Laser drilling applications in the pcb industry

Laser Drilling Applications in the PCB Industry Two kinds of laser technique can be used in laser drilling of PCB. CO2 laser wavelength is within far ultrared-ray waveband and ultraviolet laser wavelength is within UV band. CO2 laser is widely used in micro-vias production of printed circuit board, requiring micro-vias’ diameter greater than 100μm (Raman, 2001) . It has high productivity in large-size hole manufacturing since it needs very short time to finish punching by CO2 laser. UV laser technique is widely applied in micro-vias production, with diameter less than 100μm even 50μm. It has very high output when produce holes with diameter less than 80μm. Therefore, more and more manufacturers have brought in double-ended laser hole-drilling system in order to meet the increasing demand of micro-vias production.

Below are the three main types of double-ended laser hole-drilling system in current market: 1) double-ended UV drilling system; 2) double-ended CO2 laser drilling system; 3) mixed laser drilling system (with both CO2 and UV) .

Each type has its merit and demerit. Laser drilling system includes double-ended single wavelength system and double-ended dual-wavelength system. Two parts influence the drilling quality: 1) laser power / pulse energy; 2) beam positioning system.

Laser pulse energy and transmission efficiency of beam decide the drilling time, which means the period of time needed to drill a micro-via by laser drilling machine.

Beam positioning system decides the speed of movement between two holes. The speed of laser drilling machine is determined by all the factors together.

Double-ended UV drilling system is ideally suited to drilling less than 90μm in integrated circuit, with high aspect ratio.

Double-ended CO2 laser drilling system makes use of Q-switch RF-excited CO2 laser.

It’s principle advantages include high repetition rate (which reaches to 100kHz), short drilling time and wide interface.

In the HDI manufacturing process, mixed laser drilling system is the most popular system composed of UV laser head and CO2 laser head. The comprehensive application of mixed laser drilling methods allows both copper and dielectric medium drilling occur at the same time. Ultraviolet radiation drill the copper to form a particular size and shape, and CO2 laser drill the uncovered dielectric adjacently. The drilling process is to drill a 2inX2in area, which is called domain.

CO2 laser can remove dielectric effectively, even the non-uniform glass-reinforced dielectric. However, it is unable to drill hole less than 75μm and remove copper, with an exception of pre-treated thin copper foil (lustino, 2002) less than 5μm. UV laser can drill very small holes and erase every ordinary copper (3-36μm, 1oz, even electro copper foil). It can also erase dielectric material by itself but it is quite slow. And the result is not good on non-uniform materials like glass-reinforced FR-4. It is because

that only when the energy density is improved to some extent, the glass can be successfully removed but bonding pad will be destroyed in the meantime. Since mixed laser drilling system contains both CO2 and UV laser, it can achieve the best result in the two fields, with UV laser in copper foil and small hole and with CO2 laser in fast drilling on dielectric.

At present, the distance between the two heads is fixed in most double-ended laser hole-drilling system, with stepping-repeat beam positioning technology.

Stepping-repeat laser remote controller has large regulating range, reaching to 50X50 Îźm. Its disadvantage is that the laser remote controller has to move in the fixed area, and the separation distance between the two heads is fixed as well. Typical separation distance between the two heads is about 150Îźm. It is impossible to get the optimum allocation operation like programmable head when it comes to different panel sizes.

Nowadays, there are various specification of performance for double-ended laser hole-drilling system, which can apply to small printed circuit board manufacture as well as mass production.

Ceramic aluminum oxide is used in printed circuit board manufacture for its high dielectric constant. But it is frangible and the mechanical pressure in drilling need to reduce to the minimum, which is an advantage for laser drilling. Rangel had proved in 1997 that Q-switch Nd – YAG laser drilling can be applied in aluminum oxide baseboard and aluminum oxide baseboard covered with gold and anchor. Laser with

short pulse, low energy and high-peak power can protect sample from mechanical pressure and make high quality hole less than 100Îźm.