Etchback Control for Reliable Plated Through Holes

Etchback is the deliberate removal of resin from the wall of a drilled hole so that the copper of the inner layers protrudes slightly into the hole. Plating then wraps around that exposed copper, which produces a connection with more contact area than plating alone can achieve.

It is closely related to <a href="https://www.gopcba.com/pcb-desmear-process-control/” title=”desmear”>desmear, and the two are often discussed as if they were the same operation. They are not. Desmear removes drilling smear; etchback goes further and removes sound resin, and the amount removed is a specification rather than a by product.

What Etchback Is

The process removes resin from the hole wall to a controlled depth, typically expressed per side and measured from the surface of the inner layer copper. At the end of the operation the inner layer copper stands proud of the surrounding resin by that amount.

The visible result in a microsection is a small step at each inner layer, with the copper protruding into the drilled hole. The plating then follows the contour, and the electrical connection includes the end face of the inner layer as well as the wall.

Why Some Designs Need It

Connections that must survive severe thermal cycling, high current or repeated rework benefit from the additional contact area and from the way plating wraps the inner layer copper. The wrap also reduces the reliance on a single, small interface between the plating and the layer edge.

Where a design uses many inner layers, heavy copper or a high aspect ratio, the mechanical load on each connection is larger, and the extra interface that etchback creates is worth having. Where the board is thin and lightly loaded, the same benefit is much smaller. The requirement is also more common on products that will see repeated thermal cycling in service, where a slow crack at an inner layer connection is expensive to find in the field.

Chemistry That Produces Etchback

Etchback is normally produced in the same permanganate line that performs desmear, by extending the dwell or increasing the aggressiveness of the oxidation stage, or by adding a dedicated stage. Plasma processing can also be used, with oxygen and fluorine bearing gases, and it is favoured where a very uniform result is needed across a high aspect ratio hole.

Whichever chemistry is used, the neutraliser and the subsequent stages must be able to cope with the extra material removed. Residues that are not neutralised become sites where plating fails to adhere, which is a much worse outcome than no etchback at all.

Measuring Etchback

Measurement is made on a microsection at high magnification. The depth of resin removed is measured at each inner layer connection, usually on both sides of the hole, and reported as a range rather than a single value.

Typical requirements fall in the region of 0.025 to 0.05 mm per side, with the exact figure set by the product class and the customer. The measurement needs a section that passes through the hole axis, since an off centre cut makes the protrusion look larger than it is. Two figures are recorded: the depth of resin removed and the thickness of copper at the inner layer connection, because the second decides how much current the joint can carry.

Etchback Compared With Desmear Only

A desmear only process removes smear and lightly textures the resin, leaving the inner layer copper flush with the wall. The plating then meets the layer edge at a right angle, and the connection is formed at that narrow band.

An etchback process removes sound resin as well, so the plating can wrap the protruding copper. The difference in reliability is most visible in thermal stress testing, where the wrapped connection distributes stress over a larger area instead of concentrating it at a single interface. In a section, that connection appears as copper grown around the protruding inner layer, forming a fillet on both sides, and that shape is what resists the z axis movement during soldering.

Effects on Plating and Barrel Quality

Etchback changes the shape that the plating has to fill. The stepped profile creates a local change in current density, and the plating is thinner over the protruding copper than on the adjacent wall unless the process is adjusted.

This is the reason etchback has to be qualified together with the plating recipe rather than added as a separate specification. A board that meets the etchback requirement but shows thin copper at the inner layer connection has not gained the reliability the specification was intended to deliver. Thickness targets are set out in the plating thickness guide.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Electrical-Testing.jpg" alt="Plated through hole wall after desmear and etchback on a PCB” />

Risks of Excessive Etchback

Too much etchback weakens the wall. The resin that is removed was supporting the glass fibres and the inner layer copper, and beyond a certain point the copper begins to stand free, with voids forming around it during plating.

Excessive removal also creates wedge shaped voids beside the inner layer, since the recess is difficult for the plating solution to fill completely and for the rinse to clear. These voids are a recognised reliability hazard, and in a microsection they are hard to distinguish from the etchback itself without careful measurement.

Specifying Etchback on the Drawing

The specification should state the amount required per side, the method of measurement and the layers to which it applies. Stating only the words etchback required leaves the shop to choose a value, and the value it chooses may be at the top of the range for reasons of its own process control.

Where the design does not need etchback, saying so explicitly is equally useful. Many boards perform better with a clean desmear and a well controlled plating recipe than with an unspecified etchback that adds process risk without a corresponding benefit. Where the drawing does call for etchback, the plated through hole geometry should be reviewed at the same time, because removing resin changes the wall profile that the plating has to cover.

Verification and Records

Verification uses a coupon that travels with the panel, sectioned after plating and measured at the inner layer connections. Recording the measured range per lot, rather than a pass or fail, shows whether the chemistry is drifting before the value reaches the edge of the tolerance.

The record should also carry the chemistry parameters that produced the result, so that a change in etchback depth can be traced to the dwell, the temperature or the bath age. Without that link, a shift in the measurement prompts changes to the wrong part of the process.

FAQ

Is etchback the same as desmear? No. Desmear removes drilling smear and prepares the wall, while etchback deliberately removes sound resin so that inner layer copper protrudes into the hole for plating to wrap.

How much etchback is normal? Around 0.025 to 0.05 mm per side is a common requirement, depending on the product class. The value should be stated on the drawing together with the measurement method.

Can etchback be excessive? Yes. Removing too much resin leaves the inner layer copper unsupported and creates recesses that plating may not fill, producing voids beside the inner layer that are worse than the problem etchback was meant to solve.

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