Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-11-30
2004-06-15
Zarneke, David A. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S261000, C174S267000
Reexamination Certificate
active
06750404
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a high density printed wiring board having blind vias configured as surface mounting pads for surface mount electrical/electronic components, and to a method of manufacturing said wiring board.
BACKGROUND OF THE INVENTION
It is known to employ blind vias in multi-layer wiring boards as a means of electrically connecting circuits formed in copper foil layers on opposite faces of laminae comprising the board. This can be better understood by reference to FIGS.
5
(
a
) to
5
(
g
) which illustrate a method of forming a conventional printed wiring board having blind via electrical connects.
FIG.
5
(
a
) shows a two-sided printed wiring board lamina
10
before lamination with similar laminae to form a printed wiring board. The lamina
10
has a dielectric resin substrate
12
with copper foil layers (
14
,
16
) formed on opposite surfaces thereof. In a first process step for forming a conventional printed wiring board, a through hole
18
is drilled in the lamina
10
, as illustrated in FIG.
5
(
b
). As a second step (FIG.
5
(
c
)), the through hole
18
is plated to form an electrical connect layer
20
extending between the surface copper foil layers (
14
,
16
) of the lamina
10
. As illustrated in FIG.
5
(
c
), the through hole plated layer
20
extends a short radial distance l over the surface copper foil layers (
14
,
16
) of the lamina
10
in the proximity of the through hole
18
and, as such, sits higher than the surface copper layers (
14
,
16
). Also as illustrated in FIG.
5
(
c
), the lower (as shown in the drawing) surface foil layer
16
, which will become a buried inner layer of the printed wiring board when formed, is imaged and etched to form a electrical circuit
22
thereon.
FIG.
5
(
d
) illustrates how a conventional printed wiring board
24
can be formed from laminae
10
of identical or similar form to that illustrated in FIG.
5
(
c
). The laminae
10
are placed together with a prepeg separation layer
26
inserted therebetween at the time of lamination in a conventional manner. The lamination process results in the innermost openings of the through holes
18
in the laminae
10
being covered thus constituting blind vias
28
which electrically connect circuits formed on the opposing surfaces of their respective lamina. The lamination process may also result in some prepeg material being forced into the holes
18
of the blind vias
28
. After the lamination process, a through hole
30
to electrically connect the outer copper foil layers
14
and the buried inner circuit layers
22
is drilled in the printed wiring board and plated also in a conventional manner. The outer surface copper layers
14
of the printed wiring board
24
can then be imaged and etched to form outer electrical circuits
32
. In particular, re-flow pads
34
may be formed for mounting electrical components on the surfaces
14
of the printed wiring board
24
. As more readily seen in
FIG. 6
, the re-flow pads
34
are electrically connected to the through hole plated layers
20
by narrow conductor lines
36
. A re-flow pad is used only for mounting a surface mount component and a narrow conductor line is used only for connecting this to a through hole plated layer of a blind via. Flat areas are necessary for mounting surface mount components. Consequently, it can be seen from
FIG. 6
that the surface area occupied by a dog bone contact pad means consisting of a re-flow pad, a narrow conducting line and a blind via plated layer is considerably greater than the area occupied by only the blind via plating layer. A consequence of this is illustrated by
FIG. 7
which is a side sectional view of an upper portion of a printed wiring board
24
on which electrical component
38
has been mounted. The contact elements
40
of the component
38
are attached to reflow pads
34
of dog bone contact pad means which are spaced inwardly from corresponding blind vias
28
. It will be appreciated that the surface envelope E surrounding the component and the contact pad means mounting it on the board has a surface area substantially greater than that of the component itself.
The amount of surface area occupied by the dog bone contact pad means is particularly acute with respect to mounting electrical components have a ball grid array (BGA) surface contact arrangement. As illustrated in
FIG. 8
, the surface area (bounded by broken line El) occupied by the contact pad means
42
is such that it dictates a minimum spacing between BGA contact pads (not shown). This would still be the case if only a small number of the BGA electrical connections are to be made through blind vias to other circuit layers of the printed wiring board.
Accordingly, the maximum mounting density for surface mount components and, in particular, surface mount components having a BGA mounting arrangement is relatively low.
Another conventional printed wiring board having blind holes formed by laser is described below with reference to FIGS.
9
(
a
) and
9
(
b
).
More specifically, a printed wiring board
100
includes a dielectric resin substrate,
102
, copper conductor pads (inner layer pattern)
104
and
109
on opposing surfaces of the substrate
102
, dielectric resin layers
101
and
103
laminated on opposing surfaces of the substrate
102
, outer layer copper reflow pads
108
and
113
, and outer layer copper conductor lines
105
and
110
. Blind holes
106
and
111
which have copper plating layers
107
and
112
are formed in the dielectric resin layers
101
and
103
. Plated through holes
114
and
115
are formed through the layers
101
and
103
and the substrate
102
and have through hole copper plating layers
116
and
117
for connecting outer layers with the inner pads
104
and
109
. The reflow pads
108
and
113
which mount electric components of the surface are connected to the copper layers
107
and
112
of the blind holes
106
and
111
by narrow conductor lines
105
and
110
. The inner layer conductor pads
104
and
109
are connected to the narrow conductor lines
105
and
110
, and thus the outer layers
108
and
113
, by the copper plating layers
107
and
112
of the blind holes
106
and
111
.
FIGS.
10
(
a
) to
10
(
g
) illustrate a conventional printed wiring board manufacturing process which uses a laser beam.
In the first step of the process, the inner layer conductor pads
104
and
109
are etched and formed on both surfaces of the dielectric resin substrate
102
as shown in FIG.
10
(
a
). In the second step of the process, the dielectric resin layers
101
and
103
, and copper foil layers (outer layers)
105
a
and
110
a
are laminated and pressed onto the substrate as etched and formed in the first step shown in FIG.
10
(
a
), in a heated environment. In the third step of the process, special windows
118
and
119
, through which a laser beam can be radiated, are etched as shown in FIG.
10
(
c
). In the fourth step of the process, laser beams
120
are radiated through the windows
118
and
119
to form blind holes
106
and
111
which reach the inner layer conductor pads
104
and
109
, as shown in FIG.
10
(
d
). In the fifth step of the process, through holes
114
and
115
are drilled as shown in FIG.
10
(
e
). In the sixth step of the process, blind holes
106
and
111
, and through holes
114
and
115
are plated with plating layers
107
,
112
,
116
and
117
as shown in FIG.
10
(
f
) for connecting the copper foil layers
105
a
and
110
a
with the inner layer pads
104
and
109
. In the last step of the process, the copper foil layers
105
a
and
110
a
are etched to form narrow outer layer conductor lines
105
and
110
and reflow pads
108
and
113
, as shown in FIG.
10
(
g
).
However, as for the first conventional printed wiring board as described above with respect to
FIGS. 5
to
8
, on the second conventional printed wiring board
100
, the reflow pads
108
and
113
are used only for mounting surface mount components, and the narrow conductor lines
105
and
Anslow Philip Andrew
Fuller Geoff A
Tourne Ing. Joan
Norris Jeremy
Nortel Networks Limited
Zarneke David A.
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