Why a 10-Meter Oven for CCL Prepreg? The Role of IR Heating Tubes in Electronic-Grade Glass Cloth Impregnation & Drying

Author:

xiaolanjing

I. What Is Copper Clad Laminate (CCL), and How Does It Relate to PCB?

Copper Clad Laminate (CCL) is the base material of printed circuit boards (PCBs): glass fiber cloth is impregnated with epoxy resin, dried into semi-cured sheets (commonly known as "PP" or prepreg, i.e., prepreg/B-stage), then copper foil is stacked on top and pressed into boards under high temperature and high pressure. PCB manufacturers etch circuits, drill holes, and mount components on the CCL — this is what becomes the circuit boards inside our phones, servers, and automotive electronics.

The first checkpoint of CCL performance lies not in the final hot pressing, but in the preceding impregnation and drying stage — whether the glass cloth absorbs resin uniformly, how much solvent residue remains, and which step of B-stage the resin reaches directly determine whether the subsequently pressed board suffers from delamination, warpage, or dielectric loss.

II. How Does the Prepreg Drying Process Actually Work?

To produce a qualified FR-4 prepreg, the glass cloth must pass through this line:

What equipment makers use is a vertical impregnator: the drying oven is about 10 m tall, split into 5 independently temperature-controlled sections, with the glass cloth traveling vertically up and then back down. A typical temperature profile is four zones of 80 → 110 → 140 → 150°C, letting the resin progress gradually from a solvent-diluted state to the B-stage at a gel content of 30–50% — it must be neither too dry (or the resin won't flow during the subsequent hot press) nor too wet (or residual solvent causes bubbles and delamination).

3. Why add infrared heating tubes in a hot-air drying oven?

Traditional vertical ovens rely on hot-air circulation, which gives even temperature but has large thermal inertia and slow heating-up. Once the web speed of the glass cloth is increased, the solvent inside the resin can't diffuse from within to the surface fast enough to evaporate, causing:

solvent residue exceeding the limit, producing bubbles in the later hot press;
surface resin skinning first, trapping the solvent inside;
uneven temperature between the top and bottom faces when thin cloth (1080/1027 styles) runs at high speed.

Adding medium-wave infrared heating tubes at the top of the preheating and main drying sections delivers these benefits:

Direct radiation on the cloth surface: medium-wave 2–5 μm matches the absorption bands of the resin solvents (acetone, DMF, MEK), so surface solvent evaporates within seconds without preheating the air;
Gradient heating-up: the IR quickly pulls the cloth surface to 80–100°C, then the hot-air section maintains the main drying, forming an "IR surface drying + hot-air through-drying" combination;
High-speed adaptation for thin cloth: when the web speed of electronic-grade thin cloth such as 1080/1027 is raised, the IR section supplements energy to keep the gel time stable;
Fast start/stop: temperature profiles can be adjusted much faster than with hot air alone when switching cloth types or resin formulations.

4. Lanjing Optoelectronics' two positions in this supply chain

Position one: IR heating elements for the drying section (mid-stream equipment)

Medium-wave carbon fiber heating tubes (2–5 μm): match the resin solvent absorption bands, used in the preheating and main drying sections of vertical impregnators;
Short-wave tungsten halogen tubes (0.76–2 μm): fast surface drying of the cloth;
Gold-plated reflector tubes: even energy coverage across glass cloth wider than 1 m;
Made from 4N8 high-purity quartz tubes (the most upstream product): IR transmittance of 94%+, stable in long-term solvent-vapor environments.

Position two: upstream quartz reinforcement materials (the most upstream)

Lanjing's 4N8 high-purity quartz rods (SiO₂ purity 99.998%) are drawn into quartz fiber yarn, then woven into quartz cloth;
Quartz cloth is a key reinforcement substrate for high-frequency, high-speed CCL (M8/M9 grades, low-Dk materials for AI servers) — with lower dielectric constant and lower dielectric loss than ordinary E-glass cloth;
Lanjing has already demonstrated quartz fiber yarn (up to 5,100 m) drawn from self-produced quartz rods and woven into cloth; customer trials have succeeded, awaiting M9-grade CCL validation.

From "drawing rods into yarn, weaving yarn into cloth, impregnating and drying the cloth, and heating the drying section with Lanjing IR tubes," Lanjing plays both the upstream material role and the mid-stream heating-element role in the CCL supply chain.

Summary

CCL is made by impregnating glass fiber cloth with epoxy resin, drying it into B-stage semi-cured prepreg, and then laminating with copper foil under heat and pressure; it is the base material of PCBs. Prepreg drying is the first performance gate of CCL: after impregnation (resin solid content 55–65%, viscosity 300–600 cps), the glass cloth enters a vertical impregnator about 10 m tall with 5 temperature-control zones; the preheating section at 80–100°C removes surface solvent, the main drying section at 120–140°C (150–200°C on high-speed lines) drives the resin to B-stage at a gel content of 30–50%, with a total residence time of 4–12 min and solvent residue ≤0.3%. Because traditional hot-air ovens have large thermal inertia, adding medium-wave IR heating tubes (2–5 μm) to the preheating and main drying sections directly radiates the cloth surface, quickly evaporates solvent, provides gradient heating, and adapts to high-speed travel of thin cloth.
Lanjing Optoelectronics has two businesses in the CCL supply chain: first, it supplies IR heating elements for impregnators — medium-wave carbon fiber tubes, short-wave halogen tubes and gold-plated reflector tubes; second, it draws 4N8 high-purity quartz rods into quartz fiber yarn and weaves it into quartz cloth as a low-Dk reinforcement substrate for high-frequency, high-speed CCL (AI-server M9 grade), forming an integrated package from upstream materials to mid-stream heating elements.

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Lanjing Optoelectronic Debuts at the 20th Zhuhai International Office Equipment & Consumables Exhibition, Deepening Cooperation and Seizing New Market Opportunities

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Lanjing Optoelectronic Debuts at the 20th Zhuhai International Office Equipment & Consumables Exhibition, Deepening Cooperation and Seizing New Market Opportunities