Laser Drilling vs Mechanical Drilling

Almost every hole in a circuit board is made by one of two processes. Mechanical drilling uses a rotating bit to cut through the stack, and laser drilling uses a focused beam to remove material by ablation. They are not interchangeable: each has a range of diameters and aspect ratios where it is the only sensible choice, and the boundary between them moves as designs become denser.

How Mechanical Drilling Works

A carbide bit rotates at high speed and is driven through a stack of panels at a controlled feed rate. The bit cuts and clears the material, and the quality of the hole depends on the bit geometry, the feed rate, the spindle speed and the number of panels in the stack.

The process is fast and produces a hole with a defined wall that can be plated directly after desmear. It is limited by the minimum practical bit diameter, by the aspect ratio it can reach and by the mechanical damage it leaves in the surrounding resin.

How Laser Drilling Works

A laser removes dielectric material by vaporising it, usually with an ultraviolet source that is absorbed by the resin but reflected by copper. That selectivity is what makes the process useful for blind vias: the beam stops when it reaches the copper beneath.

The hole it produces is smaller than anything a bit can make, and there is no mechanical cutting force on the panel. The trade is a slower process, a wall with a different surface condition, and a depth that has to be judged by the process rather than by the tool reaching a defined point.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/zuzhuang2.png" alt="Cross section of a laser drilled microvia in a dielectric layer” />

Diameter and Aspect Ratio Limits

Mechanical drilling is practical down to about 0.15 millimetres and becomes uneconomical below that, because bits break and the cost per hole rises sharply. Its aspect ratio limit, the board thickness divided by the hole diameter, is around ten to one for reliable plating.

Laser drilling handles holes from about 0.05 to 0.15 millimetres and is used for microvias with aspect ratios up to roughly one to one. Beyond that the beam cannot clear the debris from the bottom of the hole effectively. The constraints on plating the result are described in our article on aspect ratio.

Wall Quality and Desmear

A mechanically drilled hole has a resin wall that is smeared with epoxy during cutting, and the smear has to be removed chemically before plating or the adhesion will fail. The process also leaves a certain amount of fibre protrusion and resin recession that has to be controlled.

A laser drilled hole has a different wall condition. The ablation leaves a rougher surface with a thin heat affected region, and the desmear requirement differs. Using the same desmear chemistry for both without validation can produce inadequate preparation on one of them.

Registration and Stack Up

Mechanical drilling drills a stack of panels at once to increase throughput, and the registration accuracy degrades with stack height because the bit wanders and the panels can shift. Thin panels and small holes call for a shorter stack and a slower feed.

Laser drilling is done panel by panel with optical registration to the copper beneath, which gives excellent alignment to the features below. That alignment is part of the reason the process is used for stacked and staggered microvia structures, where each via has to land precisely. The via structures themselves are described in our article on blind and buried vias.

Through Holes and Blind Holes

A through hole can be made either way, but mechanical drilling is almost always used because it is faster and can produce the larger diameters a through hole needs. Laser drilling is reserved for blind holes and microvias where the depth must be controlled.

Where a design needs both, the process becomes sequential: mechanical for the through holes and laser for the microvias, in a defined order with plating steps between them. That sequence determines how many lamination cycles the board has to survive.

Cost and Process Selection

Cost follows the process and the count. Mechanical drilling costs scale with the number of hits and the bit consumption, while laser drilling costs scale with the number of holes and the time each one takes. A design with many microvias pays for them individually.

The selection therefore starts with the via structure the design requires, not with the drilling method. Once the structure is fixed, the method is largely determined, and the cost estimate follows. The structural choices that drive it, including the through hole and microvia distinction, are described in our article on hole types.

Checks Before Release

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

Points to Confirm at First Article

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

FAQ

Can a laser drill a through hole? It can, but it is slow and the aspect ratio is limited. Mechanical drilling is the practical method.

Is a laser drilled wall weaker? Not inherently, but the wall condition differs and the desmear has to suit it. The plating adhesion is what matters, and it must be verified.

Why is a microvia limited to a one to one aspect ratio? Because the beam has to remove debris from the bottom of the hole. Once the hole is deeper than it is wide, the removal becomes incomplete.