Basics of lasing high-aspect-ratio holes

When laser drilling holes in any material, the ratio of the hole’s depth to its diameter—the aspect ratio—must be taken into account. The reason has to do with the inevitability of taper on laser-cut materials. Usually taper is oriented such that the beam entry point is larger than the exit point (or bottom of a blind feature).

Taper limits how deep a laser-drilled hole can be. Further complicating matters is that ablated material can be deposited on the sides of holes, and at high repetition rates, ejected plasma and liquid interferes with the laser-drilling process.

Conditions improve as laser pulse length shortens and/or pulse energy rises because, with either change, peak power increases. The best results are obtained when drilling in a vacuum, which, albeit, is an unrealistic option for most users.

Going deep

What is the definition of a high-aspect-ratio hole? I consider 10:1 to be the low end and 20:1 to be the practical upper limit when laser drilling.

Taper is inherent with laser processing. Usually, the entrance of the cut is larger than the exit or bottom of the cut.

High-aspect-ratio holes in this range are laser-drilled by the automotive and aerospace industries, among others. Turbine aircraft engine manufacturers drill millions of holes per engine for cooling, efficiency, aerodynamics and noise reduction. Fundamental to the use of lasers in these applications is optimizing process parameters to achieve acceptable hole quality and to drill at a high speed. The holes are typically cylindrical, from about 300µm to 1mm in diameter, and are normal to the surface or, more likely, at an angle of 15° to 90°.

Circular holes can be laser-drilled with various techniques, including single shot, percussion and trepanning. These processes can be performed with a fixed-beam laser. (To learn about these methods, see the May/June 2010 LASERpoints column.)

There is also a need for high-aspect-ratio “shaped” holes. These typically are noncircular inlets for air flow that have “round” holes set inside them. Shaped holes require the use of a galvanometer-controlled laser. Drilling rates are several holes per second.

Millions of holes with aspect ratios around 10:1 have been laser-drilled. The smallest attainable hole diameter depends on the laser and optical setup. In principle, holes from 20µm to 50µm can be drilled. In practice, though, most of the holes fall in the hundreds-of-microns range.

Moving material

A laser expels material by either vaporizing or melting it. It takes about 75 percent more energy to vaporize than melt a material, so, in principle, vaporization is faster. The expulsion mechanism—vaporization or melt—is dependent on laser pulse energy and laser length.

Vaporization dominates when a short-pulse (nanosecond or shorter) laser is used; peak powers are on the order of tens of megawatts per square centimeter. Melt expulsion dominates with longer-pulse lasers (hundreds of microseconds to milliseconds) and peak powers are less than 1 megawatt per square centimeter.

Excimer lasers have both high-pulse energy (hundreds of millijoules per pulse) and fairly short-pulse lengths (~20 ns). This combination makes them a good choice to drill high-aspect-ratio holes, especially in polymers. An excimer laser working in 1mm-thick acrylic can drill clean, round holes with an exit diameter of 25µm and less than 1° of taper, which translates to a 40:1 aspect ratio.

The taper inherent in laser drilling can be used to advantage, as my shop has learned from drilling a polyimide drug-delivery device. We have found that a 15µm-dia. entrance hole yields a 0.5µm, or smaller, exit hole. The taper enhances the drug’s flow.

When laser drilling this way, it’s recommended that an end-point detector be used to ensure that all holes have the same diameter. (Pulse counting, or applying a specific number of pulses per hole, doesn’t yield predictable enough results with holes this small.) Assuming the holes are through-holes and there is no physical limitation imposed by part geometry, sensitive UV detectors can indicate when breakthrough is about to occur. We run a laser at a 100-kHz repetition rate, which is high, and are able to stop within two pulses of the end point once it’s detected.

Going deeper

Earlier, I said the upper practical limit for laser drilling was 20:1. However, very deep, small-diameter holes—those with aspect ratios up to 100:1—can be laser-drilled with the proper laser and optical setup. Drilling such high-aspect-ratio holes is of growing interest, especially among manufacturers of engines and fuel injectors.

Engine injector holes often are laser-drilled at an angle. Photo courtesy Oxford Lasers.

Laser-drilling techniques under development are intended to improve next-generation fuel injectors by making them more fuel-efficient and to reduce the amount of noxious emissions from engines.

Other applications being considered for laser drilling include the spinnerets used by the textile industry and the injection nozzles of locomotive diesel engines. The latter application would involve laser drilling 50µm to 100µm holes through 2mm-thick metal, which translates to aspect ratios of 40:1 and 20:1, respectively. µ

About the author: Ronald D. Schaeffer is CEO of PhotoMachining Inc., a laser job shop in Pelham, N.H. E-mail: rschaeffer@photomachining.com.