Laser Via Drilling: Energy Density and Taper Control

Laser via drilling makes the small holes that connect the layers of a high density board. The beam removes dielectric by vaporising it, and the process stops on the copper of the target layer, which acts as a natural etch stop. That stopping behaviour is what makes the process practical: the dielectric thickness can vary, and the hole still lands on the pad below as long as the energy is set within its window.

Where Laser Drilling Is Used

The process is used for microvias, generally defined as holes of 0.15 mm diameter and smaller, and for the small blind vias that connect adjacent layers in a sequential build. It is not used for through holes, where mechanical drilling is faster and the depth is too great for the process to control.

Two beam types are common. Ultraviolet lasers remove material by a combination of ablation and photochemical break-up of the polymer, which gives a clean wall. Carbon dioxide lasers remove material thermally and are faster, but they leave more residue and are usually used with a conformal mask rather than directly on copper.

Energy Density and Pulse Shape

Energy density is the pulse energy divided by the spot area, and it determines whether the dielectric is removed cleanly or violently. Too low and the hole does not open to the target copper; too high and the copper below is damaged or spattered upwards around the rim of the hole.

Pulse shape matters as much as the total energy. A short, high-intensity pulse removes material quickly but can leave a rough wall, while a longer pulse train with a lower energy per pulse produces a smoother hole. Many processes use a first pulse to break through the surface and a series of lower-energy pulses to clean the bottom, which is described as a trepanning or a step pattern.

Via Taper and Diameter Control

Laser-drilled vias are tapered because the beam diverges as it travels and because the ablation efficiency falls with depth. The entrance diameter is larger than the exit diameter, and the difference is the taper. The figure matters because the plating has to cover the wall along its full length, and a steep taper concentrates the deposit at the top.

Taper is controlled by the beam optics, the focal position and the energy. A focal position that is too high produces a wider entrance and a steeper taper; one that is too low concentrates energy on the target copper and damages it. The focal point should be set relative to the top of the dielectric for each stack-up.

Conformal Mask and Copper Direct Drilling

A conformal mask is a layer of resist opened at the via positions, which defines the entrance diameter and protects the surrounding copper. The dielectric is then removed inside the opening. The resist thickness and the opening size control the usable process window, and the mask has to be removed afterwards.

Direct drilling on copper relies on the reflectivity of the copper to protect it and on the energy threshold to stop the ablation. It removes the mask step and its cost, but it demands tighter control of the energy, because the same pulse that cleans the bottom of one via will damage the copper of another. The requirements for the blind via in the stack-up have to be compatible with whichever method is used.

Wall Quality and Desmear

A laser-drilled wall carries residue. Ultraviolet drilling leaves less than carbon dioxide drilling, but neither leaves a wall that can be plated directly. The residue is a mixture of decomposed polymer and carbon, and it has to be removed before the plating step or the copper will not adhere.

The removal is done by the same desmear chemistry used for mechanically drilled holes, usually with a permanganate step. The chemistry has to reach the bottom of a small, tapered hole, so the flow and the immersion time matter more than they do on a through hole.

Entrance and Exit Diameter Measurement

The entrance diameter is measured optically on the surface, and the exit diameter is measured on a section. The difference between them is the taper, and it is the figure that the process is controlled against. A change in entrance diameter with a constant exit diameter indicates a change in focus or in energy rather than in the dielectric.

Measuring only one of the two is not sufficient. An entrance diameter inside the specification with an exit diameter that has grown means the beam is reaching further than intended and the process is close to damaging the target pad. Both dimensions should be recorded for each stack-up.

Defects and Their Causes

Four defects are common. An incomplete hole that does not reach the copper indicates insufficient energy. A hole with a copper rim or a ring of spatter indicates excessive energy. A rough wall indicates a pulse shape that is too aggressive. A hole with an enlarged exit diameter indicates focus or energy drift over the batch.

<img src="https://www.gopcba.com/wp-content/uploads/2026/04/EMI.jpg" alt="Microvia cross section showing wall taper” />

Each is seen in the same section, and the pattern across a panel distinguishes a machine drift from a material variation. Where the defect appears at one position on the panel, the beam delivery or the panel flatness is the cause; where it is spread across the panel, the energy setting or the optics are.

Process Window and Control

The window is described by the energy density, the pulse pattern, the focal position, the number of pulses and the dielectric thickness. It is established for each stack-up and each dielectric, because the absorption characteristics of different materials differ. A window established for one laminate cannot be applied to another without a trial.

In production the window is verified by drilling a test coupon at the start of each run and measuring the entrance and exit diameters. The section that confirms the wall quality is taken less often, at a defined interval, and the results are recorded against the machine settings. The aspect ratio of the via is the figure that describes the difficulty of the job, and a higher ratio needs a wider window and more frequent verification.

Records and Verification

The record should carry the laser energy, the pulse pattern, the focal position, the number of pulses, the dielectric and copper thicknesses and the measured diameters. With those fields, a change in via quality can be traced to the beam or to the material without a trial batch.

Verification of the finished via is done on a plated hole section, which shows the wall, the plating thickness at the bottom and the connection to the target pad. The section is the final evidence that the drilling and the plating steps worked together, and it is the record that a customer will ask for on a new stack-up.

FAQ

What diameter counts as a microvia? Holes of about 0.15 mm and below are generally described as microvias. Larger holes in the same build are usually made by mechanical drilling.

Why are laser-drilled vias tapered? Because the beam diverges with depth and the ablation efficiency falls as the hole gets deeper. The entrance diameter is therefore larger than the exit diameter, and the difference is the taper.

Is desmear needed after laser drilling? Yes. The wall carries decomposed polymer and carbon residue that must be removed before plating, and the chemistry has to reach the bottom of a small tapered hole.