PCB Board Thickness Selection

Board thickness looks like a free choice, and it is constrained from several directions at once. It sets the aspect ratio of every plated hole, the stiffness of the finished assembly, the height available between the board and the enclosure, and whether a standard connector will fit. It is normally fixed before the stackup is designed, because the stack has to add up to it.

This article covers what the thickness affects, how it is chosen, and how it interacts with the drills and the stack.

Why Thickness Matters

The most immediate effect is on the drilling. The aspect ratio of a plated hole, which is the board thickness divided by the hole diameter, sets how difficult the plating is. A thicker board with the same hole size needs a plating chemistry that can throw copper further into the barrel, and a hole that is easy to plate in a thin board may be at the limit of the process in a thick one.

The second effect is mechanical. Stiffness rises with the cube of the thickness, so a board that is slightly thicker is considerably stiffer, which reduces flexing during assembly and in service. The third is dimensional: the finished assembly has to fit the space the product allows, and the board is one of the largest contributors to that dimension. The mechanical arrangement that surrounds these requirements is described under board outline and mounting design.

Common Thicknesses And Panel Sizes

Fabricators stock laminates in a small number of standard thicknesses, and a design that uses one of them avoids the cost and the lead time of a special build. The common figures range from a fraction of a millimetre for a thin product to a few millimetres for a backplane, and the exact list depends on the supplier and on the laminate type. Working from the available list rather than from a preferred number is a habit worth keeping.

The panel size also interacts with the thickness. A thin panel is more difficult to handle and to keep flat through lamination and plating, and some shops limit the panel size for thin material. A very thick panel is heavy and may exceed the handling limits of the equipment. Where the product needs an unusual combination, it is worth confirming with the fabricator before the layout is committed. The way a stack is arranged within a given thickness is described under layer stackup from one to eight layers.

Drilling And Aspect Ratio

The aspect ratio is the parameter that most directly limits the choice. A small via in a thick board is the worst case, and it is where drilling and plating defects are most likely. A typical maximum for a standard process is around eight to ten to one, and higher ratios are possible with a controlled process and a slower plating bath.

The aspect ratio also limits which via types are practical. A microvia is drilled through one thin dielectric layer, so it is unaffected by the board thickness, but the through holes that remain in a thick HDI board still have to cross the full thickness. That is why a thick board with a fine pitch device often needs the through holes to be drilled after the microvias, and why the choice of layer arrangement matters as much as the total thickness. The related dimensional behaviour of the laminate is treated under PCB dimensional stability and expansion.

Mechanical Requirements

A board that will be mounted on standoffs at its corners and carry a heavy connector in the middle will flex under its own weight during assembly and under the cable load in service. Flexing damages joints, particularly those of ceramic components and fine pitch devices, so the thickness is often chosen to keep the deflection within a limit rather than to satisfy any electrical requirement.

Where a thin board is required for other reasons, the mechanical requirement is met by a stiffener rather than by the board. The stiffener is bonded locally, which allows a thin board in the areas that need flexibility and a rigid region where the load is applied. The decision between a thicker board and a stiffener is made on cost and on the space available, and it is normally settled at the mechanical design stage.

Connectors And Cards

A plug in card has to fit the socket, and the socket is made for a specific board thickness with a small tolerance. The thickness at the gold finger area is therefore specified more tightly than the rest of the board, and the chamfer and the finger plating are dimensioned for it. A board that is at the top of its tolerance can require excessive insertion force, and one that is below it may not make reliable contact.

Where a standard connector is used, the board thickness is chosen from the connector rather than the other way round. Where a custom connector or a press fit part is used, the thickness and its tolerance become part of the interface specification, and the mating part has to be ordered to match. Confirming that interface before the stack is designed avoids a redesign later. The qualification process that a new construction goes through is described under multilayer prototype requirements.

Choosing The Thickness

The order of decisions is: the interface or the connector first, because it sets a range; the mechanical requirement second, because it may exclude the thin end of the range; the drilling requirement third, because it may exclude the thick end for a given hole size; and the available laminate fourth, which may narrow the range again. What remains is usually a small set of options.

The tolerance on the finished thickness should be stated, because it is not simply the sum of the laminate and the copper. The press cycle compresses the resin, the plating adds thickness, and the measurement is taken at a defined point. A tolerance that is tighter than the process can hold adds cost without adding anything to the product, so it is worth asking the fabricator what it holds as standard before specifying a figure.

The thickness also decides the smallest hole the board can carry reliably, and that in turn decides the smallest via. A design that needs a very small via and a thick board is asking for two things that conflict, and the usual resolution is to move the fine pitch routing into a build-up layer where the microvias are shallow, and to keep the through holes for the power and the coarse signals. Seen that way, board thickness and via strategy are one decision rather than two.

FAQ

Can any thickness be ordered? Within the standard laminates the supplier stocks, yes, and outside them the laminate has to be made or the stack adjusted. Working from the available list saves cost and lead time.

Does a thicker board always mean a stronger one? Stiffness rises sharply with thickness, so a small increase helps. The limit is set by the aspect ratio of the holes and by the weight and height of the assembly.

Why is the thickness held tighter at a card edge? Because the socket is made for a specific thickness and the insertion force and the contact reliability both depend on it. The rest of the board can be looser.