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44 The PCB Magazine • November 2016 ADVANCED UV LASERS FOR FAST, HIGH-PRECISION PCB MANUFACTURING geometric shapes in a thin polyimide sheet which may be loosely adhered to a paper back- ing. The coverlay itself is then adhered to a flex circuit as a protective layer, and its purpose is functionally analogous to that of solder mask for rigid PCBs. In some cases the polyimide and adhesive are attached to a release paper, and it is important to have non- thermal ablation of a UV laser source to avoid burning the paper. To cut through the materials, high energy levels are not necessarily required because the sheets are very thin which allows a very small focus spot to be used (smaller focus spots are also more divergent and hence less suit - able for cutting thicker materials). If this energy can be applied at very high PRFs, then high pat- terning speeds can be achieved accordingly. As UV laser power is varied from 6 W upwards to 30 W, different patterning speeds can be achieved with either a single beam, high-PRF approach or a more advanced beam splitting tool design. At lower power levels, the increase in process - ing speed is approximately linear with increas- ing average power. However, when transitioning to 20 W UV a stronger throughput advantage is realized. This is because lasers at 20 W UV and higher are typically designed for a higher PRF- nom (100 kHz vs. 50 kHz), which means even higher energy is still available at the very high PRFs. Furthermore, the advantage of beam split- ting becomes increasingly significant because several multiples of the required pulse energy are available at the higher powers. Higher power lev- els allow for more beam splits, such as 2× beams for 20 W and 3× beams for 30 W. Conclusion Pulsed ns UV DPSS lasers have been rapidly making inroads into high-volume advanced high-density PCB manufacturing for many years, and the drive to thinner, more flexible PCBs is likely to maintain if not accelerate the trend. For most laser product offerings on the marketplace, however, the output power is sufficiently high for only a small range of pulse output frequen - cies, which limits the flexibility of the laser and hence narrows its application space. More recent- ly, with new UV laser technology, a substantial broadening of the application space is achieved due to the ability to maintain high power levels at high PRFs. The technology is also conducive to lower cost of manufacturing which can further expand the serviceable application space. With continued technology development for UV DPSS lasers expected to further increase the availability of higher power, low cost, reliable laser products, further miniaturization of electronic circuitry and packaging thereof should lead not only to the improvement of current electronic devices, but may also help accelerate more nascent indus - tries such as the manufacturing of wearables. Acknowledgements The author would like to thank Robert Spo- sili and Andrew Tian for their technical contribu- tions and Raj Patel for his editorial guidance and support. PCB Jim Bovatsek is applications engineering manager with Spectra-Physics, a division of MKS Instruments. Figure 6: Coverlay patterning—high-speed cutting of thin polyimide, can benefit greatly from a split-beam system configuration.