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AUGUST 2020 I PCB007 MAGAZINE 71 tional scheme used for cutting thicker materi- als. Specifically, this involves making a series of laterally displaced scribes to produce a "v- groove." The "v-groove" geometry is necessary to avoid clipping the beam as it penetrates fur- ther into the material when making a high as- pect ratio cut. This would reduce its power and thus limit ablation efficiency. But the AVIA LX, coupled with this novel pulse timing approach, can utilize pulse energies as high as ~400 µJ to repeatedly scribe along the same line (no lateral displacement, or "v-groove"). The re- sult is faster cutting and significantly reduced kerf width. Higher pulse energy also increases the laser focus tolerance at the work surface. Specifical- ly, when using a lower-pulse energy laser, it is necessary to shift the focus of the beam as the material is penetrated so that the minimum fo- cused spot size is always maintained precisely at the depth at which cutting is occurring. This is necessary to achieve sufficient laser fluence to get above the material ablation threshold. However, doing this in practice requires either physically shifting the PCB up, which slows the process, or employing a three-axis scanner (one which has a focusing capability), which increases equipment cost and complexity. The higher pulse energy of the AVIA LX makes it possible to simply focus the laser at a point mid-way through the PCB and perform cutting. This is because there is sufficient laser Advances in UV Laser Depaneling While laser depaneling clearly delivers nu- merous benefits, PCB manufacturers are al- ready pushing this technology to the limits to meet ever more stringent size, materials, and cost challenges. In particular, achieving further reductions in HAZ and debris formation, and improving the cut quality obtained with nano- second pulse width UV DPSS lasers, is an ac- tive area of development. To aid in this effort, applications research at Coherent Inc. has explored the outcomes and process space of using a nanosecond pulse width, high-pulse energy, UV DPSS laser (AVIA LX) for cutting a variety of PCB materials and material combinations. Based on this work, the Coherent team developed a new PCB cut- ting method that has already proven to deliv- er a reduced HAZ, a higher-quality cut edge, reduced kerf width, and increased production throughput. One key element of this technique is a pro- prietary method for controlling the timing and spatial positioning of laser pulses delivered to the work surface in such a manner that heat buildup is avoided. Because thermal damage is absent in this approach, it's possible to uti- lize a laser with substantially higher pulse en- ergy when cutting thicker materials (1 mm and above). The advantage of a higher pulse energy is that it eliminates the need to employ the tradi- Figure 1: Cross-sections of a 1.6-mm PCB cut using (left) a competitor's UV DPSS laser and (right) a high-pulse energy UV DPSS laser (AVIA LX) employing the Coherent technology. The latter delivers a better quality edge, and much cleaner cuts of the copper traces.