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40 The PCB Magazine • March 2017 mask. However, even in these circumstances, in order to ensure a consistent process, it is impor- tant to have found the focus point accurately before purposely defocusing the beam by a known amount. Tooling Motion Choice While every laser tool manufacturer may have slightly different options and names for tooling motions, common choices include punches, circles, spirals, and routs, as seen in Figure 5. Each tooling motion has unique characteristics that result in different typical uses. Punches may be used when the feature size is approximately equal to the laser spot size and are generally the fastest via formation method when it is possible to use them. Circles can be used effectively when the via sizes are larger than the laser spot size. Since circle tooling motions only ablate the perimeter of the circle, it is often necessary to follow up with inset circle processes to remove more of the internal material to achieve a robust pro- cess. Such multi-circle processes are described in more detail later. Spiral tooling motions can similarly be used when the via sizes are larger than the laser spot size. Spirals can be used in place of multiple in- set circle processes for more process flexibility. Finally, rout tooling motions are best used for cutting out or ablating any non-circular shapes. Pulse overlap and material removal rate are two important characteristics to consider for routs. Special process development attention should be given to areas with small turn radii, given the tendency for heat to accumulate in those areas. Depth-Limited vs. Through Processes There are distinct differences in process development best practices for depth-limited processes such as blind vias and soldermask removal versus processes that cut completely through a material, such as through-vias and excising parts. For depth-limited processes, one needs to be very careful about cutting too deep into the material and damaging the underly- ing substrate. On the other hand, for through processes, it is possible to develop much more aggressive processes since one does not have to worry about damaging the underlying material. These differences result in greater difficulty to develop optimal depth-limited processes—one needs to weigh cycle time vs. yield/quality trad- eoffs between aggressive and more conservative processes. In general, to ensure the most robust and high-yield process, it is important to completely clear the top copper using an in-focus spot be- fore proceeding to the dielectric clearing step. Another best practice is to develop blind via and multilayer processes one step at a time. Test out and evaluate process parameters for each step of the via formation process. For example, perhaps you have a three-step blind via process, cutting the top-copper perim- eter with a circle tool first, then removing the STEPPING UP TO LASER PROCESSING FOR FLEX, PART 5: PROCESS DEVELOPMENT Figure 5: Common tooling motion choices.