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Description  |
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FIELD OF THE INVENTION
The present invention relates to printed circuit boards and more
particularly to printed circuit boards including various devices or
components to be coupled with individual passive circuit components such
as resistor/conductors, inductors or dielectric/capacitors and compound
circuit components (networks) formed from combinations of passive circuit
component.
BACKGROUND OF THE INVENTION
The present invention is directed toward printed circuit boards (PCBs) and
the like. These circuit boards typically include large numbers of
electronic devices which are commonly surface mounted and also additional
component which may be present in the form of active layers within or on
each PCB. The requirement for the devices and components in such printed
circuit boards are subject to conventional electronic design restraints.
More specifically, many of the surface mounted devices and other components
on such PCBs commonly require coupling with individual passive circuit
components such as resistor/conductors, inductors or dielectric/capacitors
in order to achieve their desired function.
The solution to this problem in the prior art has been the use of
individual discrete passive components commonly surface mounted on the
PCBs. PCB design has further required the provision of though-holes in
order to properly interconnect the passive circuit components. In this
regard, the passive circuit components may be interconnected between any
combination of surface devices or component, active circuit components or
layers formed on or within the PCBs.
Accordingly, the provision of such discrete or individual passive circuit
components has increased the complexity of the PCBs and at the same time
either decreased the available surface area of the PCBs for other devices
or else resulted in an overall increase in the size of the PCBs to
accommodate necessary surface devices and components including passive
circuit components.
A more recent solution to this problem in regard to resistive circuit
components in the prior art has been the provision of planar components,
typically resistors, preferably formed on layers of the PCBs to replace
prior art surface mounted resistors as described above, thus making
surface portions of the PCBs free for other uses.
Although such planar resistors provide advantages in certain applications
over discrete surface mounted resistors, they have still tended to result
in relative increases in the complexity and space demands on the PCBs. For
example, if the planar resistors are formed on a surface layer of the PCB,
it is of course possible to arrange an active surface device over the
resistor. However, that surface portion of the PCB occupied by the planar
resistor must be dedicated to the planar resistor itself. Accordingly,
that portion of the board is not available for mounting pads,
through-holes or the like. At the same time, it is also necessary to
provide conductive couplings for interconnecting the surface formed planar
resistors in order couple them with active devices or components in the
PCBs. Here again, plated through-holes have commonly been employed for
this purpose and further increase complexity and space demands in the
PCBs.
Planar resistors of the type described above have also been formed on
internal layers or planes of the PCBs. Such a configuration permits the
use of standard subtractive PCB techniques, for example, to produce
conductor patterns and resistor elements suited for high speed and high
density circuit applications. However, even with planar resistors formed
on internal layers of the PCBs, it is still necessary to provide plated
through-holes or other conductors extending in a Z direction through the
PCBs in order to provide the necessary couplings for the planar resistors
with various surface mounted devices or components in the PCBs.
Thus, there has been found to remain a further need for improvements in the
provision of passive circuit components and compound circuit components
formed by combinations of passive circuit components for use in PCBs.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an improved
design for PCBs and the like including circuit components on external
surfaces or internal layers of the PCBs.
It is particularly contemplated in connection with the present invention
that the circuit components be formed from generally conventional
materials exhibiting desired characteristics.
The circuit components may include, for example, resistors/conductors,
inductors, dielectrics/capacitors, combinations of the above and possibly
additional components as well. Within the scope of the present invention,
it is to be noted that all resistors necessarily exhibit conductance and
similarly all conductors necessarily exhibit some resistance. Accordingly,
resistors and conductors are considered as a generally constant spectrum
dependent only upon the specific resistance and conductance of the
component of the component. Similarly, a dielectric component may function
as a true dielectric or isolating component or as a capacitor depending
upon the specific dielectric constant for the component. Accordingly,
dielectrics and capacitors are also considered as a generally continuous
spectrum in the present invention depending upon the specific dielectric
constant.
However, it is a more particular object of the invention to form the
circuit components of the present invention in combination with
through-holes in the PCB.
The term "circuit board" is employed herein to include printed circuit
boards and other device substrates such as integrated devices, multi-chip
modules and similar devices having signal traces on different layers.
The term "through-hole" is employed herein to refer to any Z directional
conductor formed in the PCB for interconnecting a surface device or PCB
component with a conductive layer on or in the PCB. For example,
through-holes are commonly employed for interconnecting surface mounted
devices on the PCB either with an internal conductive layer or plane
within the PCB or even a surface conductive layer on the PCB. In the
latter case, the most common arrangement would be a through-hole
interconnecting a surface device on one side of the PCB with a conductive
layer or plane on the opposite surface of the PCB. At the same time,
through-holes of the type defined above are also employed for
interconnecting surface mounted devices or components arranged on or in
the PCB with conductive layers or planes formed either on or in the PCB.
Accordingly, the present invention preferably contemplates formation of
its circuit component or components in combination with through-holes as
defined above. The through-hole described above may also be replaced by a
central conductor, the central conductor thus serving as a means for
completing a circuit including the circuit component assembly of the
present invention and the surrounding conductive layer.
More specifically, it is an object of the invention to provide a
resistor/conductor assembly in a PCB, the assembly including a conductive
through-hole formed in the PCB for interconnection with a surface device
or other PCB component, a conductive pad surrounding and conductively
interconnected with the plated through-hole, a conductive layer
surrounding and generally coplanar with the conductive pad and spaced
apart from the conductive pad to form an annular recess, a
resistor/conductor assembly being arranged in the annular recess and
formed from a conductive material having a selected resistivity and outer
and inner perimeters respectively conductively interconnected with the
conductive layer and the conductive pad whereby the resistor/conductor
assembly is electrically coupled along with the plated through-hole
between the conductive layer and the surface device or component.
Broadly, the present invention contemplates such a resistor/conductor or
resistor/conductor assembly wherein the annular recess forms a continuous
channel or separation between the conductive layer and the conductive pad
or plated through-hole. At the same time, the resistor/conductor assembly
is arranged in the annular recess and is preferably at least co-extensive
with the annular recess. In such a co-extensive arrangement, the
resistor/conductor assembly either extends just between the outer and
inner perimeters or may even overlap the outer and inner perimeters as
described in greater detail below.
It is a further related object of the invention to provide such a
resistor/conductor assembly wherein the outer and inner perimeters of the
resistor/conductor assembly are each formed with a substantially constant
radius with the resistor/conductor assembly being at least generally
coextensive with the annular recess whereby the operative resistance of
the resistor/conductor assembly may be simply determined from the radii of
its outer and inner perimeters, and effective thickness, of the assembly
and its resistivity. It is even more preferably contemplated that the
conductive layer and conductive pad have generally equal thicknesses at
their respective interconnections with the resistor/conductor assembly for
establishing the effective thickness of the assembly. In this case, the
resistor/conductor assembly is assumed to have a thickness approximately
equal to those of the conductive layer and conductive pad.
Within such a configuration, the resistor/conductor assembly may readily be
formed, for example, by deposition of a liquid precursor or by other
methods of formation which will be apparent from the following
description.
It is more broadly an object of the invention to provide such a
resistor/conductor assembly at the juncture of a plated through-hole in a
PCB with a surrounding conductive layer, the assembly having an outer
perimeter conductively interconnected directly with the conductive layer
and an inner perimeter conductively interconnected with the plated
through-hole. Here again, the outer and inner perimeters of the
resistor/conductor assembly are preferably formed with substantially
constant radii so that the operative resistance of the assembly may be
simply determined from the radii of its outer and inner perimeters, an
effective thickness of the resistor assembly and its resistivity.
It is further contemplated in connection with the objects set forth above
that the operative resistance of the resistor or resistor/conductor
assembly be capable of estimation in the manner summarized above. More
specifically, a typical method for estimating the operative resistance of
the resistor/conductor is set forth immediately below.
At least in a preferred embodiment of the present invention with the outer
and inner peripheries of the resistor/conductor assembly having constant
radii, the effective resistance of the assembly may be determined as
follows, having reference to FIG. 5B.
If R equals resistance, then it may be calculated as
##EQU1##
where R equals resistance in ohms, p equals resistivity of the resistor in
ohms centimeters, L equals the length of the resistor in centimeters, w
equals the width of the resistor in centimeters and H equals the height of
the resistor in centimeters (wH thus being the effective cross-sectional
area of the resistor for purposes of calculating its resistance).
Referring briefly to FIG. 5B, as described below, the resistor body 62 is
graphically illustrated with substantially constant radii forming its
outer and inner peripheries 56 and 58. Further, since the resistor body 62
forms a resistor between the conductive pad 60 and the conductive layer or
upper surface 22, then the effective length of the resistor body 62 is
equal to the radial dimension of the resistor body, that is r.sub.2
-r.sub.1. The effective width of the resistor body is thus the mean
circumference of the resistor body, that is the circumference of the
resistor body generated from a point mid-way between the outer and inner
peripheries 56 and 58. Thus, the effective width of the resistor body may
be stated as follows:
##EQU2##
These effective values for length and width may then be substituted into
the basic equation set forth above for resistance. It may readily be seen
from FIG. 5B that the overall resistance of the resistor body will be
proportional to the differential radius, that is r.sub.2 -r.sub.1. At the
same time, resistance is inversely proportional to the effective width of
the resistor body as stated above.
The above equations can readily be employed for adjusting the radii of the
outer and inner peripheries of the resistor body in order to provide any
desired resistance, at least given the effective height (H) for the
resistor body. It is also possible of course to permit variation of the
effective height of the resistor body for purposes of determining overall
resistance.
The equations set forth above thus readily facilitate the calculation of
resistance relative to dimensions for a preferred embodiment of the
resistor body as illustrated in FIG. 5B. At the same time, variations in
the configuration of the resistor body may similarly be included in such
mathematical determinations, but possibly with increased complexity
relative to the equations set forth above.
The summary includes numerical labels described in greater detail below but
set forth here for the purpose of facilitating application of the
summarized equations with the preferred embodiments described below.
In a preferred embodiment of the present invention with the outer and inner
peripheries of the resistor or resistor assembly having constant radii,
the effective resistance of an annular resistor or resistor/conductor
assembly may be more precisely determined as follows, having continued
reference to FIGS. 5B and 5D.
Generally, the macroscopic quantities voltage (V), current (i), and
resistance (R) apply to a particular body or extended shape. The
macroscopic quantities are determined from the corresponding microscopic
vector quantities (point quantities) electric field (E), current density
(j), and scalar quantity resistivity (.rho.). The microscopic quantities
are expressed as,
E=.rho.j (1)
and the corresponding macroscopic quantities are expressed as,
V=iR (2)
The resistance of a material between points a and b (of any material shape)
can be expressed in microscopic tens by the following relationship,
##EQU3##
In this expression, the line integral d1 defines the line ab along the path
E, and a closed loop path is defined by the surface integral dS, which
corresponds to the area enclosed by a current i.
The above microscopic expression applied to a rectangular resistor body
with dimensions (h, w, l), as shown in FIG. 5D, upon integration, readily
yields the macroscopic quantity which states that the resistance of a
rectangular resistor body is directly proportional to its length and
indirectly proportional to its cross-sectional area,
##EQU4##
The error analysis for this relationship (regarding the rectangular
geometry) is straight forward and is expressed as,
##EQU5##
A model of an annular resistor is shown in FIG. 5B. The model has been
simplified in that the resistor body does not overlap the top of the
conductor plane. Overlapping the conductor plane reduces the resistance of
the resistor per unit volume by exposing more resistor contact area to the
copper foil. If overlap cannot be avoided, the expression can be modified
to account for overlap as shown by equation (7) below.
Referring to FIG. 5B, an annular resistor body 62 is graphically
illustrated with substantially constant radii forming its outer and inner
peripheries 56 and 58. The resistance value of the annular geometry can
only be approximated and is not accurately described by the above
expression relating to the rectangular resistor geometry. The
cross-sectional area of the annular resistor is a continuously and
smoothly changing function of the radii. This functionality is described
by the logarithmic ratio of the outer to inner diameter. Assuming that the
radial symmetry between the inner contact pad 60 and the annular resistor
body 62 is controlled and maintained during processing, then, the
resistance of the annular resistor geometry is exactly expressed as:
##EQU6##
Further, if overlap cannot be avoided, the above expression can be modified
as follows:
##EQU7##
In this expression, d.sub.2 /d.sub.1 describes the ratio of the outer to
inner diameters.
The error function for the annular geometry equation for resistance is
given by:
##EQU8##
The above equations can readily be employed for adjusting the radii of the
outer and inner peripheries of the annular resistor body in order to
provide a desired resistance, at least given the effective height (h) for
the resistor body. It is also possible of course to permit variation of
the effective height of the resistor body for purposes of determining
overall resistance.
The equations set forth above thus readily facilitate the exact calculation
of resistance relative to dimensions for a preferred embodiment of the
resistor body as illustrated in FIG. 5B. The function
R(.rho.,h,d.sub.1,d.sub.2) and likewise the error function of R have four
dependent variables, with the dependency of those variables relating to
processing conditions such as control over ring thickness, ring dimensions
and planarization. In the annular geometry of the preferred embodiment,
the logarithmic function (of the cross-sectional area of the annulus)
controls the resistive value of resistor body. In order to visualize
resistance tolerance of a processed annular ring resistor, a plot of the
error function equation (8) against the ratio of the outer to inner
diameters is illustrated in FIG. 13. In order to provide an example of
what can be expected from the resistive tolerance of the annular ring
geometry, the parameters and tolerances chosen for this evaluation are as
follows: d1=20.+-.1.0 mil., d2=20 to 68.+-.1.0 mil., h=1.25.+-.0.25 mil
and .rho.=1000.+-.100.OMEGA.mil. These particular parameters and
tolerances were chosen for purpose of example only, and are not intended
to be limiting in the present invention.
By consideration of a permutation of the tolerance examples of the four
variables, the above figure defines an envelope for the tolerance range of
R. The tolerance range is bounded to all values inside this envelope. The
tolerance envelope of FIG. 13 illustrates that the annular resistor
tolerance is dominated by the logarithmic dependence of the radii (the log
function relating to the cross-sectional area of de annulus), that
logarithmic function being non-linear. However, FIG. 13 indicates that the
tolerance range does generally increase, allowing better tolerance control
in processing when the annular geometry is taken into consideration. In
this regard, the error function indicates that the outer ring diameter
should be not more than 2 times the inner diameter in order to keep the
resistance tolerance low (<20%).
It is further contemplated by the present invention to provide inductor
components of simplified design with reduced surface requirements.
It is particularly contemplated in connection with the present invention
that the inductor components be formed from generally conventional
materials such as iron ferrite disposed in a matrix structure to provide a
selected inductance. Accordingly, the inductance of each inductor
component of the present invention is determined by its dimensions and the
permeability of the ferromagnetic material.
Just as with the resistor element of the present invention, it is a more
particular object of the invention to provide a simplified design for the
inductor components while reducing surface requirements within the PCB by
forming the inductor components in combination with through-holes in the
PCBs for interconnecting surface devices or components with conductive
layers on or in the PCB.
More specifically, it is an object of the invention to provide an inductor
assembly in a PCB, the assembly including a conductive through-hole formed
in the PCB for interconnection with a surface device or other PCB
component, a conductive pad surrounding and conductively interconnected
with the plated through-hole, a conductive layer surrounding and generally
coplanar with the conductive pad and spaced apart from the conductive pad
to form an annular recess, an inductor assembly being arranged in the
annular recess and formed from ferromagnetic material having a selected
inductance and outer and inner perimeters respectively conductively
interconnected with the conductive layer and the conductive pad whereby
the inductive assembly is electrically coupled along with the plated
through-hole between the conductive layer and the surface device or
component.
Broadly, the present invention contemplates such an inductor or inductor
assembly wherein the annular recess forms a continuous channel or
separation between the conductive layer and the conductive pad or plated
through-hole. At the same time, the inductor assembly is arranged in the
annular recess and is preferably at least co-extensive with the annular
recess. In such a co-extensive arrangement, the inductor assembly either
extends just between the outer and inner perimeters or may overlap the
outer and inner perimeters due to processing restraints.
It is a further related object of the invention to provide an annular
inductor assembly wherein the outer and inner perimeters of the annular
inductor assembly are each formed with a substantially constant radius
with the annular inductor assembly being at least generally coextensive
with the annular recess whereby the operative inductance of the annular
inductor assembly per unit length to may be simply determined from the
cross sectional area of the inductor assembly, the effective thickness of
the inductor assembly and its permeability. It is even more preferably
contemplated that the conductive layer and conductive pad have generally
equal thicknesses at their respective interconnections with the annular
inductor assembly for establishing the effective thickness of the inductor
assembly. In this case, the annular inductor assembly is assumed to have a
thickness approximately equal to those of the conductive layer and
conductive pad.
Within such a configuration, the annular inductor assembly may readily be
formed, for example, by deposition of a liquid precursor or by other
methods of formation which will be apparent from the following
description.
It is more broadly an object of the invention to provide such an annular
inductor assembly at the juncture of a plated through-hole in a PCB with a
surrounding conductive layer, the annular inductor assembly having an
outer perimeter conductively interconnected directly with the conductive
layer and an inner perimeter conductively interconnected with the plated
through-hole. Here again, the outer and inner perimeters of the inductor
assembly are preferably formed with substantially constant radii and the
inductor is generally continuous between its outer and inner perimeters
forming an annular ring structure. It is further contemplated in
connection with the objects set forth above that the operative inductance
of the inductor or inductor assembly be capable of calculation in the
manner summarized above. More specifically, a typical and preferred method
for calculating the operative inductance of the annular inductor or
inductor assembly is set forth immediately below.
In a preferred embodiment of the present invention with the outer and inner
peripheries of the annular inductor or inductor assembly having constant
radii, the effective inductance may be determined as follows, having
reference to FIG. 5B.
In FIG. 5B, an annular body is graphically illustrated with substantially
constant radii forming its outer and inner peripheries. As with the
annular resistor of the present invention, the cross-sectional area of the
annular inductor is a continuously and smoothly changing function of the
radii. This functionality is described by the logarithmic ratio of the
outer to inner diameters. Assuming that the radial symmetry between the
inner contact pad and the annular inductor body is controlled and
maintained during processing, then the functional relationship for the
annular inductor body may be expressed by the following:
Let:
.mu.=permeability (H m.sup.-1) of the ferromagnetic core
h=thickness of the ferromagnetic core
and the inductance (L) per unit length for an annular core inductor having
an inner radius of r.sub.1 and an outer radius of r.sub.2 may be
determined in the following manner:
##EQU9##
The above equation can readily be employed for adjusting the radii of the
outer and inner peripheries of the annular inductor body in order to | | |
