Controlled Impedance: What Sets It and How It Is Verified

Impedance is the property that links the physical layout to the electrical behaviour of a trace, and it is the one parameter that cannot be adjusted after the board is made. Understanding what sets it makes the constraint manageable rather than mysterious.

What Determines the Impedance

The impedance of a trace is set by its width, the thickness of the dielectric between it and its reference plane, the dielectric constant of that material and the thickness of the conductor. Of those, the dielectric thickness has the strongest effect.

That is why the stack-up is a design document rather than a manufacturing detail. A change of fifty micrometres in the dielectric moves the impedance more than a substantial change in trace width. Our laminate notes describe the material properties.

Single Ended and Differential

A single ended line references a plane and has one impedance. A differential pair has two traces and two impedances: the odd mode, which is what a differential signal sees, and the even mode, which is what a common mode signal sees.

The odd mode impedance depends on the coupling between the pair, so the spacing is part of the specification. A pair that is drawn with a different spacing from the one the impedance was calculated for is a pair with the wrong impedance.

Reference Planes and What Counts as One

A plane acts as a reference if it is continuous beneath the trace and if it is connected to the return of the signal. A plane that is split, or that is a power plane not connected at the relevant frequency, does not act as a reference in the same way.

Where a trace passes over two different planes, the return current has to jump between them, which requires a stitching capacitor. Without one, the impedance rises at the transition and a reflection results.

Calculating and Verifying

The calculation uses a field solver or a closed form approximation, and it produces a width for a given stack. The result is a starting point, because the process achieves a slightly different geometry from the drawn one owing to etching and plating.

The verification uses a test coupon with the same geometry, measured with a time domain reflectometer. The measurement is what the acceptance criterion applies to, not the calculation. Our test coupon notes describe the patterns.

Etching and the Achieved Width

Etching removes copper from the sides as well as from the top, so the finished trace is narrower than the drawn one and its cross-section is trapezoidal. The effect is larger for thicker copper and for wider spaces.

The shop compensates by drawing the artwork wider, using a factor that depends on the copper weight and on the process. The compensation is part of what the coupon verifies, and it should be confirmed rather than assumed.

Tolerance and What It Costs

The impedance tolerance is set by the system requirement, and a tighter tolerance costs more because it requires tighter control of the dielectric thickness and of the etching. Half of the designs that specify a five per cent tolerance would work with ten.

The tolerance is normally quoted for the coupon rather than for every trace, because the coupon samples one geometry from one panel rather than every line on every board.

Where Impedance Is Not the Right Model

At low frequency, or over a short distance, the impedance is not a useful concept because the return current is not confined beneath the trace and the line behaves as a simple conductor. The transition is gradual rather than sharp.

The practical test is the electrical length. Where the trace is a small fraction of a wavelength at the highest frequency of interest, the impedance matters less than the resistance, and the design effort belongs elsewhere.

Documenting the Requirement

The drawing should state the target impedance, the tolerance, the layer and the width for each controlled line, and the coupon that will be used for verification. A single impedance figure quoted for a whole board is not usable.

Where several targets exist, they should be tabulated. The table is also what allows a change to the stack to be evaluated, because the effect of the change on each target is visible. Our fabrication notes guidance describes the entries.

Process Control and Verification

On a design of this kind, etch compensation is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Process Control and Verification

On a design of this kind, etch compensation is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Process Control and Verification

On a design of this kind, etch compensation is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

FAQ

Can impedance be measured on the product board? It can be measured where the geometry is accessible, and the measurement is easier on a coupon because the pattern is designed for it. The coupon is preferred because it does not disturb the product.

What happens if the stack changes after the layout is complete? Every controlled line changes impedance, so the layout has to be re-checked. Keeping the stack fixed after the release is one of the reasons it belongs early in the review.

What does gopcb provide for an impedance controlled board? We provide the stack with the dielectric thickness and constant at the frequency of interest, the calculated width for each target, the etch compensation applied, and a coupon measurement with the shipment.