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26 PCB007 MAGAZINE I SEPTEMBER 2025 F E AT U R E A RT I C L E by N o l a n J o h n s o n , I - C o n n e ct 0 07 S ubstrates, by definition, form the base of all electronic devices. Whether discussing silicon wafers for semiconductors, glass-and-epoxy materials in printed circuits, or the base of choice for interpos- ers, all these materials function as substrates. While other substrates have come and gone (polyimide paper, unreinforced PTFE, epoxied paper, and Bakelite for printed circuits, and germanium, selenium, and gallium arsenide for semiconductors), silicon (for semiconductors), and FR-4 (for printed circuits) have remained the de facto standards for the industry. These represent the majority of sub- strates in use over the past 25 years or more. The industry seemed content with these substrate choices as long as Moore's Law held. But now, after years of relative stability in substrates across all aspects of the electronics industry, multiple simulta- neous advances in materials are breaking through. With Moore's Law No More, Now What? With Moore's Law having reached its practical limits in monolithic silicon, new approaches (het- erogeneous integration, advanced packaging) and new substrate materials are in demand to keep the miniaturization momentum. For both semiconduc- tors and PCBs (or interconnects), glass is one contender that may be far less complex than some of its predecessors. Planarity is a key advantage of glass. Glass delivers such extreme planarity because, as an amorphous material, it has no underlying weave like FR-4. The glass fiber weave comes with local- ized unevenness and a tendency to deform under heat—traits that become even more apparent on traditional substrate materials as PCB feature sizes get significantly smaller and larger. Table 1, compiled from multiple technical resources, summarizes key findings in the glass vs. FR-4 com- parison. Thermal cycling does not affect glass flat- ness. Furthermore, glass manufacturers can deliver smooth sheets that, if needed, can be polished down to a sub-micron tolerance. On the downside, glass is very brittle and difficult to work with, mak- ing processing it into a substrate both challenging and potentially expensive. There is some indication that R&D and pilot Is Glass Finally Coming of Age?