Aspect ratio

Of a PCB via, the ratio of board thickness to via diameter. Typically less than 8, the aspect ratio provides a guide to the manufacturability of a board. (For higher aspect ratios, it is more difficult to achieve reliable plating.)

Back-drilling

Removing the plating from the "stub" of a through plated hole by drilling out the plated copper up to (but not including!) the lowest layer which is used for interconnect. This minimises the resonance caused when a stub is present because the stub of the via will otherwise see the signal propagate to the base of the stub and back and any stub length appears as twice its length because of this. Stubs will cause a dip in S21 at their resonant frequency.

Backward crosstalk

See near-end crosstalk.

beta

Letter of the Greek alphabet used to describe the imaginary part, the phase, of the propagation constant; also known as the phase constant.

Bikini Build

In flex rigid construction, the flex coverlay can either extend fully across the rigid section, or just extend a small way into the rigid with "no flow" - more correctly "reduced flow" pre preg for the remaining rigid part of the stackup. When the coverlay is only partially into the rigid stackup this is referred to as a "Bikini build" flex rigid construction.

Boundary Element Method (BEM)

A numerical computational method of solving linear partial differential equations. Conceptually, it works by constructing a "mesh" over the modeled surface and calculating only boundary values rather than values throughout the volume of space.

Broadside-coupled

On a PCB, differential traces that are physically above and below each other.

Common impedance

The impedance of a pair of conductors when driven by identical signals, i.e. the impedance of two single lines in parallel. If the signal conductors are identical and balanced, the common impedance is half the even-mode impedance. See even-mode impedance.

Complex electric permittivity

The full, real and imaginary description of a material property that is better known by its component parts. The real part of the complex electric permittivity is usually known as the dielectric constant. In the PCB industry, the imaginary part of the complex electric permittivity is usually combined with other terms and quoted as the loss tangent.

Conductivity

A measure of a material's ability to conduct an electric current. Electrical conductivity is defined as the ratio of the current density to the electric field strength. Copper has a typical conductivity of 59.6 x10⁶ Siemens/meter. Conductivity is the reciprocal of resistivity.

Conductor Loss (copper loss)

The energy lost in the conductors in the signal and return path.

Controlled Impedance

A specific impedance that is maintained by design along the length of the transmission line. See impedance.

Coplanar

(With respect to transmission lines). A transmission line structure where the return paths are on the same electrical layer as the signal path. Useful for transmission lines on single sided PCBs and where used on flex has the added benefit of more mechanical flexibility than a microstrip structure - and as a win win it removes the need to use crosshatch or meshed ground planes.

Core

In PCBs, a thin piece of dielectric (cured fibreglass-epoxy resin) with copper foil bonded to both sides.

Crosshatch (Mesh)

Crosshatching (meshing) ground planes is a technique to increase flexibility on multilayer flex constructions. It has a secondary benefit of increasing the line width for a given impedance making thin construction of impedance controlled traces more viable. Care should be taken with mesh size so the upper frequency of operation is well below 1/4 wavelength of the mesh size. An alternative to using mesh planes is to use a single sided coplanar transmission line structure.

Crosstalk

The effect whereby a signal on one conductor will create an effect on an adjacent "victim" conductor due to mutual capacitance and mutual inductance. In general, crosstalk will increase with higher frequencies or faster risetimes.

Delta-L

Insertion loss test method based on long line / medium line length test coupon.

Dielectric Constant

In the PCB industry, this refers to the relative dielectric constant, represented as ε_r or Dk, describing the property of a material that determines the electrostatic energy that can be stored relative to a vacuum. The dielectric constant affects the properties of transmission lines. The dielectric constant of a material varies as a function of frequency, temperature & moisture absorption. The dielectric constant typically quoted only refers to the real portion of the complex dielectric constant. See also complex electric permittivity.

Dielectric Loss

The energy lost by dissipation in the dielectric, e.g. through leakage current and dipole reorientation.

Dielectric materials

Insulators that are used to provide separation between conductors. Examples are air and FR-4. Their inherent dielectric constant affects how the conductors behave electrically.

Differential impedance

The series impedance of a pair of conductors when driven by a differential signal. If the signal conductors are identical and balanced the differential impedance is twice the odd-mode impedance. See odd-mode impedance.

Dispersion

The phenomenon in cables and PCBs whereby the propagation speed of the signal along the length of the conductor varies with frequency and with different propagation modes. This causes signal distortion as the phase relationship of harmonics change along the conductor.

Edge-coupled

On a PCB, refers to differential traces that are side by side.

Electrical length

The total time it takes for a signal to traverse a conductor. See also propagation delay.

Even-mode

A signal pattern on a of pair conductors, where each line is driven with exactly the same signal. See propagation mode.

Even-mode impedance

Perhaps more correctly "even impedance" The impedance of a single line in a coupled-line pair when a common signal is applied to the conductor pair. See even-mode and common impedance.

Far end

In a transmission line, refers to the non-driven end of the transmission line (the receiver end).

Far-end (forward) crosstalk

The crosstalk measured near the end opposite the driven end of the active signal line(s) caused by the difference in velocity of the different modes of propagation. See also propagation mode.

FEXT

A unitless coefficient of the maximum magnitude of far-end (forward) crosstalk. This coefficient can be approximated by its relationship with K_f, coupled line length and aggressor signal risetime.

Flex-rigid

Or in equal use "Rigid-flex" the use of flexible PCBs in place of traditional connectors to produce a one piece Rigid flex Rigid PCB all laminated together as a single component. The variety of rigid flex is diverse, from flex to fit designed for only a few flex cycles to fully flexible designed for repeated flexing. Flex rigid can be labor intensive to build and can incorporate features such as bookbinder construction where some of the flex interconnects are longer than others to allow the bend radius sufficient room for folding as in the cover of a book or magazine.

Fiber weave mitigation

Running the PCB traces at a slight angle across the warp and woof of the glass cloth in the base material to reduce the effect of dielectric constant variation because of the differing Er of Glass and resin. Alternately spread glass cloth is being used to the same effect, or albeit at higher cost - low Er glass.

Forward crosstalk

See far-end crosstalk.

Goal seek Er

A practice of attempting to deduce the "true" Er of a material by goal seeking the Er until the measured impedance meets the modeled value. Great care should be taken with this as Er is only a second order driver of impedance, and the practice risks masking dimensional errors or measurement errors which have a first order impact on correlation.

Green’s functions

Named after 19th century British mathematician and physicist, George Green who introduced several important concepts in electricity and magnetism, including the idea of potential functions used to solve inhomogeneous differential equations subject to boundary conditions.

Groisse

An empirical technique from the for estimating losses owing to surface roughness which is valid for designs up to around 10GHz and sits between Hammerstad and Huray methods. Groisse has been shown to yield a good estimation up to around 7 to 10GHz without the demanding input criteria of Huray

Ground bounce

The "raising" or "lowering" of the voltage on a ground plane or ground pin due to the inductance of the ground path. This can cause the receiving circuitry to misinterpret a 1 or 0.

Group velocity

The rate that changes in amplitude (known as the envelope of the wave) will propagate.

Hammerstad

An empirical technique from the 1940s for estimating losses owing to RMS surface roughness which is still valid for designs up to around 4GHz.

HDI

Generically a PCB with higher wiring density than a traditional PCB, though specifically HDI PCBs tend to refer to those PCBs which have additional outer layers "built up" by repeated runs through the production process and using laser microvias to interconnect to the inner layers. HDI differs from Sequential lamination, in that a Sequential laminated PCB may not have laser microvia layers built up on the outers.

Huray

A more recent methodology for estimating losses owing to surface roughness based on more accurate SEM imaging of the deposited copper topology and appropriate up to 50GHz +. Huray does require more complex input parameters or an additional estimating tool to generate the Huray input parameters from RMS roughness data.

Impedance

The instantaneous ratio of voltage to current at a point along a conductor, measured in ohms. The complex impedance of a transmission line (Z₀) is calculated from the resistance, inductance, capacitance and conductance of the line.

Insertion loss

The losses resulting from the insertion of a transmission line between source and load (usually expressed in dB).

K_b

A unitless coefficient of the backward, or near end, crosstalk.

K_f

A coefficient of the forward or far-end crosstalk measured in seconds/ distance. This coefficient needs additional terms of coupled length and aggressor risetime to be useful in assessing the magnitude of the crosstalk. This approximating coefficient can be calculated by a generalising formula either using the difference between capacitance and inductive terms or using the difference between odd and even mode velocity. For further information on the validity, constraints and usefulness of this coefficient refer to any textbook on signal propagation and coupling. Polar Application Note AP194 includes a list of recommended reference books.

Launch Point Extrapolation (LPE)

Characteristic impedance measurement technique for fine line and series resistive traces. LPE projects the TDR trace back to an imaginary "launch point" where the impedance is predicted untouched by the sloping effects on the trace of series resistance.

Loss angle

The angle described by the imaginary part of the complex dielectric constant to the real part of the dielectric constant.

Loss tangent

The tangent of the loss angle, and generally referred to as tanD.

Maxwell

Nineteenth century scientist, widely regarded as having had the greatest influence on twentieth century physics, making important contributions to expressing the basic laws of electricity and magnetism. The Maxwell RLGC matrices are derived as a result of solving Maxwell’s equations. They can be easily transformed into SPICE values that may be required for common circuit simulators.

Microstrip

A specific transmission line on a PCB where the signal trace is on an outside surface of the PCB and is spaced above a ground plane by the dielectric material, such as FR4.

Mutual capacitance and mutual inductance

The capacitances and inductances between two conductors in close proximity to each other. These can cause interaction between the signals on the conductors in the form of crosstalk.

Near end

Refers to the driven end of a transmission line.

Near-end (backward) crosstalk

The crosstalk measured on the victim line near the driven end of the active signal line(s) and is a measure of the difference in even-mode and odd-mode impedance.

Odd mode

A signal pattern on a of pair conductors, where each line is driven with an equal but opposite transitioning signal. See propagation mode.

Odd-mode impedance

The impedance of a single line in a coupled-line pair when a differential signal is applied to the conductor pair. See odd mode.

Phase velocity

The velocity at which the phase of any one frequency component of the wave will propagate. This is not the same as the group velocity of the wave.

Prepreg

In PCBs, uncured fibreglass-epoxy resin composite that cures (hardens) when heated and pressed during PCB fabrication.

Propagation constant

The propagation constant, occasionally represented by the Greek letter (insert small letter gamma), is a complex quantity describing the behavior of an electromagnetic wave along a transmission line. The real part of the propagation constant is denoted by the Greek letter alpha, and the imaginary part is denoted by the Greek letter beta.

Propagation mode

A signal pattern on conductors that will propagate down the transmission line undistorted. For differential pair signalling, there are two modes of propagation, called odd mode and even mode. For multi-conductor systems there are even more modes of propagation.

Propagation delay

The speed or rate at which a signal travels between the input and output of a transmission line. See also electrical length.

Reflection

In electricity, the result of impedance mismatch in electrical signals. When a signal encounters a discontinuity, some energy is reflected. Impedance discontinuities cause attenuation because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver. Reflection coefficient is the ratio of the reflected voltage wave to the incident voltage wave at a junction. It is dependent on material and geometry, and can be positive or negative.

Resin starvation

Inadequate quantity of epoxy resin to allow complete fill of the voids between the copper tracks and features of the adjacent stackup layers, resulting in a PCB which does not bond completely and is therefore prone to delamination. See also spread glass.

Return loss

Of a load, is the magnitude of the reflection coefficient expressed in decibels.

Sequential lamination

A method of allowing more dense via interconnect by having vias only running partially through the PCB. Conventional PCBs run the vias from the top to the bottom and the whole via is plated. By in effect making two PCBs (or more) and mechanically drilling and plating each then laminating the sub PCBs together it is possible to make PCBs with vias on the sub PCBs and through the final laminated PCB itself. Still further layers may be added and on these extra outer layers the interconnect can be added using laser microvias, and the PCB is then referred to as an HDI PCB. Sequential laminated PCBs themselves do not strictly contain laser microvias.

SET2DIL

Insertion loss measurement methodology for PCB fabrication which works best on thin or backdrilled PCBs and outer layers. The measurement is made on a test coupon specifically designed for SET2DIL test.

Single-ended transmission

Electrical transmission method that uses a single conductor to carry the signal and a common plane to provide the return path.

Skew

The difference in the time that a pair of identical signals takes to get from point A to point B travelling down two different paths. Skew is the result of the different electrical lengths of each signal path.

Skin effect

The phenomenon whereby the signal travelling through a conductor will be conducted only on the outer surface (skin) of the wire as the frequency increases. At lower frequencies the current travels through the entire cross section of the conductor. At 1 GHz the signal travels only on the outer 2 microns (0.00008 inches) of a copper wire.

Smith Chart

A graphical aid to assist solving problems with transmission lines, as it demonstrates how many RF parameters behave at one or more frequencies, an alternative to using tabular information. The Smith Chart can be used to represent many parameters including impedances, admittances, reflection coefficients, scattering parameters, noise figure circles, constant gain contours and regions.

S-parameters

S-(scattering) parameters are properties used to describe the electrical behavior of linear electrical networks when undergoing various steady state stimuli by small signals. In general, for practical networks, S-parameters change with the measurement frequency and system impedance. Many useful electrical properties of networks or components may be expressed using S-parameters such as gain, return loss, voltage standing wave ratio (VSWR) and reflection coefficient. The S-parameters calculated by the Polar Si9000 employ a driver and line termination of 50 ohms.

Spread glass

Glass cloth mechanically manipulated by various processes to reduce the "holes" between the warp and weft (woof). This type of cloth is easier and more predictable for laser drilling and has the added benefit of less Er variation so too benefits signal integrity. Users must take care to allow adequate resin levels for bonding as unlike traditional weaves, spread glass does not have the apertures that allow resin to flow from one side of the cloth to the other.

SPICE

One of the most common electronic circuit simulators in use, simulating how a circuit with chosen components will perform in a defined situation. A model of a transmission line may contain the inductance, capacitance, impedance and time delay and their changing relationship with frequency. The RLGC SPICE matrices are the numbers generally required as input by SPICE models. See Maxwell.

SPP

Insertion loss measurement technique which uses "long line / short line" measurements to determine the insertion loss of a PCB substrate. The measurement is made on a test coupon specifically designed for SPP test.

Stackup

Sometimes "buildup" the sequence of ink / soldermask mask / copper foil / pre preg / core etc that comprises the multilayer structure of a PCB.

Stripline

A specific transmission line on a PCB where the signal trace is buried within the PCB and is spaced between two planes by the dielectric material, such as FR-4.

Surface resistance

Calculated as 1/(conductivity x skin depth)

TDR (time domain reflectometer)

A combination of a sampling oscilloscope and a fast risetime signal source that can be used to examine the step response and measure the impedance of a transmission line. Post processing of TDR measurements can also transform the results into the frequency domain, for example for the measurement of insertion loss.

Transmission line

Commonly used to denote a controlled impedance conductor path. Geometry and dielectric materials determine many of the properties of a transmission line.

VNA Vector Network Analyser

Measurement instrument for making frequency domain measurements. Combined with mathematical post processing these can also be transformed into the time domain.

Velocity of propagation

The speed at which the signal travels from one point to another along a conductor. The materials surrounding the conductor and the conductor geometry affect this speed. In air the velocity of propagation is 300 million meters per second and in FR-4 printed circuit boards it is about half that speed.