Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2001-06-26
2003-07-29
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
Reexamination Certificate
active
06599617
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a printed wiring board formed by adhering a metallic foil such as a copper foil to both sides of a laminate comprising a paper substrate, for example, a paper-phenol material as a base, and to a method of manufacturing the same.
Hitherto, printed wiring boards utilized for wiring in various electronic apparatuses include those ones in which copper foil lands for a plated-through hole provided on both sides are electrically connected in order to enhance the mounting density.
As one of techniques for connecting by a plated-through hole, there is a technique in which a crude hole for plated-through hole is filled with a conductive paste, whereby copper foil lands provided on both sides of the printed wiring board are electrically connected. Among others, those printed wiring boards in which a plated-through hole is formed by using a copper paste obtained by kneading a fine copper powder and a thermosetting resin as the conductive paste are called, for example, copper paste plated-through hole printed wiring boards.
In the case of producing the copper paste plated-through hole printed wiring board as mentioned above, for example, copper foil lands for plated-through hole are provided on both sides of a base substrate to be a base of the printed wiring board, and thereafter a crude hole for the plated-through hole is formed in the copper foil lands. Then, the crude hole is filled with the copper paste, followed by drying and curing to obtain a copper paste plated-through hole.
However, the copper paste plated-through hole printed wiring board as mentioned above has had the problem that the surfaces of the copper foil lands provided on both sides are flat and, accordingly, the adhesion strength between the copper foil lands and the copper paste is weak.
In consideration of the above situation, the present applicant et al. have proposed a printed wiring board in which the adhesion strength between a copper paste and copper foil lands is enhanced such as by forming a hollow portion in the surface of the copper foil lands for plated-through hole (Japanese Patent Laid-open No. Hei 10-206277).
In ordinary printed wiring boards, for example, a paper-phenol substrate, a paper-epoxy substrate, a paper composite substrate and the like can be utilized as a base substrate. However, in copper paste plated-through hole printed wiring boards, the paper-phenol substrate cannot be utilized because of the following production problems.
For example, in production process of a copper paste plated-through hole printed wiring board, as shown in
FIG. 15
, copper foil lands
101
for plated-through hole are provided on both sides of a base substrate
100
, and a crude hole
102
for the plated-through hole is bored in the copper foil lands
101
. Then, the crude hole
102
is filled with a copper paste
103
, followed by a drying step at about 60 to 100° C. and a thermal curing step at 150 to 160° C., thereby forming a copper paste plated-through hole.
In such a production process, however, when a paper-phenol substrate is used as the base substrate
100
, volatile components contained in the base material resin of the paper-phenol substrate such as methanol, 1-butanol, 2-methyl-1-propanol, formaldehyde, toluene, salicylaldehyde and the like and water contained in the paper material are generated as out gases in the drying step and the thermal curing step.
The out gases generate a burst (blister)
104
shown in
FIG. 15
or bubbles
105
shown in
FIG. 16
in the copper paste
103
filling the crude hole
102
of the base substrate
100
, causing substrate defects.
In consideration of the above situations, the present applicant et al. have studied intensively for preventing the burst
104
and the bubbles
105
which would be generated in the copper paste
103
, and have proposed a printed wiring board in which, for example, a through-hole for discharging the gases is provided in the vicinity of a via hole formed by filling with the copper paste
103
(Japanese Patent Laid-open No. Hei 11-177497).
In recent years, attendant on reductions in the cost of various electronic apparatuses, there has been an increasing demand for reduction in the cost of copper paste plated-through hole printed wiring boards.
Therefore, also in the copper paste plated-through hole printed wiring boards, it is desired to form the base substrate from a paper-phenol based substrate material which is available most inexpensively and easily.
In consideration of the above situations, the present applicant et al. have again studied whether or not the copper paste plated-through hole printed wiring board can be formed by use of the paper-phenol substrate
100
.
As a result, the present applicant et al. have found that, in the case of producing a copper paste plated-through hole printed wiring board by the previously proposed invention (Japanese Patent Laid-open No. Hei 10-206277), a sufficient adhesion strength has not yet been obtained between the copper foil lands
101
and the copper paste
103
for the following reasons.
FIG. 17
shows an example of structure obtained where a copper paste plated-through hole printed wiring board is formed by use of a paper-phenol substrate as a base substrate.
The copper paste plated-through hole printed wiring board shown in
FIG. 17
comprises a base substrate
100
formed of a paper-phenol substrate, and copper foil lands
101
provided on both sides of the base substrate
100
. Then, a crude hole
102
for plated-through hole is bored in the copper foil lands
101
, and the crude hole
102
is filled with a copper paste
103
, followed by drying and curing to produce a copper paste plated-through hole.
In the case of producing the copper plated-through hole printed wiring board in the manner as mentioned above, a destructive test of solder heat resistance and the like after moisture absorption conducted as evaluation of reliability of the wiring board results in that exfoliation
106
is generated between the copper foil lands
101
formed in the periphery of the crude hole
102
and the copper paste
103
.
Particularly, when the case where a both side copper-clad laminate with a paper substrate such as a paper-phenol substrate as a base is used as the base substrate
100
was compared with the case where a both side copper-clad laminate with a glass substrate such as a glass-epoxy substrate is used as the base substrate
100
, it was found that the exfoliation
106
at the interface between the copper paste
103
and the copper foil lands
101
is generated more easily in the case of the both side copper-clad laminate using the paper substrate as a base.
The exfoliation
106
generated at the interface between the copper paste
103
and the copper foil lands
101
is said to be due to the difference in thermal expansion and contraction characteristics between the both side copper-clad laminate with the paper substrate as a base and the both side copper-clad laminate with the glass substrate as a base.
Here, the thermal expansion and contraction characteristics of the paper-phenol substrate are shown in
FIG. 13
, whereas the thermal expansion and contraction characteristics of the glass-epoxy substrate are shown in FIG.
14
.
The thermal expansion and contraction characteristics shown in
FIGS. 13 and 14
are measured by the so-called TMA method in which a specimen is heated up from room temperature at a rate of 10° C./min and coefficient of thermal expansion in the thickness direction is measured on a thermal analysis apparatus.
When the thermal expansion and contraction characteristics shown in
FIGS. 13 and 14
are compared with each other, it is seen that the paper-phenol substrate shown in
FIG. 13
shows a greater size change coefficient in the thickness direction relative to the heating temperature change, as compared with the glass-epoxy substrate shown in FIG.
14
.
This means that the glass-epoxy substrate has a lower coefficient of thermal expansion in the thickness direction and a smaller moisture absorption of base mate
Blackwell-Rudasill G. A.
Jones Deborah
Maioli Jay H.
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