Multiple layer printed circuit board with unplated vias

A printed circuit board (PCB) stack-up has a signal via configured to transmit a signal through at least two different layers of the PCB stack-up, a reference structure that is at least a portion of a return path for the signal; and an unplated via disposed in an area surrounding the signal via. The unplated via is disposed in the area surrounding the signal via to improve the characteristic impedance of the signal via.

FIELD

The present embodiments relate to printed circuit boards and, more particularly, to printed circuit boards having multiple layers.

BACKGROUND

Electrical signals may be transmitted on a transmission line of a printed circuit board (PCB). The transmission line may be a single trace or may be a differential pair of traces. The transmission line may extend over multiple layers of the printed circuit board. Plated vias may connect parts of transmission lines on the different layers.

DETAILED DESCRIPTION

Overview

A printed circuit board (PCB) stack-up includes a signal via configured to transmit a signal through at least two different layers of the PCB stack-up, a reference structure that is at least a portion of a return path for the signal; and an unplated via disposed in an area surrounding the signal via. The unplated via is disposed in the area surrounding the signal via to improve a characteristic impedance of the signal via.

DESCRIPTION OF EXAMPLE EMBODIMENTS

The present disclosure describes a PCB stack-up that includes one or more unplated vias positioned in areas surrounding a signal via. The signal via may connect signal traces disposed in or on different layers of the PCB stack-up. The unplated vias may extend at least partially through the PCB stack-up and may be disposed in between or substantially in between the signal via and a reference structure that functions as a return path. In addition or alternatively, the unplated vias may be disposed in between or substantially in between two signal traces, such as a differential pair of signal traces. The unplated vias may reduce an effective dielectric constant in the area surrounding the signal vias, which may increase the characteristic impedance of the signal vias, and in turn, improve impedance matching between the signal vias and the signal traces.

FIG. 1illustrates a cross-section of an example apparatus that includes a printed circuit board (PCB)100having multiple layers. The PCB100having multiple layers may be referred to as a stack of PCB layers or a PCB stack-up. A layer of the PCB stack-up may be a planar structure of insulating or partially insulating material having opposing planar surfaces where at least one of the opposing planar surfaces is adjacent to and/or facing a ground reference. Insulating or partially insulating material may include, but is not limited to including, fiberglass, epoxy, glass, resin, or a combination thereof. In addition, a layer of the PCB stack-up may be a substrate between a transmission line carrying a signal and one or more ground references. The ground references may be planar structures that are aligned or substantially aligned with the PCB layers. Further, the ground references may be parallel or substantially parallel with each other. The ground references may be made of an electrically conductive material, such as copper, gold, silver, platinum, or any other conductive material.

The example PCB stack-up100may include six layers102, including a first layer102a, a second layer102b, a third layer102c, a fourth layer102d, a fifth layer102e, and a sixth layer102f. The number of layers that may be included in the PCB stack-up100may vary, with a minimum number being two layers. In addition, the layers102a-102fmay be adjacent to and/or separated by one or more ground references. The first layer102ahas a first surface104aopposing a second surface104b. The second surface104bis adjacent and/or facing a first ground reference116. The second layer102bhas a first surface106aopposing a second surface106b. In addition, the first surface106ais adjacent and/or facing the first ground reference116. The second surface106bis adjacent and/or facing a second ground reference118. The third layer102chas a first surface108aopposing a second surface108b. In addition, the first surface108ais adjacent and/or facing the second ground reference118. The second surface108bis adjacent and/or facing a third ground reference120. The fourth layer102dhas a first surface110aopposing a second surface110b. In addition, the first surface110ais adjacent and/or facing the third ground reference120. The second surface110bis adjacent and/or facing a fourth ground reference122. The fifth layer102ehas a first surface112aopposing a second surface112b. In addition, the first surface112ais adjacent and/or facing the fourth ground reference122. The second surface112bis adjacent and/or facing a fifth ground reference124. The sixth layer102fhas a first surface114aopposing a second surface114b. In addition, the first surface114ais adjacent and/or facing the fifth ground reference124.

The first layer102aand the sixth layer102fmay be considered outer layers. An outer layer may be a layer where substantially only one of the opposing planar surfaces (e.g., only one of the first surface and the second surface) is adjacent a ground reference. The other of the opposing planar surfaces, i.e., the surface that is not adjacent to a ground, may be exposed to the environment surrounding the PCB stack-up, such as air. For example, the second surface104bof the first layer102ais adjacent the first ground reference116, and the first surface104aof the first layer102ais exposed to the surrounding environment (e.g., air, soldermask, etc.). As another example, the first surface114aof the sixth layer102fis adjacent the fifth ground reference124, and the second surface114bof the sixth layer102fis exposed to the surrounding environment (e.g., air, soldermask, etc.).

The second layer102b, the third layer102c, the fourth layer102d, and/or the fifth layer102emay be considered inner layers. An inner layer may be a layer where both of the opposing planar surfaces (e.g., both the first surface and the second surface) are adjacent a ground reference. For example, the first surface106aof the second layer102bis adjacent the first ground reference116and the second surface106bof the second layer102bis adjacent the second ground reference118. The first surface108aof the third layer102cis adjacent the second ground reference118, and the second surface108bof the second layer102bis adjacent the third ground reference120. The first surface110aof the fourth layer102dis adjacent the third ground reference120, and the second surface110aof the fourth layer102dis adjacent the fourth ground reference122. The first surface112aof the fifth layer102eis adjacent the fourth ground reference122, and the second surface112aof the fifth layer102eis adjacent the fifth ground reference124.

The PCB stack-up100may include one or more transmission lines that are configured to carry one or more signals. A signal may be an analog or a digital signal and/or comprise an analog or digital waveform. In addition or alternatively, the signal may be an alternating current (AC) signal, a direct current (DC) signal. The AC signal may be a radio frequency (RF) signal having any suitable frequency, as for example a frequency of about 3 kHz or higher. The DC signal may be a power signal used to power one or more active devices (not shown) in communication with the PCB stack-up100. An active device, such as the active device described below with reference toFIG. 7, may include an integrated circuit (IC), such as a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC), a transistor, a controller, a memory chip, a diode, an operational amplifier, or any other electronic device, circuit, or component that may require power, such as DC power, to operate.

A transmission line may comprise at least two conductors, including a signal conductor, as for example conductive traces126and128in connection with a signal via148, and a return conductor, as for example a ground via146. In the PCB stack-up100, the signal conductor may include one or more conductive traces, as for examples the conductive traces126,128. The conductive traces may comprise any conductive material, such as copper, gold, silver, or platinum, as examples. The conductive traces may be manufactured using masking and etching techniques or milling techniques, as examples. The one or more conductive traces may be disposed on or in a layer of the PCB stack-up100. For example, a conductive trace may be disposed on a first surface or a second surface of an outer layer of the PCB stack-up100. In addition, the first surface or the second surface may be a surface of the outer layer that is exposed to the environment (e.g., air, soldermask, etc.). For example, inFIG. 1, a conductive trace126is disposed on the first surface104aof the first layer102a. Where the conductive trace is disposed on a surface of an outer layer and is exposed to the environment (e.g., air, soldermask, etc.), a transmission line comprising the conductive trace may have a microstrip configuration. A microstrip configuration may refer to a transmission line configuration where the conductive trace is separated from a ground reference by the substrate, and where the conductive trace is exposed to the environment (e.g., air, soldermask, etc.). To illustrate, the conductive trace126, the first layer102a, and the first ground reference116may form a microstrip transmission line configuration.

In addition, the one or more conductive traces included in PCB stack-up100may be disposed in a layer of the PCB stack-up100. For example, the one or more conductive traces may be disposed in between two of the ground references. InFIG. 1, a conductive trace128is disposed in the third layer102cand in between the second ground reference118and the third ground reference120. In addition, inFIG. 1, two conductive traces130a,130bare disposed in the second layer102band in between the first ground reference116and the second ground reference118. Also, inFIG. 1, two conductive traces132a,132bare disposed in the third layer102cand in between the second ground reference118and the third ground reference120. Where one or more conductive traces are disposed in a layer and in between two ground reference planes, the one or more conductive traces, the layer, and the two ground references planes may be configured in a stripline transmission line configuration.

A transmission line that includes only one conductive trace as the signal conductor may be a single-ended transmission line and the single conductive trace may be a single-ended trace. A single-ended transmission line may be configured in a microstrip configuration or a stripline configuration. Alternatively, a transmission line may include two conductive traces, or a pair of conductive traces, and may be a differential transmission line. The conductive traces of the may be differential conductive traces. A pair of signals may be transmitted over the differential transmission line and together may be referred to as a differential signal. The pair of signals of the differential signal may be equal and opposite in magnitude, or substantially equal and opposite in magnitude, and/or equal or substantially equal in phase. In addition, the differential transmission line may be configured in a microstrip configuration or a stripline configuration.

FIG. 1shows two separate portions, a first portion134and a second portion136, of the PCB stack100. The first portion134may include transmission lines configured in a differential configuration. A first differential transmission line may include the pair of traces130aand130band may be configured to transmit a differential signal. The pair of traces130aand130bare disposed in the second layer102band in between the first ground reference116and the second ground reference118. The pair of traces130aand130bbeing disposed in the second layer102band in between the first and second ground references116,118may comprise a stripline configuration. A second differential transmission line may include the pair of traces132aand132band may be configured to transmit a differential signal. The pair of traces132aand132bare disposed in the third layer102cand in between the second ground reference118and the third ground reference120. The pair of traces132aand132bbeing disposed in the third layer102cand in between the second and third ground references118,120may comprise a stripline configuration.

The second portion136of the PCB stack-up100may include transmission lines configured in a single-ended configuration. A first single-ended transmission line may include the single-ended conductive trace126. The single-ended conductive trace126is disposed on the first surface104aof the first layer102aand separated from the first ground reference116by the first layer102a. The single-ended trace126being disposed on the first surface104aof the first layer102aand being separated from the first ground reference116by the first layer102amay comprise a microstrip configuration. A second single-ended transmission line may include the single-ended conductive trace128. The single-ended conductive trace128is disposed in the third layer102cand is disposed in between the second ground reference118and the third ground reference120. The single-ended conductive trace128being disposed in the third layer102cand being disposed in between the second ground reference118and the third ground reference120may comprise a stripline configuration.

Alternative embodiments of the PCB stack-up100may include substantially only the first portion134, substantially only the second portion136, multiple first portions134, multiple second portions136, or one or more combinations thereof. Other alternative embodiments of the PCB stack-up100may include alternative configurations or different combinations of the transmission lines shown inFIG. 1. For example, an alternative PCB stack-up may include a differential transmission line configured in a microstrip configuration. As another example, an alternative PCB stack-up may not have a transmission line, single-ended or differential, that is configured in a microstrip configuration. As another example, an alternative PCB stack-up may not have a transmission line, single-ended or differential, that is configured in a stripline configuration. The alternative PCB stack-up may include one or more transmission lines that are configured in microstrip configurations, where one or more of the conductive traces, configured as single-ended traces and/or differential traces, are disposed on planar surfaces of the outer layers that are exposed to the environment (e.g, air, soldermask, etc.). An alternative PCB stack-up may include any number of layers, where there are at least two layers. In addition, conductive traces may be disposed in any of the layers. For example, inFIG. 1, in addition to the conductive traces128,130a,130b,132a,132bbeing disposed in the second and/or the third layers118,120, one or more of the conductive traces128,130a,130b,132a,132b, or alternatively other conductive traces may disposed in other layers, such as the first layer102a, the fourth layer102d, the fifth layer102e, and/or the sixth layer102f.

Conductive traces that are disposed in or on different layers, or alternatively on different opposing surfaces of the same layer, may be interconnected through one or more plated vias. Similarly, two different ground references may be connected to each other by each being connected to one or more plated vias. Herein, the phrase “connected to” or “connected with” is defined to mean directly connected to or indirectly connected through one or more intermediate components/conductive materials, unless otherwise specifically described.

A via may be a hole, an opening, or a spacing that extends partially or completely through one or more layers of the PCB stack-up100. A via that extends completely through at least one layer may be referred to as a through-layer via. In one example, the via may extend completely through all of the layers of the PCB stack-up100. The via may extend perpendicular or substantially perpendicular to one or both of the opposing planar surfaces of one or more of the layers. A plated via may be a via that is electrically conductive over an entire length of the via. Alternatively, the plated via may be electrically conductive over a length that is less than the entire length of the via and non-conductive over a remaining length of the via. Alternatively, the plated via may have two or more non-contiguous electrically conductive portions that are separated by non-conductive portions. The via may be electrically conductive by having inner walls that are plated with an electrically conductive material, such as copper, gold, silver, or platinum. Other electrically conductive materials may be used. A non-plated via may be a via that does not have electrically conductive inner walls, or at least does not have an amount of conductive plating that is sufficient to connect together any two conductive structures, such as traces and/or ground references, that are disposed in different layers or separated by one or more layers.

In one example of the PCB stack-up100, at least one plated via may be configured to connect two or more different ground references together and/or two or more different conductive traces together. Alternatively or in addition, the plated vias may be configured to not connect any ground reference with any signal conductor of a transmission line. A via that connects two or more ground references may be referred to as a ground via. A via that connects two or more signal conductors or conductive traces may be referred to as a signal via.

The PCB stack-up100includes a plurality of plated vias. At least one of the plurality of plated vias comprises a signal via. In addition, at least one of the plated vias comprises a conductive structure that is in a proximity of the signal via to be a reference for a signal propagating through the signal via. As previously mentioned, a transmission line may include at least two conductors, such as a signal conductor and a return conductor. For a signal to propagate along the transmission line, the signal conductor and the return conductor may be connected in a loop to form a circuit. Additionally, a complete path for transmission of a signal may include multiple transmission lines connected by one or more signal vias, such as in a multi-layer PCB stack-up. A signal propagating along the path may propagate along a first signal conductor of a first transmission line that is in or on one layer, may travel through and/or enter into a signal via connected to the first signal conductor, and may exit from the signal via and/or transition to a second signal conductor that is connected to the signal via, the second signal conductor being part of a second transmission line in or on a different layer, and propagate along the second signal conductor. The signal via may have an input or “signal in” point, position, or location, where the first signal conductor is connected to the signal via and the signal enters the signal via. Similarly, the signal via may have an output or a “signal out” point, position, or location, where the second signal conductor is connected to the signal via and the signal exits the signal via. An alternative multi-layer PCB stack-up may comprise a multi-drop configuration, which may include one input or “signal in” point, position, or location and multiple outputs or “signal out” points, position, or locations. In the multi-drop configuration, the signal may enter the via at the “signal in” position of the signal via and exit the via at the multiple “signal out” positions of the signal via.

A complete path, similar to the individual transmission lines, may include a signal conductor path and a return conductor path. The signal conductor path may include the signal conductors of the transmission lines and the one or more signal vias that the signal conductors are connected to. The signal conductor path and the return conductor path may be connected in a loop to form a circuit. A signal propagating along the signal conductor path may couple to a conductive structure, or one or more of a plurality of conductive structures, adjacent the signal via to form or create at least part of the return conductor path. The conductive structure may be the nearest or one of the nearest conductive structures to the signal via. In addition or alternatively, the conductive structure may extend in substantially the same direction as the signal via. The conductive structure may or may not be connected to one or more ground references. Whether or not the conductive structure is connected to one or more ground references, the conductive structure may provide a reference structure for the signal propagating along the signal conductor path. A conductive structure that is in the proximity of the signal via may be a conductive structure that is at least part of the return conductor path because the conductive structure is a sufficient distance to the signal via for the signal propagating along the signal via to substantially couple to the conductive structure. Alternatively, a conductive structure that may not be a conductive structure in the proximity of the signal via is a sufficient distance away from the signal via for the signal to substantially couple to the conductive structure. The conductive structure that is in the proximity of the signal via may be a ground via. A ground via may be a plated via that is connected to one or more ground references. However, the conductive structure may be of a different type other than a ground via, such as a via that is configured to supply a power signal, for example. Hereinafter, a conductive structure that may be disposed in the proximity of the signal via to be at least a portion of the return conductor path may be referred to as a reference structure.

The PCB stack-up100inFIG. 1shows six plated vias, including a first via138, a second via140, a third via142a fourth via144, a fifth via146, and a sixth via148. The number of vias in the PCB stack-up100may generally vary. It should be appreciated that additional vias may include, and that such vias may include a signal via and a reference structure that is in the proximity of the signal via such that a return path is effectively formed. The second via140, the third via142, and the sixth via may be signal vias. The second via140may be connected to the trace130adisposed in the second layer102band to the trace132adisposed in the third layer102c. The trace130amay be connected to the second via140and in connection with the trace132aby being connected to conductive plating150that is disposed along an inner wall152of the second via140. Similarly, the third via142may be connected to the trace130bdisposed in the second layer102band to the trace132bdisposed in the third layer102c. The trace130bmay be connected to the third via142and in connection with the trace132bby being connected to conductive plating154that is disposed along an inner wall156of the third via142. Likewise, the sixth via148may be connected to the trace126disposed on the first surface104aof the first layer102aand to the trace128disposed in the third layer102c. The trace126may be connected to the sixth via148and in connection with the trace128by being connected to conductive plating158that is disposed along an inner wall160of the sixth via148.

The first via138, the fourth via144, and the fifth via146may be reference structures that are in proximity to signal vias140,142, and/or148such that the vias138,144,146are at least part of the return conductor paths. In the example PCB stack-up100, the first via138, the fourth via144, and the fifth via146are ground vias. However, in other example PCB stack-ups, the first via138, the fourth via144, and/or fifth via146may be different types of reference structures that are in the proximity of the signal vias140,142, and/or148, such as power supply vias as previously described.

Additionally, there may be more or fewer reference structures than the first via138, fourth via144, and the fifth via146that are shown. For example, in the first portion134, there may be a single reference structure, e.g., either the first via138or the fourth via144, that is at least part of the return path for a differential signal propagating along differential traces130a,130band/or differential traces132a,132b. In addition, there may be more reference structures than the first via138and the fourth via144that are return paths for the differential signal. Similarly, in the second portion136, there may be more reference structures other than the fifth via146. The other reference structures may comprise ground vias, other types of reference structures such as power supply vias, or combinations thereof.

The first via138, the fourth via144, and the fifth via146, configured as ground vias, may each be connected to at least one of the ground references116-124. In the PCB stack-up100shown inFIG. 1, the first, fourth, and fifth vias138,144,146are each connected to all of the ground references116-124. The ground references116-124may be connected to the first via138and in connection with each other by being connected to conductive plating162that is disposed along an inner wall164of the first via138. Similarly, the ground references116-124may be connected to the fourth via144and in connection with each other by being connected to conductive plating166that is disposed along an inner wall168of the fourth via144. Likewise, the ground references116-124may be connected to the fifth via146and in connection with each other by being connected to conductive plating170that is disposed along an inner wall172of the sixth via148.

FIG. 1shows the conductive platings162,166,170of the first, fourth, and fifth vias138,144,146extending an entire length of the PCB stack100, from the first surface104aof the first layer102ato the second surface114bof the sixth layer102f. In other example PCB stack-ups, the conductive plating162of the first via138, the conductive plating166of the fourth via144, and/or the conductive plating170of the fifth via146may extend less than the entire length of the PCB stack-up100. Where the conductive platings162,166, and/or170extend to a surface of an outer layer that is exposed to the surrounding environment, the conductive plating may extend to surface pads disposed on the surface. A surface pad may comprise a conductive material, such as copper, gold, silver, platinum, or any other conductive material, that is the same or a different conductive material as the conductive material of the conductive plating. For example, inFIG. 1, the conductive plating162of the first via138extends and/or is connected to a first surface pad174adisposed on the first surface104aof the first layer102a. The conductive plating162of first via138also extends to and/or is connected to a second surface pad174bdisposed on the second surface114bof the sixth layer102f. Similarly, the conductive plating166of the fourth via144extends and/or is connected to a third surface pad174cdisposed on the first surface104aof the first layer102a. Also, the conductive plating166of fourth via144extends and/or is connected to a fourth surface pad174ddisposed on the second surface114bof the sixth layer102f. Likewise, the conductive plating170of the fifth via146extends and/or is connected to a fifth surface pad174edisposed on the first surface104aof the first layer102a. Also, the conductive plating170of fifth via146extends and/or is connected to a sixth surface pad174fdisposed on the second surface114bof the sixth layer102f. Similarly, the conductive platings150,154,156of the second, third, and sixth vias140,142,148may extend to an exposed surface of one of the outer layers and to a surface pad. The conductive plating150may extend and/or is connected to a seventh surface pad174g. The conductive plating154may extend and/or is connected to an eighth surface pad174h.

In some example embodiments, the conductive plating of the signal vias, e.g., the conductive plating150of the second via140, the conductive plating154of the third via142and/or the conductive plating158of the sixth via148, may extend a length that is less than an entire length of the PCB stack-up100, from the first surface104aof the first layer102ato the second surface114bof the sixth layer102f. For example, inFIG. 1, the conductive plating150of the second via140extends from the first surface104aof the first layer102ato a position within the PCB stack-up100that is in between the conductive trace132ato the second surface114bof the sixth layer102f. Similarly, the conductive plating156of the third via142extends from the first surface104aof the first layer102ato a position within the PCB stack-up100that is in between the conductive trace132bto the second surface114bof the sixth layer102f. Also, the conductive plating160of the sixth via148extends from the first surface104aof the first layer102ato a position within the PCB stack-up100that is in between the conductive trace132bto the second surface114bof the sixth layer102f. In alternative example embodiments, one or more of the conductive platings150,154may extend from the second surface114bof the sixth layer102fto a position in between the conductive traces130a,130band the first surface104aof the first layer102a. In other alternative example embodiments, one or more of the conductive platings150,154may extend from a position in between the first surface104aof the first layer102aand conductive traces130a,130bto a position in between the second surface114bof the sixth layer102fand the conductive traces132a,132b.

During manufacture of the PCB stack-up100, a plating process that is used to plate the vias may plate the signal vias150,154,158for the entire length of the PCB stack-up100, from the first surface104aof the first layer102ato the second surface114bof the sixth layer102f. Depending on the disposition of the conductive traces130a,130b,132a,132b,126, and/or128in the PCB stack-up100, one or more portions of the conductive plating150,154,158that extend to the first surface104aof the first layer102aand/or to the second surface114bof the sixth layer102fmay not be needed to connect to or more conductive traces. For example, inFIG. 1, the conductive platings150,154of the second and third vias140,142connect the differential traces130a,130bdisposed in the second layer102bwith the differential traces132a,132bdisposed in the third layer102c. Because there are no traces disposed in the other layers (i.e., the first layer102a, the fourth layer102d, the fifth layer102e, or the sixth layer102f) and/or on the first surface104aof the first layer102aand/or on the second surface114bof the sixth layer102f, portions of the conductive plating150extending from the differential pair of traces130a,130bto the first surface104aof the first layer102aand/or portions of the conductive plating154extending from the differential pair of traces132a,132bto the second surface114bof the sixth layer102fmay not be needed for a differential signal to propagate from the differential traces130a,130b, through the second and third vias140,142, and to the differential traces132a,132b. Similarly, the conductive plating158of the sixth via148connects the trace126disposed on the first surface104aof the first layer102ato the trace128disposed in the third layer102c. Because there are no traces disposed in the other layers (i.e., the fourth layer102d, the fifth layer102e, or the sixth layer102f) and/or on the second surface114bof the sixth layer1021, portions of the conductive plating158extending from the trace128to the second surface114bof the sixth layer102fmay not be needed for a signal to propagate from the trace126, through the sixth via148, and to the trace128.

Portions of the conductive platings150,154, and/or158that are not needed for signal propagation may be referred to as via stubs. In addition to via stubs not being needed for signal propagation, via stubs may also be undesirable elements of the PCB stack-up100. To illustrate, a differential signal may propagate along differential traces in one layer, transition through a pair of vias, and propagate along differential traces in a different layer. As the differential signal transitions from the vias to the differential traces in the other layer, some of the energy of the differential signal may travel into one or both of the via stubs. Via stubs may create an impedance mismatch in the signal path, which may result in a reflection and/or energy loss of the signal. Electrically, a via stub may effectively function as a capacitor and serve to introduce a capacitance in the signal path, which may couple energy from the differential signal. The amount of signal loss of the differential signal may be dependent upon, but is not limited to being dependent upon, a length of the via stub. For example, as a length of the via stub increases, more loss of the differential signal may occur.

To reduce the length of a via stub, a conductive plating removal process such as a back drilling process may be performed. The back drilling process may be performed after plating and may involve drilling partially through the plated vias to remove unnecessary and/or unwanted portions of the conductive plating. In some situations, the back drilling process may not remove all of the via stub material due to imprecision of the back drilling process. For example, the back drilling may avoid drilling too close to the signal path in order to avoid damaging or destroying the signal path. At least some of the via stubs may remain after performing the back drilling process.

FIG. 1shows unplated portions176a,176b,176cand via stubs178a,178b,178c, which may be illustrative of unplated portions and via stubs, respectively, of signal vias after plating and back drilling processes are performed on the PCB stack-up100. InFIG. 1, the unplated portion176aof the second via140extends from the second surface114bof the sixth layer102fto a position in between the second surface114bof the sixth layer102fand the conductive trace132a. Also, the via stub178aof the second via140extends from the position in between the second surface114bof the sixth layer102fand the conductive trace132ato the conductive trace132a. Similarly, the unplated portion176bof the third via142extends from the second surface114bof the sixth layer102fto a position in between the second surface114bof the sixth layer102fand the conductive trace132b. Also, the via stub178bof the third via142extends from the position in between the second surface114bof the sixth layer102fand the trace132bto the trace132b. Additionally, the unplated portion176cof the sixth via148extends from the second surface114bof the sixth layer102fto a position in between the second surface114bof the sixth layer102fand the conductive trace128. Also, the via stub178cof the sixth via148extends from the position in between the second surface114bof the sixth layer102fand the trace128to the trace128.

In other example embodiments, one or more of the signal vias140,142, and/or148may not have an unplated portion, for example, because a back drilling process may not be performed. Alternatively, unplated portions of the signal vias176a,176bmay also include one or more unplated portions extending from the first surface104aof the first layer102ato a position in between the first surface104aof the first layer102ato one or more of the traces130a,130b. As will be appreciated by those skilled in the art, other configurations may be possible.

Conductive plating of a signal via may extend from one layer to one or more other layers. As a result, the conductive plating of the signal via may extend through one or more areas of the PCB stack-up100that is coplanar or otherwise occupied by one or more ground reference planes. If the signal via is connected to one or more of the ground reference planes, the signal via may be shorted to ground. To avoid the signal via being shorted to ground, portions of the ground reference planes intersecting with the signal vias may be removed or “cutout.” The portions to be removed or cutout may be removed using a removal process, such as etching or milling before the PCB layers are configured in the stack-up, such as before being laminated together. Cutout portions180a, b, c, dand182a, b, c, d(also referred to as anti-pads) of the second ground reference118and the third ground reference120are shown inFIG. 1. Cutout portions of the first, fourth, and fifth ground references116,122,124are similarly shown. The substrate material of the PCB layers adjacent to the ground references, in addition to an adhesive material such as a laminate material used to bind the layers together, may fill in the cutout portions during manufacturing of the PCB stack-up100. Cutout portions may also be removed to prevent differential signal traces130a,130band132a,132bfrom being shorted together. Cutout portions184and186of the second ground reference118and the third ground reference120are shown inFIG. 1. Cutout portions of the first, fourth, and fifth ground references116,122,124are similarly shown.

Although the cutout portions may prevent the signal vias from being shorted to the ground references, energy fields such as electric fields or magnetic fields may be generated between reference structures surrounding the cutout portions, such as portions of the ground references still connected to the signal via after the cutout portions have been removed and parts of the remaining ground references. Energy fields may also be generated between the signal vias and/or the portions of the ground references still connected to the signal vias after the cutout portions have been removed, and other reference structures or portions of other reference structures, such as ground vias, other signal vias carrying other signals, or power vias carrying a DC power supply signal, as examples. The energy fields may generate a coupling effect, which may remove at least some of the energy of a signal propagating through the signal via. Alternatively or in addition, the coupling effect may introduce noise into the signal conductor path. Coupling from the signal via to another reference structure not in the signal conductor path, or coupling between the signal via and the other reference structure not in the signal conductor path, may be referred to as mutual coupling or crosstalk. An electric energy field may produce a capacitance, referred to as mutual capacitance. A magnetic energy field may produce an inductance, referred to as mutual inductance. An amount of mutual capacitance and/or an amount of mutual inductance that is generated may be proportional to an amount of energy of the signal that is coupled away from the signal conductor path. It may be desirable to reduce the amount of mutual capacitance and/or mutual inductance that is generated in order to reduce the amount of mutual coupling, crosstalk, signal loss, and/or noise introduced into the signal conductor path.

FIG. 1shows signal conductor paths that include two signal conductors in two different layers. In the first portion134of the PCB stack-up100, a first signal conductor path may include the differential traces130a,130band the differential traces132a,132b. The second and third vias140,142may be connected to the differential traces130a,130band132a,132band may provide a transition from the differential traces130a,130bto the differential traces132a,132b, or vice versa. For example, a differential signal may propagate along differential traces130a,130band enter into the second and third vias140,142at inputs or “signal in” points, positions, or locations of the second and third vias140,142. The inputs or “signal in” points, positions, or locations may be where the differential traces130a,130bare connected to the second and third vias140,142. After propagating through the second and third vias140,142, the differential signal may exit the second and third vias140,142at outputs or “signal out” points, positions, or locations of the second and third vias140,142. The outputs or “signal out” points, positions, or locations may be where the second and third vias140,142are connected to the differential traces132a,132b. Alternatively, the inputs and outputs of the second and third vias140,142may be reversed. For example, the differential signal may propagate along the differential traces132a,132b, enter the second and third vias140,142, propagate through the second and third vias140,142, exit the second and third vias140,142, and propagate along the differential traces130a,130b.

An alternative PCB stack-up may include a multi-drop configuration in which one or more signal vias have multiple outputs or “signal out” points, positions, or locations. For example, using the PCB stack-up100to illustrate, a third pair of differential traces may be disposed in or on a layer that is different in which the layers that the differential traces130a,130band/or differential traces132a,132bare disposed, such as in or on the first layer102a, the fourth layer102d, the fifth layer102e, and/or the sixth layer102f. In addition, the third pair of differential traces may be connected to the second and third vias140,142. A differential signal may propagate along the differential traces130a,130b, enter and propagate through the second and third vias140,142, and exit the second and third vias140,142at outputs of the second and third vias140,142where the differential traces132a,132bare connected to the second and third vias140,142, and also where the third set of differential traces are connected to the second and third vias140,142. In alternative PCB stack-ups having a multi-drop configuration, there may be more than two outputs of the signal vias connected to more than two traces. For example, a fourth pair of differential traces may be disposed in or on a layer that is different than the layers that the differential traces130a,130b, differential traces132a,132b, and/or the third pair of traces are disposed.

In the second portion136of the PCB stack-up100, a second signal conductor path may include the single ended trace126and the single ended trace128. The sixth via148may be connected to the single ended trace126and the single ended trace128and may provide a transition from the single-ended trace126to the single-ended trace128, or vice versa. For example, a signal may propagate along single ended trace126and enter into the sixth via148at an input or “signal in” point, position, or location of the sixth via148. The input or “signal in” point, position, or location may be where the trace126is connected to the sixth via148. After propagating through the sixth via148, the signal may exit the sixth via148at an output or “signal out” point, position, or location of the sixth via148. The output or “signal out” point, position, or location may be where the sixth via148is connected to the single ended trace128. Alternatively, the input and output of the sixth via148may be reversed. For example, the signal may propagate along the single ended trace128, enter the sixth via148, propagate through the sixth via148, exit the sixth via148, and propagate along the single ended trace126. An alternative PCB-stack up having a single-ended trace may comprise a multi-drop configuration, where the signal comprises multiple outputs or “signal out” points, positions, or locations, as previously described.

In other example PCB stack-ups, a signal conductor path may include more than two signal conductors and/or more than one signal via connecting the more than two signal conductors. For example, the first signal conductor path in the first portion134may further include one or more other differential traces disposed in the first layer102a, the fourth layer102d, the fifth layer102e, the sixth layer102f, on the first surface104aof the first layer102aand/or on the second surface114bof the sixth layer102f. One or more signal vias may connect the one or more other differential traces to the differential traces130a,130band/or the differential traces132a,132b. A differential signal may propagate along the differential traces130a,130b, through the second and third vias140,142, propagate along the differential traces132a,132b, and then transition through a second pair of vias and transition to one of the other differential traces, or vice versa. Alternatively or in addition, the one or more other differential traces may include one or more signal conductors disposed in a layer that is the same as one of the layers that the differential traces130a,130band/or132a,132b, but in a different part of the PCB stack-up100. For example, a third pair of differential traces may be disposed in the second layer102b. A differential signal may propagate along the differential traces130a,130b, may transition through the second and third vias140,142and to the differential traces132a,132b, and may then transition through another pair of vias and to differential traces disposed in the second layer that are not connected to the differential traces130a,132bin the second layer. Similar configurations may be possible for the single-ended transmission lines in the second portion136.

A transmission line may have an associated impedance. The associated impedance may be referred to as a characteristic impedance or impedance, and may be dependent upon various parameters, including a dielectric constant of the substrate material, a width or widths of one or more conductive traces of the transmission line, a thickness of the layer, and/or a spacing between the conductive traces if there is more than one conductive trace, such as a differential pair of traces. The parameters may be predetermined to configure the transmission line to have a desired characteristic impedance. The characteristic impedance may be any value or any range of values that is suitable for propagation of signals over the transmission line. In one example, a characteristic impedance of a single-ended transmission line may be approximately 50 ohms. In another example, a characteristic impedance of a differential transmission line may be approximately 100 ohms. Other characteristic impedances for single-ended and differential transmission lines may be determined.

One or more signal vias may also have an associated impedance. The associated impedance of a signal via may be dependent upon the dielectric constant of the substrate material surrounding the via, a size or sizes or one or more cutout portions of one or more ground references, a distance from the one or more signal vias to one or more reference structures, and/or a length of the signal via. The associated impedance of the signal via may be less than the associated impedances of the transmission lines that the signal via may be connected to. As an example, a single-ended transmission line may have an impedance of approximately 50 ohms. An impedance of a signal via in connection with the single ended conductor of the single-ended transmission line may be between approximately 35 ohms and approximately 45 ohms. As another example, a differential transmission line may have an impedance of approximately 100 ohms. An impedance of a pair of vias in connection with the differential signal conductors of the differential transmission line may be in a range of between approximately 75 ohms and approximately 90 ohms.

Two different transmission lines of a path may be configured to have the same or substantially the same characteristic impedances. In addition, the characteristic impedance of the signal via connecting the different transmission lines may be lower than the characteristic impedances of the transmission lines. For example, in the first portion134, the transmission line having the differential traces130a,130bmay have the same or substantially the same characteristic impedance of the differential traces132a,132b, such as approximately 100 ohms. The plated portions of the second and third vias140,142, in connection with the differential traces130a,130b, and132a,132bmay have a characteristic impedance that is less than the characteristic impedance of the differential traces130a,130b, and132a,132b. As an example, where the characteristic impedance of the differential traces130a,130band132a,132bis approximately 100 ohms, the characteristic impedance of the plated portions of the second and third vias140,142may be in the range of between approximately 75 ohms and approximately 90 ohms. Other ranges or a range that is larger or smaller than between approximately 75 ohms and approximately 90 ohms are possible. Similarly, in the second portion136, the transmission line having the single-ended trace126may have the same or substantially the same characteristic impedance as the single-ended trace128, such as about 50 ohms. The plated portion of the sixth via148in connection with the single-ended traces126and128may have a characteristic impedance that is less than the characteristic impedance of the single-ended traces126and128, such as in the range of between approximately 35 ohms and approximately 45 ohms.

Different or substantially different impedances in a signal path may be referred to as an impedance mismatch. Impedance mismatch may have an effect of signal reflection, in which at least some of the energy of a signal propagating from an input to an output may be reflected back toward the input. This may result in signal loss, in which less than all of the energy transmitted in a signal is not transmitted from the input to the output. Where the impedances of the signal path over the entire signal path, such as through one signal conductor, through a signal via, and to another signal conductor, is the same or substantially the same, the amount of energy reflection may be nothing or negligible, resulting in negligible or substantially negligible signal loss.

FIG. 2shows an example PCB stack-up200that includes the PCB stack-up100, and further includes at least one unplated via that is disposed a distance from a signal via that is less than or approximately equal to a distance from the signal via to a reference structure that is at least part of the return conductor path. The at least one unplated via may be disposed in between or substantially between the signal via and the reference structure. The distance from the signal via may be determined by center positions of the vias, by the inner walls of the vias, or a combination thereof. Also, the distance may be a distance that is perpendicular or substantially perpendicular to the reference structures, such as the first via138, the second via140, the third via142, the fourth via144, the fifth via146, and/or the sixth via148. In addition or alternatively, the distance may be a distance that is parallel or substantially parallel to the planar layers102a,102b,102c,102e,102fand/or the ground references116,118,120,122,124.

As an example, in the first portion134of the example PCB stack-up200inFIG. 2, the at least one unplated via may include two unplated vias, namely a first unplated via202and a second unplated via204. The first unplated via202may be disposed at a distance from the signal via140that is less than or approximately equal to a distance from the signal via140to the ground via138, which is at least a portion of the return conductor path for a differential signal propagating along differential conductive traces130a,130band132a,132b, as previously described. In addition, as shown inFIG. 2, the first unplated via202may be disposed in between or substantially in between the signal via140and the ground via138. Similarly, the second unplated via204may be a distance that is less than, equal to, or substantially equal to a distance from the signal via142to the ground via144, which is at least a portion of the return conductor path for the differential signal propagating along differential conductive traces130a,130band132a,132b, as previously described. In addition, as shown inFIG. 2, the second unplated via204may be disposed in between or substantially in between the signal via142and the ground via144. In other example PCB stack-ups, only one of the unplated via202and the unplated204via may be included. In addition or alternatively, there may be other unplated vias that are disposed at a distance that is greater than the distance from the signal via140and the ground via138and/or from the signal via142to the ground via144.

Similarly, in the second portion136, a third unplated via206may be disposed at a distance from the signal via148that is less than, equal to, or substantially equal to a distance from the signal via148to the ground via146, which is at least a portion of the return conductor path for a signal propagating along single ended traces126,128, as previously described. In addition, as shown inFIG. 2, the third unplated via206may be disposed in between or substantially in between the signal via148and the ground via146. Alternatively or in addition, one or more unplated vias, other than the third unplated via206, may be included. One or more of the other unplated vias may be disposed at a distance that is less than, equal to, or substantially equal to a distance from the signal via148to the ground via146. For example, one or more of the other unplated vias may be disposed at a distance that is greater the distance from the signal via146to the ground via148.

The PCB stack-up200may further include one or more unplated vias disposed in between or substantially in between one or more pairs of differential traces. For example, an unplated via208is disposed in between differential traces130a,130band differential traces132a,132b.

One or more of the unplated vias, such as the unplated vias202,204,206,208, may extend at least partially through at least one layer of the PCB stack-up. In the PCB stack-up200, each of the unplated vias202,204,206,208extends an entire length of the PCB stack-up200that is perpendicular or substantially perpendicular to the opposing planar surfaces of the layers, from the first surface104aof the first layer102ato the second surface114bof the sixth layer102f. Other variations in the directions and/or lengths that the unplated vias202,204,206,208may extend through the PCB stack-up200are possible. For example, an unplated via may extend in only one layer, from one opposing planar surface to the other opposing planar surface of the only one layer, or from one opposing planar surface to a position within the layer. Alternatively, the unplated via may extend more than one layer but less than the entire length of the PCB stack-up, from one exposed outer layer surface (e.g., the first surface104aof the first layer102a) to the other exposed outer layer surface (e.g., the second surface114bof the sixth layer102f).

Alternatively or in addition, the unplated via may extend in one or more of the layers in or on which there are conductive traces. As an example, for the PCB stack-up200, in the first portion134, the unplated via may extend in the second layer102band/or the third layer102c. In the second portion136, the unplated via may extend in the first layer102aand/or the third layer102c. Alternatively or in addition, the unplated via may extend in the layers for which the signal via has conductive plating. As an example, for the PCB stack-up200, the unplated via may extend in the first, second, third, and/or fourth layers102a-102d.

The unplated via may extend in a direction that is perpendicular or substantially perpendicular to one or more opposing planar surfaces of one or more layers. Alternatively, the unplated via may extend in a direction other than perpendicular or substantially perpendicular to the one or more opposing planar surfaces of the one or more layers. In some examples, the unplated via may extend in a direction that is in a range of approximately thirty degrees to approximately ninety degrees with reference to one or more of the opposing planar surfaces. Alternatively or in addition, the unplated via may include a plurality of unplated vias that are disposed in different layers and are in alignment or in substantial alignment with each other. The different layers may be sequential layers or nonsequential layers. For example, in the PCB stack-up200, the unplated via may include a first unplated via extending in the first layer and a second unplated via extending in the third layer102c, where the first unplated via and the second unplated via are in alignment or substantial alignment with each other.

The unplated via may be referred to as an air via to indicate the environment (e.g., air) that is occupying the unplated via hole. The air via may be configured in the PCB stack-up200to reduce an effective dielectric constant of an area surrounding at least a portion of one or more signal vias, such as an area surrounding a single signal via connected to a single-ended conductive trace, an area surrounding a signal via of a pair of signal vias connected to a differential pair of conductive traces, or an area surrounding the pair of vias. The area surrounding the one or more signal vias may be defined by one or more predetermined distances from one or the signal vias or a position between the pair of signal vias. An example predetermined distance may be a distance from a signal via to a reference structure that is at least a portion of the return conductor path for a signal propagating through the signal via. Another example predetermined distance may be a distance from one signal via to the other signal via of the pair of signal vias. Another example predetermined distance may be a distance from a middle point between the two signal vias to the reference structure. It should be appreciated that other predetermined distances are possible. The predetermined distances may be exact or precise distances. Alternatively, the predetermined distances may be approximate distances used to approximately or generally define an area surrounding one or more signal vias.

The area surrounding the one or more signal vias may be circular, elliptical, rectangular, or square, although the area may defined by shapes. Alternatively, the area surrounding the one or more signal vias may be substantially amorphous having one or more boundaries defined by one or more predetermined distances. The signal via or the middle point between the pair of signal vias may be a center position of the area. As an example, for a circular area, where the predetermined distance is a distance from the signal via to the reference structure, the predetermined distance may be a radius of the circular area. As an another example, for a rectangular area, where the predetermined distance is a distance from the signal via to the reference structure, the predetermined distance may be a distance from the center of the rectangular area to one or more borders of the rectangular area. As another example, for an elliptical area, where the predetermined distance is a distance from the middle point between the pair of signal vias to a reference structure, the distance may be a radius of a major or a minor axis of the elliptical area surrounding the pair of signal vias. The air via may be disposed in the area surrounding the one or more signal vias. The effective dielectric constant in the area surrounding the one or more signal vias may be a combination of the dielectric constant of the substrate material and the dielectric constant of the air in the air via. The dielectric constant of air may be less than the dielectric constant of the substrate material. The effective dielectric constant of the area surrounding the one or more signal vias that includes the combined dielectric constants of the air and the substrate may be less than the dielectric constant of the substrate alone. As an example, the dielectric constant of air may be about 1. The dielectric constant of the substrate may be in a range of between approximately 3.5 to approximately 5. The effective dielectric constant in the area may be less than the dielectric constant of the substrate material, such as approximately 3.0, for example. Without the air via, the effective dielectric constant of the area surrounding the one or more signal vias may be the dielectric constant of the substrate material in the area. The area where the air via is present has an effective dielectric constant that is lower than the dielectric constant of the area when the air via is not present.

The characteristic impedance of the signal via may be inversely proportional to the effective dielectric constant of the area surrounding the one or more signal vias. As the effective dielectric constant decreases, the characteristic impedance of the one or more signal vias increases. As previously described, the characteristic impedance of the one or more signal vias without the air via in the area surrounding the one or more signal vias may be less than the characteristic impedance of the transmission lines that the one or more signal vias are connected to. Where the air via is included in the area surrounding the one or more signal vias, the effective dielectric constant of the area may decrease, which may increase the characteristic impedance of the one or more signal vias to an impedance that is closer to the impedance of the transmission lines. An increased characteristic impedance may reduce the impedance mismatch in the signal conductor path, which may reduce the amount of energy that is reflected in the signal and overall signal loss.

The air via being disposed in the area surrounding the signal via, such as by being disposed in between the signal via and the reference structure, may extend through one or more cutout portions of one or more of the ground references116-124. For example, inFIG. 2, the unplated via202may extend through the cutout portion180aand/or the cutout portion182a. Similarly, the unplated via204may extend through the cutout portion180band/or the cutout portion182b. Also, the unplated via206may extend through the cutout portion180cand/or the cutout portion182c. Alternatively or in addition, the unplated via208may extend through cutout portion184and/or cutout portion186. Alternatively or in addition, the unplated vias202,204,206,208may extend through other cutout portions of the first, fourth, and/or fifth ground references116,122,124. By extending through the cutout portions between the signal via and the references structure, the unplated vias may decrease the mutual coupling and/or crosstalk between the signal via and an adjacent signal via or an adjacent differential pair of signal vias (not shown).

Also, the unplated vias may extend through the PCB stack-up200in the area surrounding the signal via such that the unplated vias extend adjacent one or more via stubs. As examples, the unplated via202may be disposed adjacent the via stub178a. The unplated via208may be disposed adjacent the via stubs178aand178b. The unplated via204may be disposed adjacent the via stub178b. The unplated via206may be disposed adjacent the via stub178c. The unplated vias adjacent the via stubs may reduce the capacitance generated by the via stub, which may reduce signal loss and/or impedance mismatch in the signal conductor path.

FIGS. 3-6are top views of a layer of a PCB stack-up, such as PCB stack-up200, showing various example configurations of one or more air vias. The one or more air vias may be disposed in the area surrounding one or more signal vias, as previously described.

The one or more air vias may have various cross sections, such as circular, elliptical, obround (e.g., “pill-shaped” or oval shaped), “C”-shaped, rectangular, triangular, pentagonal, hexagonal, heptagonal, octagonal, star-shaped, or any other geometrically-shaped cross sections. Also, one or more of the air vias may have a size (e.g., a diameter) that is smaller, the same, substantially the same, or larger than a size of the signal via and/or a size of the reference structure.

InFIG. 3, an example configuration of one or more air vias may include an air via302disposed in an area surrounding a signal via340and/or an area surrounding a pair of signal vias that includes the signal via340and a signal via342. In addition, the air via302may be disposed in between or substantially in between the signal via340and a reference structure338. Alternatively or in addition, the configuration of one or more air vias may include an air via304disposed in an area surrounding the signal via342and/or the area surrounding the pair of signal vias340and342. Also, the air via304may be disposed in between or substantially in between the signal via342and a reference structure344. Alternatively or in addition, the configuration of one or more air vias may include an air via308disposed in the area surrounding the signal via340, the area surrounding the signal via342, and/or the area surrounding the pair of signal vias340,342. Also, the air via308may be disposed in between or substantially in between the pair of signal vias340,342.

InFIG. 4, another example configuration of one or more air vias may include a plurality of air vias402, including air vias402a,402b, disposed in an area surrounding a signal via440and/or an area surrounding a pair of signal vias that includes the signal via440and a signal via442. In addition, the air vias402a,402bmay be disposed in between or substantially in between the signal via440and a reference structure438. Alternatively or in addition, the configuration of one or more air vias may include a plurality of air vias404, including air vias404a,404b, disposed in an area surrounding the signal via442and/or the area surrounding the pair of signal vias440and442. Also, the air vias404a,404bmay be disposed in between or substantially in between the signal via442and a reference structure444. Alternatively or in addition, the configuration of one or more air vias may include a plurality of air via408a,408bdisposed in the area surrounding the signal via440, the area surrounding the signal via442, and/or the area surrounding the pair of signal vias440,442. Also, the air vias408a,408bmay be disposed in between or substantially in between the pair of signal vias440,442.

InFIG. 5, another example configuration of one or more air vias may include a plurality of air vias502, including air vias502a,502b,502c,502d,502edisposed in an area surrounding a signal via540and/or an area surrounding a pair of signal vias that includes the signal via540and a signal via542. In addition, the air vias502a,502b,502c,502d,502emay be disposed in between or substantially in between the signal via540and a reference structure538. Alternatively or in addition, the configuration of one or more air vias may include a plurality of air vias504, including air vias504a,504b,504c,504d,504edisposed in an area surrounding the signal via542and/or the area surrounding the pair of signal vias540and542. Also, the air vias504a,504b,504c,504d,504emay be disposed in between or substantially in between the signal via542and a reference structure544. Alternatively or in addition, the configuration of one or more air vias may include a plurality of air via508a,508b,508c,508d,508edisposed in the area surrounding the signal via540, the area surrounding the signal via542, and/or the area surrounding the pair of signal vias540,542. Also, the air vias508a,508bmay be disposed in between or substantially in between the pair of signal vias540,542.

InFIG. 6, another example configuration of one or more air vias may include an air via602disposed in an area surrounding a signal via640and/or an area surrounding a pair of signal vias that includes the signal via640and a signal via642. In addition, the air via602may be disposed in between or substantially in between the signal via640and a reference structure638. Alternatively or in addition, the configuration of one or more air vias may include an air via604disposed in an area surrounding the signal via642and/or the area surrounding the pair of signal vias640and642. Also, the air via604may be disposed in between or substantially in between the signal via642and a reference structure644. Alternatively or in addition, the configuration of one or more air vias may include an air via608disposed in the area surrounding the signal via640, the area surrounding the signal via642, and/or the area surrounding the pair of signal vias640,642. Also, the air via608may be disposed in between or substantially in between the pair of signal vias640,642. Additionally, as shown inFIG. 6, the air vias602,604, and608comprise an obround shape.

The example configurations shown inFIGS. 3-6are non-limiting and are merely illustrative. Other configurations or one or more combinations of the configurations shown inFIGS. 3-6may be used. For example, the numbers of air vias disposed in between a signal via and a reference structure and between the pair of signal vias may not be equal, may have different cross-sectional shapes, and/or may have different sizes. In addition, air vias may not be present between a signal via and an adjacent reference structure (e.g., between signal via340and reference structure338or signal via342and reference structure344) or between a pair of signal vias (e.g., signal via340and signal via342). For example, one or more of air vias302,402,502,602, one or more air vias304,404,504,604, and/or one or more air vias308,408,508,608may not be present. In addition, similar or other configurations of air vias may be used for a single-ended transmission line. In the single-ended transmission line, one or more air vias may be disposed in an area surrounding a signal via connected to a single-ended trace. Additionally, the one or more areas may or may not be disposed in between and/or substantially in between the signal via and a reference structure. Other via configurations are possible.

The air vias may be formed at any suitable time, such as before, after, or during the formation of the other vias, such as the ground vias, power supply vias, and/or the signal vias. For example, the air vias may be formed when one or more of the other vias are formed. During an imaging process, the air vias may be selected and/or separated from the other vias so that the air vias are not plated during the plating process. Alternatively, the air vias may be formed after the other vias are drilled and/or plated, and/or the PCB stack-up is manufactured, such as by laminating the layers together.

FIG. 7. illustrates a perspective view of an example PCB stack-up700(or a portion of a PCB stack-up) connected to an active device705. The example PCB stack-up700includes five layers714a,714b,714c,714c,714d,714eseparated by four ground reference planes716,718,720,722. The PCB stack-up700includes a differential signal path comprising differential signal traces710a,710bdisposed on an outer (or top) planar surface of a first layer714a, and differential traces706a,706bdisposed on the outer surface of the first layer714a. The differential signal path further includes signal vias732,734connected to the differential signal traces710a,710and signal vias740,742connected to the differential vias706a,706b. The differential signal path may further include one or more additional differential signal traces disposed in or on one or more layers of the PCB stack-up700other than the first layer714a, such as layers714b,714c,714d, and/or714e(not shown). The signal vias734,734and740,742may be connected to the one or more additional differential signal traces. In addition, the differential signal traces710a,710band the differential signal traces706a,706bmay be interconnected through the signal vias732,734,740,742and the one or more additional differential signal traces.

The differential signal traces706a,706bmay be connected to the active device705. In addition,FIG. 7shows two other traces746,748connected to the active device705. In one example, the differential signal traces706a,706bmay provide a differential input signal to the active device705and the two other traces746,748may provide an output path for an output signal of the active device705. Alternatively, the two other traces may provide the differential input signal to the active device705and the differential signal traces706a,706bprovide an output path for the output signal of the active device705. In other examples, more or fewer traces connected to the active device705may be connected to the active device705, as will be appreciated by one of ordinary skill in the art.

The signal vias732,734,740,742may transition a portion of the signal path to a layer other than the first layer714ato divert the signal path from an obstructing object, such as signal trace712, which may prevent the signal traces710a,710bfrom being directly connected to signal traces706a,706bon the outer surface of the first layer714a. In other example PCB stack-ups, one or more obstructing objects other than or in combination with the trace712may prevent a direct connection of signal paths in or on a single layer, such as an electronic component and/or an active device.

In one example, the differential traces710a,710bmay be input signal traces and a differential signal may propagate along the differential traces710a,710band enter “signal in” points of the signal vias732,734as previously described. The differential signal may propagate through the signal vias732,734, exit the signal vias732,734, and propagate along the one or more additional differential signal traces disposed in or on one or more layers other than the first layer714a(now shown). The differential signal may enter a “signal in” point of the signal vias740,742in one of the other layers, propagate through the signal vias740,742, and exit the signal vias740,742. The differential signal may propagate along the differential signal traces706a,706b, which may be input traces of the active device705, and the differential signal may be input to the active device705. Alternatively, the differential traces706a,706bmay be output traces of the active device705. In the alternative example, the differential signal is output from the active device705, propagates along the differential signal traces706a,706b, transitions through the signal vias740,742, propagates along the one or more additional differential signal traces (not shown) and signal vias732,734, exits the signal vias732,734, and propagates along the differential traces710a,710b.

Reference structures, such as ground vias730,736,738, and744, may be located near the signal vias732,734,740,742to function as return paths for a differential signal propagating along the differential signal path. In addition, unplated vias (e.g., air vias)702,704,708,724,726, and/or728may be disposed in between or substantially in between the signal vias and the reference structures. For example, unplated vias702may be disposed in between or substantially in between the signal via740and the reference structure738; unplated vias704may be disposed in between or substantially in between the signal via742and the reference structure744; unplated vias724may be disposed in between or substantially in between the signal via732and the reference structure730; and/or unplated vias728may be disposed in between or substantially in between the signal via734and the reference structure736. In addition or alternatively, unplated vias708may be disposed in between or substantially in between the signal via740and the signal via742; and/or unplated vias726may be disposed in between or substantially in between the signal via732and the signal via734. As previously described, the unplated vias702,704,708,724,726,728configured and/or positioned near the signal vias732,734,740,742and the reference structures730,736,738,744, may decrease an effective dielectric constant of an area surrounding the signal vias732,734,740,742, which may yield a characteristic impedance of the signal vias732,734,740,742that is closer to a characteristic impedance of the differential signal traces706a,706b,710a,710bthan if the702,704,708,724,726, and/or728were not part of the PCB stack-up700.

Various embodiments described herein can be used alone or in combination with one another. The foregoing detailed description has described only a few of the many possible implementations of the present invention. For this reason, this detailed description is intended by way of illustration, and not by way of limitation.