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12 PCB007 MAGAZINE I JUNE 2025 prove that the technology lives up to the theo- retical expectations. Currently, the major holdup in TVG tech- nology is how to form vias in the glass. Various glass companies have provided glass for- mulations with the desired dielectric properties in thicknesses varying from 0.3 mm (0.012 inches) to 0.9 mm (0.035 inches). Glass thicknesses less than 0.3 mm are difficult to handle and result in low yields because of breakage. Using conventional drilling techniques is out of the question; the drills cause too much trauma to the glass in the form of cracking and breakage. e only viable alternative at this point is laser drilling, but that has its problems. It takes a relatively long time to drill a TGV in glass with a laser, even for 0.3 mm glass (and we are talking about drilling millions of vias in a 550 x 510 mm [22 x 20 inch] glass sub- strate). at's okay for prototyping to prove the concept, but it's too slow for production quantities. A faster way is to use laser induc- tion to modify the glass structure through the glass where the via is to go, then etch the weakened glass to form the TGV. Unfortunately, the best etchant for glass is hydrofluoric acid (HF), which is not fun to work with. With the concentrations needed to etch glass, 5–10% by weight (concentrated HF is 49 wt%), it is not deadly but can cause painful burns. I've used a great deal of HF to etch titanium and TV and phone displays in the past 10 years. However, with the proper precautions and training, it is not hard to provide a safe working environment. Once the laser induction is complete, we etch the vias out by soaking the substrate in a tank of HF at the proper strength for eight to 10 hours. is is still very slow, and even though the unal- tered glass is more resistant to the HF, it will still etch to some extent, resulting in undesir- able glass thickness variations. Limited testing has shown that spray etching will reduce the via formation time to an hour or less, depend- ing on the thickness of the glass, with less sur- face thickness variation. I expect to see a lot more testing for etched glass vias in our lab in the near future. Oh joy! Most glass PCBs are double- sided, but there are also multi- layer prototypes. For multi-lay- ers, glass thinner than 0.3 mm is desirable for obvious rea- sons, but also because as the glass thickness approaches between 0.2 mm and 0.1 mm, it's possible to use the laser to create the holes for the vias rather than using laser induction and etch- ing to make the vias. e major downside— assuming you can handle the glass without breaking it—is that the laser shot leaves micro- cracks on its surface. One must remove these microcracks by etching a micron or two from the glass surface with HF aer the laser shot. is is the most desirable process because of its speed compared to other via formation methods, provided that thin glass handling problems are overcome. I also expect to soon see more glass thinning tests in the lab. Dou- ble oh joy! (You might get the impression from the somewhat sarcastic "Oh joys" that etching tiny holes in glass with hydrofluoric acid is not one of my favorite activities. You would be right.) If you think you're having fun with vias in FR-4 substrates now, looking at what's in store in the future will put things in perspec- tive. e advantages of glass substrates are so substantial that they will arrive probably sooner rather than later. PCB007 Don Ball is a process engineer at Chemcut. To read past columns or to contact Ball, click here.