Figure 3: example of a thin bottom copper application

ESI’s Geode system is capable of performing CO2 laser-based via drilling on the rigid PCB panels at the core of emerging 5G technologies

pulse tail (see Figure 1) is the energy that exits the laser after RF (radio frequency) excitation in the laser source has ceased. This is the water pouring out of the hose after we have shut off the valve. This can result in an over-delivery of energy versus what one is targeting. Of course, there are ways to address this potential excessive energy being delivered to the material, but these typically involve employing multiple lower power pulses, which can adversely affect drilling throughput (more pulses = more time per via). It would appear then, if presented with

materials potentially sensitive to high thermal loading over time (more time = more melting/ablation = higher Cu undercut and other effects impacting quality negatively), the desire would be to more finely contain/ control energy amplitude and duration, as well as spatial distribution. In other words, there could be an advantage found through applying the desired peak power when and where needed, without the disadvantages of a large residual pulse tail or the throughput- inhibiting lower fluence spot size needed to drill.

In UV-based via drilling, devices known as AOD’s (acousto-optic deflectors) have been

used for some time to essentially slice and dice the pulse train – in terms of amplitude and duration – and simultaneously distribute the energy spatially. Hence with a small, higher fluence spot size that is rastered over the material, vias are created. Different to the punch process already described, this rastering of vias takes advantage of the high speed of AOD deflection to accommodate complex multi-diameter toolpaths without spot recalibration. However, this has largely been limited to the UV wavelengths and the accompanying material sets thus far. With its recent acquisition of ESI, MKS

leverages substantial history with AODs in UV flex PCB drilling to benefit the HDI CO2 via drilling space with a new system, named Geode. With the system’s Hypersonix AOD- based technology, pulses can be tuned and edited for amplitude, duration and spatial distribution with a high-power laser engine (see Figure 2). The capability to finely tune energy

“These technologies help manufacturers meet new 5G material challenges with increased photonic flexibility”

and duration of the pulse creates a new toolset for managing the above-mentioned challenging base materials. In addition, the elimination of the pulse tail supports a more predictable process quality window with a more precisely dosed drilling recipe. The use of delays in the pulse execution also allows for additional leverage in managing material melt/ablation behaviour and potential benefits for reducing via undercut and bottom Cu damage on thin inner Cu layers (see Figure 3). The use of AODs, however, also brings

with it some spatial distribution benefits. In the punch process, for a given material, there are general limitations which result in ‘one spot size = one via size’ operations. But if a single spot and trepan that pulse in a given pattern is maintained, multiple via sizes may be achieved without any changes


to the optics (such as no recalibration of new apertures, etc). This helps maintain, and often increase throughput (see Figure 4), determined by how many vias per second can be drilled. The use of the technologies described

above are helping manufacturers meet new 5G material challenges with increased photonic flexibility, without any fundamental changes to existing process landscapes. Whichever material compositions will set new baselines for HDI PCB manufacturing, it is clear that having more physics in our system’s toolkit will help to meet existing and emerging technical and commercial demands. l

Christopher Ryder is the director of product marketing for ESI products, MKS Instruments


Figure 4: drilling multiple via diameters in a single pass with acousto-optic deflector technology

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