Direct wire attach

An assembly has a conductive trace on a substrate and at least one conductor electrically coupled to the trace. First and second gaps arranged such that one gap is on either side the trace, allowing control of electrical characteristics of a signal path formed of the conductor and the trace.

BACKGROUND

When mating two conductors of electricity, impedance and other parameters may become mismatched due to the transitions from one conductor to the other, as well as characteristics that vary between the conductors. Parameter mismatch may lead to signal degradation and losses, resulting in a lower quality signal path than desired. This may impact the speed and performance of electrical circuits.

An example of such a mismatch occurs when a conductor in a cable, such as a co-axial or twinaxial cable, connects to a printed circuit board trace. As the signal travels the signal path, the path characteristics of the conductor may vary as the path travels a portion of the cable that has a jacket and shield, to a portion that just has the shield, to a portion of the cable that lies bare. Further transitions occur when the conductor bends to connect to the trace on the PCB. All of these transitions may result in a characteristic mismatch that in turn has an effect on signal quality.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1shows an embodiment of an assembly formed from a coaxial cable and a printed circuit board. The coaxial cable has a jacket16, shielding18, a dielectric20and a center conductor22. The printed circuit board10has a circuit trace12and a ground trace14separate by a gap28. As can be seen in the figure, the coaxial cable is laid down on top of the circuit board and the conductor22must be bent to be connected to the circuit trace12. The conductor and shield are connected to the circuit and ground traces by solder24and26.

The signal path of the conductor22and12must go through several different environments, each of which may affect a particular signal characteristic. A signal characteristic, as that term is used here, may include such characteristics as impendence, insertion loss, return loss, cross talk, and s-parameters. S-parameters can be defined for any collection of linear electronic components. They are algebraically related to the impedance parameters and to the admittance parameters, and to a notional characteristic impedance of the transmission lines.

The signal path transitions from an environment in which is it shielded by a jacket, the shield and the dielectric, to an environment where it is just surrounded by the shield and the dielectric, to an environment where it is surrounded by the dielectric to an environment where it is surrounded by air. Further, in the environment where the signal path is surrounded only by air, the signal path curves to attach to the trace of the circuit board. It then transitions from the conductor to the trace itself. Generally, the impedance or other signal characteristic is not controlled or managed in any of these transitions, resulting in characteristic mismatches that can degrade signal quality.

It is possible to control the signal characteristics in these transitions and to manufacture the assembly such that the signal characteristics of the coax cable are minimally affected by transitions in the signal path. A side view of such an assembly is shown inFIG. 2. InFIG. 2, the substrate10has upon it a circuit trace12and a ground trace14. In embodiments of this invention, the gap28ofFIG. 1does not exist as the bottom of the condutor is planar with the top of the circuit trace. It must be noted that the substrate10was referred to as a printed circuit board inFIG. 1, but substrate10may be any substrate, including ceramic, dielectric, flex circuitry, etc.

A notch, discussed in more detail later, has been formed in the substrate, such that the coaxial cable can be positioned to control the signal characteristics in the conductor22. The jacket16has been stripped back to expose the shield18. Further, the shield is electrically coupled and planar with to the ground plane trace14, in this instance by solder26. It must be noted that the dielectric shown inFIG. 1is not exposed in embodiments of this invention because it is trimmed to the length of the shield. Similarly, the conductor22has been electrically coupled to the conductive circuit trace12, in this instance by solder24, but may be attached in many different ways, such as by electrically conductive paste or any other method of forming electrical connections.

As will be seen in further discussion, each of the transitions is modeled and manufactured with a predetermined characteristic as part of the manufacture. For example, in the assembly ofFIG. 2, the impedance may be controlled throughout each transition from jacket, to shield, to solder, to trace, allowing control of a desired level of impedance such as 50 ohms. This alleviates problems with impedance mismatch that can result in degradation of signals.

FIG. 3shows an end view of an assembly similar to that ofFIG. 2. A field solver or other such tool may used to model a conductor22, trace12and solder24and26. The resulting conductor model is then used to determine a width x for the gaps, depending upon a particular signal characteristic. Carrying through with the previous signal characteristic of impedance, the width of the gaps may determined based upon a given impedance value, such as 50 ohms. Other typical values may be 60 ohms, 100 ohms or 120 ohms, as examples.

One aspect of the assembly that allows this control is the positioning of the cable relative to the substrate. An alternative embodiment of the assembly is shown inFIG. 4, in which the shielding connects to grounds on one or more planes penetrating into the substrate10. In this embodiment, the conductor22is soldered or otherwise joined to the trace12by solder24, but the shield18is coupled to the ground planes14within the substrate10. Alternatively, the ground plane14may be the bottom layer of the substrate10.

In order to enable this connection, it may be necessary to remove selected portions of the substrate to expose a ground plane. In one embodiment, shown as a top view inFIG. 5, the other layers28of the substrate10are removed to expose the ground plane layer14in a comb/teeth configuration. The ground plane layer14may have tabs formed in it by cutting gaps such as30in the ground plane. This allows the tabs to be bent up out of their plane to allow an easier connection to be made to the shielding18ofFIG. 3, if necessary. These tabs may be formed by laser cutting, among other methods.

FIGS. 6aand6bshow top views of a substrate prepared to receive a conductor such as that shown inFIG. 2. The substrate in the example ofFIG. 6ais a ‘paddle’ board used to then insert into a receptacle attached to another printed circuit board. The substrate10has a notch32cut into it; a portion of the notch may then be covered with a conductive material. It should be noted that the gaps34and36have portions38and40at which the gaps are wider than when surrounding the trace portion44. Trace portion44is a continuation of trace12of previous figures. As will be discussed further, the control of these gaps in the transition area contributes to the control of the desired electrical characteristic to meet a targeted value. These gaps form a co-planar waveguide at the termination of the cable adjacent to the cable shield and conductor, such as the co-planar waveguides discussed in U.S. Pat. No. 6,878,901, issued on Apr. 12, 2005, but may transition to a microstrip or other structure in which the signal characteristics are controlled.

The trace44traverses the substrate10to electrically couple to the paddle finger42in the example ofFIG. 6a.The goal is to control the characteristic from the edge of the substrate10to the paddle finger42. The configuration ofFIG. 6ashows a region in which the conductor will couple to the trace that is of a varying size.

FIG. 6bshows an embodiment in which the fingers are of a uniform width, and the region of the trace to which the conductor will couple is of uniform width. In this instance, the trace on the substrate is engineered to be a constant width line with variable impedance. The impedance is varied by the width of the gaps34and36at the regions38and40. The variable impedance trace becomes a constant impedance signal path when the center conductor is connected to the trace.

This characteristic may change with the addition of the conductor, as will be discussed further. An embodiment of an assembly using direct wire attach is shown inFIG. 7. The center conductor22couples to the trace12, which cannot be seen as it is covered by the center conductor22in this view, such that region44of the trace traverses the board. The dielectric20, shield18and jacket16partially obscure the portions of the substrate that were seen inFIG. 6a.The shield may be coupled using solder as discussed previously, the solder not shown here to simplify the drawings. The variation in the width of the gaps34and36widths adjacent the region where the center conductor couples to the trace shown by reference numbers38and40allows control of the desired electrical characteristic of the signal. An embodiment of a process of manufacturing this and similar assemblies is shown inFIG. 8.

It must be noted that the use of a coax cable is for ease of discussion only. As will be discussed further, application of the embodiments of the invention may be made to twinax cables, as well as many other types of cables that have conductors with and without shielding.

FIG. 8shows a flowchart of an embodiment of a method of manufacturing an assembly using direct wire attach. Initially, both the cable and the substrate are prepared. These sub-processes may be performed simultaneously, or serially, with either sub-process being first or second.

At60, a substrate is prepared by forming traces and gaps adjacent those traces, including controlling the width of the gaps adjacent the region where the conductor and the trace are to couple. For shielded cables, a notch may be formed in the substrate at62. If a notch is formed, portions of the notch may be covered with a conductive material. For example, vertical portions of the notch may be plated with a conductive material to allow the shield to be electrically coupled to the ground plane or planes of the substrate. This is an optional process and may be unnecessary for cables without shielding.

To prepare the cable, the jacket is generally stripped back at70to expose the shield, wherein the length of the shield exposed is based upon the length of the notch and the center conductor is also a predetermined length. At72, the conductor is exposed of a predetermined length to match the desired electrical characteristic for which the variable impedance line was formed. If the cable does not have shielding, the jacket is merely stripped to expose the conductor. As previously mentioned, the dielectric of the cable is not exposed in embodiments of this invention.

The contacts are then attached at76and78, such as by soldering. While the order shown is to attach the shield and then the conductor, it is possible that these may be done in either order. Further, if the cable has no shielding, then only the center conductor is attached. Other methods of attaching the contacts are of course possible, such as conductive paste, etc.

In addition to cables having shields or not, embodiments of the invention may be applied to different configurations of conductors, including multiple conductors, differential signaling conductors, etc.FIG. 9shows an embodiment of multiple, single-ended conductors. In this instance each conductor is surrounded by gaps, between the conductors and ground. Generally, the gap dimension x will be uniform for the different gaps.

FIG. 10shows an embodiment of differential signaling conductors, with two gap dimensions. Gap dimension y is used for common mode between the conductors and ground, and gap dimension x is between the differential signal conductors. The control of these dimensions in turn controls the signal path to match the desired electrical characteristic.

Similarly, differential signaling could be configured on either side of the substrate10. In this case, the gaps are controlled between each signal trace and its dual ground references, such as between signal trace112and the ground traces110and114. This allows for differential signaling to be configured in this manner, yet still be controlled for a particular signal characteristic.

Thus, although there has been described to this point a particular embodiment for a method and apparatus for controlling signal characteristics through transitions in an assembly, it is not intended that such specific references be considered as limitations upon the scope of this invention except in-so-far as set forth in the following claims.