OPTIMIZATION OF THROUGH PLANE TRANSITIONS

This application is a continuation of, and claims priority to, US Provisional

Application No. 60/701,138, filed July 20, 2005, and is incorporated herein by reference.

BACKGROUND

Printed circuit boards (PCBs) or other circuit substrates are often constructed of multiple layers, with connections from the surface of the substrate being connected

to inner layer traces of the substrate. For signal integrity, the impedance of the signal

path from one point to another should be a constant as possible. With transitions

between layers in a substrate, there is a high probability of impedance mismatch

between a signal path through a first layer, the transition to a second layer and the signal path through the second layer. This causes overall impedance mismatch in the

signal path from end to end, resulting in degraded signal integrity at the receiving end.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention may be best understood by reading the

disclosure with reference to the drawings, wherein:

Figure 1 shows a three-dimensional view of a circuit substrate.

Figure 2 shows an alternative three-dimensional view of a circuit substrate.

Figure 3 shows an embodiment of an annular ring at a layer transition.

Figure 4 shows an embodiment of annular rings at an inner layer. Figure 5 shows a flowchart of an embodiment of a method of designing a substrate.

Figure 6 shows a cross-sectional side view of a circuit substrate with clad vias.

Figure 7 shows a top view of a circuit substrate having a signal via and reference vias.

Figure 8 shows a top view of a circuit substrate showing alternative placements of reference vias around a signal via.

Figure 9 shows a cross-sectional side view of a circuit substrate having an interlayer transition.

DETAILED DESCRIPTION OF THE EMBODIMENTS Figure 1 shows three-dimensional view of a circuit substrate. The board as shown has five layers of conductive material 11, 14, 13, 15 and 19 such as copper clad

and four layers of dielectric 16, which is one layer on the side without the conductive

layer 14, and two layers 16a and 16b on the side with the conductive layer 14, 17 and

18 between them. The conductive layers may take the form of traces. However, it

must be noted that this is merely an example and the embodiments disclosed here may

apply to any number of layers. The dielectric may be any typical dielectric material

used in substrates of this nature. Generally, lower dielectric constant (k) materials are

becoming prevalent as circuit substrate dielectrics.

The circuit substrate 10 has a top surface 11, which may include traces. An

example of a trace is shown at 12. The circuit substrate has layers 16, 17 and 18,

shown here as a dielectric. In this example, layer 16 is a single layer on the left side of

the diagram and divided into two sublayers 16a and 16b on the right side. On the left

side, the layer 16 may actually be formed of two different dielectric materials, one on the top of the strip line 14 and the other below, but on the left side, they form one

layer of dielectric between conductive layers. The strip line 14 is connected to the

trace 12 by a via 20 that has been back drilled or stub drilled to minimize the stub effect of the via, discussed in more detail later.

It must be noted that the transition shown here is from a microstrip through a

plated via to an internal stripline. Application of the invention is not restricted to this occurrence. The transition could be from a microstrip or other surface trace to another

microstrip or surface trace coplanar waveguides. Alternatively, the transition could be

from a stripline in one layer to a stripline in another layer completely internal to the circuit substrate. For ease of discussion, here, however, the transition will be from a

surface microstrip to an internal stripline, with the understanding that the via 20 may traverse a dielectric layer to form a connection between two metal layers.

The metal stub of the signal via 20 may be formed from a metal-plated via

through the substrate 10. Currently, metal stubs such as 20 are typically formed as a metal-plated via through the substrate which may then be optionally back-drilled to

minimize the stub beyond the stripline trace, from the bottom of the substrate in the

orientation shown in Figure 1, leaving a significantly reduced metal stub 20. The depth of the metal stub beyond the strip line is currently not optimized with regard to

particular signal characteristics in light of manufacturing process limitations and may

form reflections in the path that affects the signal integrity due to the reflected energy trapped in the metal stub.

The signal path via 20 is electrically connected to the microstrip 12 and the

strip line 14, allowing signals traveling through the microstrip 12 to transition into the

layers of the circuit board and into strip line 14. The signal via 20 may transition

through the layers having apertures such as 28. These transitions, as well as the

differences between the microstrip 12, the signal path via 20 and the strip line 14, may

result in mismatches or irregularities in the signal path characteristics.

Signal path characteristics as used here means measurable qualities of an

electrical signal in the path. These include but are not limited to impedance, including

components of impedance of inductance, capacitance, resistance, and conductance; return loss; insertion loss; cross talk; and attenuation.

In situations where impedance mismatch arises, there is a disturbance in the

electromagnetic (EM) field around a signal path. This can affect the signal strength,

causing loss in the signal, hi more extreme cases, for example, a signal that has a voltage level associated with a logic level ' 1 ' may experience enough loss that when it reaches the other end of the signal path is has a voltage level associated with a logic

level O.'

Return loss is generally affected by the location of the ground plane relative to

the signal path due to reflections associated with the mismatch of impedance between

signal vias and references vias. As can be seen in Figures 1 and 2, reference vias such as 22 surround the signal path via 20 from Figure 1. The placement of these reference

vias relative to the signal path via 20 may have a drastic effect on the signal integrity

in the signal path. hi addition to the placement of the reference vias, annular rings used in the

manufacturing process may be controlled for the signal characteristics as well. An

example of such a ring at a surface of a substrate is shown at 26 in Figure 3. Figure 4

shows an annular ring 26 on an interlayer of the substrate. The term 'annular ring' is a

term used in manufacturing of the substrate. The presence of an annular ring allows

more consistent connection to the drilled and plated of the hole forming the via. The

annular ring and the other portions of the structure that provide electrical connection may also be referred to as the 'pad.'

In future embodiments, it may be possible and desirable to eliminate the

annular ring, in which case the connection would be directly to the metal lining the

via, without the annular ring. For example, if via size and drill size were small

enough, the via may be drilled such that the circumference of the via is contained within the trace, with no need for an annular ring.

It is possible to optimize the formation of the annular ring 26, the placement of

the reference vias 22 and the aperture 28 to eliminate or mitigate mismatches and

irregularities in the signal path characteristics. As mentioned above, currently substrate manufactures are concentrating on the annular ring and optimizing the

formation of that to minimize impedance in the outer layer. The formation of the

annular ring in this example, as well as any other apertures in any other layers, may be tuned to a particular electrical characteristic, such as impedance, of the signal path in

that layer.

A method of designing a signal path to manage a selected signal characteristic

is shown in Figure 5. For ease of discussion, a cross-section through a substrate, such

as along line A-AA in Figure 3 is shown in Figure 6. For ease of discussion and better

understanding, the process will focus first on a simple substrate having a via from a

trace on one side of the substrate to a trace on the other side of the substrate.

In Figure 6, the signal via 50 has reference vias 52 and 54 on either side of it.

The size of the signal via, the number of reference vias, the distance between the

signal via and the reference vias, the application of the via, whether for single signals

or differential signals, are determined at 30 in Figure 5. The determination may take

into account the size of the substrate, the nature of the connectors, the design rules

used in designing and laying out the circuitry, etc.

The number of reference vias may be guided by the size of the area provided

for the vias, the application and geometry of the signal vias, the circuit requirements,

etc. The placement of the reference vias relative to the signal via and each other may

be used to control the desired signal characteristics as will be discussed further.

In 34, the via size is selected based upon the determinations made in 30. If a drill is used to form the via, the drill size is selected based upon the geometry of the

via. It must be noted that in current implementations, other means may be used to

make the hole such as by laser drilling and are considered to be included in this discussion. Therefore, the drill size selection is considered to be an optional process. hi 36, an aperture, sometimes referred to as an anti-pad, is set. Referring back

to Figures 3 and 4, the aperture 28 is the area around the via that is 'outside' the

annular ring 26. hi one embodiment, using a three-dimensional, electromagnetic

(EM) solver tool, a 'port' maybe defined to be in the area of the aperture with a

particular signal characteristic. This process is iterated until the structure being tested meets the desired characteristic.

In 42, the aperture on each layer may be dealt with differently depending upon

the signal via, reference vias, dielectric thickness and characteristic above and below

the trace, the stub, the annular ring, etc.

For the embodiment under discussion here, once the aperture is set at 36, the

process moves to controlling the trace topology, hi one embodiment, the trace is

treated as a co-planar waveguide for modeling purposes. A co-planar waveguide is a

trace topology that has two reference traces on either side of the signal trace, separated

by a gap, typically air on the same plane.

Using a co-planar waveguide model, it is possible to determine the layout of

the surface topology. Referring to Figure 7, it can be seen that the surface can be

viewed as a metal pad 62 encompassing the area around the via 20, the annular ring

26, if there is one, and the aperture or air gap 28. This surface is modeled as a co-

planar waveguide and adjustments are made to the topology to ensure that the signal characteristics are maintained.

As can be seen in Figure 8, the position and number of the reference vias such

as 22a-i may change depending upon the application. Referring to Figure 5, the

number of reference vias available for adjustment is generally determined prior to this process within 30. However, there is no limitations to a particular number of vias

being used, so alternative arrangements are presented. During the setting of the

aperture 28, typically also done previous to this process, the aperture may have been

adjusted to many different possible positions, including those shown by the dotted

circles. The aperture may intersect with the reference vias, be smaller than a circle defined by the reference vias, etc.

During the process of adjusting the trace topology at 38 of Figure 5, the position of the reference vias may be shifted slightly. In the example of Figure 8, the

reference via 22a may be shifted slightly to the position of 22b to adjust for the

presence of the co-planar waveguide. Similarly, the position of the via 22c may be

adjusted such as shown at 22d. The arrangement of the other reference vias 22e-i may

or may not be symmetrical, depending upon the effects of their positions on the desired signal characteristic.

Once the trace topology is set based upon the co-planar waveguide, there may

be further adjustments due to the presence of the annular ring, if one is used, at 40.

Typically, in current manufacturing processes, the presence of an annular ring ensures

that the plating of the via is complete with no disconnects. However, in future

implementations, it maybe possible to drill into the via with a drill small enough that

the trace itself will form the connection to the via, without use of an annular ring.

Therefore, the process of adjusting for the annular ring may be optional.

Having discussed application of the embodiments of the invention for a

substrate having one layer of dielectric between two metal layers, it is possible to

discuss a multi-layer substrate in which there are interlayers. A cross-section of such

a substrate is shown in Figure 9.

In Figure 9, the multi-layer substrate has five layers, which is just an example.

It should be noted that the 'layers' referred to here are metal or conductive layers. The interposing layers of substrate dielectric are not counted as part of the layers. Layer 1

(Ll) is the surface trace. The vias have been plated in this cross-section, resulting in

metal cladding such as 70 and 78 on the inner walls of the vias.

Layer 2 (L2) is a reference layer, connecting to the reference via 52, but not to

the signal via 50. Layer 3 (L3) is the signal layer connecting to the signal via 50. For purposes of discussion here, the reference layer 2 will be referred to as being above

the signal layer. Similarly, layer 4 (IA), which is another reference layer, will be

referred to as being below the signal layer. In this particular embodiment, layer 5 (L5)

is the layer on the opposite surface of the substrate from the incoming signal trace.

In the interlayers, layers 2-4, the apertures of the reference layer relative to the

signal via are to be set and controlled similar to the surface aperture referred to

previously. The apertures may differ in each layer, however, because the effective

dielectric constant is different due to the air dielectric at the surface. The aperture 82,

for example, of the reference layer 2, is controlled and adjusted to maintain the desired signal characteristic. The apertures 74 and 76 may be of different sizes, due to the

dielectric constant of the material used, or the thickness of the dielectric, as examples.

Li the embodiment of Figure 9, where there is a second reference layer, the

aperture in the second reference layer, the one below the signal layer, is also manipulated to maintain the desired signal characteristic. The apertures involved may

depend upon the relationship between the signal via, reference vias, the signal trace

and the annular ring. For example, the signal via may provide connection between a

surface microstrip and an interlayer stripline, between two surface microstrips as in

the previous example, but through either a 'simple' substrate or a multi-layer substrate, or between two interlayers of the substrate. Controlling any apertures

through which the signal path passes allows finer control of the properties of the

signal path to maintain the desired signal path characteristic. Further, controlling the

depth of the back drilling process, the resulting position of which is shown at 80,

contributes to this finer control.

In this manner, the signal transition portions of the substrate are tuned and controlled so as to make the transitions have a particular target characteristic. For

example, if the target characteristic is an impedance for the entire signal path of 50

ohms, the signal transitions from stripline to the various levels of the signal via to the

other stripline are tuned and controlled such that the entire signal path has an

impedance of 50 ohms. This may sometimes be referred to as an electrically

'invisible via' as any testing done shows no impedance variations at the via.

Thus, although there has been described to this point a particular embodiment

for a method and apparatus for manufacture of a circuit substrate, 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.