Substrates and PCBs

On TSMC's July 2026 earnings call, chief executive C.C. Wei said packaging capacity is now so tight that it limits customers' growth^[1]. The silicon interposer wires chips together inside a package. Past its edge the wiring turns to epoxy and glass: a build-up substrate, a printed circuit board of thirty layers, then a rack.

How it works

How a substrate fans out

1. The package. An AI chip is more than the chip itself. It sits in a package: the chips on a silicon base, and under them the substrate, which connects everything to the circuit board.
2. Tight to wide. The connections on top are packed tight. The circuit board below is wired in millimeters. The substrate has to join the two.
3. Layer by layer. The substrate does it in layers, built outward from a stiff core. Each layer carries the copper a little further out.
4. Connected. Stacked together, the layers fan the tight grid out to the board’s wide one. An AI chip needs about 18 layers, against six for a PC, and every layer is another chance for a flaw, which is what makes these substrates hard to build.

Simplified. Eight layers are drawn, and every thickness is exaggerated.

The substrate maker drills the core and plates it with copper before the build-up starts. Each cycle then presses on a film of epoxy, burns holes through it with a laser to connect the layers, and adds copper wiring on top^[2]. Each film is about 10 micrometers thick, and a human hair is about 80 micrometers across^[3].

Silicon and epoxy swell at different rates when heated, so the larger the package, the more it warps. Ajinomoto counts 22 layers of its film in a 2026 AI package substrate about 100 mm square, against 18 on a high-performance computing substrate about 70 mm square around 2023, and six when PCs first adopted it in 1999^[2]. Nitto Boseki says its low-expansion T-glass cloth has been highly evaluated as an effective solution to that warping, and its adoption in substrates has increased significantly^[4].

Variants and trade-offs

BT substrates

Bismaleimide-triazine laminate, or BT, is the other main family. Mitsubishi Gas Chemical, which makes BT materials, says they go mostly into thin packages for smartphone processors, radio modules and memory^[5]. The company also lists flip-chip substrates for computer CPUs and GPUs among BT's uses, where its laminate forms the core between layers of build-up film^[5]. In an October 2024 investor presentation, it estimated that its BT resins held about 40 percent of the substrate materials market^[5].

ABF build-up substrates

Ajinomoto Build-up Film, or ABF, is the insulating film between the layers of package substrates for logic chips, used in PCs, network equipment, servers and AI equipment^[2]. Ajinomoto first developed it in 1999, and says major chipmakers have used it as the de facto standard ever since^[6]. Asked on the company's own site whether its share was nearer 30 or 50 percent, the electronic materials division answered: "More like 95 percent market share", and said practically all high-performance computers and servers rely on it^[3]. In June 2026 Ajinomoto told investors that the film has held a share of over 95 percent since its launch^[2].

Layers of build-up film in one package substratelayers

Layers of build-up film in one package substrate

PC, 1999	6 layers
HPC, about 2023	18 layers
AI package, 2026	22 layers

Source: Ajinomoto, Growth Strategy for the Electronic Materials Business, June 2026

Glass core substrates

A glass core in place of resin expands less with heat and stays flatter, which is why chipmakers and substrate makers are both trying it^[7].

• Absolics, a subsidiary of Korea's SKC, has a final Commerce Department award of up to $75 million for a 120,000-square-foot glass substrate plant in Covington, Georgia, and as of October 2026 NIST's project page expected first deliveries to customers in 2025 and production capacity to come online in 2027^[8].
• Intel announced glass substrates in September 2023 for the latter part of this decade^[9].
• Samsung Electro-Mechanics signed a joint venture agreement with Sumitomo Chemical's subsidiary Dongwoo Fine-Chem in July 2026 to make glass core substrates, with a supply system planned by the second half of fiscal 2027^[10].

All three timelines reach into the later years of this decade.

The materials underneath

Ajinomoto's film is a small line in a food company's accounts and a chokepoint for high-end chip packages. The Healthcare and Others segment that carries electronic materials lifted business profit 45.1 percent to JPY 66.2 billion in the year to March 2026, which Ajinomoto attributes mainly to higher electronic materials revenue and higher profit in Bio-Pharma Services & Ingredients^[11]. Ajinomoto has set an investment budget of JPY 25 billion between 2023 and 2030 to expand production, and says there are no concerns about its current supply chain overall^[2]. In May 2026 it announced the acquisition of land in Gifu Prefecture for a third production site, scheduled to start operating in 2032 to meet demand after 2030^[2].

The core one layer down also has a dominant supplier. Nitto Boseki's T-glass is stiff and expands little when heated, and the company calls it essential for making high-density package substrates^[4]. Laminate makers such as Mitsubishi Gas Chemical coat the glass cloth with resin and press it with copper foil into the core laminate that substrate makers build on^[5]. Nitto Boseki claims an overwhelming advantage in quality and capacity for low-expansion glass, says market demand seems to have risen about two to three times since its fiscal 2024 to 2027 plan was set, and planned about JPY 80 billion of capital spending over those four years^[4]. In August 2025 it decided to expand glass fiber cloth production at its Fukushima Enterprise Center, with production scheduled to begin in the fourth quarter of its fiscal 2026^[4]. Apple and Qualcomm are among the customers worried about Japanese glass cloth supply^[12].

Chinese firms are entering this layer, and Nitto Boseki says Chinese and Taiwanese companies are aggressively catching up with it in the special glass sector^[4]. At stake is a chip materials market Nikkei sizes at $73 billion^[13].

Who makes it

The plants that build substrates for other firms are an Asian oligopoly. The White House's 2021 hundred-day supply chain review recorded build-up substrate among the materials commenters called vulnerable, and named key substrate makers including Ibiden, Nanya, Shinko, Samsung, Unimicron, Shennan Circuits, Zhuhai Yueya and AKM. None of the eight is American^[14]. A May 2024 report by the Semiconductor Industry Association and Boston Consulting Group counted the major vendors of package substrates by headquarters and found two in Japan, two in Taiwan, one in mainland China and none in the United States^[15]. TTM Technologies, an American circuit board maker, says it also offers IC substrates^[16].

• Ibiden will invest about JPY 500 billion from fiscal 2026 to fiscal 2028, starting with about JPY 220 billion, mainly at its Gama plant, and planned mass production from fiscal 2027^[17]. In May 2026 management said it typically takes close to 100 percent of a new generation of interposer-type substrates at launch, and that this share may decline by 20 to 30 percent after about three to six months^[18]. On silicon-bridge substrates, it said, wiring the underside is hard enough that it expects to keep its technical lead at least through fiscal 2030^[18].
• Unimicron took NT$42.9 billion of sales in the second quarter of 2026, 52 percent of it build-up substrates and 61 percent of it AI data center work^[19].
• Samsung Electro-Mechanics grew package solution sales 37 percent year on year to KRW 771.6 billion in the second quarter of 2026, on substrates for AI accelerators, server processors and cars^[20].

A company owned mainly by funds of JIC Capital, a subsidiary of Japan Investment Corporation, launched a tender offer for Shinko's other shares in February 2025 as part of a deal to take it private, and Fujitsu agreed to sell its 50.02 percent stake back to Shinko^[21]. The tender offer succeeded in March 2025^[22]. In September 2026 Shinko agreed to buy a plant in Mobara, Chiba Prefecture, from Japan Display, a display maker, and said it would make the site a new core plant for advanced chip packages while also expanding capacity at its plants in Nagano^[23]. Production there is scheduled to start in fiscal 2028^[23].

Unimicron sales by technology, second quarter 2026%

ABF substrate52%HDI27%BT substrate9%PCB9%FPC2%Other1%

Source: Unimicron 2026 Q2 earnings conference

The boards under the GPU

TTM Technologies says it regularly builds printed circuit boards of more than 30 layers and can go past 70^[16]. Those layers carry power to the GPUs and route fast signals between them, so how much signal the laminate absorbs becomes a main design constraint. Nitto Boseki also makes NE-glass, a low-dielectric glass cloth that absorbs less signal, used in switches and routers in data centers^[4].

Data center computing was 24 percent of TTM's $2.91 billion of fiscal 2025 revenue, against 14 percent two years earlier^[16]. It says its circuit board competitors are mostly based in China and Taiwan^[16]. In its annual report for 2025, TTM said it was building a plant in Syracuse, New York, to make ultra-high-density circuit boards for national security needs, with volume production expected in the second half of 2026^[16]. Thailand aims to be the biggest circuit board hub outside China as makers race to diversify production^[24].

The chokepoint

Money can expand an oligopoly of substrate plants, and it is being spent. The weakest points are one layer down, where one Japanese company dominates the film and another dominates the low-expansion glass cloth in the core. Both are adding capacity on plans that run for years.

Packages lose flatness as they grow. TSMC has told investors that its CoWoS packaging roadmap goes beyond 14 times the area a lithography machine can print in one shot^[1]. Bigger bodies mean more warping, more layers, more holes that can break the circuit, and lower yield on a part that is already expensive before a die touches it.

Key evaluation criteria

• Layer count sets how much current and how many signals the substrate can carry. Each added layer is another chance to lose the part.
• Body size drives everything else. Once a body is much larger than the dies it carries, warping and handling take over from the wiring in deciding what can be built.
• Coefficient of thermal expansion, how much a material swells when heated, must be close enough to silicon that the package survives repeated heating and cooling, which is what T-glass provides and why its supply matters.
• Dielectric properties, how much signal the core laminate absorbs, set the speeds the substrate and the board underneath can run.
• Qualification time protects the incumbents. A second supplier's film or laminate has to be approved by each substrate maker and chip customer before it can be used.