Material Considerations for High Speed PCB Design

September 27, 2018 , in Blog

It’s not that I’m any good at it, but I love the game of tennis. I have been a fan for a while and would not miss an opportunity to watch John McEnroe finesse his way through a game. Although his antics drew significant attention, it was magical watching him outplay bigger and stronger players. He managed to do this with a wooden racket, which suited his style of play. Today, with hard-hitting players like Serena Williams and serves commonly exceeding 100 mph, that equipment would be woefully inadequate.

If you want to achieve the best results in any endeavor, using the right equipment is the best thing that you can do. This is certainly true for high speed PCB design where the right equipment means proper components and the best board material. There are a number of material considerations that can affect the performance of your high speed PCB signals, including signal decay and controlled impedance. First, let’s look at the PCB material properties that impact performance; you will then be better equipped to make board material selections that align with your high speed PCB design requirements.

Inadequate equipment

Material Properties for High Speed PCB Design

In today’s world of high-frequency signals and fast switching devices, it is highly likely that you have been, or will be, called upon to do high speed PCB design. There is a bit of a gray area as to what constitutes a high speed PCB. For example, some say you’re working with high speed if your design contains several sub-circuits that operate independently within the greater overall design. Others classify high speed as having trace lengths long enough that at least ⅓ of the rise time of your signal is achieved. Most designers would agree, however, that signal frequencies determine high speed. Generally, this refers to anything over 50MHz, but boards classified as “good” for high speed are typically designed to accommodate frequencies from 500MHz to 2GHz. This is because in this approximate frequency range, signal integrity issues start to influence PCB performance and board material choices have a bigger impact on overall board design.

At high frequencies, signal integrity may be affected by board properties that are electrical or mechanical. Chief among these are the dielectric constant, dk, dissipation factor, df, dielectric loss, conductor loss, Ploss, and the PCB stackup . To ensure these properties don’t adversely affect signal integrity, the signal decay (more accurately the signal decay as a function of frequency) is limited and the impedance is controlled.

Signal decay – Typically denoted as exponential decay, this is the reduction of the signal strength over time, based on the signal type or form and frequency. The controlling element is the time constant, 𝜏, based on the impedance (R + jX) of the system, which, for our purposes, is the board material including the trace or via through which the signal passes. Since a PCB can be classified as a linear time-invariant system1, the following equations may be used to analyze the decay effect on signal integrity.

V(t) = Vo(t)exp(-t/𝜏) for fixed magnitude signal (e.g. digital switching)     (1)

where  V(t) is the signal strength at time t

Vo(t) is the initial signal strength

𝜏 is the time constant (usually denoted by RC or L/R depending on the transmission line model used for the signal path)

V(t) = Vo(t)exp(-t/𝜏) + (A/(j𝜔 + t/𝜏))(exp(j𝜔t) – exp(-t/𝜏)) for analog signal     (2)

where  A is the analog signal peak amplitude

𝜔 = 2𝜋f is the angular frequency

For both Eqs(1 and 2), the larger the time constant, the longer the time required for the signal strength to drop.

Controlled impedance – Impedance control is a critical factor in high speed PCB design. For the best signal integrity, it is desirable to maintain a consistent impedance throughout the signal path and match the impedance for differential pairs. This is done by using identical trace lengths, copper weights and fixed distance between the signal and return traces. Preventing impedance mismatch will reduce or eliminate transmission line reflections that reduce signal magnitude and cause frequency shifts.

Example of differential traces on a PCB

Differential traces

From Ohm’s Law, we know that resistance, the real part of impedance, attenuates signal strength over distance. It is also known that the imaginary part of impedance, reactance (derived from capacitance and inductance), shifts signals away from their base frequency. To mitigate the impact of these signal integrity issues, the characteristic impedance (Z0) is used to determine board properties, like dk, trace widths and layer thicknesses. Or, conversely, the impedance is determined for given board material properties.

Z0 = [(R + jωL)/(G + jωC)]0.5 basic characteristic equation     (3)

There are closed form solutions to Eq. (3) for microstrip, stripline, and grounded coplanar-waveguide (GCPW) transmission line model types, including single-ended and differential pairs. Additionally, a free downloadable tool is available from Rogers Corporation.

High Speed PCB Design Material Selection

Before selecting the material for your high speed PCB design, it is necessary to determine a value (or values) for dk and Z0 for your transmission line (or lines). Your PCB design software may allow you to set these values and include them as part of the design file(s) for your contract manufacturer (CM). If not, there are dk charts and impedance calculators online to help you arrive at the proper values. Now, you are ready to implement the 2-step solution for your high speed PCB design material selections!

STEP 1: Select board material types

Choose material type from types recommended for high-frequency PCBs. This includes selecting core, prepreg and substrate materials. You may be able to capitalize on hybrid construction, where signal layer material is chosen for high frequency, but other layers may use other materials to reduce fabrications costs.

STEP 2: Select board material thicknesses and copper weights

Use your calculated or preferred values for dk and Z0 to select thickness and copper weights. Remember to maintain impedance consistency throughout signal paths.

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Your CM should be a part of your material selection process as the board fabrication and PCB assembly stages may require modifications to your selections before your boards can be made. Tempo Automation, the industry leader in fast, precise PCB prototype manufacturing, is ready to partner with you and assist you in optimizing the material selection process.

And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on high speed PCB design or making material selections for your board, contact us.

 

 

 

 

 

 


1 Haider, Michael and Johannes A. Russer. 2018. “Computer Aided Analysis of EMI Radiated from Printed Circuit Boards.” 2nd URSI AT-RASC. http://www.atrasc.com/content/stick/papers/URSIpcbcstsummary02updated.pdf.

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