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54 The PCB Magazine • June 2015 layer to layer, the push for smaller devices, bet- ter signal integrity of high speed signals, and lower capacitance to reduce parasitic power losses is driving higher usage of blind and bur- ied microvias. Chip and circuit sizes are also shrinking, which requires smaller microvias, thinner lines and tighter spacing between sig- nals, and higher placement accuracy to process all of these features. While the flex circuit industry continues to use double-sided 12 µm Cu/25 µm PI/12 µm Cu copper-clad laminates, more and more manu- facturers are beginning to use thinner materials such as 12/12/12 and 5/13/5 (Figure 1) copper- clad laminates in production, with some com- panies experimenting with novel additive tech- nologies. For example, some suppliers are drill- ing unclad dielectrics and adding an extremely thin copper layer to achieve even smaller geom- etries at even lower costs. This trend toward in- creasingly thinner material drives lower mate- rial cost and higher flexibility, while increasing micromachining difficulty. Processing Methods and the Latest Trends In the discussion so far, we have seen how end-user market requirements have begun to impact the use of flexible circuits as well as the complexity and difficulty of their microvia drilling and circuit cutting needs. Let us now turn to the three methods currently most fre- quently used to drill and cut printed circuit boards in general—mechanical processing, CO 2 laser processing, and UV laser processing—and explore some of the reasons why UV laser tools are uniquely suited to addressing flexible circuit processing needs as well as keeping up with the latest market trends. While mechanical processing methods are typically the most cost-effective means of pro- cessing material, they have several issues in keeping pace with the latest trends. Die punch- ing or routing the outlines of flexible circuits and associated coverlay material become im- possible as part sizes and curve radii shrink be- yond the capability of the die manufacturer or below the size of the routing tool. Mechanical via drilling becomes excessively expensive as via sizes shrink due to the higher cost and breakage of small-diameter drill bits. Mechanical drills also suffer from lower accura- cy. This is due to two factors. First, at smaller di- ameters, these machines suffer from "drill wan- der" caused by the higher length-to-diameter ratio of the drill bit. Second, typical multi-head mechanical drill benches drill the exact same locations on all panels and do not compensate for the small scaling inaccuracies and deforma- tions that each individual panel typically suf- fers from. Furthermore, mechanical drilling has insufficient depth control accuracy to robustly drill blind vias in typical flex circuit construc- tions. Typical flexible circuit copper thicknesses are on the order of (or thinner than) the depth- control accuracy of the drilling tool. These problems will be challenging for mechanical technology to overcome as the flexible circuit roadmap progresses towards smaller circuits, higher accuracy, and smaller microvias. CO 2 laser processing has found a solid place in high-volume manufacturing of relatively large vias in rigid board processing, but has sev- eral issues that have limited CO 2 tools from be- ing widely used in processing flexible circuits. CO 2 technology is often used in blind via pro- cessing of rigid copper-clad laminates due to the fact that the CO 2 laser's far infra-red wavelength (typically between 9.4 and 10.6 µm) is poorly absorbed by copper. Once the top layer of cop- Figure 1: 150 µm blind via drilled in 5/13/5 material. FeAtuRe STAYING CURRENT continues

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