Board Thickness Tolerance and Control

The board thickness on a drawing is a finished dimension with a tolerance, and it is the sum of several contributions that are controlled by different processes. The laminate thickness, the copper weight, the plating and the finish all add to the total, and each of them varies. Understanding which contribution dominates is what allows a thickness problem to be corrected rather than tolerated.

How the Total Is Built

The starting point is the laminate, which is supplied at a nominal thickness with its own tolerance. That tolerance is a property of the material and of the press that made it, and it is usually a few percent of the nominal value.

The copper layers add their weight in the form of foil and plating. An inner layer contributes its foil thickness, while an outer layer contributes the foil plus the plating that is added during the process.

The surface finish contributes a small amount, and the solder mask over the copper contributes a little more. For most products the finish and the mask are negligible, while a heavy finish on a thick copper outer layer is measurable.

Where the Variation Comes From

The lamination is the largest single contributor. The resin flows during the press cycle, and the finished thickness depends on the resin content, the copper density and the press parameters.

A board with a large area of copper in one region and little in another will have a different thickness in the two regions, because the resin has less distance to travel where the copper is dense. That variation is within the panel and it is not described by the tolerance on the drawing.

Plating contributes a variable amount that depends on the current distribution across the panel. A panel with a large isolated copper area plates thicker in that area and thinner elsewhere, and the difference appears in the total thickness of the outer layers.

Why the Tolerance Is Specified

The thickness matters mechanically. A board that has to fit a connector with a defined slot, a card guide or a chassis with a fixed gap has a limited range that it can accept.

It matters for the assembly. A very thin board flexes during printing and placement, while a very thick one has a different thermal mass and a different reflow profile. The printer, the placement machine and the oven are all set for a range of thicknesses, and a board outside that range forces the settings to be revisited.

It matters electrically. The dielectric thickness between a signal layer and its reference plane sets the impedance, so a thickness variation at the panel level produces an impedance variation across the panel.

Impedance and Thickness

The relationship is direct: the impedance of a microstrip or a stripline depends on the dielectric thickness, and a variation of a few percent in the thickness produces a comparable variation in the impedance.

For this reason an impedance controlled stack is usually specified with a tighter thickness tolerance than a general board. The tolerance is not cosmetic; it is what allows the impedance to be held inside the specified band.

The correct measurement is the dielectric thickness between the layers, not the overall board thickness. A section at a representative position gives the value that the impedance calculation needs.

Measurement

The overall thickness is measured with a micrometer at several positions on the panel. The measurement should be made at the same positions each time and recorded with the panel identification.

The measurement is affected by the solder mask and by any raised feature, so the position should be chosen in an area without components or markings. A measurement over a masked feature is a measurement of the mask. Where the drawing calls for a measurement on bare laminate, the mask should be excluded or the measurement taken before the mask is applied.

The dielectric thickness is measured on a section. The section shows the layer build up, the resin rich areas and the copper distribution, and it is the only way to confirm that the thickness corresponds to the intended stack.

Specification and Control

The specification should state the finished thickness with a tolerance, the measurement method and the positions at which the measurement is taken. A tolerance without a method is a number that two parties will interpret differently.

The laminate specification should be consistent with the finished requirement. If the finished board has to be within a narrow band and the laminate is ordered at the standard tolerance, the process has no room to absorb the variation.

The press parameters should be recorded with each lot so that a change in the finished thickness can be traced to a change in the cycle. The copper distribution of the design itself should also be known, because it sets the expectation.

Copper Balance and Thickness Uniformity

A uniform copper distribution produces a uniform thickness. Where the design has a large empty area, thieving is added to give the resin somewhere to flow and to even out the pressure. A design that is unbalanced in this way shows it in the thickness measurement before it shows it anywhere else.

The thieving pattern should be designed rather than left to the fabricator, because a pattern that is too dense changes the local dielectric constant and one that is too sparse does not solve the problem.

The balance is also relevant at the panel level. A panel that carries several boards should be arranged so that the copper density is reasonably even across the whole area, otherwise the pressure distribution varies from one side to the other.

Consequences of a Thickness Error

A board that is too thin for a connector will not make reliable contact or will sit loose in the guide, and the fault appears as an intermittent connection that moves with the board.

A board that is too thick will not fit, and the discovery usually happens at the assembly stage after the boards have been built. That is the most expensive place to find a thickness problem.

A thickness that is within the drawing tolerance but at the edge of the range will produce an impedance that is at the edge of its band. The two tolerances should be considered together rather than independently.

Practical Rules

Specify the finished thickness with a method and a position, keep the laminate tolerance consistent with the requirement and control the copper balance. Measure the dielectric thickness on a section for an impedance controlled board.

Record the thickness data with the build records and the impedance stackup design, and review the laminate properties and the plating thickness guide when a stack is specified.

FAQ

What builds the finished board thickness? The laminate, the copper foil, the plating and, to a small extent, the finish and the solder mask. The laminate and the lamination process are the largest contributors.

Why does thickness vary across a panel? The resin flows differently depending on the local copper density and the plating current varies across the panel, so the thickness is not uniform.

How is dielectric thickness measured? By a microsection at a position that represents the average copper density. It is the figure that the impedance calculation needs.