Issue link: https://iconnect007.uberflip.com/i/746712
38 The PCB Magazine • November 2016 ADVANCED UV LASERS FOR FAST, HIGH-PRECISION PCB MANUFACTURING the small sizes necessary for the fabrication of in- creasingly small microvias. Since the mid-1990s, pulsed UV DPSS lasers with nanosecond (ns) pulse durations have been commercially avail- able for industrial/OEM use. While during the early days of the technology the relatively high cost and troubling reliability issues limited their appeal, today's products are vastly improved in both areas. Indeed, over the past decade, the cost per Watt for such lasers has fallen by an order of magnitude, and product lifetimes have improved dramatically, in some cases surpassing 20,000 op- erating hours at high power levels. Today's UV DPSS Laser Technology Typically, a UV DPSS laser begins with a high- power laser source at a fundamental infrared (IR) wavelength of ~1 mm which is focused into non - linear optical crystals to generate the UV output, a phenomenon known as harmonic conversion. The IR to UV conversion efficiency is dependent on the IR pulse energy, among other factors. The pulse energy is equal to the laser's average power divided by the pulsing frequency or pulse repeti - tion frequency (PRF). For IR wavelengths, the average power is rela- tively constant above some particular frequency, and therefore higher PRFs result in lower pulse energies, and vice-versa. As for the converted UV light, the maximum average power is achieved at some nominal frequency, PRFnom, the specific value of which is determined by the design of the laser. The average power at the UV wavelength decreases with operation at higher PRFs since the IR pulse energy, and hence the IR to UV conver- sion efficiency, diminishes. If elevated UV power levels can be main- tained for a wide range of operating PRFs through a careful laser design consideration then this of- fers a high level of machining flexibility for the end user. Higher energies can be used to machine large features and deep cuts, but if lower energies are required for precision drilling, cutting and mi- cromachining, operation at higher PRF levels can be used to proportionally scale up throughput. In short, the ability to have relatively high power levels for an extended PRF range would make for a highly flexible tool with a large application space. And if one laser can operate across this continu - um, then tool builders can reap the cost savings inherent in having both a single equipment in- terface (mechanical, electrical, optical, communi- cation) and a single equipment supplier (higher volume orders reducing cost per unit). Increasingly, lasers incorporating such ad- vanced design and UV conversion technology (including Spectra-Physics' latest industrial UV lasers) are becoming available on the market- place. While the exact techniques for achieving this performance are generally proprietary and closely guarded, they typically require strong ex- pertise in harmonic conversion methods as well as access to advanced optical coating technology . Compared to other UV DPSS nanosecond lasers (including Spectra-Physics' older genera- tion technology), the new harmonic conversion technologies allow elevated power levels to be maintained at an extended range of PRFs—well beyond PRFnom—as shown in Figure 1. Indeed, going beyond 3× PRFnom, the power output ad- vantage approaches a factor of 2. Furthermore, the pulse energy remains remarkably stable (well below 5%) out to 5× PRFnom. More conventional UV laser technologies technically do allow opera- tion at such higher PRF values, but beyond about 2–3× the PRFnom, the pulse energy stability de- grades very rapidly. PCB Microvia Drilling with UV Lasers A common laser via drilling application is microvia formation in Ajinomoto Build-up Film- Figure 1: Advanced UV light conversion leads to high power and high-pulse energy stability well beyond the nominal pulse frequency.