenaka Toshiaki
Yamane Shigeru
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Wilson Gregory
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