Laser drilling

Laser and it’s applications.

With laser drilling, nearly all solid materials, from metals to diamonds, can be drilled by selection of a suitable laser. Minute holes, impossible to create with mechanical processes, are possible. Laser drilling is performed contact-free, and thanks to extremely small optical systems, drilling is possible at locations that otherwise would be very difficult to reach.
Hole quality is subjective to the criteria of the design's form, fit, and function. Process time or cost may also determine the level of quality required. Laser produced hole quality is assessed in terms of roundness, taper, entrance and exit diameter tolerances, oxidation (i.e. metals), burring or recast layer, and micro-cracking (i.e. glass).
Single pulse drills
This method permits the greatest flow-rate and the highest processing speed. Each laser pulse creates a hole. The maximum frequency / repeat rate determines the throughput and the efficiency. Depending on the material and its strength, it is possible to achieve over 1000 drillings per second.
Percussion holes
Depending on the material, this method is used for hole diameters of 20 – 1200 µm. Aspect ratios (diameter / hole depth) of up to approx. 1:200. Depending on the material, hole depths of up to 40mm can be achieved.
Trepanning
If the hole diameter is significantly larger than the focus, it is trepanned, in other words, the hole is cut out. This can be achieved by either a rotating optic or by moving the work piece. Depending on the material, depths of up to 25mm can be trepanned.

The principle of laser drilling
Laser drilling of metals is based on a face-heating phenomenon. The absorbed intensity is transformed into heat within the penetration depth of laser radiation. And when the illuminated spot on the surface reaches boiling temperature, material removal starts due the processes of  vaporisation and melt expulsion. Laser enables drilling of a diamond die in a few minutes as against 20 hours taken by conventional methods. There is  no wastage in the process and the saving in terms of the cost of diamond dust helps in recovering the financial outlay on such a drilling system.
Laser light energy is primarily applied in effecting microopenings in rubies and diamonds. Without heating the entire machined unit, now possible to drill filament canals in refractory materials. Laser drilling of holes in a diamond takes 2 to 3 minute as against 2 to 3 days taken by conventional drilling. A laser installation in Russia drills holes with diameters and depths from 0.005 to 0.8 mm and depths from 1 to 3 mm, respectively, in diamonds of any size and shape. The plus point about laser drilling is that it does not cause any damage to the diamond or  any other processed material. A typical hole drilled in a ruby disc is shown in Fig: 5.1.
For laser drilling, usually pulsed carbon dioxide, Nd:YAG or alexandrite laser is used. Special operations like drilling of holes with diameter less than 0.5 mm using conventional techniques are difficult.  However, the pulsed laser microdrilling is quite successful for such operations, both for metals and non-metals. The Nd:YAG laser emits at 1.06 µm and the alexandrite laser is tuned at 755 nm. For some metals, the alexandrite
laser consistently gives a cleaner entrance hole with more roundness and finer edges than is obtained with Nd:YAG laser. The superior performance of alexandrite laser is due to its shorter wavelength and continuous spiking of its output. The absorption of most materials increases with shorter wavelengths, giving alexandrite laser an edge over the Nd:YAG or laser in many material processing applications.