These are not bottles of milk, but bobbins of glass yarn.

This time not chips. not memory. A cloth: a woven glass fabric, made mostly by one company in Japan. Story 1 showed that glass cloth is the slowest input to fix in the 2026 circuit board market, even though it is the smallest share of the cost.

This story explains why a fabric decides what a computer can do. Eight minutes. Here is the whole board.

📖 In plain words: the terms you need

Laminate (CCL). The raw sheet a board is cut from: glass cloth, hardened resin, copper foil.

Prepreg. Glass cloth soaked in resin, not yet hardened. The glue between a board's layers.

Dk (dielectric constant). How much a material slows down a signal passing near it.

Df (dissipation factor). How much of a signal's energy the material soaks up and turns into heat. Low Df is the expensive kind. Where to check it: laminate makers publish Dk and Df on each product's data sheet, for example Panasonic's Megtron series.

Low-Dk glass. Glass cloth made from a special recipe that slows and absorbs signals less. Used in fast circuit boards. NE glass is one type.

T-glass. A different special glass that barely expands when heated. Used in the small boards chips are mounted on, where warping is the enemy.

Package substrate. The small board a chip is mounted on, which then sits on the main circuit board.

Fibre weave effect. Timing errors caused when a copper line runs over glass yarn and its partner runs over resin.

🎯 THE WHAT

At AI speeds, the board material decides how far a signal can travel, which helps decide where chips can sit. The best material depends on specialty glass cloth, and the dominant supplier is choosing customers rather than raising prices.

Start with sand, which becomes glass. The glass is melted and pulled into filaments thinner than a hair, twisted into yarn, and woven on a loom into fabric, like any other cloth. That fabric is soaked in resin, baked stiff and clad in copper. Story 1 has the step-by-step flowchart.

A toaster's board might have two layers. A car's engine controller, a handful. AI server boards can have dozens: Würth Elektronik puts their layer counts at three to five times conventional designs. Prismark reported that boards with 18 or more layers grew 40% in value in 2024, the fastest-growing part of the market.

Underneath the most advanced machine we build, there is a textile.

At the speeds modern chips run, the signal is carried by an electromagnetic field that spreads from the copper line into the glass and resin around it. That material absorbs some of the energy as it goes. That is what Df measures.

Ordinary FR-4 has a Df of roughly 0.02. A widely used low-loss material, Panasonic's Megtron 6, is about 0.002 at 1 GHz and about 0.004 at higher frequencies. So the expensive material absorbs roughly five to ten times less, depending on frequency.

Cheap material, short distances. Expensive material, longer ones. A property of baked plastic and woven glass helps decide the physical layout of a computer.

Glass slows signals more than resin does. In woven cloth, the glass is bunched into yarns with resin-rich gaps between them. Fast signals travel in matched pairs, and if one line sits over glass and its partner over resin, they arrive at slightly different times. Engineers call this the fibre weave effect (explainer).

One standard fix sounds like a joke: rotate the whole design on the manufacturing panel by about ten degrees, so every line crosses the weave at an angle. The IPC, now the Global Electronics Association, has published technical work on this. It reduces the problem rather than eliminating it.

The other fix is better glass: flatter, spread yarns and a different glass recipe. That is where the shortage lives.

Low-Dk glass is for the circuit board itself. It slows and absorbs signals less, so AI servers and network switches need it. T-glass is for the package substrate under a chip. It barely expands when hot, so a big, hot AI package stays flat and its connections do not crack.

Nittobo is reported to hold about 90% of T-glass and 60% to 70% of the leading low-Dk segment. As AI packages grow, they soak up T-glass. Goldman Sachs has warned, as reported by TrendForce, that makers of BT substrates, a different package used widely in memory and smaller chips, could face double-digit T-glass shortfalls.

Nittobo is expanding. Its current medium-term plan budgets about ¥120 billion of capital investment over the four years to March 2028, up from about ¥80 billion in the 2024 plan. New lines are going into Fukushima and Taiwan. New facilities arrive from 2027, and it aims for three times its March 2026 capacity by the year ending March 2029.

Challengers exist. Taiwan Glass was certified for low-Dk cloth, reportedly only the third company to get there. Fulltech has been certified too. Asahi Kasei announced in April it would enter AI glass cloth. Nan Ya Plastics says it will weave about a fifth of Nittobo's specialty fabric by 2027.

Certification is slow, because laminate makers, board makers and their customers each have to prove a new cloth performs identically.

The most revealing fact is about price. Nittobo raised T-glass prices, then said in August it had no further increases planned, unless energy costs force a rethink. Instead, by all reports, it is deciding who gets supplied.

The scarcity at the top of the chain is being managed by rationing, not auctioning. That is why, as Story 1 showed, ordinary buyers feel it as waiting and quotas, not only as price.

The newest ultra-low-loss laminate grades, sometimes called M9, need glass that absorbs even less. The industry's answer is quartz cloth, woven from nearly pure silica. Fulltech began certifying M9 quartz cloth this summer.

High-purity silica is the same family of material used for the crucibles that grow silicon wafers. It is now heading into the boards those chips sit on. A scarcer material, not a more common one.

As of September 2026.

Unlike many chokepoints in the chip supply chain, most of this part of the board chain is publicly listed.

The investor read is analysis of industry structure, not investment advice. Nothing here is a recommendation to buy or sell any security. I hold no positions in the companies covered and take no payment from them. Do your own work.

🧭 THE WHERE

1. TrendForce: what is glass fibre fabric, and why is T-glass critical for AI servers? [8 min]

2. Altium: how the fibre weave effect influences signal integrity, the engineering behind the ten-degree trick [10 min]

3. The Real AI Shortage Is Memory, our own piece on the other AI bottleneck that sits in the package [7 min]

PCB Story 1: Why Your PCB Quote Went Up
The price on your desk, and why buyers outside AI are waiting in a queue.

PCB Story 2: Your AI Cluster Runs on Cloth (you are here)
Why a woven glass fabric decides what an AI computer can do.

PCB Story 3: Nobody Is Getting Rid of the Circuit Board
Why the board keeps losing work to the chip package, and gets harder to make anyway.

PCB Story 4: The Map
Who makes the board, where, and how the map came to look this way.