Controlled Impedance

For Increasing Impedance Modeling Requirements

With the increasing speeds of modern circuitry, the demand for high quality controlled impedance

printed circuit boards continues to grow. Today’s PCBs are not just simple electrical interconnection

devices; they are complex, highly specified components in their own right. As the demand for

controlled impedance PCBs has risen, there has been a subsequent increase in requirement to verify

these board designs prior to manufacture.

·         Differential Impedance PCB Structure

·         Single Ended Impedance Modeling

·         Microstrip and Stripline Constructions

·         All new field solvers called Boundary Element Method (BEM)

·         Model soldermask thickness between and adjacent to tracks

 

High speed PCBs require accurate controlled impedance traces to operate reliably.

Impedance Simulator handles most common types of characteristic impedance requirements. Users

define formulas based on their processes. Both the Impedance Control and Stackup modules are tied

to the design. After users modify an impedance tolerance, the system automatically modifies the

associated line width and stackup.

As PCB signal switching speeds increase today’s PCB designer needs to understand and control the

impedance of PCB traces. With the short signal transition times and high clock rates of modern digital

circuitry, PCB traces need to be considered not as simple connections but as transmission lines.

What is controlled impedance?

Probably the most common example of a controlled impedance component is the download (or

feeder) connecting a receiving aerial to a wireless or television set. Aerial feeder leads usually take the

form of “flat twin” cable (commonly supplied with VHF broadcast receivers) or low-loss coaxial cable.

In both cases the physical dimensions and material of the cable control the impedance of the feeder.

You can think of PCB traces as short cables, precisely constructed, connecting the devices mounted

on the board, where the PCB trace, like the coax inner conductor, carries the signal and is insulated

from its return path (in this case a ground plane) by the board laminate. This is shown in cross section

in the microstrip configuration. The dimensions for trace width W1 and W2, thickness T1 and laminate

height H1 and H2 and the dielectric constants Er1 and Er2 must be strictly controlled. Solder resist on

the surface reduces the impedance slightly so the more predictable stripline configuration is often

used.

 

So why do we need to control impedance?

The receiving aerial possesses a natural, or characteristic, impedance. Electrical theory shows that for

the aerial to transfer maximum power to the set (and to ensure the integrity of the electrical signal),

the impedance both of the feeder and the receiver should match that of the aerial. In other words, the

signal should ideally be presented with a constant impedance as it travels from its source to its

destination. Where a mismatch occurs, only part of the signal will be transmitted and the rest will be

reflected toward the source (which degrades the signal). Cable designers, therefore, take great care to

ensure the accuracy and consistency of the cable dimensions and material characteristics. At high

signal switching speeds the electrical properties of the cable (such as the capacitance and

inductance) must be taken into account, and cables can no longer be considered as simple wires.

Cables designed for high signal speeds, where these factors are taken into consideration, are referred

to as transmission lines.

Controlled impedance on PCBs

Similarly, as the speed of signal switching on a PCB increases, the electrical properties of the traces

carrying signals between devices become increasingly more important. The impedance of a PCB trace

is controlled by its configuration dimensions (trace width and thickness and height of the board

material) dielectric constant of the board material.

As with a cable, when the signal encounters a change of impedance arising from a change in material

or geometry, part of the signal will be reflected and part transmitted. These reflections are likely to

cause aberrations odn the signal (e.g. low gain, noise and random errors), which may degrade circuit

performance. In practice board designers will specify impedance values and tolerances for board

traces and rely on the PCB manufacturer to conform to the specification.

Testing the PCBs

Most controlled impedance PCBs undergo 100% testing. However, it is not uncommon for the actual

PCB traces to be inaccessible for testing. In addition, traces may be too short for accurate

measurement and may well include branches and vias which would also make exact impedance

measurements difficult. Adding extra pads and vias for test purposes would affect performance and

occupy board space. PCB testing is therefore normally performed not on the PCB itself, but on one or

two test coupons integrated into the PCB panel. The coupon is of the same layer and trace

construction as the main PCB and includes traces with precisely the same impedance as those on the

PCB, so testing the coupon affords a high degree of confidence that the board impedance will be

correct.

 

Measuring controlled impedance

Impedance measurements are usually made with a time domain reflectometer (TDR). The TDR applies

a fast voltage step to the coupon via a controlled impedance cable and probe. Any reflections in the

pulse waveform are displayed on the TDR and indicate a change in impedance value (this is known as

a discontinuity). The TDR is able to indicate the location and scale of discontinuity. Using appropriate

software the TDR can be made to plot a graph of the impedance over the length of the test trace on

the coupon. The resulting graphical representation of the trace characteristic impedance allows

previously complex measurements to be performed in a production environment.