Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2000-10-31
2002-11-26
Talbott, David L. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S251000, C174S261000, C174S262000, C174S266000, C361S749000, C361S803000
Reexamination Certificate
active
06486408
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to electrical systems and in particular to reliable multiple path flexible circuit connections.
BACKGROUND
In many industrial systems, such as computer and telecommunications systems, there is a need for making a large number of electrical interconnections between a plurality of points of interest. It is generally desirable to achieve reliable connections (i.e. with low connection failure rates), with good electrical properties, and to be able to dispose such connections within a small area to address space limitations. Various prior art approaches have been attempted which experience limitations in one or more of the above-mentioned characteristics.
One prior art approach is that of Printed Wire Board (PWB) flex circuits. PWB flex circuits are generally easy to manufacture if they have wide lines and traces. However, such circuits are generally difficult to manufacture in volume when fine lines, high density, and/or tight impedance controls are required. Moreover, if tight impedance controls are required, yield losses are generally high in volume production due primarily to variations in the etch processes.
FIG. 1
is a section view of a strip line flex circuit
100
for high speed high density applications according to a prior art solution. Conductive traces
102
are shown with a dashed line. Ground planes
101
and
103
are shown running parallel to traces
102
.
FIG. 2
is a top view of a center line cross section of the strip line flex circuit depicted in
FIG. 1. A
number
200
of parallel traces
201
may be seen in the top view of FIG.
2
. Generally, the width of and spacing between traces
201
may be important features in determining electrical properties of the structure such as characteristic impedance, resistance, skin effect losses, and crosstalk between the traces.
FIG. 3
depicts sectional views
300
of four possible conductive trace geometries. The three columns separated by ideal spaces
306
are generally equivalent. Accordingly, the four trace geometries depicted in the rightmost column
301
will be discussed herein. Trace
302
generally represents an ideal trace geometry, which although highly desirable, is very difficult to achieve in a typical etch process. Trace
303
generally represents a desirable trace geometry which may be achieved with careful process control and with some yield loss. Trace
304
depicts a trace from which an excessive amount of conductive material, such as copper, has been removed. Trace
304
will generally experience excessive direct current (d.c.) resistance, increased skin effect losses, and undesirably high characteristic impedance. Element
305
depicts an under-etched trace. Such an under-etched trace will generally have excessive crosstalk and lower than ideal characteristic impedance.
High speed data cables have been employed to provide interconnection having superior electrical signal characteristics. However, the use of such cabling is generally more expensive to implement for transmission of a given set of signals than is printed wire board flex circuit. Moreover, connectors used to connect such data cables to a board generally provide lower signal density than do flex circuits. Furthermore, such cable connectors are commonly the cause of impedance mismatches, crosstalk, and skew.
In certain cases, multiple rigid PCBs (printed circuit boards) are used to establish electrical connections to system or subsystem units which are not on the same plane. As a result, multiple connectors may be introduced into the signal path, the addition of which generally degrades the quality of signal transmission. In addition, implementing a plurality of rigid PCBs generally adds to system cost, and takes up additional space.
FIG. 4A
is a section view
400
of a via connected to a wire according to a prior art multiwire connection arrangement. Employing this arrangement, via
401
is generally formed onto the end of wire
402
. Wire
402
may be coated with TEFLON® (polytetrafluoroethylene) for insulation purposes. In a connection employing the arrangement depicted in
FIG. 4A
, the available area for establishing a connection between wire
402
and via
401
is generally limited to the cross-sectional area of wire
402
. Some additional contact area may become available where the polytetrafluoroethylene coating is etched back for a finite distance along wire
402
from the outside diameter of via
401
.
FIG. 4B
is a top view of the same connection.
Although the embodiment depicted in
FIGS. 4A and 4B
generally provides the superior electrical properties of discrete wiring, the attachment of wire
402
to via
401
provides a weak mechanical connection between wire
402
and via
401
which is subject to failure if wire
402
is pulled or moved in any direction.
FIG. 5A
is a section view
500
of a via
501
connected to a trace
502
in a printed circuit board arrangement. Employing this arrangement, a hole is drilled in a conductive pad connected to trace
502
, and plating material added to create via
501
.
FIG. 5B
is a top view of the connection depicted in FIG.
5
A. This connection generally provides for a 360 degree connection between plating on via
501
and the copper of trace
502
. This trace-via connection offers a more robust mechanical connection than the connection between the discrete wire and via depicted in FIG.
4
. However, the trace-via connection of
FIG. 5
is subject to the inconsistency in dimensional tolerance and electrical properties discussed in connection with FIG.
3
.
FIG. 6
is a section view of a rigid circuit employing discrete wiring
602
between stripline shield layers
601
according to a prior art embodiment.
FIG. 7
is a top view
700
of a center line cross section of wires
602
depicted in FIG.
6
. Returning to
FIG. 6
, generally, the cross sectional area between stripline shield layers
601
is filled with rigid dielectric material
603
, such as FR
4
or G-Tec. The rigid circuit embodiment of
FIG. 6
benefits from the advantageous electrical performance properties of discrete wiring. However, the embodiment employs the mechanically vulnerable wire to via connection discussed in connection with FIG.
4
A.
Accordingly, it is a problem in the art that PWB flex circuits are difficult to manufacture in high volume and experience and high yield losses due to etch variations.
It is a further problem in the art that it is difficult to generate consistent trace geometries, resulting in inconsistent electrical properties for conductive traces.
It is a still further problem in the art that cables are generally more expensive to implement than flex circuits for the same number of signals.
It is a still further problem in the art that connectors used to attach cables to boards offer lower signal density than PWB flex circuits, and commonly cause impedance mismatches, crosstalk, and skew.
It is a still further problem in the art that the deployment of multiple PCBs cause added cost, take up additional space, and cause degraded performance because of the implementation of multiple connectors along individual signal paths.
It is a still further problem in the art that the connection of discrete wires to plated vias generally mechanically weak and subject to failure when the wire is pulled or moved.
SUMMARY OF THE INVENTION
The present invention is directed to a system and method which provides consistently high quality electrical performance characteristics in combination with robust mechanical attachment between conductors and electrical junctions. Preferably, discrete wires are connected to conductive pads employing either laser welded or ultrasonically bonded wire joints between the wires and pads. A plurality of layers having conductive pads may be combined to form a multiple layer flex circuit. Holes may then be drilled through the conductive pads in the traditional manner and then plated to create vias.
Preferably, electrically reliable discrete wires may be employed and still benefit from the mechanically robust connection created b
Hewlett--Packard Company
Patel I B
Talbott David L.
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