Laser Ablation for Solder Mask Openings: Depth and Edge Quality
Laser ablation removes solder mask from a defined area with a focused beam, without a phototool and without a development step. It is used to open windows on a finished board, to repair a mask that has covered a pad, and to create openings on materials and thicknesses that are difficult to image photographically. The process removes material by delivering enough energy to break it down, and the margin between removing the mask and damaging the copper beneath it is narrow.
The two parameters that matter are the depth to which the mask is removed and the quality of the edge that is left. Both are set by the energy density at the surface, the number of passes and the wavelength, and both are measured on the panel rather than inferred from the machine setting.
Why Laser Ablation Is Used
The process removes the need for a phototool, which makes it attractive for small batches and for rework. It also removes the development step, so a mask that has already been cured can be opened, which is not possible photographically. For a board that needs a window in a specific place, the laser is a direct route.
Its limits are equally clear. Ablation is a serial process, so the time increases with the number of openings and the area to be removed. It also leaves debris and a tapered wall, both of which have to be managed, and the energy has to be controlled closely enough that the copper is not damaged.
How the Material Is Removed
The beam is absorbed by the mask, which heats locally and breaks down. Depending on the material and the wavelength, the removal may be a true ablation, where the material is converted to vapour, or a combination of ablation and thermal decomposition. The second leaves more debris and a larger heat affected zone around the opening.
The mask is a filled organic material, and the filler is what makes the process difficult. A filler that does not absorb the wavelength well requires more energy to remove, and the extra energy is delivered to the copper beneath. Choosing a wavelength that the mask absorbs strongly is therefore the first step in making the process controllable.
Energy Density and Depth Control
Depth control comes from the energy density and the number of passes. A single pass at low energy removes only part of the mask thickness, and repeated passes remove more, so the depth can be built up incrementally. That is the safe way to work, because it allows the process to stop at the mask and copper interface rather than overshooting into the copper.
The energy density is set by the power, the spot size and the scanning speed. A smaller spot gives a higher energy density for the same power and a narrower kerf, while a faster scan reduces the energy delivered per unit length. The parameters interact, and the process window is defined for a specific mask material and thickness rather than generally.
Edge Quality and Taper
The wall of an ablated opening is tapered rather than vertical, because the beam is not perfectly collimated and because the material is removed progressively from the top. The taper is a property of the optics and the process, and it has to be included in the design of the opening: the surface opening has to be large enough that the opening at the copper is the size the pad requires.
The edge itself may show a rim of re-deposited material and a slight charring where the energy was highest. Both affect the appearance and the adhesion of the mask at the opening, and the rim is removed during the cleaning step that follows. A ragged edge usually indicates that the energy was too low and the material was torn rather than removed.
Copper Exposure and Damage
The copper beneath the mask is the point at which the process has to stop. A small amount of copper removal is tolerable on a pad that will be soldered, while damage to a fine conductor changes its width and therefore its resistance and impedance. The visible sign of over-ablation is a bright, polished copper surface with a roughened texture, and in the worst case a partial cut into the trace.
The safe margin is achieved by stopping the ablation above the copper and removing the last of the mask by a controlled cleaning step, or by using a wavelength and a pulse length that the mask absorbs much more strongly than the copper. Both approaches require the depth to be verified rather than assumed, and the verification is described below.
Debris and Cleaning
Ablation produces debris: material that was vaporised and re-condensed, and particles that were ejected mechanically. The debris settles on the mask around the opening and inside it, and if it is not removed it interferes with soldering and can be mistaken for contamination from the assembly process. The cleaning step after ablation is therefore part of the process rather than an optional extra.
The cleaning method has to remove the debris without attacking the mask or leaving a residue of its own, which is the same requirement as any other cleaning step on a coated board. The verification methods used for cleaning elsewhere in the plant apply here as well, and the cleanliness discipline of a coated assembly is described in the notes on the surface preparation of the board before coating.
Comparing With Photoimaging
Photoimaging exposes the mask through a film and develops the unexposed material away. It is a parallel process, so the time is independent of the number of openings, and it produces a wall whose taper is set by the exposure profile. For a production board with many openings, imaging is faster and more uniform than ablating each window individually.
Ablation wins where the opening is needed on a cured board, where the number of openings is small, or where the mask is too thick to image. The decision is a cost model rather than a quality model, because both processes can meet a thickness and a definition requirement if they are controlled. The imaging route is described in the notes on exposure energy, and the alternative deposition route in the notes on solder mask jetting.
Verification and Measurement
Verification has three parts: the depth, the opening size and the condition of the copper. The depth is measured on a section or with a depth gauge, the opening size is measured optically at the surface and, where the taper matters, at the copper, and the copper condition is examined under magnification for discolouration or removal.
The measurement should be taken on the first opening of a run and on a sample thereafter, with the result recorded against the machine parameters. Where the mask thickness varies across the panel, the same energy density produces a different depth, so the thickness map from the coating step is a required input to the ablation program. That relationship between mask thickness and the process is described in the notes on solder mask thickness control.
Process Control and Records
The controls are the beam power, the spot size and focus, the scanning speed and pattern, the number of passes, the mask material and thickness, and the cleaning step. Each is recorded with the program revision, because a change in the program changes the depth without any change in the machine settings.
The records that matter over time are the measured depth and the copper condition against the program revision and the mask lot. Where an opening is found to be too deep, the combination of records identifies whether the program, the focus or the mask thickness changed, and it prevents the correction from being made by reducing the power alone, which would leave the mask unopened elsewhere on the same panel.
FAQ
Can a laser open a mask window without touching the copper? Yes, if the depth is built up in passes and the process stops at the interface. The last of the mask is then removed by cleaning.
Why is the wall tapered? The beam is not perfectly collimated and the material is removed from the top down. The design of the opening has to account for the taper.
Is ablation suitable for production? For a small number of openings and for rework it is. For a board with hundreds of openings, photoimaging is faster and more uniform.
