Laser Drilling Microvias: Parameters and Limits

Why Laser Instead of Mechanical

A mechanical drill below about 0.15 mm is impractical: the bit is fragile, the spindle speed needed to keep the surface speed reasonable becomes extreme, and the drill tends to wander. A laser removes material without contact, so the hole size is set by the beam rather than by a tool, and a hole of 0.1 mm or smaller becomes routine. That is the foundation of HDI. Every microvia in a build-up layer is a laser-drilled hole, and the parameters that produce it determine the hole shape, the pad it lands on and the reliability of the connection.

Wavelengths and Materials

Two wavelengths dominate. A carbon dioxide laser at around 9 to 10 micrometres is absorbed strongly by the resin and the glass but is poorly absorbed by copper, which is exploited in the conformal mask process. An ultraviolet laser at 355 nm is absorbed by both copper and resin, which allows it to open the copper directly but makes it slower for bulk resin removal. In practice, shops use the CO2 laser for the dielectric and the UV laser for the copper, either in sequence on the same machine or on separate systems, and the choice affects both the achievable hole shape and the throughput.

The Conformal Mask Process

In the conformal mask process the outer copper is patterned with a small opening over each via site, and that copper becomes the mask that defines where the laser can remove dielectric. The beam passes through the opening, ablates the resin down to the target pad and is then stopped by the copper of the pad. Because the copper opening is smaller than the beam, the hole takes the shape of the copper window rather than of the beam, which gives a consistent diameter and a controlled pad capture. The copper window also has to survive the ablation, so the process relies on the difference in absorption between copper and dielectric.

Pulse Energy, Taper and Recast

The energy per pulse, the pulse length and the number of pulses set how much material is removed per shot and how much heat is left behind. Too much energy per pulse removes resin quickly but leaves a wider entry and a more tapered hole, and it can damage the target pad. Too little energy means more pulses, which is slower and can leave a residue in the bottom of the hole. The result is a hole that is wider at the top than at the bottom: the taper angle is a real property of the process and it has to be accounted for in the pad design, because the pad has to be large enough to catch the bottom of the hole plus the registration error of the laser and the pattern.

Aspect Ratio and Pad Rules

The aspect ratio of a laser-drilled microvia is the dielectric thickness divided by the entry diameter, and it is limited by the ability of the beam to reach the bottom and by the subsequent plating. A practical range is 0.75 to 1.0, so a dielectric of 60 micrometres supports a via of roughly 60 to 80 micrometres. The pad beneath must be large enough to tolerate the taper and the registration, which is why the capture pad is typically 1.5 to 2.5 times the via diameter. The clearance from the via to the surrounding copper follows from the dielectric thickness and the reliability requirement, since the resin between the via wall and a neighbouring feature is thin.

Desmear and Plating After Drilling

Ablation leaves a residue of decomposed resin on the walls and on the target pad, and it has to be removed before plating or the connection will be weak. The desmear step is therefore part of the laser process rather than an unrelated operation, and its aggressiveness has to be matched to the hole: too little and the residue remains, too much and the resin is etched back from the glass, which creates the interface that moisture and CAF can follow later. Plating then has to fill the small hole reliably, and because the hole is shallow and small, the plating chemistry behaves differently than it does in a deep mechanical barrel.

Common Defects

The characteristic laser defect is an incompletely opened hole, where a thin film of resin or residue remains over the target pad and the via connects poorly or not at all. The second is a hole that has punched through the target pad, caused by excess energy or by a pad that was thinner than expected. The third is an oversized or misshapen hole where the copper window was damaged or the beam was misaligned. The fourth is a target pad that is too small for the taper, so the via lands partly off the pad. All four are visible in cross section, which is why microvia qualification is normally done by section rather than by optical inspection.

Laser Drilling and the Stack

Laser parameters are not independent of the stack-up, because the dielectric thickness, its resin content and the glass style all change how the beam is absorbed and how much energy is needed. A build-up layer that is thicker than the process window, or a laminate whose glass weave is coarse relative to the via diameter, forces a change in parameters that in turn changes the taper and the achievable diameter. The practical consequence for the designer is that a microvia rule cannot be agreed in isolation: the fabricator has to confirm the dielectric thickness that the laser can open reliably and the diameter that results, and both numbers belong in the stackup document. Where a design needs the smallest possible via, the answer is usually a thinner dielectric rather than a more aggressive laser recipe, since thin dielectric also improves the plating and the reliability of the connection.

FAQ

What is the smallest microvia a laser can drill? Below 0.1 mm is achievable; the practical limit is set by the dielectric thickness and the aspect ratio rather than by the beam alone.

What is a conformal mask? A patterned copper opening that defines where the laser removes dielectric, so the hole takes the shape of the copper window.

Which wavelength is used? CO2 for resin removal and UV for copper, often in the same process.

Why do laser vias taper? Because the beam loses energy as it goes deeper and the entry is exposed longer than the bottom, so the hole is wider at the top.

How is the process verified? By cross sectioning a sample, since the critical defects are inside the hole rather than on the surface.

Conclusion

Laser drilling is a beam process with a shape, a taper and a residue, and the design has to allow for all three. Size the capture pad from the taper and the registration error, keep the aspect ratio inside the process window, treat desmear as part of the drilling step and verify the result by cross section. Microvia capability is described under PCB capabilities, the laser and plating sequence belongs to PCB manufacturing, and the pad and escape geometry is set in PCB design and layout. Dense boards of this kind are normally proven through prototype PCB assembly in 2026.