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March 2017 • The PCB Magazine 41 Finally, make sure your alignment points are high contrast, highly consistent, and have a well-defined center at the small scale commen- surate with the accuracy that you are trying to achieve. Patterned butterfly-type targets best meet these criteria. However, when used appro- priately, cross-hair, circle, and donut targets can be used effectively as well. Summary Optimizing PCB laser processing for produc- tion requires a holistic view of the laser process- es and their place in the production line. Tak- ing the time to understand how those factors interact allows you to make informed decisions about where to focus your efforts and helps you to create a production process that effectively supports your organization's unique set of op- erational goals. But since process development is iterative and always ongoing, you will need to devote the necessary time to test, document, adjust and improve your process library. Fol- lowing basic process development principles and best practices with a focus on constant im- provement will not only allow you to be more flexible in response to changes in requirements but the resulting production processes will be better aligned to your business goals. PCB Patrick Reichel is product manager for ESI's flexible circuit micromachining tools. To read past columns by ESI, or to contact the authors, click here. STEPPING UP TO LASER PROCESSING FOR FLEX, PART 5: PROCESS DEVELOPMENT interior top-copper slug with a second circle tool, and finally cleaning up the polyimide di- electric with a spiral tool. In such a case, develop and evaluate all three steps individually. Verify that the first perimeter cut fully cuts through the top copper without penetrating the bottom copper. Then, after optimizing the first step pro- cess, verify that the central copper slug removal is complete. Finally, after optimizing the second step process, verify that the dielectric removal does not leave any residue and does not cause bottom copper damage. Alignment Points and Geometry Transforms There are many factors beyond the scope of this article that affect the registration accuracy between via holes and the landing pad. How- ever, a few common-sense best practices can be followed to improve your registration today. First, use the same alignment points for all processes, such as laser drilling, patterning, and drilling the tooling holes for layup—this results in the least amount of inconsistency. Second, use the most accurate tool/process to create the alignment points—in many cases, this will be your patterning process or laser drill. While laser drills will often take more time, and have a higher cost per hole than a punch or me- chanical drill, the registration accuracy will be higher. Third, use the same scaling methods and geometry transforms (e.g., parallelogram, trap- ezoid, etc.) for your drilling and your patterning processes—this again results in the least amount of inconsistency. Figure 6: Common types of alignment targets.

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