Impedance Controlled Stackup Design Guide

An impedance controlled stackup is a build in which the dielectric thickness of each layer is chosen so that a defined trace geometry gives a defined impedance. It is a joint decision between the layout and the shop, because the impedance depends on dimensions that the shop controls as much as on the width that the designer draws. A stackup that is released without that agreement will be built to a default and the impedance will be whatever the default produces.

What the Stackup Has to Deliver

The stackup has to deliver a defined impedance for every controlled line on every signal layer, including the single ended and the differential lines and the lines that run on the outer layers.

It also has to deliver the total board thickness and the mechanical properties that the product needs, and those requirements can conflict with the electrical ones. A thin dielectric is good for the impedance control and poor for the mechanical stiffness.

The number of layers comes from the routing and from the plane requirements, and the layers then have to be filled with available material combinations. The result is a set of dielectric thicknesses that are close to the targets but not exactly equal to them.

The stackup drawing should therefore show the target impedance and the actual dielectric thickness for each layer, rather than an idealised build that cannot be purchased.

The Variables That Set Impedance

The impedance of a trace depends on its width, its thickness, its distance to the reference plane and the dielectric constant of the material around it. Those four variables are the whole of the calculation.

The trace width and the dielectric thickness are the two that the layout and the shop control directly. The copper thickness is set by the foil and the plating, and the dielectric constant is set by the material.

The structure also matters, because a microstrip has one dielectric layer below it and air above, while a stripline is embedded between two layers. The same width gives a different impedance in the two structures.

The routing guide for microstrip and stripline describes both, and the choice is usually made with the layer count rather than separately.

Choosing the Dielectric Thickness

The dielectric thickness is the primary lever, because the impedance falls as the trace gets closer to the plane. A thin dielectric allows a narrow trace for a given impedance, which is useful in a dense area.

A thin dielectric also gives a tighter field and a lower loss per unit length, which is good for a high speed line. It is harder to manufacture consistently, because the tolerance of the thickness becomes a larger fraction of the total.

The available thicknesses come from the material combinations, and a shop can only build what the sheet suppliers offer. A stackup that requires an unusual thickness will be built with a substitute or will cost more.

The dielectric thickness is also the term that varies most from panel to panel, because the press compresses the stack. A nomimal value with a wide tolerance produces an impedance with an equally wide spread, which is why the tolerance of the stackup is quoted together with the impedance target.

The thickness also sets the mechanical properties of the finished board, so a very thin dielectric between two planes reduces the stiffness and can make the panel harder to handle.

Trace Width and the Fabrication Window

Once the dielectric thickness is fixed, the trace width is calculated from the target impedance. The result is a nominal width that the shop has to hold within a tolerance.

The width is affected by the etching process, which removes copper from the sides of the trace as well as from the spaces. A wide trace loses proportionally less than a narrow one, so a fine line is harder to hold.

The copper thickness also affects the width, because a heavier foil needs more etching and undercuts more. A design with a fine line and a heavy copper is a combination that the shop will struggle with.

The practical approach is to state the impedance and the tolerance, and to let the shop adjust the width to suit its process. The width on the drawing is then a starting point rather than an absolute.

Tolerance and Shop Capability

The impedance tolerance is usually quoted as a percentage, and a common figure is ten per cent. The figure has to be achievable, because a tolerance that is tighter than the process can hold becomes an inspection problem rather than a design feature.

The tolerance is made up of the width tolerance, the thickness tolerance and the material variation. Each contributes, and the combination is what the shop has to control.

A shop that has a proven process will quote a capability, and the design should be checked against it rather than against a generic figure. The high speed design rules give the context for the values.

Where a tighter tolerance is required, the shop can measure and adjust, but the cost rises because each panel has to be checked. That is a legitimate choice for a critical link and not for a general purpose board.

Test Coupons and Verification

A coupon is the only way to prove that the impedance was achieved. It carries traces of the same geometry as the product, on the same stackup, and it is measured with a time domain reflectometer.

The coupon is a test coupon in the formal sense: a separate structure that is not part of the product and that exists only to be measured. It costs almost nothing to add to the panel and it removes the argument about whether the impedance was met.

The coupon should include a single ended line and a differential pair for each controlled layer, and it should be placed where it sees the same plating and etching as the product.

The measurement is usually made on a sample basis, and the result is recorded with the panel documentation. A trend across a batch is a process signal and should be investigated before the whole batch is delivered.

The coupon design is described in the coupon material, and the layer arrangement in the stackup guide. Both are worth reading before the stackup is released.

Practical Rules

Agree the stackup with the shop before the layout is finished, and use the thicknesses that the shop can actually buy.

State the impedance target and the tolerance on the drawing, and let the shop adjust the trace width within the tolerance.

Keep the coupon on the production panel, with the same geometry and the same stackup as the product.

Record the measured result with the stackup, so that a later revision or a reorder can be compared with the same data.

FAQ

What tolerance is normal for controlled impedance? Around ten per cent is common, and a tighter value costs more because the panels have to be measured individually.

Why does the shop adjust the trace width? Because the etching and the plating change the width that the artwork defines. The shop tunes the width to hit the impedance, which is the number that matters.

Does a thin dielectric always help? It gives a tighter field and a lower loss, and it makes the impedance easier to control at a narrow width. It also makes the thickness tolerance a larger fraction of the total.