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38 The PCB Magazine • March 2017 ferent thermal response of the smaller pads. If that patterned material is scarce, evaluate small- er process shifts using selective processing of in- dividual circuits on the panel. Process Robustness Verification At this point, we can introduce key process development concepts surrounding process ro- bustness. Process robustness can be defined as the process' ability to meet the process quality specifications across the design tolerances in system characteristics as well as slight changes to system and material characteristics due to environment changes, handling, and contami- nation over time. Different manufacturers and even different product models each might have different tolerances on collimated beam size, laser spot size, focus accuracy, power control, and work table flatness as a few key examples. Similarly, the system, the material, and the pro- cess will be impacted by extrinsic factors such as facility temperature and humidity, facility vacuum, and compressed air pressure and flow rate, etc. that can impact process quality if not managed properly. In short, process robustness is a key factor in ensuring high yield processes with consistently high quality. The alternative to spending time developing robust processes is accepting lower yields, system downtime and the expense associated with constant system adjustment and cleaning, and the higher expense associated with more stringent environmental control. Process Windows Process windows are a key method of quantifying the process robustness. Process window tests typically measure how much laser fluence (laser energy per unit area) change the process can tolerate before the process no longer meets quality specifications. Fluence is used as an evaluation metric due to its vital role in material ablation (removal). Furthermore, each of the system tolerances listed above (e.g., spot size, focus accuracy, power control, etc.) have an impact on either the laser energy or area over which that energy is delivered. Some manufacturers measure process window by varying laser focus above and below the process setpoint for the specified application. Some manufacturers measure process window by varying laser energy above and below the process setpoint for the specified application. Others do so by varying both laser focus and laser energy. No matter the precise method, it is critical to validate that the chosen process can withstand real world manufacturing conditions. Laser Focus For most laser processes, it is critical to find laser focus accurately. The reason for this is again related to the importance of laser fluence for material ablation (Figure 4). Not only will an out-of-focus laser spot be larger and therefore lower fluence, most lasers also suffer from lower beam circularity and higher beam distortion the further the laser spot is out of focus. This can result in poorer-quality and less predictable pro- cesses. Note that it is equally critical to find and verify laser focus accurately both during process development and during production. If the pro- cess was developed in focus, but processed out of focus—or vice versa—the process quality will suffer significantly in production. There are times that it is acceptable and even desirable to process out of focus, such as in clearing dielectric material from a blind via using the top copper opening as a conformal Figure 4: Laser focus has a significant impact on both effective spot size and laser fluence. STEPPING UP TO LASER PROCESSING FOR FLEX, PART 5: PROCESS DEVELOPMENT