A changing landscape

Christopher Ryder, of MKS Instruments’ ESI products group, discusses how CO₂ lasers can address new material challenges in the manufacture of 5G technologies

5G is more than a buzzword for anyone directly involved with the materials and components needed to make it a reality. While not necessarily requiring a paradigm shift in terms of manufacturing, this higher frequency bandwidth brings with it some significant new challenges, and with 5G set to ‘go live’ in the next few months around the world, finding robust, impactful solutions to these challenges has become the focus of various tiers of the micro-electronics supply chain. In this article I will explore how CO2 lasers can be used to perform via drilling on rigid PCB panels featuring complex base materials – which are at the core of emerging 5G technologies. Typically, rigid PCB panels consist of

copper-cladded FR4 dielectrics comprising of woven glass fibres and a resin compound, which ensures the required panel flatness and stiffness, as well as the desired inter- layer electrical/signal insulation. Given the CO2 laser’s ability to process these materials effectively and quickly, it remains a popular solution for high-volume manufacturing. The power to the work surface is an order of magnitude larger than what one would typically see with a UV laser used for PCB via drilling, making it highly versatile for the target HDI (high density interconnect) materials. Nonetheless, as base materials become more complex, new ways of approaching these laser processing applications may become necessary. For our purposes, we should understand the main material challenges stemming from the need for high frequency applications for electrical interconnect. These applications, as such, are not particularly new, and neither are the various material solutions


and processes used in manufacturing. But with the rising applications and production volumes associated with applications such as 5G, IoT (Internet of Things), and autonomous driving, a whole new emphasis is needed to characterise expected base material performance in terms of its processability. In particular, we see a convergence of lower Df/Dk (dissipation factor and dielectric constant) and lower profile Cu (copper) foils, which help to meet functional requirements, with generally reduced material thicknesses (both Cu and dielectric) driven by smaller form factors. These smaller, thinner form factors potentially complicate high-frequency demands for multilayer PCBs. To address the need for lower Df/Dk and the concurrent need for thinner base material stack-ups, base material suppliers can offer products that are modified from existing recipes, which is typically an FR4 with high and specialised resin filler content designed to better insulate signals between Cu layers. Issues may arise in processing these due to the differing resin melt properties onset from the filler combinations and the effects this may have on material ablation.

Figure 1: a typical CO2 laser pulse used to ‘punch’ through the copper material in rigid PCB panels

Materials such as PTFE (Teflon) offer

excellent Df/Dk properties with very low thicknesses, but also tend to have a higher CTE (coefficient of thermal expansion) and typically lack the structural integrity (rigidity) of an FR4. Therefore, the high thermal loading of a CO2 pulse train can negatively affect via quality (barreling, residues, etc) and panel scaling. Some base material manufacturers offer composite dielectric materials, which offer great performance potential, but can present process challenges given the variations in constituent glass-transition temperature (Tg) – meaning each material is melting/ablating at a different rate. When we add to these complications to the demand for thinner inner layer low- profile Cu that can be easily damaged or penetrated with excessive power, the CO2 process faces some setbacks, largely driven by overall thermal load, the temporal displacement of that load and the spatial distribution of that energy. To understand the difficulty, we need to take a look at the pulse and how it is delivered to the work surface. In essence, a round imaged beam is formed through various apertures, which provide the desired spot size for the via diameter in question. However, the larger the spot size, the lower the fluence of that beam. This is sometimes called a ‘punch’ process, as we’re using the full-imaged spot to punch through the Cu and dielectric down to the bottom Cu. Power and duration of the pulse

can be attenuated, and under normal circumstances we see delivery of the pulse with a desired peak power, followed by a usually less-desired residual pulse tail. The

Figure 2: pulse amplitude and temporal modification with acousto-optic deflector technology


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