Copper Balance And Etch Uniformity On Inner Layers

An inner layer is not etched as a collection of individual traces. The whole panel passes through the etcher at once, and the etchant removes copper at a rate that depends on how much copper it has to dissolve in each area. A layer that carries a dense processor bus in one region and a nearly empty region beside it will be etched unevenly, and the difference shows up as a variation in trace width, in copper thickness, and in the electrical behaviour of the finished board.

This article explains the etching mechanism, why the distribution of copper matters, how a layer is balanced, and how the fabricator compensates for the effect.

What Happens During Etching

Etching is a chemical reaction between the etchant and the copper, and the reaction consumes the etchant as it proceeds. In a conveyorised spray etcher the etchant is sprayed against the panel and then drains away, and fresh etchant reaches the surface by flow and diffusion. In an area where most of the copper has to be removed, the local concentration of active etchant falls and the reaction slows, while in a large open area the etchant is replenished easily and the copper is removed quickly.

The lateral component of the reaction is the reason the effect matters. Etching proceeds downwards towards the laminate, but it also proceeds sideways under the edge of the resist, and the ratio between the depth removed and the sideways undercut is called the etch factor. A high etch factor means a nearly vertical sidewall, and a low factor means a trapezoidal trace that is narrower at the top than at the base. Thick copper has a lower etch factor than thin copper, which is why a two ounce layer cannot hold the same minimum line width as a one ounce layer.

Why Uniformity Matters

The most direct consequence is a change in conductor width. A trace that is specified as a tenth of a millimetre and comes out five hundredths narrower has lost half of its cross section, which raises its resistance and reduces its current capacity, and on a controlled impedance line it changes the impedance by several percent. On a fine pitch layer the same variation decides whether two adjacent traces touch, which is a short, or whether one of them breaks, which is an open.

The distribution of copper also changes the way the layers laminate. Resin has to flow around the copper features and fill the spaces between them, so a region with heavy copper and a region with none behave differently under pressure, and the resulting laminate thickness varies across the panel. That thickness variation changes the dielectric spacing, which changes the impedance of lines that run through both regions, and it contributes to the warpage of the finished board. The same reasoning applies to the through hole plating that follows, because the current density in the plating bath is affected by the copper already present.

How To Balance A Layer

The standard measure is to fill the empty areas with copper, either as a solid pour or as a pattern of bars and dots that is connected to a net. Adding this dummy copper brings the local copper density closer to the average across the layer, which evens out both the etching and the resin flow. The pattern is usually placed a defined distance from the signal traces, because copper that is too close changes the impedance of the trace and couples noise into it, and the spacing rule follows the same logic as the clearance between any two conductors.

The net to which the dummy copper is connected is a decision in itself. Copper that is tied to ground gives a shield and a return path, provided it is stitched to the ground plane with vias at intervals, while a floating pattern behaves as a resonator and can radiate or couple energy. Isolated slivers of copper, which are pieces of metal with no connection and no purpose, should be removed altogether, because they can lift during thermal cycling and they can act as an antenna. A mesh pattern is often preferred to a solid pour in thick or high layer count designs because it lets the resin flow and reduces the risk of delamination, which is the trade described under copper flooding options.

Balancing Across The Stack

Balance is not only a within layer question. Two adjacent layers with very different copper distributions will laminate differently and can bow the stack, so the distribution is normally arranged so that a heavy layer is mirrored by another heavy layer on the other side of the centre line. A similar rule applies to copper weight, because a two ounce layer beside a half ounce layer is an asymmetric stack that will bend when it is heated.

The construction of a balanced stack is discussed under layer stack up from one to eight layers, and the fabrication sequence that turns the artwork into the finished layer under PCB design and fabrication.

Etch Compensation And Process Control

Compensation is applied by the fabricator rather than by the designer. The etcher removes a predictable amount of copper from each side of a trace, so the artwork is drawn slightly wider than the finished dimension and the process brings it back to the nominal value. The compensation factor depends on the copper weight, the etchant chemistry, the conveyor speed, and the pattern density, which is why the fabricator asks for the finished width and why a change of supplier can change the etched width without any change to the drawing.

Process control comes from coupons. A test pattern on the panel carries traces of the same width as the product, and the fabricator measures them after etching, either optically or by cross section, and adjusts the line if the width drifts. Where the design carries controlled impedance lines, a coupon with the same trace geometry and the same dielectric thickness is measured with a time domain reflectometer, and the result is the evidence that the impedance is within tolerance.

Verifying The Result

The verification of a copper distribution decision is the measured feature, not the design intent. A cross section shows the trace geometry and the copper thickness at the top and the base of the sidewall, an optical measurement shows the width from above, and an impedance measurement shows the combined effect of the width, the thickness, and the dielectric spacing. Those measurements belong on a coupon that travels with the panel and that is built with the same artwork, because a coupon that is not representative proves nothing about the boards.

Where a design has a very uneven distribution and the fabricator raises the point in a design for manufacture review, the answer is usually to add dummy copper rather than to accept a wider tolerance. The same review often identifies isolated slivers and traces that are too close to a copper pour, so the review is worth reading carefully rather than treating it as a formality.

Additional Considerations for This Build

Practical attention to copper balance pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating copper balance explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to etch uniformity pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating etch uniformity explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Does dummy copper change the electrical behaviour? It can, if it is placed close to a signal trace or if it is left floating. Copper that is spaced adequately and connected to ground improves the return path, while copper that is too close changes the impedance and couples noise.

Why does heavy copper limit the minimum trace width? Because the etchant has to remove more material vertically, and the sideways undercut grows with the depth. A thicker layer therefore needs a wider minimum line and a larger compensation factor.

Who compensates for the etch factor? The fabricator, who draws the artwork larger than the finished dimension. The designer should specify the finished width and the tolerance, and should not attempt to compensate in the design.