Chemical Etching vs Laser Etching for PCBs

Etching is the process that turns an image on a copper clad panel into a circuit. The two families that matter in practice are chemical etching, which dissolves the metal through a mask, and laser etching, which removes it with focused energy or which patterns a resist that later guides a chemical step.

The two are often presented as competitors, and for the finest geometries they are, but in most production they cooperate. Understanding where each one is used, and what limits it, is what allows a design to specify a process it can actually achieve.

What Each Process Does

Chemical etching immerses the panel in a solution that attacks exposed copper while the resist protects the pattern. The etchant is chosen for the copper thickness and the required resolution, and the process is run in a conveyorised spray machine in the common case.

Laser etching removes material by ablation, using a focused beam that vaporises or ejects the copper. It can also expose a resist so that a subsequent chemical step does the bulk removal, which combines the fine resolution of the laser with the throughput of a wet process.

Chemical Etching Fundamentals

The chemistry attacks the copper in all directions, so the etchant works sideways beneath the resist while it works downwards. That lateral attack is undercut, and it is the fundamental limit on the resolution of a wet process, because the trace becomes narrower than the image on the artwork.

Etch rate rises with temperature and with the strength of the solution, and it varies with the copper thickness. A heavy copper board needs longer in the etchant, which increases the undercut and forces the designer to compensate the artwork.

Undercut and Etch Factor

The ratio of the etch depth to the lateral undercut is the etch factor, and it is a characteristic of the chemistry and the machine rather than of the operator. Vertical spray etching with a well controlled conveyor gives a better etch factor than immersion, because the fresh solution reaches the surface uniformly.

Designing for the process means applying a compensation factor to the trace width so that the finished width matches the requirement. Where a design is transferred from a prototype shop to a high volume line, the etch factor changes and the compensation has to be recalculated rather than reused.

Laser Etching and Ablation

Laser ablation is a serial process: the beam follows the pattern rather than the panel moving through a bath. That makes it flexible, because the pattern is a data file, and precise, because the beam does not attack sideways. It is also slower over a large area and it leaves a recast layer that has to be considered.

The heat affected zone around an ablated feature can change the copper and the dielectric beneath it, and the surface finish of the etched area is rougher than a chemical etch. Where the feature is a fine gap in a plane rather than a trace, that roughness is usually acceptable; where it forms a controlled impedance structure, it is not.

Resolution and Tolerance Compared

Chemical etching with a photoimageable resist reaches line and space dimensions of around 50 to 75 micrometres in volume production, and finer geometries are possible with careful control. The limit comes from the resist thickness, the etchant and the copper thickness together.

Laser processing reaches smaller dimensions, because the removal is directed and the undercut is negligible. The practical limit is set by the beam spot size and by the throughput, so it is used for the finest features, for thin materials and for organic substrates that a wet process cannot handle.

Thermal and Material Effects

Chemical etching is essentially isothermal at the panel level, because the panel is immersed or sprayed at a controlled temperature. The material effects are chemical: the etchant can attack the resin at the edge of a feature if the resist adhesion is poor, producing an undercut that is invisible until cross section.

Laser processing is a localised thermal event. It can carbonise the resin beside a feature, create a conductive residue on the surface and leave a tapered wall. Those effects are managed by the pulse parameters, by an assist gas and, where necessary, by a cleaning step after the laser.

Cost and Throughput

Chemical etching is a parallel process and the cost per panel falls with volume. The chemistry has to be replenished and analysed, and the waste has to be treated, so the operating cost has a significant chemical and environmental component.

Laser processing has a capital cost and a per feature time cost. It is usually the more expensive option per panel where a wet process can do the job, and it becomes competitive where the geometry, the material or the prototype timeline rules the wet process out.

Where Each Belongs

Chemical etching remains the default for rigid boards in volume, for inner layers, for fine line patterns on standard laminates and for most high speed circuits. It is the process that the design rules in a fabrication capability document usually describe.

Laser processing appears where the geometry is beyond a wet process, where the substrate is not compatible with a bath, and where a modification has to be made to an existing panel without a new artwork set. In a production flow the two are frequently combined, with a laser defining the finest features and the chemistry removing the bulk of the copper.

Process Control on the Etching Line

Chemical etching is monitored through the chemistry and through the product. The etchant is analysed for concentration and for dissolved copper, the temperature and the conveyor speed are recorded, and a coupon on the panel is measured after the resist is stripped to confirm that the trace width is inside the tolerance. When the copper concentration rises, the etch rate falls and the undercut increases, so the compensation that was correct at the start of a campaign may be wrong several thousand panels later.

Laser processing is monitored through the tool rather than through the chemistry. Pulse energy, focus position, pulse overlap and the assist gas flow all change the geometry of the ablated feature, and the optics degrade with contamination from the ablated material. A regular test pattern cut in a scrap coupon confirms that the beam is still producing the designed feature, and that check is worth more than an occasional visual inspection of the finished board.

Choosing a Process for a New Design

The starting point is the finest feature in the artwork and the copper weight on the layer that carries it. Where the feature is inside the capability of a wet process, chemical etching is the cheaper and faster route, and its tolerance behaviour is well characterised. Where the feature is at or beyond that limit, the design should either be relaxed or the process moved towards laser or a combined flow.

The second consideration is the volume and the change rate. A laser process absorbs a pattern change without a new film set, which is valuable during development, while a wet process is more economical once the design is stable. Many projects therefore begin with the flexible process and migrate to the cheaper one when the layout freezes, and the design rules are written so that both can build the same board.

FAQ

Why does the finished trace come out narrower than the artwork? Because the etchant attacks sideways as well as downwards. The compensation applied to the artwork is chosen to cancel that undercut for a given process and copper thickness.

Is laser etching suitable for controlled impedance traces? Usually not for the trace itself, because of the recast layer and the surface roughness. It is more often used for the fine gaps and features around a controlled impedance structure.

Can both processes be used on one board? They can, and they often are. The bulk of the pattern is etched chemically while the finest features are defined by a laser before or after the wet step.

Related reading: PCB manufacturing processes, PCB manufacturing tolerances, impedance discontinuity analysis, and PCB routing precision.