Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2000-10-11
2004-03-09
Vo, Peter (Department: 3729)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C029S829000, C029S830000, C029S831000, C156S233000, C156S272200, C156S272800, C156S272800, C156S289000, C156S345420, C174S264000, C174S265000
Reexamination Certificate
active
06703565
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing printed wiring boards (PWBs), especially double-side PWBs or multilayer PWBs.
Recently, a low cost multilayer PWB that enables high-density mounting of semiconductor chips such as LSIs has been desired for industrial machines as well as home electronic appliances. It is important for such multilayer PWBs to provide high quality electric connections between plural layers of wiring patterns formed at a fine spacing or pitch.
The conventional PWB that is produced by drilling, etching and plating a copper-foil laminated board cannot satisfy the above-mentioned need anymore for sophisticated electronic equipment having a number of functions. To solve such a problem, some methods are under development for producing PWBs that have a new structure and a high density of wiring.
One of the methods is a recent technique for forming a fine pattern that can be applied to a high-density surface mounting. This method for producing PWBs utilizes a plating technique and a transferring technique for the wiring pattern. Two metal sheets are prepared, each of which has a surface with a wiring pattern formed by electroplating of copper. A semi-hardened resin sheet such as a prepreg is placed between the wiring patterns of the metal sheets. Heat and pressure are applied to the outer surfaces of the metal sheets. Thus, the copper wiring patterns are transferred from the surfaces of the metal sheets to the surfaces of the resin sheet. After removing the metal sheets, through holes are formed by drilling in the resin sheet, and copper plating is performed inside the through holes to connect the wiring pattern on one side with that of the other side electrically (Naoki Hukutomi et al. “Development of Fine Pattern Wiring Technique”, The Institute of Electronics, Information and Communication Engineers, C-II, Vol. J72-C-II, No. 4, PP243-253, 1989). This method provides a line width and a line space of 20 microns each.
There is another technique called “ALIVH” (a trademark of Matsushita Electric Industrial Co., Ltd.), that is a resin-based multilayer PWB having an inner via hole (IVH) structure for all layers. In this multilayer PWB, a conductive material is filled in the inner via holes instead of copper plating inside the through holes that is a major method for electric connection between layers of a multilayer PWB in the prior art. This “ALIVH” PWB thus improves the reliability of the electric connection between layers, and facilitates forming inner via holes under lands for mounting components or between any layers (U.S. Pat. Nos. 5,346,750 and 5,481,795).
An example of the method for producing the “ALIVH” PWB is explained below, referring to
FIGS. 5A-5F
that show cross sections in the producing process. As shown in
FIG. 5A
, via holes
502
are perforated by using a laser beam machine at predetermined positions in an adhesive insulator sheet
501
that comprises an aramid-epoxy prepreg made of a non-woven aramid sheet impregnated with an epoxy resin. Then, as shown in
FIG. 5B
, the via holes
502
are filled with a fluid conductive paste
503
. Then, as shown in
FIG. 5C
, the adhesive insulator sheet
501
with via holes filled with the conductive paste is placed between copper foils
504
, and heat and pressure are applied to the outer surfaces of the copper foils. Thus, the adhesive insulator sheet (the prepreg)
501
and the conductive paste
503
are hardened, the copper foils
504
adhere to the surfaces of the adhesive insulator sheet
501
, and electrical connections are formed between the copper foils by the conductive paste
503
packed into the via holes
502
. The copper foils
504
are etched by a conventional photolithography method to form wiring patterns
505
a
,
505
b
. Thus, a double-side PWB
506
is obtained as shown in FIG.
5
D.
In the next step shown in
FIG. 5E
, the double-side PWB
506
is used as a core, and on both sides of the core PWB
506
other adhesive insulator sheets
501
a
and
501
b
are placed with proper registration. These adhesive insulator sheets
501
a
,
501
b
have been made previously according to the step shown in
FIG. 5B
, and each of them has via holes filled with the conductive paste at predetermined positions. On the outer surfaces of the adhesive insulator sheets
501
a
and
501
b
, copper foils
507
a
and
507
b
are placed. Heat and pressure are applied to both outer surfaces of the copper foils
507
a
,
507
b
for lamination. Then, similarly to the step of
FIG. 5D
, the outer copper foils
507
a
,
507
b
are etched by the photolithography method. Thus a four-layer PWB is obtained having the outer wiring patterns
508
a
,
508
b
as shown in FIG.
5
F. This method for producing PWBs enables via-connections (electric connections between layers) with very small via holes since via holes are formed by a laser beam and filled with the fluid conductive paste for the electric connection.
However, in the above-mentioned transferring technique of the wiring pattern, there is a limit for reducing the size of through holes since they are perforated by machining. On the other hand, in the above “ALIVH” PWB, there is a limit on the fineness of patterns with respect to pattern density such as a line pitch and a line width since the outer and inner copper patterns are formed by the conventional photolithography method. These limits are obstacles to producing high density mounting of surface mount components, especially small electronic components such as recent chip components or LSI bare chips.
SUMMARY OF THE INVENTION
The present invention provides a method for producing fine pattern PWBs that enables high density mounting of components by combining the advantage of the conventional transferring method of wiring patterns and the advantage of the conventional “ALIVH” structured multilayer PWB.
The method for producing PWBs according to the present invention comprises the steps of perforating through holes at predetermined positions in an adhesive insulator sheet, filling the through holes with a conductive material, forming conductive wiring patterns on surfaces of releasable supporting sheets, and transferring the conductive wiring patterns from the surface of the releasable supporting sheets onto surfaces of the adhesive insulator sheet so as to form the wiring patterns on the surfaces of the adhesive insulator and perform electric connections between the wiring patterns of plural layers.
According to the present invention, PWBs can be produced with fine patterns defining very fine wiring pitches and electric connections by very small via holes. In addition, such fine PWBs can be produced at a low cost since the method according to the present invention is simple compared with other conventional methods.
Another method according to the present invention uses the PWB produced by the method mentioned above as a core. Second adhesive insulator sheets are prepared that have through holes filled with a conductive material. These second adhesive insulator sheets are placed on surfaces of the core PWB. On the outer surfaces of the second adhesive insulator sheets, second releasable supporting sheets are placed, whose surfaces facing the second adhesive insulator sheets are provided with second conductive wiring patterns. The second conductive wiring patterns are transferred from the surfaces of the second releasable supporting sheets onto the surfaces of the second adhesive insulator sheets so as to form surface wiring patterns and perform electric connection between the surface wiring patterns and the inner wiring patterns. Thus, multilayer PWBs with fine patterns can be produced inexpensively. By repeating the steps mentioned above, PWBs having more layers can be produced easily.
Another method according to the present invention uses a double-side or multilayer PWB produced by a conventional method as a core. Adhesive insulator sheets that have via holes filled with a conductive material are placed on surfaces of the core PWB. Conductive wiring patterns on surfaces o
Hasegawa Masanaru
Hatanaka Hideo
Tsukamoto Masahide
Kim Paul
Vo Peter
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