Issue link: https://iconnect007.uberflip.com/i/1539509
28 PCB007 MAGAZINE I SEPTEMBER 2025 production of glass as a substrate material will continue through 2026. As the manufacturing chal- lenges are addressed, glass could see commercial use in high-performance systems as early as 2027. Looking Deeper Into Glass Research Happy Holden has been collecting data and research on glass as an IC substrate since 2012, when Georgia Tech first started publishing re- search on glass substrates. Back then, it seemed there was much to appreciate about glass as a substrate material, though admittedly, there were some challenges to overcome. Happy described working with glass substrates at Gentex when glass substrates were on the "to be funded" list as early as 2013. "It may still be there, but we were successful in developing laser drilling of thin glass, metallizing it with our Indium tin oxide (ITO), and then plating it up with 5 microns of copper," he said. "We de- veloped a plasma fusion technique for laminating our two-sided glass structures into multilayers and then singulating them into flip-chip packages." There has been a tipping point in the past three years, with Taiwan and Korea showing interest in glass as a next-generation option for flip-chip packaging. "When AMD and Intel signaled glass as the pre- ferred flip-chip substrate, that gave glass a boost," Happy says. There are considerable downsides to working with glass, particularly its tendency to crack easily, and Happy says that prototypes have not solved all the fabrication and reliability issues. "The primary issue is that OSATs are encounter- ing glass cracking and sometimes adhesion issues during assembly. Still, I think they are getting very close (Table 1)," he says. "Mostly, the companies showing great interest are still predominantly in Asia, but interest is growing across the spectrum." Here are the companies he's been following closely (Table 2): • Japan's Asahi, now part of AGC Multi Material, is the leading supplier. In 2018, AGC also acquired U.S.-based Nelco • AMKOR of Korea is investing a lot in glass substrates and is building a factory for the CHIPS Act • Corning (and AGC's former Nelco unit,) are the only U.S. suppliers. Both are currently supplying Intel and AMD. • Taiwan's COMPEQ, NanYa, Kensus, and Unimicron are all working on the process and have published several papers • Unless Gentex has taken an interest again, no one in the U.S. is investing in new capabilities Regarding conditions or constraints that might make glass the preferred material over other, more traditionally used materials, Happy says glass has excellent thermal and dielectric proper- ties, better than organics. "It's very smooth," he says. "It's easy to get down to 1 micron trace/space using glass. We were doing 5-micron trace/space back in 2013, and R&D experts are currently talking about features as small as 0.1 micron. The LCD industry has been working with glass substrates for a very long time, so Ta b l e 1 : P l a n a r i t y : G l a s s vs . F R-4 Property Glass FR-4 (Fiberglass Epoxy Laminate) Surface flatness Extremely high (≤1 μm/mm typical) Moderate: Can vary due to fiber weave and resin shrinkage Surface roughness (Ra) < 1 nm (polished glass) 100s of nm (typical untreated FR-4) Warp/bow tolerance Very low; highly stable Higher, especially across temperature cycles Dimensional stability Excellent: Low CTE and isotropic Varies with temperature and humidity; anisotropic Ta b l e 2 : C o m p a n i e s c u r re nt l y i n R & D a n d p i l ot p ro d u ct i o n st a g e s fo r u s e of g l a s s a s a s u b st rate m ate r i a l Company Current Stage Product Focus Corning Early production/sample RDL substrates, panel packaging SCHOTT Production-ready Thin glass, chiplet interposers AGC Pilot/early stage Flat panel glass substrates NEG Pilot Glass cores for advanced packaging HOYA R&D Photonics and interposers