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June 2015 • The PCB Magazine 55 per is removed, subsequent laser pulses used to remove the dielectric material pose effectively zero risk to damaging the bottom copper layer. Typical industry practice to enable copper- direct drilling with CO 2 lasers is to pre-treat the top copper layer using a variety of special meth- ods including black oxide, inter-granular micro- etching, and special laser direct drill (LDD) foils. This pre-treatment process adds additional cy- cle time and capital cost and typically enables drilling copper layers only up to approximately 12 µm. Given this limitation, CO 2 lasers are ef- fectively prohibited from processing through vias in copper-clad laminates as well as L1 to L3 (and beyond) blind vias in multilayer stack-ups. Another complication is that CO 2 lasers typically leave dielectric residue behind after drilling. And since the standard dielectric ma- terial in flexible circuits is polyimide and the desmear process for polyimide either requires very caustic chemicals or aggressive plasma etch processes, most manufacturers avoid CO 2 lasers for flexible circuit drilling. Furthermore, due to the high energies and long pulse widths in- volved in CO 2 processing, a CO 2 -processed ma- terial generally results in significant carboniza- tion, which must also be removed via aggressive post-processing to avoid quality and reliability issues later in the circuit lifecycle. Finally, due to the CO 2 laser's long wavelength, such laser systems cannot keep up with the flexible circuit industry's push to smaller and smaller via sizes. UV laser processing overcomes many of the limitations that both mechanical and CO 2 laser processing face in meeting the evolving needs of the flexible circuit industry. The UV wavelength (typically around 355 nm for the most common Nd:YAG diode pumped lasers and dropping to below 200 nm for more specialized excimer la- sers) has several beneficial attributes that help address these needs. First, its wavelength is absorbed very well by most common flexible circuit materials, such as copper, polyimide, and various adhesives and resins. This enables UV laser micromachining to be extremely versatile – processing blind and through vias through thick and thin copper- clad laminates, unclad materials, and multilay- er stack-ups without any costly and often toxic pre-treatment steps. An example of a multi- layer through via is shown in Figure 2. UV la- ser tools also can be used to remove layers of copper, excess adhesive, and improperly placed coverlay material, as well as excising complex and fine-featured shapes of circuits and cover- lay material. Another benefit of using UV lasers is that most smear and carbonization can be avoided when drilling vias and cutting circuits. As such, much less aggressive desmear or other post- processing steps are needed before plating (if drilling) or reliability testing (if excising the panels). Given UV's short wavelength, typical focused laser beam "spot sizes" are in the 15–25 µm range. That means that with precise beam positioning, features sizes can range from the size of the entire processing area down to that 15–25 µm spot size. Finally, with proper precision engineering techniques, a UV laser tool can achieve ex- tremely accurate placement of vias and other features. At this time, the most typical tool accu- racy specification is ±20 µm, although specialty shops can and do achieve better than ±10 µm with that same tool by maintaining tight con- trol over the tool's and the processing materials' temperature and humidity, as well as enforcing strict alignment, calibration, and pre- and post- processing procedures. Figure 2: Through-vias drilled by uV laser in a multilayer stack-up. FeAtuRe STAYING CURRENT continues

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