Title: Printed circuit board trace routing method
Abstract: An I/O routing pattern method is disclosed, for use with heterogeneous printed circuit boards (PCBs), such as those embedded with a reinforcement material, for example, a fiberglass weave. Traces are routed on the PCB so as to reduce sensitivity to changes in the dielectric constant (Dk), which are brought about by the strands of reinforcement material contained within the PCB laminate. The method minimizes the local variations, such as the Dk, time of flight, and capacitance variations, that are observed with traditional routing methods on heterogeneous PCBs.
Patent Number: 7,022,919 Issued on 04/04/2006 to Brist, et al.
| Inventors:
|
Brist; Gary A. (Yamhill, OR);
Long; Gary B. (Aloha, OR);
Alger; William O. (Portland, OR);
Miller; Dennis J. (Sherwood, OR)
|
| Assignee:
|
Intel Corporation (Santa Clara, CA)
|
| Appl. No.:
|
610147 |
| Filed:
|
June 30, 2003 |
| Current U.S. Class: |
174/260; 174/256; 174/261; 174/255; 716/15 |
| Current Intern'l Class: |
H05K 1/16 (20060101) |
| Field of Search: |
174/258,260,261,250,256
361/760
257/778
428/209,298.1
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Cuneo; K.
Assistant Examiner: Patel; Ishwar (I. B.)
Attorney, Agent or Firm: Boone, P.C.; Carrie A.
Claims
We claim:
1. A system, comprising:
a printed circuit board comprising a reinforcement material embedded within a
resin-based material, wherein the reinforcement material comprises strands arranged
in a weave pattern, the strands of the weave pattern having a predetermined distance
between any two strands; and
a trace disposed on the printed circuit board, the trace for supplying a signal
path between circuits residing on the printed circuit board, the trace comprising
a number of mini-traces coupled so as to form a zigzag pattern, wherein one mini-trace
of the number of mini-traces is disposed at an angle relative to one of the strands,
the angle being selected so that the one mini trace crosses over a desired number
of strands, the one mini-trace further having a calculated length defining a residual
trace length for the one mini-trace length and selecting the number of mini-traces
so that the residual trace length multiplied by the number is approximately an
integer value, wherein
the calculated length, the residual length, and the angle satisfy the following relationship:
r(
L, d, φ):=
d/(sin(φ))×fract(
L x (sin(φ)/
d)),
where L is the calculated length, d is the predetermined distance, φ is
the angle and r is the residual length.
2. The system of claim 1, further comprising:
a second trace for supplying a signal path between circuits residing on the printed
circuit board, the second trace being routed to be substantially equidistant from
the trace at all points along the traces, wherein the trace has a first dielectric
constant and the second trace has a second dielectric constant, wherein the first
dielectric constant is substantially similar to the second dielectric constant.
3. The system of claim 2, further comprising:
a third trace disposed on the printed circuit board so as to be substantially
parallel to the second trace, the third trace being routed to be substantially
equidistant from the second trace at all points along the traces, the third trace
having a third dielectric constant, wherein the second dielectric constant is substantially
similar to the third dielectric constant.
4. The system of claim 1 the number of mini traces further comprising: a second
mini-trace having a second length and a second angle, wherein the length is not
identical to the calculated length and the angle is not identical to the second angle.
5. The system of claim 4, wherein each of the pair of traces traverses at least
a predetermined number of strands of the reinforcement material, wherein the predetermined
number is a factor by which an unknown component of the dielectric constant of
each trace is reduced.
6. The system of claim 5, wherein each of the pair of traces traverses a residual
portion of a strand of the reinforcement material, wherein the dielectric constant
of the residual portion is equal to the unknown component.
7. A system, comprising: a printed circuit board comprising a reinforcement material
embedded within a resin-based material, wherein the reinforcement material comprises
strands arranged in a weave pattern, the strands of the weave pattern having a
predetermined distance between any two strands; and a trace disposed on the printed
circuit board, the trace for supplying a signal path between circuits residing
on the printed circuit board, the trace comprising a number of mini-traces coupled
so as to form a zigzag pattern, one mini-trace of the number of mini-traces having
a length, the length being selected so that the one mini trace crosses over a desired
number of strands, wherein the number of mini-traces is obtained by calculating
a residual trace length for one mini-trace length and selecting the number of mini-traces
so that the residual trace length multiplied by the number is approximately an
integer value,
the one mini-trace further having a calculated angle relative to one of the strands
of the printed circuit board, wherein calculated angle is based on the length,
the predetermined distance between the strands in the printed circuit board, and
the desired number of strands, wherein
the calculated length, the residual length, and the angle satisfy the following relationship:
r(
L, d, φ)=
d/(sin φ)) x fract(
L x (sin(φ)/
d)),
where, L is the calculated length, d is the predetermined distance, φ is
the angle, and r is the residual length.
8. The system of claim 7, further comprising:
a second trace for supplying a signal path between circuits residing on the printed
circuit board, the second trace being routed to be substantially equidistant from
the trace at all points along the traces, wherein the trace has a first dielectric
constant and the second trace has a second dielectric constant, wherein the first
dielectric constant is substantially similar to the second dielectric constant.
9. The system of claim 8, further comprising:
a third trace disposed on the printed circuit board so as to be substantially
parallel to the second trace, the third trace being routed to be substantially
equidistant from the second trace at all points along the traces, the third trace
having a third dielectric constant, wherein the second dielectric constant is substantially
similar to the third dielectric constant.
10. The system of claim 7, the number of mini-traces further comprising: a second
mini-trace having a second length and a second angle, wherein the length is not
identical to the second length and the calculated angle is not identical to the
second angle.
11. A method, comprising:
identifying a distance between two strands of reinforcement material within a
printed circuit board;
specifying an angle between a trace and one of the two strands;
calculating a length of the trace when the trace is disposed at the specified
angle over strands with the identified distance there between, such that a desired
integer number of strands is traversed by the trace at the specified angle and
the calculated length;
routing the trace on the printed circuit board at the specified angle in relation
to one of the two strands and at the calculated length;
obtaining a residual trace length for the trace; and routing a number of traces
on the printed circuit board in a zigzag pattern, wherein the residual trace length
times the number of traces is approximately an integer value, wherein,
the calculated length, and residual length and the angle satisfy the following relationship:
r(
L, d, φ)=
d/(sin φ)) x fract(
L x (sin(φ)/
d)),
where L is the calculated length, d is the predetermined distance, φ is
the angle, and r is the residual length.
12. The method of claim 11, further comprising:
obtaining a residual length of the trace that does not traverse the integer number
of strands; and
multiplying the residual length by the number, wherein the residual length of
the trace has a dielectric constant that is the unknown component.
13. A method, comprising:
identifying a distance between two strands of reinforcement material within a
printed circuit board;
specifying a length of a trace;
calculating a angle between the trace and one of the two strands for the specified
trace length over strands with the identified distance there between, such that
a desired integer number of strands is traversed by the length of the trace at
calculated angle;
routing the trace on the printed circuit board at the calculated angle in relation
to one of the two strands and at the specified length;
obtaining a residual trace length for the trace; and routing a number of traces
on the printed circuit board in a zigzag pattern, wherein the residual trace length
times the number of traces is approximately an integer value, wherein,
the calculated length, residual length and the angle satisfy the following relationship:
r(
L, d, φ)=
d/(sin φ)) x fract(
L x (sin(φ)/
d)),
where L is the calculated length, d is the predetermined distance, φ is
the angle, and r is the residual length.
14. The method of claim 13, further comprising:
obtaining a residual length of the trace that does not traverse the integer number
of strands
multiplying the residual length by the integer number, wherein the residual length
of the trace has a dielectric constant that is the unknown component.
Description
FIELD OF THE INVENTION
This invention relates to printed circuit boards and, more particularly, to
techniques for routing traces on printed circuit boards.
BACKGROUND OF THE INVENTION
Printed circuit boards (PCBs), also known as printed wiring boards, are used
to interconnect and assemble electronic circuits. The operating temperature, mechanical
strength, and other characteristics of a PCB, may vary according to the application
in which the PCB is used. PCBs provide electrical conductor paths between the circuits
disposed upon them.
Some PCBs consist of paper or woven glass impregnated with an epoxy resin. PCBs
may include materials such as copper, iron, aluminum, or ceramic. Flexible PCBs
may have polyester or polyimide bases. Ultimately, PCBs typically include at least
a resin-based material, a reinforcement material, and one or more conductive foils.
FR-4 is the most common printed circuit board material, and is used in the majority
of computer-based applications. The reinforcement material in FR-4 is typically
a woven fiberglass material that is impregnated with an epoxy resin, which may
vary in composition.
One of the measured characteristics of the PCB is its dielectric constant. The
dielectric constant of a material relates to the velocity at which signals travel
within the material. The dielectric constant is actually variable, and may change
with a modification in frequency, temperature, humidity, and other environmental conditions.
Traces etched or otherwise routed on the PCB carry signals between circuits
at a certain speed. The propagation of the signal between the circuits, known as
its "time of flight," is proportional to the length of the trace. Thus, board layout
designers typically route straight-line traces between the circuits on the PCB.
The speed of a signal propagating along a trace is inversely proportional to
the square root of the dielectric constant of the PCB upon which the trace is formed.
Thus, the dielectric constant of the PCB affects the speed of all signals propagating
on the PCB. Since most PCBs are not homogeneous, but a blend of materials, the
dielectric constant measured on the PCB is slightly different when taken over a
strand of woven glass, as compared to a measurement taken between two strands of glass.
The woven glass of the PCB is typically aligned at right angles within the PCB
material, forming a familiar weave pattern. For those PCBs that are rectangular
or square in shape, the weave pattern within the PCB is thus substantially orthogonal
to two sides of the PCB and parallel to two sides, although deviations from this
alignment may be found. Likewise, signal traces, such as input/output (I/O) buses,
and other electrical interconnect traces, are typically routed in a direction parallel
to the sides of the PCB, taking right angle turns where a change in direction is needed.
The routing practices, as well as the material properties of the PCB, result
in a condition where traces may be disposed parallel to the glass strands or orthogonal
to the glass strands. Those that run in parallel will have a random probability
of being routed directly over a parallel glass strand or between a set of parallel
glass strands. Even when a trace runs over a glass strand, their relative positions
may change, due to a skewing of the underlying glass strands. Those traces that
run orthogonal to the glass strands will intermittently be disposed over glass
strands all along the trace. These varying conditions make it difficult to ascertain
the dielectric constant of the PCB beneath the trace, thus making the signal propagation
speed along the trace difficult to successfully predict.
When a trace is disposed over a parallel glass strand, the trace achieves a
relatively higher dielectric constant, D
k, than when the trace is disposed
between two glass strands. Thus, two traces that are parallel to one another and
identical in length on the same printed circuit board may propagate signals at
different speeds, based upon the relative position of the underlying material within
the PCB. In addition to the speed difference, impedance variations between the
traces are likely to occur. These phenomena are particularly troublesome for bus
signals, in which multiple traces for each I/O line of the bus, while having identical
lengths between circuits, do not have identical impedances and may not propagate
at the same speeds. The variation in impedance and speed are dependent on the type
and size of the glass weave, the size of the trace, and the orientation of the trace.
Thus, there is a need for a method to route traces such that the dielectric
constant can be predicted and, thus, the speed and impedance of signals along the
trace can be more accurately known.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of this document, reference is made to the
following descriptions taken in connection with the accompanying drawings in which:
FIG. 1 is a perspective view of a printed circuit board including an inset depicting
a reinforcement matrix, according to the prior art;
FIGS. 2A and 2B are diagrams of printed circuit boards in which a trace is
disposed upon a glass fiber and between glass fibers, respectively, according to
the prior art;
FIG. 3 is a diagram of a printed circuit board in which a trace is disposed
in a non-orthogonal manner, according to some embodiments;
FIG. 4 is a diagram of a system including traces routed using a trace routing
method, according to some embodiments;
FIG. 5 is a flow diagram for illustrating the trace routing method, according
to some embodiments;
FIGS. 6A and 6B are diagrams of printed circuit boards in which zig-zag traces
for differential and bus signals, respectively, are routed, according to some embodiments;
FIG. 7 is a diagram of a printed circuit board in which circuits are disposed
in a non-orthogonal manner, according to some embodiments; and
FIG. 8 is a diagram of a printed circuit board in which the reinforcement material
is disposed in a non-orthogonal manner, according to some embodiments.
DETAILED DESCRIPTION
In accordance with some embodiments described herein, an I/O routing pattern
method
is disclosed, for use with heterogeneous printed circuit boards (PCBs), such as
those embedded with a reinforcement material, for example, a fiberglass weave.
Traces are routed on the PCB so as to reduce sensitivity to changes in the dielectric
constant (D
k), which are brought about by the strands of reinforcement
material contained within the PCB laminate. The method minimizes the impact of
local variations, such as the D
k, time of flight, and capacitance variations,
that are observed with traditional routing methods on heterogeneous PCBs.
In some embodiments, traces are routed between circuits on the PCB in a zig-zag
pattern, such that multiple strands of reinforcement material are disposed beneath
each portion of the zig-zag trace. The strands are embedded into the PCB in a known
manner, such as with an FR-4-type PCB, in which glass strands are aligned at right
angles to one another in a weave pattern. For a square or rectangular cut of the
PCB, the weave pattern is orthogonal to two sides of the PCB and parallel to two
sides. The zig-zag traces, including multiple mini-traces disposed at predetermined
angles, cross over multiple strands of the reinforcement material. For multiple
traces, such as differential signal traces and bus signal traces, the dielectric
constant for each trace is substantially similar.
In some other embodiments, circuits are disposed on the PCB in a non-orthogonal
configuration while traces are routed in straight lines and/or right angles between
the circuits. Again, the strands of the reinforcement material are embedded within
the PCB material in a weave pattern, which is orthogonal and parallel to the sides
of the PCB. The straight-line and/or right-angle traces cross over multiple strands
of the reinforcement material. For multiple traces which are disposed in parallel
and equidistant from one another, the dielectric constant of each trace is substantially similar.
In still other embodiments, the strands of reinforcement material that form the
weave pattern are impregnated into the resin-based material such that the weave
pattern is not orthogonal to and parallel to the sides of the PCB. As another option,
a sheet of PCB laminate, such as FR-4, is cut such that the sides of the cut PCB
are not orthogonal or parallel to the weave pattern of the impregnated glass fibers.
Using such a non-orthogonally cut PCB, circuits are disposed upon the PCB in a
typical arrangement, that is, the circuits are laid orthogonal to one of the sides
of the cut PCB. Likewise, straight-line and/or right-angle traces are routed between
the circuits, such that the traces cross over multiple strands of reinforcement
material. For differential signal traces and bus traces routed in this manner,
the dielectric constant of each trace is substantially similar.
In the following detailed description, reference is made to the accompanying
drawings,
which show by way of illustration specific embodiments in which the invention may
be practiced. For convenience, a printed circuit board reinforced with glass material
is described in order to illustrate the properties of heterogeneous printed circuit
boards. However, it is to be understood that other printed circuit board materials,
now known or yet to be developed by those of ordinary skill in the art, may be
used in practicing the principles of the invention. The following detailed description
is, therefore, not to be construed in a limiting sense, as the scope of the present
invention is defined by the claims.
In FIG. 1, a printed circuit board (PCB)
10 is depicted, according to
the
prior art. The PCB
10 includes a resin-based material (not shown), such
as an epoxy resin, a reinforcement material
12, such as woven fiberglass
strands, and a conductive foil (not shown), such as copper, for etching or otherwise
routing a trace
18 between circuits
30 and
32 disposed on
the PCB
10. The resin-based material has a different, typically lower, D
k,
than the reinforcement material or glass strands
12.
An enlarged inset of the glass strands
12 provides a closer view of the
weave pattern, which is typical for PCB laminates such as FR-4. As illustrated
by the cross-hatch pattern, the glass strands
12 are disposed orthogonal
to the sides of the PCB
10. As also depicted in FIGS. 2A and 2B, the glass
strands
12 includes vertically disposed strands
12A and horizontally
disposed strands
12B, interwoven so as to enhance the strength of the PCB
laminate. The vertically disposed strands
12A are substantially parallel
to the side
10A of the PCB while the horizontally disposed strands
12B
are substantially parallel to the side
10B of the PCB.
FIGS. 2A and 2B provide a closer view of the PCB
10, also according
to the prior art. The PCB
10 is embedded with vertically and horizontally
disposed strands
12, just as in FIG. 1. In FIG. 2A, a trace
18A is
positioned over a vertically disposed strand
12A. In FIG. 2B, a trace
18B
is positioned between two vertically disposed strands
12A.
When an individual circuit trace lines up along a strand, such as with trace
18A of FIG. 2A, the trace achieves a relatively higher D
k compared
to the trace
18B, which is between two strands. The variation in D
k
between the traces
18A and
18B results in differences in signal
propagation speeds, causing timing problems that are difficult to predict. Particularly
when the traces
18A and
18B are differential mode signals, or are
two of several bus signals, the different propagation speeds can cause an undesirable
mode conversion problem. Additionally, the traces
18A and
18B may
have different impedances resulting from the variation in D
k.
Practically speaking, the trace
18A might not be disposed over
the strand
12A for the entire length of the trace. The weave pattern of
the glass strands
12 may be skewed somewhat from being parallel to the sides
of the PCB along its entire length or width. Likewise, the trace
18B may,
at some point along the trace, be disposed directly over the glass strands
12,
causing its D
k to be somewhat higher at that point. This uncertainty
limits the ability to successfully predict the speed of signals propagating over
the traces.
Returning to FIG. 1, the trace
18 disposed between the circuits
30 and
32 has a physical length, L
0. Its "electrical length"
is defined as the physical length of the trace
18 multiplied by its dielectric
constant, D
k. The electrical length term reflects the effect that the
D
k of the trace has on the speeds of signals traveling along the trace
18. Traces disposed directly over a glass strand (e.g., trace
18A
of FIG. 2A) will generally have a longer electrical length than traces disposed
between glass strands (e.g., trace
18B of FIG. 2B), such that signals traversing
the trace
18A take longer to arrive at their destination than signals traversing
the trace
18B.
Because the relationship between trace placement and the location of the
glass strands or other reinforcement material is random, the D
k for
the trace
18 can only approximately be known. The dielectric constant, D
k,
can thus be thought of as having two components, a known component (k
1)
and an unknown component (k
2), where D
k=k
1+k
2.
So, in ascertaining the electrical length of the trace
18, some tolerance
exists, namely, ±k
2×L
0.
Suppose that the trace
18, instead of being disposed over or next
to a glass strand, is orthogonal to the glass strands. (The traces
18A and
18B are, in fact, orthogonal to several of the horizontally disposed strands
12B.) If the trace
18 crosses over a single strand or fifty strands,
the trace will have the same average D
k. In other words, the known component,
k
1, of D
k, is obtainable where the trace is disposed over
any integer number of strands. Practically, though, the trace could be disposed
over a fraction of a glass strand, say, a half or a third of a strand, at its end.
The D
k for this residual portion is uncertain (the unknown component,
k
2) and, thus, makes the electrical length of the entire trace
18 uncertain.
One way to reduce the effect of the unknown component, k
2, is to increase
the incidence of the known component, k
1. If, for example, the trace
18 is disposed orthogonal to 100.5 glass strands
12, the length of
the trace that is subject to uncertainty (the portion that is disposed over half
a strand) has been reduced by a factor of 100, as compared to a trace disposed
over just a half of a strand. By increasing the number of integer strands traversed,
the effect of the unknown component, k
2, is diminished by a factor equal
to the integer value, known herein as the reduction factor. Thus, the trace
18
is preferably routed on the PCB
10 so as to cross over as many glass strands
as possible, so as to increase the reduction factor.
As FIGS. 2A and 2B illustrate, while the trace may be disposed orthogonal to a
number of glass strands, it also may be placed directly over a glass strand, between
two glass strands, or partially over one or more glass strands. Currently, traces
are generally disposed in the same direction as either the vertically disposed
glass bundles
12A or the horizontally disposed glass bundles
12B.
The trace routing technique of the prior art thus inhibits the ability to minimize
the effect of the unknown component, k
2, in establishing the D
k
of the trace.
As an alternative, in FIG. 3, according to some embodiments, a trace
18C
is disposed in a manner so as to cross a number of glass strands, both vertically
disposed strands
12A and horizontally disposed strands
12B. The trace
18C is disposed at an angle, φ, relative to one of the horizontal
glass strands
12B. A right triangle
38 is formed with the side opposite
the angle, φ, having a length d, where d is the distance between the center
of two horizontal glass strands, and a hypotenuse of length L. The distance, d,
is a constant, based on the density of the glass strands
12, known herein
as the strand separation.
The trace
18C is neither orthogonal to the vertical strands
12A
nor to the horizontal strands
12B. Instead, the trace is disposed over a
number of strands and, being disposed at the angle, φ, relative to one of
the strands, there is little likelihood that the trace
18C will be disposed
above a single glass strand, such as in FIG. 2A, as long as the angle, φ,
is not 0° or a multiple of 90°. The right triangle
38 indicates
how many glass strands are traversed by a trace of length L, disposed at the angle,
φ, where the distance between glass strands, or strand separation, is d.
Using basic trigonometry, the length, L, of the trace
18C can be determined
for a given angle, φ, and distance between strands, or strand separation,
d. Likewise, for a given length, L, the angle, φ can be obtained. In FIG.
3, the trace
18C traverses a predetermined number, p, of strands. The more
glass strands traversed by the trace, the lower the uncertainty in the D
k
of the trace. The predetermined number, p, thus provides the reduction factor of
the trace
18C of length, L.
Once the number of glass strands traversed by the trace
18C of length,
L, at angle, φ, is known, the principles can be scaled for a trace whose
length is multiples of L. So, where the predetermined number, p, is one-fifth the
reduction factor, a trace of length
5L can be disposed on the PCB, at the
same angle, φ, relative to one of the glass strands, and will cross over
five times as many strands as the trace
18C. The unknown component, k
2,
of the dielectric constant, D
k, of the trace is reduced by a factor
equal to the number of strands traversed.
Using this information, a trace between two circuits can be routed so as to
preferably reduce the impact of the unknown component, k
2, of the dielectric
constant, D
k of the trace. In FIG. 4, according to some embodiments,
a PCB
100 is depicted, including a zig-zag trace
20 for single-ended
mode transmission of a signal between circuits
30 and
32. The shape
and size of the zig-zag trace
20 is mathematically determined with reference
to the position of the glass strands
12 within the PCB
100.
The zig-zag trace
20 includes multiple mini-traces
24 which are
routed at a glass strands
12" with predetermined angle relative to the glass
strands
12 (see angle φ in FIG. 3). Two mini-traces
24 meet
at an endpoint
26. The angle φ and length of the mini-traces
24
are determined, using a trace routing method
300, described in FIG. 5, below,
so as to reduce the uncertainty of D
k by a desired reduction factor.
The traces
20 are then routed in a zig-zag pattern between the circuits
30 and
32, incorporating the calculated angle φ and mini-trace
24 length.
The use of a zig-zag trace over a straight-line trace involves a tradeoff between
having the shortest traces between circuits (there is a practical limit to how
long the traces can be before excessive loss occurs) and having traces in which
the signal propagation speed can be successfully predicted. In some embodiments,
using the trace routing method
300, the improvements timing accuracy are
greater than an order of magnitude while the added loss at high frequencies (due
to the longer trace length) is less than ten percent.
The trace routing method
300 ensures that each mini-trace
24 passes
over a predetermined number of strands in the reinforcement material
12,
wherein the predetermined number is equal to the desired reduction factor. In passing
over any integer number of strands, the average deviation of the dielectric constant,
D
k, is a constant. Only the last portion of the mini-trace
24,
which passes over a fraction of a strand, results in a deviation of the D
k
for that mini-trace. The errors can be thought to accumulate at the endpoints
26
of the zig-zag trace
20. The deviation for each mini-trace
24 can
be accumulated over the number of mini-traces
24 in the trace
20,
to yield a total deviation or error. The error can be adjusted by changing the
angle φ and/or the length of the mini-trace
24, resulting in a change
of relative position between the zig-zag trace
20 and the weave pattern
of the reinforcement material
12.
In FIG. 4, each mini-trace
24 of the trace
20 has the same length
and angle φ. However, the trace
20 can be formed of mini-traces of
varying length and disposition, relative to the underlying glass strands. For example,
one mini-trace might have twice the length of another, and, thus, may be disposed
at a more acute angle, relative to the glass strands. Various arrangements for
the trace
20, other than that depicted in FIG. 4, are possible.
The routing methods commonly used on printed circuit boards in the computer industry
essentially ignore the weave characteristics of the PCB laminate. (One exception
is in the microwave industry, in which PCBs may be routed with traces disposed
at 45° angles relative to the underlying reinforcement material.) The result
is that measured signal velocities, impedances, and signal skews differ substantially
from those of circuit simulations. The trace routing method
300 reduces
the errors in timing, skew, and/or common-mode generation, by more than an order
of magnitude, in some embodiments, such that much greater timing accuracies on
the PCBs can be obtained.
In the flow diagram of FIG. 5, the trace routing method
300 is described,
for optimizing the route path of a trace on a heterogeneous PCB. The trace
20
of FIG. 4, for single-ended mode transmission, is used to illustrate the method
300, although the principles described herein can equally be applied to
differential signal traces, bus signal traces, and any other traces that are routed
on a heterogeneous PCB.
The trace routing method
300 uses a formula to mathematically relate the
worst-case accumulation of periodic variations in dielectric properties (caused
by the heterogeneous composition of the PCB material) to an angle made between
the trace
20 and the strands of the reinforcement material
12 within
the PCB
100. In some embodiments, the method
300 is implemented using
commercially available software for executing mathematical functions.
In some embodiments, the function, r, is used in the trace routing method. Function
r provides the residual length of a trace, that is, the part of the trace that
does not cross over an integer number of strands. The function, r, has three arguments,
L, d, and φ, and is given below:
r(
L, d, φ):=
d/(sin(φ))×fract(
L×(sin(φ)/d)) (1)
where L is the length of a trace portion, d is the distance between the glass
strands within the PCB, and φ is the angle between the trace portion and
the strands. The trace
18C of FIG. 3, which has these characteristics, may
be referred to in understanding the function. Additionally, fract(n) is a function
used to convert a number into its fractional part.
The function, r(L, d, φ), has two parts, a first quantity and the fractional
part of a second quantity. The first quantity provides the length of the trace
portion, L, which covers the distance from the center of one glass strand to the
center of an adjacent glass strand, at the angle, φ. The second quantity
is the fractional part of the total length of the trace, L, divided by the first
quantity. The second quantity indicates how many sections of length, L, can be
fit into a trace. The function takes the fractional result and multiplies it by
the length of the hypotenuse. The result, r, the residual trace length, is how
much length of the trace remains after one or more trace portions fits into the
total trace length. The equation (1) thus determines how much length of the trace
remains when the rest of the trace traverses an integer number of glass strands.
In FIG. 5, the trace routing method
300 is used to obtain the residual
trace length for the trace
20 of FIG. 4. The trace routing method
300
commences by specifying various design parameters for the execution of the residual
trace length function (block
302). Some design parameters are based on the
physical characteristics of the PCB laminate. In particular, the distance, d, between
the glass strands
12 is obtained. Values are assigned to these parameters,
such as by performing measurements. In some embodiments, the values assigned are
reasonable approximations.
Also, the reduction factor, i.e., the factor by which the uncertainty (k
2)
in the D
k is to be reduced is determined (block
304). In other
words, the minimum number of integer strands to be traversed by each mini-trace
24 is selected. For a reduction factor of fifty, the trace portion traverses
fifty glass strands.
The flow diagram of FIG. 5 shows two paths for obtaining the residual trace length.
As one option, the length of a mini-trace
24, L, is selected (block
306).
Using trigonometry, the angle, φ, between the mini-trace and a glass strand,
which yields the desired number of strand crossings is calculated (block
308).
As an alternative, the routing angle, φ, can be selected (block
310),
such that the length, L, of the mini-trace that yields the desired number of strand
crossings is calculated (block
312). Since the optimal trace angle and mini-trace
length for the PCB are not yet known, several selections of the angle, φ,
and the length, L, may be evaluated.
Once the routing angle and mini-trace length are known, the residual trace length,
r, is obtained (block
314), in some embodiments, by executing the function
(1), above. The residual trace length is then multiplied by the number of mini-traces
24 that are in the zig-zag trace
20 (block
316), the number
being selected so that the product is approximately an integer.
The trace routing method
300 thus enables a system designer to design
a trace
20 with an angle, φ, and length, L, such that each mini-trace
24 crosses an integer number of traces, as nearly as possible, for the given
characteristics of the glass strands
12. The trace routing method
300
may be repeated, as many times as desired, by using a different trace angle, φ,
or by using a different mini-trace length, L, until an optimum trace angle and
mini-trace length are obtained. Various embodiments of the trace routing method
300 may also utilize fewer or more steps, and this method may be performed
using a number of different implementations, depending on the application.
In FIG. 6A, the circuits
30 and
32 are interconnected by a pair
of zig-zag traces
50 and
52, for differential mode transmission of
signals. In FIG. 6B, the circuits are connected by a plurality of zig-zag traces
60, for transmitting bus signals. Like the trace
20 for single-ended
mode transmission, the differential and bus signal traces are disposed on the PCB
100 using the trace routing method
300. The trace routing method
300 ensures the optimum placement of traces in relation to the location
of the glass strands within the PCB, such that the dielectric constants for each
of the differential and bus traces are substantially similar.
In a differential signaling environment, the signal travels along multiple signaling
paths. Differential pair traces are typically routed as close to one another as
possible, are the same length, and are equidistant at all points along the traces.
These properties can be achieved with zig-zag traces. The receiver (such as the
circuit
30) is generally sensitive to the differential mode and less sensitive
to the common mode. This is because noise interfering with the signal tends to
impact both (or all) signal traces equally, in other words, in the common mode.
Physical mechanisms, such as varying impedances and signal speeds along the multiple
traces, can cause "common mode conversion" to occur, such that common mode signals
unintentionally become differential mode signals, and vice-versa.
By averaging the D
k for each trace, signal propagation speeds and
impedances
can be controlled. Phenomena, such as common mode conversion, can likewise be minimized
using the trace routing method
300. The method
300 can thus be used
for single-ended mode, differential mode, and bus mode transmission of signals.
The trace routing method
300 determines an optimal routing path for a
trace over a heterogeneous PCB. The zig-zag pattern of the trace
20 ensures
that each mini-trace passes over several strands of the underlying weave pattern,
such that the unknown component of the D
k is minimized. However, the
benefits of the trace routing method
300 can be achieved without zig-zag traces.
For example, in FIG. 7, a heterogeneous printed circuit board
110 is depicted,
which includes a reinforcement material
112, such as woven fiberglass strands,
embedded within a resin-based material
114, and an overlying conductive
material that forms traces
120 and
122. Two circuits
130 and
132, are disposed upon the PCB
110. The circuits
130 and
132
are square or rectangular in shape. These circuits are laid upon the PCB
110
in a non-typical manner. Instead of being laid out orthogonal to the sides of the
boards, the circuits
130 and
132 are disposed such that the sides
of the circuits are not parallel to the sides of the PCB
110. However, the
traces
120 and
122 are orthogonal to the circuits
130 and
132. In other words, the traces are routed in a direction parallel or perpendicular
to the sides of the circuits
130 an
132 (rather than to the sides
of the PCB
110), taking right-angle turns, where needed, in providing an
electrical connection path between the circuits.
The PCB
110 is a typical printed circuit board, with the reinforcement
material
112 typically being aligned at right angles within the PCB material
in a weave pattern. (A cutout of the PCB
110 illustrates the orientation
of the reinforcement material
112.) The weave pattern is substantially orthogonal
to two sides of the PCB and parallel to two sides, although some deviation from
this alignment may occur. By positioning the circuits and routing the traces as
shown in FIG. 7, the trace portions are likely to pass over several strands of
the reinforcement material, such that the unknown component, k
2, of
the D
k is minimized. Further, the D
k for the two parallel
traces
120 and
122 are substantially similar, so as to mitigate common
mode conversion, impedance mismatches, and other undesirable phenomena.
As another possibility, FIG. 8 depicts a printed circuit board
200 in
which
circuits
160 and
162 are disposed in a typical manner, that is, orthogonal
to the sides of the PCB. A cutout of the PCB
200 shows that the orientation
of the embedded reinforcement material
168 is not orthogonal to the sides
of the PCB, as is typically the case. The repositioning of the reinforcement material
168 may be achieved during production of the PCB or by cutting the PCB in
an atypical manner.
Traces
170 and
172 are routed on the PCB
200 as straight-line
or right-angle traces. Since the underlying reinforcement material
168 is
not orthogonal to the sides of the PCB
200, the traces
170 and
172
are likely to cross over several strands of the reinforcement material, minimizing
the effect of the unknown component of the dielectric constant. Further, the D
k
for the traces
170 and
172 are substantially similar.
While the invention has been described with respect to a limited number of
embodiments, those skilled in the art will appreciate numerous modifications and
variations therefrom. It is intended that the appended claims cover all such modifications
and variations as fall within the true spirit and scope of the invention.
*
