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March 2017 • The PCB Magazine 35 STEPPING UP TO LASER PROCESSING FOR FLEX, PART 5: PROCESS DEVELOPMENT tion of via size, material, depth, quality, and tar- get drill time. The massive diversity in process requirements and continuous evolution of the market prevents a single such library from being developed. Furthermore, developing processes for more demanding applications—whether due to sensitive materials, unusual feature char- acteristics, or stringent process quality, yield, or cycle time requirements—often requires significant trial and error to meet all the suc- cess criteria. As a result, each manufacturer will need to develop processes that best suit their needs—developing their own process libraries based on their unique set of products, customer requirements, cost profiles, market conditions, goals, and strategies. In building up such librar- ies and making use of known-good processes, process development duration can be reduced over time. Process Cycle Time Laser processing cycle time can be broken down into a few categories: time spent drilling features (drill time), time spent moving the la- ser between features (move time), time spent aligning to features (alignment time), time spent placing and removing the material on the system work table (handling time), and any time that the system spends performing addi- tional tasks. Process development will generally affect drill time, sometimes also move time, but generally none of the other factors, which are mainly characteristics of the system and han- dling methods. Process Quality and Yield Process quality specifications differ between flex manufacturers. This can be traced back to both the diversity in company priorities as well as the diversity in downstream processing. Dif- ferent downstream processing, such as types and effectiveness of patterning, desmear, etch, plating, and other processes, will all impact the laser drilling quality characteristics necessary to achieve a given end-product yield. Similarly, yield requirements—the required percentage of product output meeting quality specifications—can differ between flex manu- facturers. While all manufacturers prefer high laser processing and end-product yields, some- times the cost profiles of yield loss versus pro- cess cycle time will favor a faster process over a few percentage points in yield. Example Tradeoffs In an extreme example of the tradeoff be- tween cycle time, process development dura- tion, and process quality/yield, a process engi- neer might choose to use a single laser pulse for a large-diameter through-via process. It would be an extremely fast process (cycle time) and have been very quick to develop (process devel- opment duration), but be very unlikely to meet any of the process quality or yield requirements for this application. In an alternative extreme example, favoring process yield and extremely stringent process quality requirements, a process engineer might spend years getting closer and closer to meet- ing the necessary quality requirements, run- ning thousands of panels through the entire manufacturing process flow to understand and improve on the end-product yield. In practice, process development activities fall somewhere in between these extreme ex- amples, balancing the relative priorities of each of these key criteria. 2. Flex Laser Processing Basics Developing a Process Although you may already have some idea of a process starting point, each new material and application generally has unique, unfore- seen challenges. As a result, it is generally a good idea to generate several test grids on un- patterned materials to perform a broad sweep of the process space around that process start- ing point, as seen in Figure 2. For very new ap- plications where the starting point is less clear, that process sweep can be an extensive design- of-experiments (DoE) around laser power, laser focus, laser pulse repetition frequency (PRF), process velocity, number of process steps, and tooling motions. For more well-understood ap- plications, this process sweep might be limited to varying laser power and focus. This process sweep would typically be fol- lowed by an iterative process of verifying qual- ity, adjusting parameters, running process win-