Board Thickness Selection For PCB Designs

Board thickness looks like a simple choice, and in most projects it is made by default at 1.6 mm. That figure is a convention rather than a requirement, and the right thickness depends on the mechanical environment, on the connector system and on the impedance the design has to meet.

Thickness also interacts with the fabrication process. A thin board is harder to handle and to drill, a thick board demands a higher aspect ratio, and both of those change the price and the lead time considerably.

The Default And Why It Exists

The 1.6 mm figure became the default because it suits the standard card guides, connectors and enclosure slots that grew up around it. It also gives a comfortable aspect ratio for a typical through hole, which is why it remains the cheapest option at most fabricators.

Deviating from it costs money, because a non-standard thickness requires a different combination of prepreg and core, and sometimes a different panel size. The cost is small for a prototype but grows with volume, so the choice should be justified early rather than late.

Mechanical Stiffness And Support

Mechanical stiffness rises steeply with thickness, roughly with the cube of the value. A board supported only at its edges and carrying heavy components may need a thicker core, while a board fully supported on a chassis can be considerably thinner.

Stiffness also affects assembly. A thin board flexes during depanelisation, during connector insertion and during test, and flexing is what cracks a solder joint or a ceramic capacitor. Where a board will be handled roughly, thickness is a cheap way to reduce the risk.

Aspect Ratio And Drilling

Aspect ratio is the board thickness divided by the hole diameter. A 1.6 mm board with a 0.3 mm hole has a ratio of about 5 to 1, which most fabricators handle without difficulty, while a ratio above 10 to 1 limits the number of suppliers who can plate reliably.

A thicker board therefore pushes the minimum hole size upwards, which in turn limits the via size and the routing density. Where a design needs both a thick board and small vias, the plating process becomes the constraint that decides whether the build is possible at all. The prototyping requirements behind this are described under multilayer PCB prototype requirements.

Controlled Impedance And Stackup

Controlled impedance is set by the trace geometry and by the dielectric thickness between the trace and its reference plane. A thicker board has more room in the stackup, which makes it easier to reach a target impedance with a comfortable trace width.

The reverse is also true. A thin board that must reach 50 ohms may need a very narrow trace or a very thin dielectric, and a narrow trace then limits the current and increases the difficulty of etching. Thickness is therefore a design variable rather than a mechanical afterthought. The trace geometry that results is described under PCB routing and microstrip or stripline.

Connectors And Card Guides

Edge connectors, card guides and backplane slots are dimensioned for a specific board thickness, and a deviation of a few tenths of a millimetre changes the fit. A card that is too thin sits loosely in its guide and can vibrate, while one that is too thick may not enter at all.

Press fit connectors add a further constraint, because the pin length is designed for a range of board thicknesses. Outside that range the press fit may not form a reliable connection, so the connector supplier should be consulted before a non-standard thickness is chosen.

Thickness Tolerance And Flatness

The nominal thickness is quoted with a tolerance, and that tolerance depends on the construction. A board built from prepreg and core is typically held to plus or minus 10 percent, which on a 1.6 mm board is 0.16 mm and is enough to affect a tight connector fit.

Flatness is a separate requirement and is usually expressed as a percentage of the diagonal. A thick board is naturally flatter, while a thin board relies on balanced copper and a controlled lamination cycle to stay inside the limit. The specification should state which of the two matters for the application.

Thin Boards And Their Handling

A board below about 1 mm flexes easily and needs support throughout the assembly process. Vacuum tables, carriers and dedicated fixtures are used to hold it flat while paste is printed and components are placed, and those carriers add cost to the assembly rather than to the fabrication.

Thin boards also need care during depanelisation. Routing a thin panel without support can crack the laminate, and the same applies to a breakaway tab that is snapped by hand. The panel design should therefore include the support features that the assembly process needs.

Thick Boards And Heavy Copper

A thick board is often chosen together with heavy copper for a power application. That combination raises the lamination pressure and the press time, because a heavier copper layer is harder to encapsulate without voids, and it also reduces the achievable minimum line width.

The price reflects all of that. A 3 mm board with 2 ounce copper is a specialist build, and it should be quoted early rather than assumed to cost roughly the same as a standard 1.6 mm board with 1 ounce foil.

Choosing And Documenting

A practical sequence is to start from the mechanical requirement, check the aspect ratio at the smallest hole, and then confirm that the impedance targets can be met within the stackup. If all three conditions are satisfied, the thickness is workable.

Starting from a standard stackup rather than a bespoke one keeps both cost and lead time under control. The choice and its reasons should be recorded with the stackup, because a later change of thickness affects the impedance, the connector fit and the price. How the stackup is documented is described under layer stackup from one to eight layers.

Cost And Lead Time Effects

Thickness affects price through the material and through the process. A thicker board uses more prepreg and core, needs a longer lamination cycle and takes longer to drill, and all three of those appear in the quotation. The step between standard thicknesses is not a smooth one.

Lead time is affected as well. Standard thicknesses are common and the materials are held in stock, while an unusual thickness may require a special order of prepreg. For a prototype, staying on a standard thickness can save several days even when a non-standard value would be technically better.

The tolerance also carries a cost. A tight thickness tolerance requires closer control of the press and more measurement, and the price reflects that. Where the extra tolerance is not needed by a connector or a card guide, the looser standard tolerance is the sensible choice.

FAQ

Is 1.6 mm always the best choice? It is the cheapest and the most widely supported, so it is the right default. A different value should be justified by a mechanical, connector or impedance requirement.

What is a safe aspect ratio? Around 8 to 1 is comfortable for most fabricators, and 10 to 1 is achievable with care. Above that the plating quality and the price both suffer noticeably.

Can a thick board use small vias? Only within the aspect ratio the fabricator supports. A 3 mm board with 0.2 mm vias is a 15 to 1 ratio and will be both difficult and expensive to build.