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56 The PCB Magazine • June 2015 Understanding UV Laser Micromachining Tools Beyond having the capability of processing a flexible circuit to customer specifications, the most important factor in deciding on a produc- tion tool is cost of ownership. Cost of owner- ship means different things to different board shops. For high-volume board shops, cost of own- ership will typically mean cost per panel over the course of the system lifespan, taking into consideration upfront and maintenance costs, as well as tool productivity, part yield, and sys- tem uptime. For quick-turn shops or specialty research institutes, cost of ownership will be more influenced by how flexible the tool is in processing many different types of materials and features as well as how easy and fast it is to develop new processes for new applications. In order to achieve the best cost of ownership, UV laser micromachining tools—supported by the supplier's service and applications engineering organization—must both incorporate the most optimal laser for the end-user's application as well as be able to harness the full capabilities of the lasers. Micromachining with lasers operates by la- ser ablation, whereby material is removed by an absorptive interaction between laser photons and the material being machined when the laser fluence (energy per unit area) exceeds the mate- rial's ablation threshold. The characteristics of most UV lasers—small spot size, excellent ab- sorption in most flexible circuit materials, and relatively high pulse repetition rate (number of laser pulses per unit time)—require precise ener- gy dosing and fast and accurate beam position- ing to fully take advantage of their capabilities. That becomes even truer given industry trends to thinner materials and smaller vias with in- creasing accuracy requirements. Precise and well-distributed energy dosing is very important as shown in Figures 3 and 4 to reduce or avoid quality issues such as dam- age to the bottom copper layer and adhesive or polyimide residue in blind vias, etch back of via sidewalls, fiber protrusion in rigid-flex construc- tions, and copper splash, to name a few. Especially with the trend toward materials with thinner copper foils, precise power control is extremely important to avoid yield issues. The drill tool must employ methods that keep laser transients from reaching the material to ensure consistent process quality. Since typical via sizes are larger than the UV laser spot size, consistent beam positioning with flexible motion paths (e.g., spirals, circles, etc.) must be available for robust process development. For multi-revo - lution processes, pulse distribution methods should be used to avoid localized ablation. Fast and accurate beam positioning is equal- ly important to avoid quality issues, while also Figure 4: Minimal etch back and fiber protrusion with good energy dosing. Figure 3: Poor etchback and fiber protrusion with poor energy dosing. FeAtuRe STAYING CURRENT continues