Frequently Asked Questions
Many parameters have an influence on the laser drilling process and thereby the quality that can be achieved. Those with a significant influence are discussed below.
A beam of laser light can, for the same beam quality, be focused to a smaller spot size if the wavelength of the light is shorter. This obviously allows smaller diameter holes to be drilled. However, a shorter wavelength laser also has other advantages: the energy coupling with the workpiece is in general better, there is less absorption by a plasma and the photon energy is higher (mainly of importance in photolytic processes, where the photon energy can be high enough to allow direct breaking of many molecular bonds).
The quality of the holes tends to increase with increasing peak power and power density. This is mainly associated with the change from molten ejection to vaporisation (i.e. ablation) dominated material removal. This reduces the thickness of the HAZ and of the recast layer as less material resolidifies in the hole which, in turn, increases the repeatability of the holes. However, it should be borne in mind that, in general, vaporisation dominated drilling is slower than molten ejection.
In general, for drilling, the pulse duration (also known as pulse width) is in the range from a fraction of a microsecond, to about one microsecond (1 x 10-6 second). As shorter pulses give better hole quality (again related to the material removal mechanism), there is much interest in using lasers with pulses in the pico- (1 x 10-12 second) and even femto-second (1 x 10-15 second) range. However, with shorter pulses the increased capital cost of the laser and the longer drilling time need to be considered.
Focal length and position
Optics with long and short focal lengths have their own separate advantages and disadvantages. For example, a long focal length results in a longer focus depth but also a larger spot size, and therefore a lower power density. Although this longer depth of focus makes the process easier to control and gives less hole taper, the lower power density has its disadvantages (see above). Conversely shorter focal lengths give higher power densities but are less easy to control. With regard to the focal position, incorrect positioning of the focal point above or below the surface will generally affect the depth and taper of the hole.
Process gases can be used to assist the drilling process in a number of ways. The gas protects the lens from the material ejected from the drilling hole (mainly a problem with pyrolytic percussion drilling) and minimise the debris build-up around the hole. The gas can also be used to prevent the formation of a plasma or plume over the hole (which absorbs/scatters much of the laser light). Another advantage is that oxygen can assist the actual material removal by inducing an exothermic reaction with some materials.