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Driving Innovation: Selecting the Right Laser Source

Editor's note: Stefan Rung, technical director for laser systems at Schmoll Maschinen GmbH, contributed to this article.

When I first joined Schmoll Maschinen, I brought experience from almost every PCB process, except for laser. As I immersed myself in laser processing, I realized why it can seem so daunting to a newcomer. The complexity arises from three intersecting factors:

• A vast variety of laser sources: CO₂, UV-nano, green-pico, UV-pico, IR-pico, and others
• A diverse range of applications: Drilling, cutting, ablation, and more
• An extensive list of materials: These have vastly different absorption rates

Choosing the right machine or laser source is rarely trivial. Even for experienced engineers, answering "Which source is best?" requires examining the business's specific goals. Stefan Rung and I have written this article to help simplify that decision-making process, covering the most common scenarios from the perspective of someone building their knowledge from the ground up.

Intro to Laser Sources
In laser material processing, the laser source is only half the story. The other half is time. When we talk about nano, pico, and femto, we are discussing pulse duration, meaning the amount of time the laser beam actually touches the material. Understanding this distinction is the difference between a burned via and a surgical via (Figure 1).

      1. Microsecond (10-6 s): The Industrial Workhorse (CO₂)

Most CO₂ lasers used in PCB drilling operate in the microsecond range.

Mechanism: This is an almost purely thermal process. The pulse is long enough to vibrate the molecules of the dielectric (resin/glass) until they reach their boiling point and vaporize.

Result: High volume removal. It is the fastest way to drill a via, but because the pulse is long, heat has time to spread. This creates a heat-affected zone (HAZ) and requires a copper "stop" layer because the laser cannot easily ablate copper.

      2. Nanosecond Lasers (10-9 s): The Thermal Process

Nanosecond pulses are the traditional solid-state workhorse of the industry.

Mechanism: Photothermal. The laser heats the material until it reaches its boiling point, at which point it evaporates.

Result: Because the pulse is relatively long in molecular terms, heat spreads into the surrounding resin and glass. This creates a HAZ, which can lead to charring or carbonization on the sidewalls.

      3. Picosecond Lasers (10-12 s): The Cold Ablation

Picosecond pulses are significantly faster; they are so fast that they change the physics of the cut.

Mechanism: Mostly photolytic. The pulse duration is shorter than the "thermalization time" of the material. The laser breaks molecular bonds before heat can travel to the next molecule.

Result: Often called cold ablation. The edges are incredibly clean, there is almost no HAZ, and carbonization is nearly eliminated.

      4. Femtosecond Lasers (10-15 s): The Ultrafast Frontier

Femtosecond pulses represent the current peak of industrial laser technology.

Mechanism: Non-linear absorption. Energy is delivered so instantaneously that even materials normally transparent to light are forced to absorb the energy and vaporize.

Result: Perfection. Precision is at the molecular level, allowing for features so small and clean that they are difficult to see even under a high-powered microscope.

In current PCB manufacturing, CO₂ and UV-nano remain the industry standards for high-volume production. However, as designs push toward the miniaturization of features and the use of sensitive materials, picosecond (pico) technology is increasingly adopted to achieve cold ablation results. While femtosecond lasers represent the pinnacle of precision, they remain a rare case in the PCB industry, reserved for highly specialized R&D or ultra-high-end semiconductor packaging.

Which Wavelength Should I Choose
While the terms microsecond, nanosecond, or picosecond describe the pulse duration, the color of the light is the second main parameter making the choice of the correct laser source important. Laser sources have discrete wavelengths, which have a huge impact on the desired application. The wavelength, i.e., the “color” of the light, determines the absorption behavior of each material. We’ve learned this from our daily lives: If materials absorbed different wavelengths homogeneously, our visual impression of the world would be a greyscale between black and white.

However, for most of us, the world is colorful. Due to the reddish appearance of copper, we see that red is predominantly reflected at the surface while the other colors of our spectral eyesight are absorbed more strongly. From the perspective of a laser application, we prefer to use wavelengths outside the red spectrum to deposit laser energy for removal purposes, such as green or blue light. On the other hand, we can use the reddish spectrum to prevent copper from absorbing the laser energy and being damaged by it. By selecting the appropriate laser wavelengths, we can directly influence the targeted removal and protection of different materials. The most common laser wavelengths utilized in the PCB industry are:

• 355 nm (UV)
• 532 nm (Green)
• 1064 nm (IR)
• 9600 nm (IR/CO₂)

For each application that requires laser processing, selecting the laser wavelength is important. This makes it mandatory to consider the entire application spectrum of current and future materials before deciding the laser wavelength.

Drilling Methods: Navigating the Possibilities
Laser drilling involves several distinct methods for creating holes. The choice depends on the target diameter, the material stackup, and the required pulse diameter.

• Trepanning: The laser beam follows a spiral path to cut out the hole. This is used when the desired hole is larger than the beam diameter (Figure 2).

• Pulsing: Also known as "percussion drilling," where the beam diameter matches the desired hole diameter, allowing the via to be created in one or more pulses without moving the beam (Figure 3)

• The Combi Method (Figure 4): A multi-step process typically involving: 

1. UV-nano or pico-green to trepan (ablate) the top copper layer.
2. CO2 t_o pulse and remove the dielectric material.
3. Optional: A de-focused UV or pico-green pass for "bottom cleaning" to ensure a reliable connection to the inner layer.

There are two methods of direct CO₂ drilling, which differ from one another only in the first step.

• Direct copper pulsing: Uses only a CO₂ laser. Since CO₂ cannot ablate shiny copper, the copper surface is chemically treated (blackened or browned) to increase absorption and thin it down so the CO₂ can penetrate it (Figure 5).

• Conformal mask: Copper is either pre-etched or the process is used on panels where no copper exists on the top layer (common in build-up processes) (Figure 6).

When Should You Use Which Method?

For prototyping and mid-size production: The Combi process is the gold standard for flexibility. It combines the high CO₂ productivity (which naturally stops at the target copper pad) with the precision of UV or pico lasers (which can ablate copper and achieve smaller diameters).

For mass production: Standard mass-market HDI often utilizes pure CO₂ configurations to maximize throughput on pre-defined stack-ups.

For ultra-small vias (<75 microns): CO₂ becomes physically limited at these sizes. One must switch to UV-nano, pico-green, or pico-UV. These sources offer the precision needed for microvias but require careful energy management and process fine-tuning (Figure 8):

• Risk of low energy: Not reaching the target copper pad (open circuit)
• Risk of high energy: Penetrating through the target copper (damage to the inner layer)

Cutting Applications
Cutting performance is strictly material dependent. While any laser can cut, the best match ensures the cleanest edge and fastest speed:

• Flex materials: UV nano is a classical solution or pico-green lasers for minimizing the HAZ/carbonization and achieving the clean edge
• PTFE: CO₂ or pico-green lasers have the best absorption threshold for PTFE
• Ceramics: Pico is often the best match for brittle ceramic substrates

Ablation (Depth Routing by Laser)

Depth routing with a laser is used, for example, to reach contact pads on inner layers.

• Constant thickness: If the layer to be removed is uniform and you need to protect the layer beneath, an ultra-short pulse laser is ideal for cold ablation.
• Variable thickness: If the layer to be removed is dielectric and its thickness is inconsistent, a CO₂;or IR laser is better, as these sources are reflective of copper, which acts as a natural safety stop.

Combination of the Applications
In high-volume environments, machines are usually dedicated to one task. However, in prototype and mid-series shops, the goal is often versatility.

When making a decision, you must prioritize:

1. Budget and space: Can one machine do the work of two?
2. Technological margin: Do you buy for today’s needs or tomorrow’s requirements?
3. The "good enough" factor: A pico-green + CO₂ combination is the most flexible machine configuration, but is it overkill for your specific product mix?

Summary
The choice of a laser source is a business decision as much as a technical one. The answer lies in your priorities, so ask yourself:

• Should I spend more now for a technological margin in the future?
• Which specific PCBs bring the highest margin to my company, and which laser do those boards require?
• Universal machine or dedicated specialist?

We have examined the most common and critical use cases for laser micro-machining. However, in a real-world manufacturing environment, variables are far more diverse. Every project presents a unique combination of materials, specialized applications, and demanding process requirements, such as extreme aspect ratios (AR), specific taper control, or unique blind hole geometries. Furthermore, the success of the laser process often depends on the synergy between pre- and post-processing steps, which must be carefully calibrated to ensure final product integrity.

This column originally appeared in the April 2026 issue of I-Connect007 Magazine.