U.S. patent application number 10/870506 was filed with the patent office on 2004-11-25 for pcb design and method for providing vented blind vias.
Invention is credited to Boggs, David W., Dungan, John H., Paek, Gary I., Sato, Daryl A..
Application Number | 20040231886 10/870506 |
Document ID | / |
Family ID | 32927449 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040231886 |
Kind Code |
A1 |
Boggs, David W. ; et
al. |
November 25, 2004 |
PCB design and method for providing vented blind vias
Abstract
An apparatus and method for providing a vented blind via in pad
of a printed circuit board (PCB). A vent in the blind via in pad to
allow gases formed during reflow soldering to escape from the
solder joint. In one embodiment, the vent extends from the outer
edge of the pad to the blind via. In another embodiment, a method
includes forming a blind via in pad having a vent.
Inventors: |
Boggs, David W.; (Hillsboro,
OR) ; Dungan, John H.; (Hillsboro, OR) ; Paek,
Gary I.; (Banks, OR) ; Sato, Daryl A.;
(Portalnd, OR) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD
SEVENTH FLOOR
LOS ANGELES
CA
90025-1030
US
|
Family ID: |
32927449 |
Appl. No.: |
10/870506 |
Filed: |
June 17, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10870506 |
Jun 17, 2004 |
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10442834 |
May 20, 2003 |
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6787443 |
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Current U.S.
Class: |
174/262 |
Current CPC
Class: |
H05K 3/027 20130101;
Y10T 29/49165 20150115; H05K 3/3452 20130101; H05K 2203/1178
20130101; Y02P 70/50 20151101; H05K 1/116 20130101; H05K 3/3436
20130101; H05K 2201/09381 20130101; H05K 2201/09472 20130101; H05K
2201/0989 20130101; Y02P 70/613 20151101; H05K 2201/09509 20130101;
H05K 2201/09463 20130101; H05K 1/112 20130101; H05K 3/421
20130101 |
Class at
Publication: |
174/262 |
International
Class: |
H05K 001/11 |
Claims
What is claimed is:
1. An apparatus, comprising: a substrate; and a via in pad
positioned in the substrate, the via in pad including a pad having
a vent to provide an escape path for gas from the via in pad, and a
blind via disposed in the pad.
2. The apparatus of claim 1, wherein the vent to include a first
opening at the outer edge of the pad.
3. The apparatus of claim 2, wherein the vent to include a second
opening at the blind via.
4. The apparatus of claim 2, wherein the vent is a wedge.
5. The apparatus of claim 3, wherein the vent is a groove.
6. The apparatus of claim 1, wherein the pad has a plurality of
vents.
7. The apparatus of claim 2, wherein the vent to include a second
opening at the outer edge of the pad, the vent to not intersect the
blind via.
8. The apparatus of claim 1, wherein the vent has a depth that
exposes the substrate.
9. The apparatus of claim 1, wherein the blind via to electrically
couple the pad to a trace on an inner substrate layer, wherein the
substrate is an outer substrate layer of a multilayer printed
circuit board having one or more inner substrate layers.
10. An apparatus, comprising: a first substrate to form an outer
layer of a printed circuit board (PCB); a second substrate to form
an inner layer of the PCB, the second substrate having a trace; and
a via in pad positioned in the first substrate electrically coupled
to the trace, the via in pad including a pad having a vent to
provide an escape path for gas from the via in pad, and a via,
disposed in the pad, having a first end at the pad and a second end
at the trace.
11. The apparatus of claim 10, wherein the vent to include a first
opening at an outer edge of the pad.
12. The apparatus of claim 11, wherein the vent to include a second
opening at the via.
13. The apparatus of claim 12, wherein the vent is a groove.
14. An apparatus, comprising: a substrate; and one or more via in
pads in the substrate, the one or more via in pads each to include
a pad and a blind via positioned in the pad, wherein the pad of at
least one of the one or more via in pads having a vent to provide
an escape path for gas from a gas pocket in the at least one of the
one or more via in pads.
15. The apparatus of claim 14, wherein the vent to include a first
opening at the outer edge of the pad.
16. The apparatus of claim 14, wherein the vent to include a second
opening at the via.
17. The apparatus of claim 16, wherein the vent is a groove.
18. An apparatus, comprising: a substrate; a via in pad positioned
in the substrate, the via in pad to include a pad and a blind via
disposed in the pad; and means for allowing a gas to escape from a
solder joint coupled to the pad.
19. The apparatus of claim 18, wherein the means for allowing a gas
to escape includes the pad having a vent, the vent to include a
first opening at the outer edge of the pad.
20. The apparatus of claim 19, wherein the vent to include a second
opening at the via.
21. The apparatus of claim 20, wherein the vent is a groove.
22. A system, comprising: a printed circuit board (PCB),
comprising: a substrate; and a via in pad positioned in the
substrate, the via in pad including: a pad having a vent to provide
an escape path for gas from the via in pad, the vent to include a
first opening at the outer edge of the pad; and a blind via
disposed in the pad; and a memory device coupled to the PCB at the
via in pad.
23. The system of claim 22, wherein the vent to include a second
opening at the blind via.
24. The system of claim 23, wherein the vent is a groove.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of, and claims priority
from, U.S. patent application Ser. No. 10/442,834, filed May 20,
2003, and currently pending.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to printed circuit
boards (PCBs) and, more specifically, the present invention relates
to a method an apparatus for a vented blind via in pad structure of
a printed circuit board.
[0004] 2. Background Information
[0005] A PCB typically includes a number of insulation and metal
layers selectively patterned to provide metal interconnect lines
(referred to herein as "traces"), and a plurality of electronic
components mounted on one or more surfaces of the PCB and
functionally interconnected through the traces. The routing traces
typically carry signals that are transmitted between the electronic
components mounted on the PCB. Some PCBs have multiple layers of
routing traces to accommodate all of the interconnections.
[0006] Traces located within different layers are typically
connected electrically by vias formed in the board. A via can be
made by making a hole through some or all layers of a PCB and then
coating or plating the interior hole surface with an electrically
conductive material. A via that connects all layers of the PCB,
including the outer layers, is called a "through via." A via that
connects one or more inner layers to an outer layer is a "blind
via."
[0007] In order to fabricate PCBs in which electrical components
are mounted in higher densities, a via in pad structure is often
used. FIG. 1A is a perspective view diagram illustrating a prior
art blind via in pad. A blind via 12 is configured in pad 10. The
pad is formed on a multilayer PCB 16. Blind via 12 has an opening
at pad 10 and extends into PCB 16. The walls of the blind via 12
are an electrically conductive material, such as copper. A solder
mask (not shown) surrounds the pad 10.
[0008] One of the conventional ways of mounting electrical
components on a PCB is called surface mount technology (SMT). SMT
components have terminations or leads (generally referred to as
"contacts") that are soldered directly to the surface of the PCB.
The solder joint forms the physical and electrical connection
between the component and the PCB. One conventional type of SMT
component utilizes a ball grid array (BGA) to connect to the PCB. A
BGA component has a plurality of solder balls on one surface, each
of which represents an electrical contact.
[0009] The electrical contacts of an SMT component, such as a BGA
component, are coupled to corresponding metallized mounting or
bonding pads (also referred to as "lands") on the surface of the
PCB. Ordinarily one pad is dedicated to one SMT electrical
contact.
[0010] Prior to mounting the SMT component on a PCB, the PCB is
selectively coated with solder paste, using a mask (also referred
to in the art as a stencil) that permits solder paste to coat just
the pads. To mount an SMT component to a PCB, the component is
carefully positioned or "registered" over the PCB so that its
electrical contacts are aligned with the corresponding pads.
Finally, the entire package is heated to a temperature that melts
the solder paste (e.g., reflow soldering), to form a solder joint
that is an electrical and physical connection.
[0011] During the heating process, one or more gas pockets can form
within the soldier joint. Such gas pockets can be formed by
expanding air bubbles trapped within the solder paste and/or blind
via (also referred to as "outgassing"). FIG. 1B is a
cross-sectional view diagram illustrating a prior art blind via in
pad structure coupled to an electrical component. Blind via 12 is
configured in pad 10. The blind via 12 extends into multilayer PCB
16 and is electrically coupled to an inner conductive layer 14.
During the heating process to affix contact 20 of electrical
component 22 to pad 10 by way of solder joint 24, a gas pocket 26
was formed by trapped air within the blind via 12. A solder mask
(not shown) surrounds the pad 10.
[0012] Such a gas pocket can cause the solder joint to expand to
the point where it touches an adjacent solder joint and, thus,
create a short circuit. In the case of BGA components, this
phenomenon is referred to as "BGA bridging." The gas pocket may
also cause the solder joint to crack and break resulting in an open
circuit between the electrical component and the PCB. Additionally,
a solder joint with a gas pocket may experience long term
reliability issues due to repeated heating and cooling during its
operative life.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present invention is illustrated by way of example and
not limitation in the accompanying figures.
[0014] FIGS. 1A and 1B are diagrams illustrating a prior art blind
via in pad structure.
[0015] FIG. 2A is a perspective view diagram illustrating one
embodiment of a vented blind via in pad in accordance with the
teachings of the present invention.
[0016] FIG. 2B is a cross-sectional view diagram illustrating one
embodiment of a vented blind via in pad in accordance with the
teachings of the present invention.
[0017] FIG. 3A is a perspective view diagram illustrating one
embodiment of a vented blind via in pad in accordance with the
teachings of the present invention.
[0018] FIG. 3B is a cross-sectional view diagram illustrating one
embodiment of a vented blind via in pad in accordance with the
teachings of the present invention.
[0019] FIGS. 4A-4H are cross-sectional diagrams illustrating one
embodiment of constructing a vented blind via in pad structure in
accordance with the teachings of the present invention.
[0020] FIGS. 5A-5I are top view diagrams illustrating embodiments
of vented blind via in pad structures in accordance with the
teachings of the present invention.
[0021] FIGS. 6A and 6B are perspective view diagrams illustrating
embodiments of vented blind via in pad structures in accordance
with the teachings of the present invention.
[0022] FIG. 7 is a diagram illustrating a computer system for
implementing an embodiment of the present invention.
DETAILED DESCRIPTION
[0023] Methods and apparatuses for a providing a vented blind via
in pad structure for a PCB are disclosed. In the following
description numerous specific details are set forth in order to
provide a thorough understanding of the present invention. It will
be apparent, however, to one having ordinary skill in the art that
the specific detail need not be employed to practice the present
invention. In other instances, well-known materials or methods have
not been described in detail in order to avoid obscuring the
present invention.
[0024] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment. Furthermore, the
particular features, structures or characteristics may be combined
in any suitable manner in one or more embodiments. In addition, it
is appreciated that the figures provided herewith are for
explanation purposes to persons ordinarily skilled in the art and
that the drawings are not necessarily drawn to scale.
[0025] FIGS. 2A and 2B illustrate one embodiment of a blind via in
pad having a vent. Referring to FIGS. 2A and 2B, a pad 202 is
positioned on a PCB 201. The PCB 201 is a multilayer PCB having a
plurality of substrate layers and one or more inner conductive
layers.
[0026] A blind via 206 is positioned in pad 202. The blind via 206
includes walls of conductive material. The blind via 206 is
electrically connected to an inner conductive layer 210, such as,
for example, a routing trace. The combination of blind via 206 in
pad 202 is typically called a via in pad (or "microvia"), and is
well known in the art.
[0027] Surrounding the pad 202 is a solder mask 214. A contact 222
of electrical component 224 is positioned over pad 202. In one
embodiment, contact 222 includes a solder ball of a BGA. The
electrical component 224 may be, but is not limited to, a
microprocessor, a microcontroller, an application specific
integrated circuit (ASIC), an amplifier, a filter, a clock circuit,
or the like. The conductive material of pad 202, blind via 206,
contact 222, and inner conductive layer 210 may be, but is not
limited to, copper, aluminum, gold, or the like. While FIG. 2B
shows an electrical component attached to one outer layer of PCB
201, in another embodiment, electrical components can be attached
to additional outer layers of PCB 201.
[0028] It will be appreciated that blind via 206 extends partially
into PCB 201. Generally, a blind via is a via that electrically
couples one or more inner layers to an outer layer of a multilayer
PCB. Note that blind via 206 is not a through-hole via utilizing a
via cap or via plug. A via that connects all layers of a PCB,
including the outer layers, is called a "through via" or a "plated
through hole via" (PTH).
[0029] A vent 204 is positioned in pad 202. In one embodiment, the
vent 204 can have approximately parallel walls 207 to form a groove
shape. In one embodiment, the vent 204 is sized to permit nearly
all of a gas 208 to escape to the atmosphere. In another
embodiment, the approximately parallel vent walls 207 are sized to
a width (w) to meter escape of the gas 208 such that some gas 208
remains in the blind via 206. In one embodiment, the depth (d) of
vent 204 reaches the outer substrate layer of PCB 201; while in
another embodiment, the depth of vent 204 does not reach the outer
substrate layer of PCB 201 (discussed further in conjunction with
FIGS. 6A and 6B.) In the embodiment shown in FIGS. 2A and 2B, the
vent 204 runs from the outer edge of pad 202 to blind via 206.
[0030] A gap placed at the outer edge of the pad 202 allows
clearance for gas 208 to escape the vent 204. In one embodiment
shown in FIGS. 2A and 2B, surrounding the pad 202 is the solder
mask 214 where a gap (g) is between the solder mask 214 and the
outer edge of pad 202. Solder will not adhere to the outer layer of
substrate of PCB 201 or to solder mask 214. As a result, the
venting path (vent 204 plus gap g between pad 202 and solder mask
214) will not become filled or clogged with solder during a solder
reflow process. In an alternative embodiment, a gap is placed at
the outlet of the vent 204 between the solder mask 214 and the
outer edge of pad 202. Such a gap is does not surround the entire
pad 202. For example, the gap may be approximately the same width
as the width (w) of the vent 204.
[0031] In a typical solder reflow process, solder paste is applied
to pad 202. The entire assembly is heated to a temperature that
melts the solder paste (and solder balls of a BGA) so that the pad
202 and contact 222 form a physical and electrical connection via a
solder joint 226.
[0032] During solder reflow, gas 208 can form in the solder joint
226 and/or blind via 206. The expanding gas 208 will seek a path of
less resistance and travel into the vent 204. By incorporating a
vent 204 within the pad 202, a pathway is established that allows
the expanding gas 208 to escape the solder joint 226 while the
solder (and solder ball) is in the molten state. All or a portion
of the expanding gas 208 that otherwise would contribute to
expanding the solder joint 226 may now escape to atmosphere through
vent 204.
[0033] FIGS. 3A and 3B illustrate another embodiment of a vented
blind via in pad. Referring to FIGS. 3A and 3B, a pad 302 is
positioned on a multilayer PCB 301. A blind via 306 is configured
in pad 302. The blind via 306 includes walls of conductive material
and is electrically connected to an inner conductive layer 316.
Surrounding the pad 302 is a solder mask 314. A contact 322 of an
electrical component 324 is positioned over pad 302.
[0034] Within the pad 302, a vent 304 does not run continuously
from the outer edge of the pad 302 to the blind via 306. In this
embodiment, the vent 304 can be roughly shaped like a wedge having
an apex 307. With this wedge shaped vent 304, a solder joint 326
can begin to expand due to gas 308 building up within the solder
joint 326 with no venting. As the gas 308 builds up in the solder
joint 326 and the solder joint 326 continues to expand, a void 310
crosses over and contacts the wedge shaped vent 304 at the vent
apex 307. Once contact by the void 310 with the vent 304 occurs,
gas 308 can begin to release or vent to atmosphere (or a lower
pressure space). With sufficient pressure released, the void 310
can shrink until the void 310 is no longer positioned over the vent
304 or the void 310 is positioned over a smaller cross-section of
the vent 304. With venting stopped or slowed, pressure could again
build up, again increasing the size of the solder joint 326 to a
point where the vent 304 would once more begin to release pressure.
This "burping" action could repeat as long as outgassing with the
solder joint 326 was being generated with sufficient pressure.
[0035] In another embodiment, the release of gas occurs without the
burping action. The solder joint 326 could expand to a point on the
vent 304 where gas 308 builds up in solder joint 326 verses the
rate of gas 308 escaping from vent 304 is such that that the rate
of expansion of the solder joint 326 would slow or cease. Once the
heating process was complete and the assembly began to cool, the
outgassing from solder joint 326 would ultimately cease.
[0036] The embodiment in FIGS. 3A and 3B show a gap (g) surrounding
the pad 302 between the solder mask 314 and the outer edge of pad
302 to create a clearance for gas to escape the vent 304. In
another embodiment, the gap (g) does not surround the entire pad
302. For example, the gap may be approximately the same width as
the outlet of vent 304 at the outer edge of pad 302.
[0037] FIGS. 4A-4H illustrate one embodiment of a method to
fabricate a vent within a blind via in pad on a multilayer PCB.
FIG. 4A is an illustration of an outer conductive layer 402 that
has been laminated to substrate 404. Substrate 404 is laminated to
substrate 406. Inner conductive layer 408 includes a circuitry
trace pattern. The material of conductive layers 402 and 408 may
include, but is not limited to, copper, aluminum, gold, or the
like. It will be appreciated that the PCB of FIGS. 4A-4H is not
limited to the embodiments as shown and may contain additional
layers. A blind via may electrically couple one or more of these
additional layers.
[0038] FIG. 4B is an illustration of the results of creating a
window 410 in the outer conductive layer 402. In one embodiment,
the window 410 may be created through various photolithography
processes that are well known in the art. In another embodiment,
the window 410 can be created through a laser skive process of
outer conductive layer 402 that is well known in the art. During
laser skiving of outer conductive layer 402, a vent 416 (shown in
FIGS. 4E-4H) can also be created at this time through laser skiving
(described further below).
[0039] FIG. 4C is an illustration of the results of laser drilling
a blind via 412. The laser ablates the substrate 404 to expose the
inner conductive layer 408. FIG. 4D is an illustration of the
results of a plating process. The walls of blind via 412 are plated
with conductive material to allow layer-to-layer electrical
conductivity between the layers of the PCB.
[0040] FIG. 4E is an illustration of the results of trace formation
in the outer conductive layer 402. In one embodiment, a pad 414, a
vent 416 and a trace 418 is formed through photolithography. If the
vent 416 is formed here through photolithography, then the depth of
the vent is to the substrate 404.
[0041] If the vent 416 was formed by laser skiving as described in
conjunction with FIG. 4B, then the vent is not formed here by
photolithography. The vent formed by laser skiving is positioned in
the outer conductive layer 402 such that the photolithography
process creates the pad 414 with the knowledge of the placement of
the vent 416 by laser skiving. In one embodiment, a vent formed by
laser skiving will not pass entirely through the depth of the pad
414.
[0042] FIG. 4F is an illustration of the results of adding a solder
mask 420 to the remaining outer conductive layer 402 and substrate
404. Typically, the solder mask 420 covers all exposed features
except for the pad 414 (including vent 416 of pad 414). FIG. 4G is
an illustration of the results of applying solder 422 to the
exposed pad 414.
[0043] FIG. 4H is an illustration of the results of reflow
soldering to couple a contact 424 of an electrical component 426 to
pad 414. Electrical component 426 was positioned so that contact
424 was aligned with pad 414. The contact 424 was affixed to pad
414 through reflow soldering, which is well known in the art.
During the reflow soldering, a gas 430 that developed in the solder
joint 428 or blind via 412 could escape to atmosphere through vent
416.
[0044] FIGS. 5A-5I are top view illustrations of alternative
embodiments of vent design. FIG. 5A is a top view diagram
illustrating one embodiment of the present invention. A pad 510 is
formed on a substrate 516. A blind via 512 is configured in pad
510. Vent 518 extends from the outer edge of pad 510 towards the
blind via 512, but does not reaching the opening of blind via 512.
The vent 518 has generally a groove shape.
[0045] FIGS. 5B and 5C are top view diagrams illustrating
embodiments of the present invention. In FIGS. 5B and 5C, a pad 510
is formed on substrate 516. A blind via 512 is configured in pad
510. Vents 518 and 519 are formed in pad 510. Vents 518 and 519
extend from the outer edge of pad 510 to blind via 512. It is
understood that the embodiment shown in FIG. 5B is not limited to
the angle between vents 518 and 519. FIG. 5C shows vents 518 and
519 such that vents 518 and 519 are collinear along a diameter
passing through pad 510.
[0046] FIGS. 5D and 5E are top view diagrams illustrating
embodiments of the present invention. In FIGS. 5D and 5E, a pad 510
is formed on substrate 516. A blind via 512 is configured in pad
510. In the embodiment shown in FIG. 5D, a vent 518 is positioned
in pad 510 extending from the outer edge of pad 510 to the blind
via 512. The vent 518 has generally a wedge shape. In the
embodiment of FIG. 5E, the vent 518 has a half-wedge shape with the
apex of the wedge removed. In the embodiment of FIG. 5E, the vent
518 does not reach blind via 512 from the outer edge of pad
510.
[0047] FIGS. 5F and 5G are top view diagrams illustrating
embodiments of the present invention. In FIGS. 5F and 5G, a pad 510
is formed on substrate 516. A blind via 512 is configured in pad
510. A vent 518 in pad 510 extends from the outer edge of pad 510
to blind via 512. In the embodiment of FIG. 5F, the path of vent
518 has generally a round shape. In another embodiment (not shown),
the path of vent 518 in FIG. 5F has one or more angles to form a
zigzag shape. In the embodiment of FIG. 5G, the path of vent 518
has generally a spiral shape.
[0048] FIGS. 5H and 5I are top view diagrams illustrating
embodiments of the present invention. In FIGS. 5H and 5I, a pad 510
is formed on substrate 516. A blind via 512 is configured in pad
510. A vent 518 is in pad 510 extending from a first point on the
outer edge of pad 510 to a second point on the outer edge of pad
510, and not intersecting the blind via 512. In the embodiment of
FIG. 5H, the path of vent 518 has generally a straight path. In the
embodiment of FIG. 5I, the path of vent 118 has generally a curved
path. In another embodiment (not shown), the path of vent 518 in
FIG. 5I has one or more angles to form a zigzag shape.
[0049] FIGS. 6A and 6B are perspective view illustrations of
alternative embodiments of a vent design. In FIGS. 6A and 6B, a pad
610 is formed on a substrate 616. A blind via 612 is configured in
pad 610. A vent 618 is in pad 610 extending from the outer edge of
pad 610 to the blind via 612. Pad 610 has a first face 610A and a
second face 610B, where second face 610B is the face of pad 610
coupled to substrate 616. In the embodiment of FIG. 6A, the vent
618 passes from first face 610A into pad 610, but does not reach
second face 610B. The depth of vent 618 does not pass all the way
through pad 610. In the embodiment of FIG. 6B, the vent 618 passes
from first face 610A to second face 610B. In this embodiment, the
depth of vent 618 passes all the way through pad 610 to expose
substrate 616.
[0050] FIG. 7 is an illustration of one embodiment of an example
computer system 740 that can be implemented in conjunction with the
presently claimed invention. Computer system 740 includes a
processor 750, a memory 755, an input/output controller 760. Bus
765 is coupled to each of processor 750, memory 755 and
input/output controller 760. Processor 750 may be a conventional
microprocessor including, but not limited to, an Intel Corporation
x86, Pentium, or Itanium family microprocessor, a Motorola family
microprocessor, or the like. Memory 755 includes Dynamic Random
Access Memory (DRAM), Static Random Access Memory (SRAM),
Synchronized Dynamic Random Access Memory (SDRAM), Rambus Dynamic
Random Access Memory (RDRAM), or the like. An input/output device
(not shown) coupled to input/output controller 760 may be a
keyboard, disk drive, printer, scanner and other input and output
devices, including a mouse, trackball, trackpad, joystick, or other
pointing device. In computer system 740, any one of the processor
750, memory 755, and input/output controller 760 are coupled to a
PCB having at least one vented blind via in pad as described
herein.
[0051] In the foregoing detailed description, the method and
apparatus of the present invention have been described with
reference to specific exemplary embodiments thereof. It will,
however, be evident that various modifications and changes may be
made thereto without departing from the broader spirit and scope of
the present invention. The present specification and figures are
accordingly to be regarded as illustrative rather than
restrictive.
* * * * *