Plated Hole Copper Thickness Specification

The copper in the wall of a plated hole is the only connection between the layers that the hole serves, and its thickness is specified in every fabrication drawing, usually as a minimum in microns or in thousandths of an inch. The figure seems simple and it is not: it is a minimum at a specific place, measured in a specific way, on a coupon rather than on the board, and a specification that does not say those things cannot be inspected consistently.

This article explains what the thickness requirement covers, how the minimum and the average differ, where the measurement is taken, and how the aspect ratio changes what is achievable.

What The Specification Covers

The requirement is for the copper that forms the barrel of the hole, and it is usually written as a minimum of 20 or 25 microns for a class 2 product, with higher figures for a product that will be thermally cycled or used for a press fit connector. The figure is a minimum because the failure mode is a thin wall that cracks under thermal stress, and because the plating on a hole is never uniform.

The plating consists of a thin electroless layer that makes the wall conductive and a thicker electroplated layer that provides the strength and the conductivity. The electroplated layer is what the specification usually refers to, but the two are deposited in sequence and they cannot be separated on the finished board. The chemistry that controls the deposit is described under electroplating additives.

Minimum And Average

The thickness varies around the circumference of a hole and along its length. It is thicker at the surface and thinner in the middle, and thicker on the side of the hole that faces the anode than on the side that faces away. A minimum specification therefore describes the thinnest point, which is usually at the middle of the barrel, and it is the figure that governs the reliability.

An average specification is easier to meet and says less. A hole with an average of 25 microns may have 35 microns at the ends and 15 in the middle, which is a wall that will crack. Where the specification is written as an average, the minimum should be stated as well, and the measurement positions should be chosen to include the middle of the barrel.

Where The Measurement Is Taken

The measurement is made on a microsection cut through a coupon that was plated with the production panel. The section is prepared so that the copper is visible against the laminate, and the thickness is measured at the surface, at the middle and at the far end of the barrel, on both sides of the hole. The coupon carries holes of the smallest diameter and the highest aspect ratio on the product, since those are the hardest to plate.

A coupon is used rather than a product board because the section destroys the hole and because the coupon can be designed to include the worst case. The position of the coupon on the panel matters as well, since the current density is higher near the bus bar, and a coupon at the edge of the panel will read thicker than one in the middle. The requirement therefore includes where the coupon is placed as well as what is measured.

Aspect Ratio And Distribution

The aspect ratio is the board thickness divided by the hole diameter, and it is the figure that determines how much the plating will vary. A hole with an aspect ratio of 5 plates relatively evenly, while one with a ratio of 15 has a middle thickness that may be half of the surface thickness. The plating bath, the agitation and the current density are all chosen for the highest aspect ratio on the product.

The aspect ratio also limits the design. A hole that is too small for the board thickness cannot be plated reliably, and the answer is a thinner board, a larger hole or a different via structure. The limit is normally stated by the shop as a maximum aspect ratio it can plate, and it is one of the first questions to ask when a design pushes the density. The stackup that sets the thickness is described under layer stackup from one to eight layers.

Standards And The Drawing

The requirement is usually taken from a standard that defines the classes of product and the minimum thickness for each, and it is then written on the drawing together with the class. Where the drawing is silent, the shop will apply a default, and the default may be the lowest figure that the standard allows. Stating the class and the figure removes the ambiguity.

The drawing should also state whether the figure applies to the finished hole after plating, whether it is a minimum or an average, and whether a coupon is required. Where the product is a press fit or a thermally cycled assembly, the figure is higher than the default and the reason should be recorded, so that a later cost reduction exercise does not quietly remove it. The rules that make the documentation complete are described under manufacturable design guidelines.

Verification

Verification is a section on a sample from each lot, measured at the defined positions and recorded. The record should include the coupon location, the hole diameter and the board thickness, because the same thickness reading means different things at different aspect ratios. The section also reveals the other defects that the plating process can produce, such as voids and cracks.

Where a lot has to be released without a section, X-ray fluorescence can give an indication of the surface copper but not of the barrel, and it is not a substitute. The measurement that matters is the one inside the hole, and it requires a section. Related fabrication controls are described under PCB design and fabrication.

Why The Thickness Matters Under Thermal Stress

When a board is heated, the copper in the barrel expands more than the laminate around it in the axial direction and less in the radial direction, and the barrel is constrained by the layers it connects. The result is a shear stress at the interface between the plating and the laminate, and the plating has to accommodate it by yielding. A thin wall has less material to distribute that strain and cracks sooner, and the crack usually starts at the corner where the barrel meets the pad on the surface.

The number of cycles that a joint survives therefore scales with the plating thickness, which is the reason the requirement is higher for a product that will see thermal cycling in service. The same reasoning applies to a press fit connector, where the insertion applies a mechanical load rather than a thermal one, and to a heavy copper board, where the expansion of the copper imposes a larger strain on the barrel.

FAQ

What is a typical minimum thickness? About 20 to 25 microns for a general purpose multilayer board, and 25 microns or more for a product that will be thermally cycled or that uses press fit connectors.

Is the plating thicker on the surface than in the hole? Yes, always, and the difference grows with the aspect ratio. The surface copper is also thicker than the foil because it receives the same plating as the barrel.

Can the thickness be measured without cutting the board? Not reliably in the barrel. X-ray fluorescence measures the surface, and a cross section is the only method that shows the wall.