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November 2016 • The PCB Magazine 39 ADVANCED UV LASERS FOR FAST, HIGH-PRECISION PCB MANUFACTURING coated thin-rigid copper-clad substrate. The goal is to quickly and cleanly remove the material with minimal copper damage and with a small, controlled amount of sidewall taper angle. Using a pulsed nanosecond UV laser combined with high-efficiency flattop beam-shaping optics, ex - periments were performed to determine drilling throughput at varying average power levels. The ABF type was GX13 with a thickness of 30 µm, and the targeted via-diameter was in the 50–60 µm range. The power was varied within the range 2–7 W with a fixed PRF of 100 kHz, resulting in a pulse energy window of 20–70 microJoules (µJ). The number of irradiating pulses was varied for each power level, and the minimum count re- quired to cleanly expose the copper substrate was noted. Dividing this number into the 100 kHz PRF generates the maximum theoretical drilling rate in vias per second. Results of the study are summarized in Fig- ure 2. The rate of drilling is plotted on the left- hand axis and the drill rate per unit Watt of laser power, a measure of efficiency, is on the right- hand axis. As the power and hence pulse energy increases, the drilling rate initially climbs rapidly, approaching 3000 holes per second. Above ~40– 50 µJ of energy, however, a saturation regime is encountered and the drilling rate levels off which results in the downtrend in drilling efficiency. This saturation phenomenon is caused by the exponential nature of light extinction in the ma - terial, whereby beyond certain fluence (energy per unit area) levels, only marginal increases in ablation depths can be achieved even with large increases in fluence. With UV light and strongly absorbing polymeric materials, this transition can be quite abrupt. A consequence of this phe- nomenon is illustrated in Figure 2: A doubling of pulse energy from 25 µJ to 50 µJ only resulted in a 1.4× faster drilling/ablation rate, much lower than the 2× one might have expected. Since the laser used for the tests maintains high power at high PRFs, drilling rates easily exceeding 3,000 vias/sec could be achieved simply by increasing the PRF used for the process. In Figure 3, microscope images of a ~50 µm diameter laser-drilled via show that the ABF is drilled with high quality and with minimal damage to the underlying copper. For this via, 45 pulses at 150 kHz PRF were used, which equates to a drill rate of 3,300 vias per second. Since small microvias require less energy due to the higher concentration of the beam, it can be a challenge to design a system and define a pro - cess that can make efficient use of today's higher power UV sources, which in many cases are de- signed to operate nominally (i.e., with maximum output power) at lower PRFs and higher pulse energies. One simple way to maximize through- put is to increase the laser's PRF until the optimal combination of pulse energy and pulse rate is en- Figure 2: Via drilling rate and efficiency with increasing average power at a fixed PRF. NOTE: This data is at a condition of non-optimized system configuration and process parameter set. Figure 3: Top ABF surface (a) and bottom copper surface (b) views of a blind microvia drilled with a pulsed UV laser at high PRF with 3,300 holes/sec throughput. (a) (b)