Chapter12
PCB Materials and Traces
The choice of PCB materials and cable type can have a large impact on system
performance. Although any transmission medium is lossy at gigahertz frequencies, this chapter provides some guidelines on managing signal attenuation so as to obtain optimal performance for a given application.
How Fast is Fast?
Signal edges contain frequency components called harmonics. Each harmonic is a multiple of the signal frequency and has significant amplitude up to a frequency determined by Equation12-1:
f ≈ 0.35 / T
Where:
f = Frequency in GHz
T = The smaller of signal rise (Tr) or fall (Tf) time in ns
Because dielectric losses in a PCB are frequency dependent, a bandwidth of concern must be determined to find the total loss the PCB. Frequencies must start at the operation frequency and extend to the frequency in Equation12-1. For example, a 10Gb/s signal with a 10ps rise time has a bandwidth from 10GHz to 35GHz.
Equation 12-1
Dielectric Losses
The amount of signal energy lost into the dielectric is a function of the materials
characteristics. Some parameters used to describe the material include relative permittivity εr (also known as the dielectric constant) and loss tangent. Skin effect is also a contributor to energy loss at line speeds in the gigahertz range.
Relative Permittivity
Relative permittivity is a measure of the effect of the dielectric on the capacitance of a conductor. The higher the relative permittivity, the slower a signal travels on a trace and the lower the impedance of a given trace geometry. A lower εr is almost always preferred.Although the relative permittivity varies with frequency in all materials, FR4 exhibits wide variations in εr with frequency. Because εr affects impedance directly, FR4 traces can have a spread of impedance values with increasing frequency. While this spread can be less significant at 3.125Gb/s, it can be a concern at 10Gb/s operation.
RocketIO GTX Transceiver User GuideUG198 (v3.0) October 30, 2009
Chapter 12:PCB Materials and Traces
Loss Tangent
Loss tangent is a measure of how much electromagnetic energy is lost to the dielectric as it propagates down a transmission line. A lower loss tangent allows more energy to reach its destination with less signal attenuation.
As frequency increases, the magnitude of energy loss increases as well, causing the highest frequency harmonics in the signal edge to suffer the most attenuation. This appears as a degradation in the rise and fall times.
Skin Effect and Resistive Losses
The skin effect is the tendency for current to flow preferentially near the outer surface of a conductor. This is mainly due to the larger magnetic fields in higher frequency signals pushing current flow in the perpendicular direction towards the perimeter of the conductor.
As current density near the surface increases, the effective cross-sectional area through which current flows decreases. Resistance increases because the effective cross-sectional area of the conductor is now smaller. Because this skin effect is more pronounced as frequency increases, resistive losses increase with signaling rates.
Resistive losses have a similar effect on the signal as loss tangent. Rise and fall times increase due to the decreased amplitude of the higher harmonics, with the highest frequency harmonics being most affected. In the case of 10Gb/s signals, even the fundamental frequency can be attenuated to some degree when using FR4.
For example, an 8mil wide trace at 1MHz has a resistance on the order of 0.06Ω/inch, while the same trace at 10Gb/s has a resistance of just over 1Ω/inch. Given a 10 inch trace and 1.6V voltage swing, a voltage drop of 160mV occurs from resistive losses of the fundamental frequency, not including the losses in the harmonics and dielectric loss.
Choosing the Substrate Material
The goal in material selection is to optimize both performance and cost for a particular application.
FR4, the most common substrate material, provides good performance with careful system design. For long trace lengths or high signaling rates, a more expensive substrate material with lower dielectric loss must be used.
Substrates, such as Nelco, have lower dielectric loss and exhibit significantly less
attenuation in the gigahertz range, thus increasing the maximum bandwidth of PCBs. At 3.125Gb/s, the advantages of Nelco over FR4 are added voltage swing margin and longer trace lengths. At 10Gb/s, Nelco is necessary unless high-speed traces are kept very short.The choice of substrate material depends on the total length of the high-speed trace and also the signaling rate.
What-if analysis can be done in HSPICE simulation to evaluate various substrate
materials. By varying the dielectric constant, loss tangent, and other parameters of the PCB substrate material. The impact on eye quality can be simulated to justify the use of higher cost materials. The impact of other parameters such as copper thickness can also be explored.
RocketIO GTX Transceiver User Guide
UG198 (v3.0) October 30, 2009
Traces
Traces
Trace Geometry
For any trace, its characteristic impedance is dependent on its stackup geometry as well as the trace geometry. In the case of differential traces, the inductive and capacitive coupling between the tightly coupled pair also determines the characteristic impedance of the traces.
The impedance of a trace is determined by its inductive and capacitive coupling to nearby conductors. For example, these conductors can be planes, vias, pads, connectors, and other traces, including the other closely coupled trace in a differential pair. The substrate properties, conductor properties, flux linkage area, and distance to a nearby conductor determine the amount of coupling and hence, the contribution to the final impedance.2D field solvers are necessary in resolving these complex interactions and contribute to the calculation of the final impedance of the trace. They are also a useful tool to verify existing trace geometries.
A common misconception is that two 50Ω single-ended traces can be routed side-by-side to give a pair with 100Ω differential impedance. While this approximation might be true if the traces are loosely coupled, routing differential traces in a loosely coupled fashion does not maximize the noise immunity of differential mode signaling.
Tightly coupled differential pairs are required for all high-speed transceiver traces because they are more sensitive to noise than slower signals. As a general rule of thumb, tight coupling within a differential pair is achieved by spacing them no more than four trace widths apart.
Wider traces create a larger cross-sectional area for current to flow and reduce resistive losses. Use the widest traces that space constraints allow. Because trace width tolerances are expressed in absolute terms, a wider trace also minimizes the percentage variation of the manufactured trace, resulting in tighter impedance control along the length of the transmission line.
Striplines are preferred over microstrips because the reference planes on both sides of the trace provide radiation shielding. Microstrips are shielded on only one side (by the
reference plane) because they run on the top-most or bottom-most layers, leaving the other side exposed to the environment.
For best results, the use of a 2D field solver is recommended for verification.
Trace Characteristic Impedance Design
Because the transceivers use differential signaling, the most useful trace configurations are differential edge-coupled center stripline and differential microstrip. While some backplanes use the differential broadside-coupled stripline configuration, it is not recommended for 10Gb/s operation, because the P and N vias are asymmetrical and introduce common-mode non-idealities.
With few exceptions, 50Ω characteristic impedance (Z0) is used for transmission lines in the channel. In general, when the width/spacing (W/S) ratio is greater than 0.4 (8mil wide traces with 20mil separation), coupling between the P and N signals affects the trace impedance. In this case, the differential traces must be designed to have an odd mode impedance (Z0O) of 50Ω, resulting in a differential impedance (ZDIFF) of 100Ω, because ZDIFF=2xZ0O.
RocketIO GTX Transceiver User GuideUG198 (v3.0) October 30, 2009
Chapter 13:Design of Transitions
RocketIO GTX Transceiver User Guide
UG198 (v3.0) October 30, 2009
Section 3: Appendices
RocketIO GTX Transceiver User Guide
UG198 (v3.0) October 30, 2009
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