Cutting flexible copper clad laminates (FCCLs) using UV lasers is an excellent process; however, it has certain drawbacks. Multiple laser cuts are often required for double- and multi-layer FCCLs, resulting in lower productivity. In addition, laser cutting leaves debris on the product surface, which requires secondary cleaning, thereby increasing costs. Most importantly, the high temperature and high laser energy density can easily exceed the thermal damage threshold of the product, resulting in carbonization. Carbide can cause short circuits, which are the most severe defects in electronics. Moreover, process defects such as excessive heat-affected zones of FCCL caused by improper parameter selection, over-melting of the material, or cutting energy not reaching the cutting threshold result in product failure and increased power loss, which in turn increases process costs. Therefore, we designed a laser cutting process that can effectively reduce the photo-thermal effect on the surface and cutting edge of the FCCL, and cut the FCCL using minimum energy to reduce the generation of carbide. It is expected that this method can be applied to various thin-film materials that require laser processing.

The 49 μm polyimide double-sided adhesive-free flexible copper clad laminate was cut using a 355 nm UV nanosecond laser cutter as follows. First, the minimum energy for cutting the FCCL, the critical cutting energy , was determined by varying the laser scanning speed, laser scanning power, and the number of repetitive scans through pre-cutting experiments. Second, three laser cutting parameters were traversed to verify and correct the critical cutting energy value. Finally, the critical cutting energy value was used to examine the effect of the aforementioned laser cutting parameters on the surface profile, cross-sectional profile, degree of carbonization, width of the etched slit, and changes in the surface and cross-sectional composition of the FCCL, while ensuring the cutting through the FCCL. This allows the process parameters to be further adjusted to achieve high quality and low power consumption cutting.

Through the study of the intrinsic relations among the laser cutting parameters used in this experiment, the critical cutting energy value of a 49 μm FCCL was initially derived as 0.4000 J/mm using the relevant equations and experimental data. Then, the critical cutting energy value was corrected to 0.3867 J/mm by traversing each laser parameter (Fig. 3). The results also indicate that an increase in laser scanning speed is beneficial in reducing carbide generation, inhibiting the phenomenon of melt reflux due to thermal effect, avoiding slag accumulation, and obtaining a flatter and cleaner cross section, which are beneficial to improve the dimensional accuracy of the product and reduce the short circuit phenomenon of the product more effectively (Figs. 4 and 10). An increase in laser scanning power can reduce the copper content at the cross-section and increase the spacing between slits. However, the high power is likely to cause defects such as rough cross-sections and over-melting of the copper layer, whereas a low power can fail to cut through the FCCL (Fig. 6). Increasing the number of repetitive cuts can alleviate the over-melting phenomenon during cutting and improve the surface quality of the etched seam; however, it tends to increase the spacing of the etched seam. Although it does not have a significant effect on the degree of charring and carbonization, repetitive cutting results in loss of energy (Fig. 8). The optimal parameters for cutting the FCCL under the conditions of this study are a laser scanning speed of 2000 mm/s, laser scanning power of 12 W, and 70 repetitive cuts (selected at the expense of partial processing efficiency).

In this study, the influence of different process parameters on the laser-cutting quality of FCCL is examined in detail, and a method to optimize the laser cutting process is proposed. The principle behind the selection of the process parameters is to select cutting parameter values according to the evaluation parameter table used for pre-cutting experiments after calculating and verifying the required critical cutting energy value. Therefore, priority is given to moderate cutting power, followed by a high cutting speed, and finally, a minimum number of repetitive cuts. The results of this study provide a guide for achieving high-quality, low-energy laser cutting of FCCLs. In addition, the experimental method and the optimization of laser cutting parameters provided in this study are applicable to different thicknesses of FCCL, flexible circuit boards, polyimide coverlay, and reinforcing plates shaped by laser cutting.