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

On a design of this kind, board thickness is the item that decides how the rest of the board is arranged. 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.

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.

Process Control and Verification

On a design of this kind, board thickness is the item that decides how the rest of the board is arranged. 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.

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.

Process Control and Verification

On a design of this kind, board thickness is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, board thickness is the item that decides how the rest of the board is arranged. 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

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.