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As demand for portable electronics and wired telecommunications devices continue to increase, the need to reliably and rapidly produce microvias will also grow. Laser drilling utilizing the UV diode-pumped solid-state laser represents one of the compelling techniques for the production of small microvias (less than 50 µm diameter) in chip substrates and flex packages.
Laser Used:AVIA Family of DPSS Lasers
The majority of UV DPSS lasers operate at the frequency-tripled wavelength of 355 nm for microvia formation. Average output power at this UV wavelength typically exceeds 5W, but the extremely narrow pulsewidth (<40 ns) creates a peak power of more than 7 kW. This minimizes the HAZ and contributes to cleaner, higher-quality microvia formation.
Pulse repetition rates up to 100 kHz are available and beam quality is excellent (M2 <1.2). The combination of high beam quality and short wavelength makes for a very small spot size (5 to 20 µm) at the work surface and allows for the formation of correspondingly smaller microvias for HDIs. Presently, however, the demand for <25 µm microvias is limited to specialized high-performance applications.
In most laser-ready PCB shops, microvias are formed using conformal mask drilling. With this technique, the uppermost copper layer of a multi-layered board (MLB) is first etched away in the areas directly above where microvias are to be drilled. This generates a mask pattern of small apertures through which a laser beam can pass. Blind microvias are then created one at a time by positioning the laser beam over each aperture and pausing precisely long enough to remove the exposed dielectric down to the next copper layer below. Post-cleaning and -plating steps follow. The rapid beam positioning required for high-volume microvia formation is typically achieved with high-speed, accurate galvanometer-controlled mirrors, telecentric lenses, and high-speed precision-positioning tables, which allow the laser beam to be quickly positioned perpendicularly over each hole.
UV DPSS lasers can create microvias through the bare copper layers of MLBs. At currently available UV power levels, hole-formation speeds can range from 150 to 200 microvias/sec through copper and dielectric combined. Through dielectric alone, the hole-formation speed can reach 300 microvias/sec.
The smaller spot size and lower average power of UV DPSS lasers contribute to the slower formation speeds, because the beam must be trepanned to form microvias larger than 25 microns. With today’s average microvia diameters of 100 or 125 microns, this translates into a slower processing speed for a single UV DPSS laser.
Fig. 1 - SEM Photograph of FR4 hole drilled.
The UV DPSS laser is a viable tool for producing microvias of 50 µm and under in size, and also demonstrates the required reliability and cost of ownership characteristics for the production environment.