Etching Factor Control: Getting Straight Copper Sidewalls

The etching factor is a number describing how close an etched conductor comes to a rectangular cross section, and it is one of the few figures that ties imaging, plating and etching together in a single measurement. A high factor means a sidewall that is nearly vertical, and a low factor means a conductor wider at its base than at its top. It is measured from a cross section and expressed as a single ratio, which makes it useful for comparing one process with another.

That difference matters because the base width is not the dimension the designer specified. A trace drawn at a hundred micrometres and etched to a trapezoidal profile has a top width below the target and a base width above it, and the impedance of the finished trace follows the base.

What the Etching Factor Describes

The factor is the ratio of copper thickness to the horizontal undercut, measured on a cross section. A thickness of thirty micrometres with an undercut of ten gives a factor of three, and the same thickness with an undercut of five gives a factor of six. The number is dimensionless, so it can be compared across boards of different thickness.

A steeper sidewall angle is better for the electrical result and harder to achieve. The factor falls as the copper gets thicker, so a heavy copper board and a fine line board work against each other inside the same process. Two boards etched on the same line can have very different factors when their copper weights differ.

The Geometry Behind the Number

The etchant attacks copper in every direction, so it removes material sideways as well as downwards. The resist protects the top of the trace, and the region beneath the resist edge is exposed to etchant from the side, which is where the undercut forms. The lateral attack continues for as long as the copper remains exposed, so time is a direct lever on the result.

The shape of that undercut depends on the balance between the vertical and the lateral etch rate. A process that etches downwards much faster than sideways keeps the sidewall steeper, and the variables that control it are chemistry, spray and time. Temperature raises both rates, and in a well-controlled bath it raises the lateral rate proportionally less.

Copper Thickness and Etch Time

Thicker copper requires a longer etch, and a longer etch allows more lateral attack. The relationship is not linear, because the etchant loses strength as it works and the resist is exposed to the chemistry for longer. A panel ten percent heavy takes appreciably longer than one at nominal, and the extra time costs line width.

The tolerance on copper thickness therefore becomes a tolerance on line width. A plating process that holds thickness within ten percent gives the etcher a predictable starting point, and control of that step is described in the notes on current density control. That starting point is measured on a coupon rather than assumed from the plating setpoint.

Etchant Chemistry and Regeneration

The etch rate depends on the concentration of the active species, on the pH and on the temperature. An etchant that is not regenerated loses strength, and the line compensates by slowing the conveyor, which extends the exposure and increases the undercut. The correction works only while the etchant still has capacity, and beyond that point slowing the conveyor stops helping.

Regeneration keeps the rate stable so the conveyor speed can stay at the value that was qualified. The control parameters are measured on a schedule, and that measurement belongs in the same record as the line width result. Temperature is measured at the spray manifold as well as in the tank, because the two can differ.

Spray Pressure and Nozzle Condition

Spray pressure determines how quickly fresh etchant reaches the copper surface and how quickly spent solution is removed. Higher pressure improves the exchange and reduces undercut, and it also increases the mechanical load on the resist. The two effects have different optima, so the pressure is set where the exchange is adequate and the resist survives.

Nozzle condition matters as much as pressure. A blocked nozzle creates a region of poor exchange on the panel, and the resulting defect follows the nozzle pattern. Fan pattern, spacing and oscillation are checked at the same interval as the pressure.

Imaging and Resist Adhesion

The resist edge defines where etching begins, so its quality sets the upper limit on the factor that can be achieved. A resist with a ragged edge transfers that raggedness to the conductor, and an undercut resist lifts during etching and lets etchant under the feature. A resist that adheres poorly fails at the feature edge first, where the mechanical load from the spray is highest.

Exposure and development therefore belong in the same discussion. The exposure energy controls the edge profile, and it is described in the notes on dry film exposure.

Measuring Line Width and Sidewall

Line width is measured on the top surface by an optical system, because that is the dimension a specification usually names. The base width is measured on a cross section, and the two together give the factor. An optical measurement of the top width alone cannot detect a change in the base width.

A coupon placed on the panel and measured after etching is the practical way to track the factor without destroying product. Coupon placement and measurement are described in the notes on copper thickness coupons.

Common Profile Defects

The characteristic defects are a trapezoidal profile with excessive undercut, a ragged edge from a poor resist image, and a step in the sidewall where the etch rate changed during the pass. The last points to a nozzle or chemistry problem rather than to an imaging problem. A step that appears on one side of the panel and not the other points to a nozzle rather than to the chemistry.

Scumming, where residual copper remains between fine features, is the opposite failure and follows from an etch that was too short. It is a process window problem in the same way that undercut is.

Process Window and Records

The window is defined by the finest feature and the thickest copper on the product, because those two conditions together set the tightest requirement. A panel with a wide range of features is etched to satisfy the most demanding combination, and the rest of the panel is etched less than it could be. That compromise is a design consideration as much as a process one, and it is best resolved at the stack-up stage.

The record holds copper thickness, etchant parameters, conveyor speed, spray pressure and the measured factor. With those fields a change in line width can be attributed to one of the steps rather than to etching by default.

FAQ

What etching factor is achievable? Three to four is common on standard copper, and higher values are possible with thin copper and good spray exchange.

Does the factor apply to both sides of a trace? It is measured on one sidewall and applies to both when the process is symmetric, which it usually is on a double-sided etcher.

Can undercut be corrected by etching less? Only until the copper is no longer cleared, at which point scumming appears between the features.