The designs of printed circuit board (PCB) are pushing the limits of electrical performance in an effort to further the performance and miniaturization of various devices PCB manufacturing—in particular for consumer electronics such as smart phones, tablets, and GPS devices. With continual advancement in integrated circuit (IC) fabrication, the number and density of connections required to interface to the devices continues to rise.
Via drilling is the gateway application for lasers in PCB manufacturing. Implemented in the late 1980s1 and throughout the 1990s2, PCB the ability of a laser to quickly machine so-called "blind vias"—holes ending at a predefined distance in the PCB material—was a boon for PCB manufacturing. Combined with laminated buildup process advances, it enabled the multilayer PCB architectures that were needed to keep up with the ever-shrinking semiconductor device fabrication node. The increasing need for small-diameter blind vias for dense packaging applications has created a fast-growing market for lasers PCB. While through-vias in the thickest PCBs are typically drilled mechanically, and CO2 lasers are used for large-diameter blind vias, the smallest-diameter micro-vias found in the most advanced PCB architectures are created with nanosecond-pulse, Q-switched, diode-pumped solid-state (DPSS) lasers in the UV (355 nm) wavelength regime. A typical blind-via-drilling application involves removing a few tens of microns of a buildup resin (e.g., ABF or Ajinomoto buildup film) to expose an underlying copper layer PCB. For vias less than about 50–60 μm PCB, the long (~10 μm) wavelength of the CO2 laser is significantly challenged with respect to the ability to focus tightly, and the relatively large melt recast can present additional challenges. In contrast, the clean photoablation that occurs with short-nanosecond, Q-switched 355 nm pulses results in high-quality, PCB melt-free micro-via fabrication.
Because of the micro-vias' small size and the strong absorption of the 355 nm wavelength PCB, this application does not require very high pulse energies; just tens of microjoules are sufficient. However, a nonoptimal process can give poor results, leaving a copper surface that is unsuitable for subsequent plating. For best quality PCB manufacturing, short-nanosecond laser pulses and top-hat beam shaping3 are used PCB, resulting in minimal damage/oxidation to the exposed copper surface and minimal sidewall taper, both of which are important for assuring good adhesion of the subsequent copper plating layer.the high-quality micro-via was generated with a Spectra-Physics Hippo 355-5 laser system [5 W power at 50 kHz pulse repetition frequency (PRF)]. While good throughput of about 500 holes/s can be achieved with this laser, PCB manufacturing a more state-of-the-art via-drilling tool could take advantage of the higher-power UV lasers available today—such as a Spectra-Physics Pulseo 355 nm, 20 W laser—for throughputs approaching 2000 holes/s or higher, depending on via-diameter resin thickness. Although drilling blind micro-vias is the primary via-drilling application for 355 nm Q-switched DPSS lasers, a recent generation of higher-power UV lasers is also showing promise for larger-diameter vias in thicker, rigid PCB. These materials often contain glass fibers in combination with epoxy resins, so high pulse energy and short pulsewidths are beneficial. For example, a Pulseo 355-20 UV laser was recently used for drilling 200-μm-diameter vias in a 2.3-mm-thick copper-clad FR4 material PCB manufacturing.
The feature in Fig. 2 was machined at a rate of more than 10 vias/s, PCB manufacturing which was 2.5× faster than the customer's requirement. Also of note is the high aspect ratio that was achieved: With an exit- vs. entrance-hole diameter difference of just 5 μm over the 2.3 mm thickness, a nearly perfect cylindrical via is formed with a sidewall angle greater than 89.93°. To achieve the large-diameter, low-taper hole with high throughput, a multiscan trepan routine was executed with a two-axis scanner galvanometer system and a short-focal-length f-theta lens. In addition, precision focus position control was provided with a Newport IMS-V-series high-load bearing linear stage, PCB manufacturing which adjusted the positioning throughout the cutting process PCB.
