Although many types of reinforcement materials are used in substrates, plain-woven glass fabric (woven from glass fibers) is by far the most common. Other materials include paper, cellophane, nonwoven aramid fibers, nonwoven glass, and various fillers. Woven glass fabric offers several advantages, including excellent mechanical and electrical properties, suitability for manufacturing laminates in a wide range of thicknesses, and relatively low cost.
Woven Glass Fiber
Woven glass fiber is a widely used reinforcement material in PCB manufacturing. The production process involves melting inorganic raw materials, forming glass filaments through a spinneret, spinning the filaments into yarn, and weaving the yarn into glass fabric. The composition of elements in the glass significantly affects its electrical, mechanical, and chemical properties.
The most common type is E-glass, widely used in the PCB industry for its well-balanced performance and reasonable cost. NE-glass is an enhanced material that offers a lower dielectric constant and lower dissipation factor, but it comes at a higher price. L-glass, developed by other companies, offers similar performance to NE-glass and is already commercially available. S-glass provides higher strength but is more challenging to process, making it less commonly used.
Glass yarn comes in various grades. D, DE, E, and G types are the most common in the PCB industry. The glass fabric is typically woven in a plain weave. In this method, the yarn alternates over and under in both directions. This weaving improves dimensional stability. The glass fabric surface is usually treated with a coupling agent. This enhances resin adhesion and suppresses the formation of CAF (Conductive Anodic Filaments).
Glass Fabric
Variations in E-glass composition, filament diameter, yarn type, and weave pattern result in a wide range of glass fabric types. The impact of glass fabric on the substrate depends primarily on these variables. Additionally, the number of fabric layers and the number of warp and weft yarns significantly affect the properties of both the fabric and the final material. The warp direction refers to the lengthwise direction of the weave (also called the machine direction), while the weft direction runs perpendicular to the warp. The warp direction is often called the grain direction.
As previously mentioned, glass fabric used in PCBs is typically made from E-glass, and plain weave is almost exclusively used in its construction. In a plain weave, warp yarns alternately pass over and under the weft yarns. This pattern effectively prevents yarn slippage and fabric deformation. The image below shows three of the most common glass fabric types. For a given resin content, each fabric type has a different nominal thickness. The ability to control both the type and thickness of glass fabric is essential for achieving impedance control and maintaining the overall board thickness.
During the production of glass yarn and fabric, various surface treatment techniques can be applied to improve manufacturability, prevent abrasion and static buildup, and minimize yarn entanglement. For laminates and PCBs, the most critical surface treatment is the application of a coupling agent.
Coupling agents are typically organosilane compounds that enhance the adhesion between the glass fabric and the resin. These agents improve wet-out and increase overall reliability during PCB manufacturing and operation—for example, maintaining interlayer adhesion during mechanical drilling or under high humidity. Coupling agents are also highly effective at suppressing the growth of CAF (Conductive Anodic Filaments), thereby extending the service life of the circuit board. Standard types of coupling agents are commercially available, while proprietary formulations tailored to specific resin systems are often held by glass fabric manufacturers and not publicly disclosed.
Other Reinforcement Materials
While woven glass fabric is the primary reinforcement material in PCBs, other materials—or hybrid combinations of woven glass fibers—are also commonly used. Below are some of the alternative reinforcement materials:
1. Glass Mat (Nonwoven Glass)
Glass mat is a nonwoven reinforcement material with a more random fiber orientation compared to traditional woven glass fabric. It is made by quenching glass fiber yarn, stretching it into strands approximately 1–2 inches long, and then continuously laying them down in random, spiral-like directions. Glass mat is typically used as the core layer in CEM-3 substrates, making it suitable for general-purpose electronics where performance demands are lower.
2. Nylon Fiber
Unlike conventional inorganic glass fibers, nylon fibers are composed of organic compounds, such as aramid (an aromatic polyamide), offering unique performance advantages.
In certain high-performance PCBs or Multi-Chip Module Laminates (MCM-L), nylon fiber provides the following benefits:
- Easily ablated by plasma or laser, making it ideal for microvia drilling and fine-feature processing.
- Lightweight and high-strength, which enhances structural stability.
- Negative axial coefficient of thermal expansion (CTE), which helps mitigate thermal stress.
These properties make nylon fiber an effective alternative to glass fiber in specialized applications, especially in high-frequency or miniaturized circuit designs.
3. Linear Continuous Glass Filaments
Using linear, continuous glass filaments as the reinforcement material for laminate production is a unique, specialized technique. In this method, the resulting laminate consists of three layers of glass filaments, with the outer layers aligned parallel to each other. In contrast, the middle layer is oriented perpendicular to them. With an equal number of continuous glass filaments woven in each direction, the reinforcement structure significantly enhances the laminate's dimensional stability.
4. Paper
Fiber paper can also be used as a reinforcement material in substrates. It may be used on its own or combined with other materials, such as woven glass fabric, to form a composite reinforcement structure. This type of composite can be processed only by punching, not drilling, which makes it economical and practical for high-volume, low-tech consumer electronics such as radios, toys, calculators, and video game consoles.
Paper-based reinforcement is primarily used in FR-2 and FR-3 materials, as well as in the core layer of CEM-1.
5. Fillers
Fillers are primarily small, solid particles added to resins to modify their properties. These include various inorganic substances such as talc, silica (including modified silica), kaolin powder, and microscale hollow glass spheres. Fillers are typically used in special-purpose substrates to achieve specific performance goals.
For example, kaolin powder coated with palladium can be dispersed within the substrate to serve as a catalyst for electroless copper plating. Hollow glass microspheres are used to reduce the dielectric constant of the material. Other fillers are employed to lower thermal expansion, enhance reliability, improve drillability, modify electrical characteristics (such as Dk and Df), and reduce overall material cost.
Using fillers to reduce Z-axis expansion is especially common in lead-free solder-compatible materials. Because lead-free soldering involves higher temperatures, reducing Z-axis expansion helps minimize tensile stress and mechanical strain on plated-through holes (PTHs), improving the durability and performance of the final PCB.
6. Expanded Polytetrafluoroethylene (ePTFE)
Expanded PTFE (ePTFE) is a microporous material that, while not typically classified as a reinforcement material, offers exceptional performance in applications requiring ultra-low dielectric constant or low signal loss, thanks to its sponge-like structure.
ePTFE is often combined with resin to create B-stage prepreg, which is used to bond layers in multilayer PCBs. These prepregs are widely applied in high-frequency circuits, such as RF and high-speed communication devices, where superior electrical performance is critical.
Frequently Asked Questions
1. What are reinforcement materials in PCBs?
Reinforcement materials are structural supports embedded in PCB substrates to improve strength, stability, and electrical performance.
2. Why is E-glass commonly used?
E-glass offers a good balance of mechanical strength, electrical insulation, and cost-efficiency. Its plain weave structure also enhances dimensional stability.
3. When is nylon fiber used in PCBs?
Nylon fiber is used in high-performance or miniaturized designs. It supports laser drilling, offers low CTE, and reduces weight while maintaining strength.
4. What do fillers do in PCB materials?
Fillers adjust key properties—lowering dielectric constant, reducing thermal expansion, improving drillability, and cutting material costs.
5. What is ePTFE and where is it used?
Expanded PTFE is a porous material with very low dielectric loss. It’s used in high-frequency, high-speed multilayer PCBs for RF and telecom.
Conclusion
Reinforcement materials are crucial to the performance of PCB substrates, impacting electrical, mechanical, and thermal characteristics. Options such as E-glass fabric, nylon fiber, continuous glass filaments, and specialty fillers provide solutions tailored to diverse applications and requirements, including impedance control and signal integrity in advanced designs.
With industry demands for higher frequency, speed, and miniaturization increasing, selecting the right reinforcement materials is critical to developing advanced circuit boards. Deep knowledge of their properties underpins effective design for next-generation performance.
