Board Thickness Selection and Stack Up Tolerance

Board thickness is usually quoted as a single number, but that number is the sum of several layers each with its own tolerance. Understanding where the figure comes from decides whether the specification can be met and at what price.

How the Nominal Figure Is Built

The stack up is a list of cores and prepregs with their nominal thicknesses, and the pressed result is the sum. Prepreg flows during the press, so the finished figure depends on the press cycle as well as on the materials.

The designer should specify the stack up rather than only the total. Our layer count notes describe how the layers are allocated.

Where the Tolerance Comes From

Each material is supplied within a tolerance, and the press adds its own variation across the panel. The combined effect is a thickness tolerance that is wider in the middle of a large panel than at the edge.

A tight requirement on a large panel is expensive rather than impossible. Our laminate notes describe the material figures.

Effect on Drilling and Aspect Ratio

The thickness divided by the finished hole diameter is the aspect ratio, and it decides how difficult the plating is. A board that is thicker than planned moves the ratio and can put the process out of window.

The ratio should be checked against the smallest hole on the board. Our aspect ratio notes describe the limits.

Effect on Impedance

Thickness sets the dielectric height between the reference planes, so it enters every impedance calculation. A finished thickness that differs from the nominal moves every controlled trace at once.

That is why the coupon is measured on the same panel. Our coupon notes describe the check.

Rigidity, Bow and Twist

Thickness contributes to stiffness in the third power, so a small change has a large effect on how much the board flexes. Where the assembly has a heavy component, that matters.

Bow and twist are measured against the diagonal rather than the length. Our warpage notes describe the measurement.

Connectors and Card Guides

A board that has to slide into a card guide or mate with a connector needs its thickness to match the receiving slot. In that case the tolerance is set by the mating part rather than by the process.

A connector specification should therefore reach the fabricator. Our connector notes describe the alignment that follows.

Verification

The verification is a micrometer or a cross section measurement at several points across the panel, taken at the same stage as the finished board, with the stack up recorded alongside.

Our fabrication notes list the points to confirm on the drawing.

Process Control and Verification

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.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Checks Before Release

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 a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. 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.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

FAQ

Is a tighter thickness tolerance worth the cost? It is worth it where a connector, a card guide or a controlled impedance layer depends on it. Where nothing depends on it, a standard tolerance is cheaper and just as good.

Does the tolerance apply across a panel? It usually applies to the finished board, and the variation across a panel is a separate figure that the shop controls internally.

What does gopcb provide for thickness work? We provide a stack up with each material and its thickness stated, a finished thickness measured across the panel rather than at one point, the aspect ratio checked against the smallest hole, impedance calculated from the same dielectric heights, and a tolerance chosen to match the mating part where one exists.