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For more than 30 years, lasers have played a significant role in the manufacturing of PCBs. It is not a coincidence that electronic devices have, at the same time, become increasingly miniaturized. The ability to tightly focus a laser beam much smaller than a mechanical tool has been an enabler of such dense, compact circuitry; and the elimination of consumables such as drilling and routing bits has reduced manufacturing costs.
The workhorse laser technology over the years has been the carbon dioxide (CO2) laser, which has provided manufacturers with reliable, cost-effective power for various applications. The most identifiable laser process in PCB manufacturing is what is referred to as via drilling, which involves laser drilling a hole through an electrically insulating dielectric layer on a copper substrate. Generally, if the substrate is left intact, the hole is a blind via; if it is also drilled through, it is a through via. Very small holes having diameters below about 150 micrometers are commonly referred to as microvias. After a subsequent copper plating step, an electrical interconnection through the dielectric layer is formed. By arranging these vias in various two-dimensional configurations and by implementing additional build-up, drilling, and plating steps to introduce a third dimension, the high-density interconnect (HDI) and packaging needs of today’s powerful yet compact electronic devices are satisfied.
Making it Smaller
As always there is a mandate for smaller: smaller mobile devices, smaller microchips, smaller electronics packages, and smaller interconnect vias. Vias drilled with CO2 lasers are generally limited to diameters of 60–80 µm or larger due to the long (~10 µm) wavelength of the light, which has a direct bearing on how small the beam can be focused. While smaller via sizes can technically be achieved, the business case quickly vanishes due to higher overall process complexity (and therefore cost).
Here is where shorter-wavelength pulsed ultraviolet (UV) diode-pumped solid-state (DPSS) laser technology enters the picture. The short UV wavelength—about 30 times shorter compared to CO2 wavelengths—can easily be focused to the small sizes necessary for the fabrication of increasingly small microvias. Since the mid-1990s, pulsed UV DPSS lasers with nanosecond (ns) pulse durations have been commercially available for industrial/OEM use. While during the early days of the technology the relatively high cost and troubling reliability issues limited their appeal, today’s products are vastly improved in both areas. Indeed, over the past decade, the cost per Watt for such lasers has fallen by an order of magnitude, and product lifetimes have improved dramatically, in some cases surpassing 20,000 operating hours at high power levels.
To read this entire article, which appeared in the November 2016 issue of The PCB Magazine, click here.