Stackup Dielectric Thickness

A stackup is usually specified by the number of layers and by the finished board thickness, and both of those numbers can be met while the dielectric thicknesses inside the board are wrong. The inner layer spacing is what sets the impedance, the crosstalk and the coupling, so it deserves to be specified explicitly rather than left to the shop.

What Sets the Finished Thickness

The finished thickness is the sum of the core thicknesses, the prepreg thicknesses after cure and the copper thicknesses. Each of those is itself a range, and the ranges do not simply add: resin flows during lamination and the prepreg thins by an amount that depends on the copper beneath it.

That is why a stackup nominal that has been calculated from catalogue values is often outside tolerance when the board is measured. Our article on plating thickness explains the same principle for the copper that is added after lamination.

Dielectric Thickness and Impedance

Impedance is a function of the trace geometry and of the dielectric thickness between the trace and its reference plane. A change of ten percent in that thickness produces a change in impedance of roughly five percent, which is the whole tolerance in many designs.

The stackup drawing should therefore show the target dielectric thickness for the impedance layers and should state the tolerance that applies, rather than only the finished board thickness. Our article on impedance tolerance verification describes how the result is checked.

Resin Flow and the Copper Effect

Prepreg is partly resin and partly glass, and the resin flows during lamination to fill the space around the copper features. Where there is a lot of copper, less resin is available to fill the gap, and the dielectric ends up thinner than the catalogue value.

The correction is made by the shop from experience with the pattern, but the designer has to allow the shop the freedom to adjust. A stackup that is specified to the last hundredth of a millimetre and marked as non negotiable removes that freedom and often produces a worse result.

Choosing Between Thick and Thin Dielectrics

A thin dielectric gives tight coupling and controlled impedance with narrow traces, which helps routing density. It also increases the capacitance per unit length and makes the layer more sensitive to any variation in thickness.

A thick dielectric is more forgiving and cheaper to hold to tolerance, at the cost of wider traces for the same impedance. The trade should be made with the fabrication capability in mind, as described in our article on aspect ratio.

Layer Assignment and Symmetry

The order of the layers affects the result as much as their thicknesses. A signal layer sandwiched between two planes is shielded from both sides, while a signal layer next to another signal layer couples to it directly.

Symmetry around the centre line is also a manufacturing requirement, because an asymmetric stackup tends to warp. Our article on high speed design rules covers the assignment method for a controlled impedance design.

Specifying the Stackup

The drawing should list the layers in order, the copper weight, the dielectric thickness for each layer and the material where it matters. It should also state the finished thickness and its tolerance, and it should say which layers carry impedance controlled traces.

Leaving the material to the shop is acceptable for a general purpose board, but not for a high frequency design where the dielectric constant sets the result. In that case the material should be named.

Verification

The verification is a cross section on a coupon, measured on the same panel that carries the product. The dielectric thicknesses and the copper thicknesses are read from the image, and the impedance is measured on a test coupon built alongside.

The records should be kept for the stackup rather than for the order, so that a repeat build can be compared with the original. Our article on PCB test coupon describes what the coupon should contain.

Checks Before Release

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel.

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Verification and Records

A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

Points to Confirm at First Article

The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

FAQ

Can the finished thickness be achieved with any stackup? Within limits. The shop can vary the prepreg and the copper, but the dielectric thicknesses that the design needs are what constrain the choice, not the total.

Does dielectric thickness affect crosstalk? Strongly. Thinner dielectrics on the same layer increase the coupling between adjacent traces, so the spacing rules have to follow the stackup.

Should the stackup be locked to one shop? Not normally. A stackup that can be built by more than one supplier is more robust, which means stating the requirement rather than the exact build.