U.S. patent application number 11/426787 was filed with the patent office on 2006-10-19 for printed circuit board with integral strain gage.
This patent application is currently assigned to VISHAY MEASUREMENTS GROUP, INC.. Invention is credited to THOMAS P. KIEFFER, ROBERT B. WATSON.
Application Number | 20060231622 11/426787 |
Document ID | / |
Family ID | 36821631 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060231622 |
Kind Code |
A1 |
KIEFFER; THOMAS P. ; et
al. |
October 19, 2006 |
PRINTED CIRCUIT BOARD WITH INTEGRAL STRAIN GAGE
Abstract
A method for strain measurement measures strain in a printed
circuit board having an integral strain measurement layer. The
strain measurement layer includes an insulating layer having a top
surface and a bottom surface, a strain sensitive layer of metallic
foil adhered to the top surface of the insulating layer for
measuring strain associated with the printed circuit board, and a
copper coating disposed on the strain sensitive layer of metallic
foil.
Inventors: |
KIEFFER; THOMAS P.; (WAKE
FOREST, NC) ; WATSON; ROBERT B.; (CLAYTON,
NC) |
Correspondence
Address: |
MCKEE, VOORHEES & SEASE, P.L.C.
801 GRAND AVENUE
SUITE 3200
DES MOINES
IA
50309-2721
US
|
Assignee: |
VISHAY MEASUREMENTS GROUP,
INC.
P.O. BOX 27777
RALEIGH
NC
|
Family ID: |
36821631 |
Appl. No.: |
11/426787 |
Filed: |
June 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11063318 |
Feb 22, 2005 |
7094061 |
|
|
11426787 |
Jun 27, 2006 |
|
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Current U.S.
Class: |
235/449 |
Current CPC
Class: |
G01L 5/0047 20130101;
H05K 1/0271 20130101; H05K 1/16 20130101; H05K 2203/163 20130101;
H05K 2201/10151 20130101; H05K 1/0268 20130101 |
Class at
Publication: |
235/449 |
International
Class: |
G06K 7/08 20060101
G06K007/08 |
Claims
1. A method for strain measurement on a printed circuit board
comprised of a plurality of layers wherein at least one of the
plurality of layers is a strain measurement layer comprising of an
insulating layer and a strain sensitive metallic foil adhered to
the insulating layer for measuring strain associated with the
printed circuit board, comprising measuring strain associated with
the strain sensitive metallic foil.
2. The method of claim 1 further comprising locating an area of
high stress based on the strain.
3. The method of claim 1 wherein the area of high stress is
associated with solder connection failure in the printed circuit
board.
4. The method of claim 2 further comprising modifying a
manufacturing process based on location of the area of high stress.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional Application of U.S.
application Ser. No. 11/063,318 filed Feb. 22, 2005, which
application is hereby incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a printed circuit board
with an integral strain gage. Although the present invention
addresses numerous problems, specific problems related to the
manufacturing of printed circuit boards are discussed herein to
provide a background of the invention.
[0003] During printed circuit board manufacturing, it is
advantageous to measure strain associated with a printed circuit
boards. Strain measurements can be used for stress analysis
purposes. During the manufacturing processes printed circuit boards
undergo a variety of operations that can be stressful. For example
populating a board during one manufacturing operation may cause
damage to connections made in a previous manufacturing operation.
Locating and monitoring areas where strain is being produced on a
printed circuit board is desirable. Knowledge regarding strain can
be used to assess electronic component failure modes within a
printed circuit board to thereby alter or improve the manufacturing
process as necessary to reduce or eliminate defects and/or
otherwise improve quality.
[0004] This problem has been addressed in the prior art by using
conventional discrete strain gages attached to the surface of the
printed circuit board with separate signal lead wires attached to
the strain gage. In such an approach discrete strain gage sensors
are glued to the top surface of the printed circuit board and
attached to electrical circuitry via leadwires attached to the
sensors.
[0005] In the case of prior art strain gages, there is a
significant installation cost associated with placing a strain gage
on a printed circuit board. This step is performed by a technician
which is time-consuming and costly. This type of labor intensive
installation process is not consistent with the goals of an
automated assembly process or a high volume PCB manufacturing
environment. Moreover, because an automated process is not used the
results obtained from strain gages installed in this manner may not
be as accurate or useful as desired due to inconsistencies between
installations.
[0006] An alternative prior art approach has been to use
strain-sensitive material applied by metal deposition directly to
the board. This approach has allowed some use of printed circuit
board manufacturing techniques, however there are significant
disadvantages. In particular deposited metal does not provide the
requisite strain-sensitive properties that may be required in more
sensitive applications. Also, although printed circuit board
manufacturing techniques are used, the use of a metal deposition
step for the strain gage is a significant addition to the
manufacturing process that may be cost prohibitive in particular
applications.
[0007] A further problem is that with the prior art approaches
there are numerous limitations as to where strain can be measured.
Strain sensors can not be located in positions which are not
readily accessible. Every place where it may be advantageous or
appropriate to measure strain is simply not accessible with a
discrete sensor.
[0008] Thus, despite these varying approaches used in the prior
art, problems remain. Therefore, it is a primary object, feature or
advantage of the present invention to improve upon the state of the
art.
[0009] Another object, feature, or advantage of the present
invention is to provide for accurately and efficiently locating
areas of stress in a printed circuit board.
[0010] It is a further object, feature or advantage of the present
invention to provide a printed circuit board with an integral
strain gage as opposed to a discrete strain gage.
[0011] It is a still further object, feature or advantage of the
present invention is to provide an integral strain gage that does
not require deposition of material directly on the printed circuit
board.
[0012] Another object, feature or advantage of the present
invention is to provide an integral strain gage that is compatible
with a multi-layer printed circuit board.
[0013] Yet another object, feature or advantage of the present
invention is to provide an integral strain gage for use in a
printed circuit board that allows for flexibility with respect to
where in the printed circuit board the strain gage is placed.
[0014] A further object, feature or advantage of the present
invention is to provide an integral strain gage that does not
require using discrete sensors attached to the top surface of the
printed circuit board.
[0015] A still further object, feature, or advantage of the present
invention is to provide an integral strain gage in a printed
circuit board suitable for transducer purposes such as measuring
deflection or force.
[0016] Another object, feature, or advantage of the present
invention is to provide an integral strain gage in a printed
circuit board that is accurate.
[0017] One or more of these and/or other objects, features, or
advantages of the present invention will become apparent from the
specification and claims that follow.
SUMMARY OF THE INVENTION
[0018] According to one aspect of the present invention a strain
measurement layer for use in assembling a printed circuit board to
provide for strain measurement on the printed circuit board is
provided. The strain measurement layer includes an insulating layer
having a top surface and a bottom surface. There is a strain
sensitive layer of metallic foil adhered to the top surface of the
insulating layer for measuring strain associated with the printed
circuit board. There is a copper coating disposed on the
strain-sensitive layer of metallic foil. The layer may also include
a second strain sensitive layer on the bottom surface of the
insulating layer and a second copper coating on the second strain
sensitive layer. The strain sensitive layer may be patterned to
provide one or more strain gage features. The copper coating may be
patterned to provide for circuit features.
[0019] According to another aspect of the invention, a printed
circuit board includes a plurality of layers where at least one of
the layers is a strain measurement layer adapted to provide for
strain measurement of the printed circuit board. The strain
measurement layer includes an insulating layer having a top surface
and a bottom surface, a strain sensitive layer of metallic foil
adhered to the top surface of the insulating layer for measuring
strain associated with the printed circuit board and a copper
coating disposed on the strain sensitive layer of metallic foil.
The strain sensitive layer may be an outer layer or an inner layer.
Also more than one strain sensitive layer can be used. Both the
strain sensitive layer may be patterned to form various strain gage
features. Similarly, the copper coating may be patterned to provide
various circuit features.
[0020] According to another aspect of the invention, a method for
strain measurement on a printed circuit board is provided. The
method includes providing a printed circuit board having a
plurality of layers where at least one of the layers is a strain
measurement layer having an insulating layer and a strain sensitive
metallic foil adhered to the insulating layer for measuring strain
associated with the printed circuit board. According to the method,
strain is associated with the strain sensitive metallic foil as
measured. The method allows for locating an area of high stress
based on the strain. For example, the method provides for locating
an area of high stress associated with solder connection failure in
the printed circuit board. The resulting strain measurements can be
used to provide information necessary to appropriately modify
manufacturing process based on location of the areas of high
stress.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 illustrates one embodiment of a side view of a
printed circuit board according to the present invention.
[0022] FIG. 2 illustrates one embodiment of a strain measurement
layer according to one embodiment of the present invention.
[0023] FIG. 3 illustrates a view of one embodiment of a strain
measurement layer which is an internal layer according to one
embodiment of the present invention.
[0024] FIG. 4 illustrates a top view of one embodiment of a printed
circuit board of the present invention having multiple strain
measurement layers.
[0025] FIG. 5 illustrates a side view of a strain measurement layer
according to one embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] The invention provides for an integral strain-sensitive
layer in a printed circuit board. The integral strain-sensitive
layer addresses the problems of strain measurement on a printed
circuit board while allowing the use of printed circuit board
manufacturing techniques to locate the point of strain measurement
and to connect the strain sensor to the appropriate electrical
circuitry.
[0027] FIG. 1 illustrates a side view one embodiment of a printed
circuit board 10 having multiple layers. The multiple layers
include an outer layer 12, an inner layer 14 and a second outer
layer 16, the inner layer 14 sandwiched between the outer layer 12
and the second outer layer 16. Although only three layers are
shown, the present invention contemplates that printed circuit
boards may have any number of layers, one or more of which may be
strain measurement layers. To assist in describing the invention,
FIG. 2 illustrates a strain measurement layer which is an outer
layer, FIG. 3 illustrates a strain measurement layer which is an
inner layer, and FIG. 4 illustrates that a multi-layer board can
have multiple strain measurement layers.
[0028] FIG. 2 illustrates a top view of one embodiment of a strain
measurement layer 12. The strain measurement layer 12 includes a
number of different features. The features include high resistance
strain sensor features such as strain sensitive grids 18 and 26. In
addition, there are conventional features such as circuit traces
20, 21, 22, 24 for low resistance electrical signal transmission to
each of the strain sensitive grids 18, 26. Another conventional
feature includes low resistance attachment points 28, 30, 32 for
non-strain sensitive components. There is also a plurality of pads
34, 36 for low resistance attachment to non-strain sensitive
components. The features 20, 21, 22, 24, 28, 30, 32, 34, and 36 are
preferably copper features. The various features are shown are
merely illustrative as the strain measurement layer is configurable
to whatever circuit design and combination of features desired by
its users through removal processes.
[0029] FIG. 3 illustrates one embodiment of an inner layer having
strain sensor features. It should be apparent that the strain
measurement layer of the present invention can be either an outer
layer or an inner layer. As shown in FIG. 3, inner layer 14 has a
strain sensor feature 40. Also, there are conventional circuit
features such as circuit traces 42 for low resistance electrical
signal transmission to the strain sensitive grid 40. Other features
shown include low resistance drilled attachment via 44, 46 for
routing electrical signals from the strain sensitive feature 40.
Other features shown also include low resistance drilled attachment
via 48, 52 for routing electrical signals from non-strain sensitive
components. A circuit trace 50 is also shown as is a low resistance
attachment point 54 for non-strain sensitive components. A low
resistance drilled attachment via 56 for routing electrical signals
from non-strain sensitive components is also shown. The various
features shown are merely illustrative.
[0030] FIG. 4 is a top view of one embodiment of the printed
circuit board having multiple strain measurement layers. FIG. 4
illustrates both the layers shown in FIGS. 2 and 3 combined with
the features shown in FIG. 3 provided in hidden lines.
[0031] Thus, it is apparent that the present invention is
consistent with printed circuit board manufacturing processes as a
strain measurement layer can be inserted as a layer in a
multi-layer printed circuit board. The same strain measurement
layer can include strain sensor features as well as conventional
features made of a conductor such as copper and can include via for
interfacing with adjacent layers. Where the strain sensitive layer
is an inner or subsurface layer any strain sensor features on the
strain sensitive layer can be connected to electrical circuits via
holes drilled through the strain sensor feature directly under or
over electrical traces that are joined by conventional printed
circuit board manufacturing techniques using plated and solder via,
for example.
[0032] FIG. 5 illustrates a side view of a strain measurement layer
80. Each strain measurement layer 80 is formed from an insulating
layer 82 having a top surface and a bottom surface. There is a
strain sensitive layer of metallic foil 84 adhered to the top
surface of the insulating layer 82 for measuring strain associated
with the printed circuit board. There is also a conductive coating
86, preferably of copper, disposed on the strain sensitive layer 84
of metallic foil. A strain measurement layer 80 may have the strain
sensitive layer of metallic foil and the copper coating on one side
or both sides as may be appropriate for a particular use. For
example FIG. 5 illustrates a strain sensitive layer 88 adhered to
the bottom surface of the insulating layer 82 with a conductive
coating 90, disposed on the strain sensitive layer 88.
[0033] The insulating layer 82 is preferably of a polyimide
material. The conductive coating 86, 90 is preferably copper
coating. Preferably the strain sensitive layer is of a metallic
foil of a material such as a nickel chromium alloy. Examples of
suitable materials for the metallic foil include Karma or
Constantan depending upon the particular properties desired. The
metallic foil is preferably a rolled metallic foil. Preferably the
strain sensitive layer is bonded to the insulating substrate with
an epoxy adhesive, although the present invention contemplates that
other types of adhesives may be used depending upon the particular
properties desired. The strain measurement layer is typically less
than about 2 mils in thickness.
[0034] During the printed board manufacturing process, the present
invention contemplates using essentially standard printed circuit
board techniques. It is observed that the strain measurement layer
of the present invention may be somewhat more flexible than the
types of layers normally used in printed circuit boards so some
alterations in handling may be required in appropriate
circumstances. The strain measurement layers of the present
invention can be patterned using normal processes including
etching. Where the strain measurement layer is etched to form
strain sensor patterns, it is observed that typical chemicals used
for removing copper may take longer to remove the portions of
metallic foil than the copper coating, therefore other chemicals
may be desirable to speed the process.
[0035] It is further observed that the integral strain gage of the
present invention is not limited to use for detecting strains
associated with manufacturing process. The integral strain gage of
the present invention also has other applications as a sensor. For
example, force and deflection can be sensed in any number of
applications.
[0036] Therefore an integral strain gage has been disclosed. The
present invention contemplates numerous alternative embodiments,
modifications, substitutions, and additions within the intended
spirit and scope of the invention. Thus, the present invention is
not to be limited to the specific embodiments described herein.
* * * * *