7628, 1080, 2116, 3313, 1506, and 1078 are easy to mistake for laminate material model numbers when you first encounter a PCB stack-up. They are not.
In the PCB industry, these numbers generally identify specifications of electronic-grade glass fabric. Glass fabric forms the structural framework of prepreg, or PP (prepreg, also called partially cured resin). Although they appear to be simple codes, they can affect PCB thickness, resin content, lamination fill capability, dielectric properties, and even high-speed signal integrity.
What Is PP in a PCB?
A multilayer PCB is not made by stacking copper layers directly on top of one another. Insulating material is required between the copper layers, and this material is commonly called PP, or prepreg.
Prepreg can be understood simply as a combination of glass fabric and resin:
- Glass fabric: Provides mechanical strength and acts as the structural framework.
- Resin: Provides electrical insulation and fills the spaces between the glass fibers. During lamination, it flows and bonds the individual layers together.
Therefore, when a stack-up specifies 7628 PP, 2116 PP, or 1080 PP, the designation does not mean "7628 resin" or "1080 resin." It identifies the type of glass fabric used in the prepreg. This distinction is essential when evaluating the final laminated dielectric structure.
How Do 7628, 2116, and 1080 Differ?
The simplest way to understand the difference is that each number represents a different glass-fabric construction, including its weave, yarn specification, and overall structure.
As a rough engineering reference, the common designations can be understood as follows:
- 7628: Relatively thick and coarse glass fabric
- 2116: Intermediate construction
- 3313: Relatively thin and fine glass fabric
- 1080: Thinner and finer glass fabric
- 1078: Fine-weave glass fabric category
- 1506: A special glass-fabric specification
However, the numbers must not be interpreted as a simple thickness scale. A larger number does not automatically mean a thicker glass fabric. Treating the designation as "the larger the number, the greater the thickness" is a common mistake.
Common Glass-Fabric Types: 7628, 2116, 3313, and 1080
If only the most frequently encountered types need to be remembered, the following summary is useful:
- 7628: Thick and coarse, with a relatively high glass-fiber content; commonly used for thicker dielectric layers and general multilayer boards.
- 2116: A general-purpose prepreg with an intermediate construction.
- 3313: Relatively thin and fine; often encountered in high-density and high-speed PCB designs.
- 1080: Thin and fine, with a relatively high resin proportion; commonly considered for HDI, high-speed, and high-density designs.
The final laminated thickness is not determined by the glass-fabric designation alone. Different material suppliers, resin systems, and RC values can produce different results. RC means Resin Content, or the percentage of resin in the prepreg.
For example, two prepregs that both use 1080 glass fabric may have different laminated thicknesses if their RC values are different. For this reason, engineers generally do not specify only "1080" when defining a stack-up. They may specify 1080 together with a particular resin content, and the final value may still need to be confirmed against the actual material system and lamination process.
Why Do High-Speed PCBs Pay Special Attention to 1080, 1078, and 3313?
The importance of these glass-fabric types becomes more apparent in high-speed PCB design. Glass fabric is not used only to improve board strength; it can also affect signal propagation.
The reason is that glass fibers and resin have different dielectric constants, or Dk values. A high-speed trace embedded in a PCB does not always see a perfectly uniform dielectric environment. Some portions of the electromagnetic field may pass primarily through resin, while other portions may be more strongly influenced by the glass fibers. As a result, the effective dielectric environment can vary along the trace, causing small changes in propagation velocity.
This effect may be insignificant for many low-speed signals. At higher data rates, however, it can become relevant to USB 3.x, PCIe, high-speed SerDes, DDR, and high-speed differential routing.
The issue can be more serious for a differential pair. If one trace passes through a region with more glass influence while the other passes through a region with more resin influence, the two traces may experience different propagation velocities. This difference is commonly referred to as the fiber weave effect.
Velocity mismatch within a differential pair can contribute to intra-pair skew and can affect differential signal behavior. The practical impact depends on the material construction, trace geometry, routing direction, and signal rise time. Therefore, high-speed PCB designs may use finer and more uniform glass fabrics, or apply techniques such as routing-angle optimization and spread glass to reduce the influence of the weave pattern.
What Are 1506 and 1078?
These two designations can be confusing because they are less familiar to many new PCB designers than 7628, 2116, and 1080.
1506
1506 is a special glass-fabric specification. It should not be interpreted simply as a fabric that is slightly thicker than 1080. It is an independent glass-fabric designation, and its actual application must be evaluated together with the PCB manufacturer's material system, RC value, target laminated thickness, and stack-up requirements.
1078
1078 is generally associated with a relatively fine glass-fabric construction. However, 1078 is not the same as 1080. Although both belong to the fine-weave category, their detailed material parameters may differ. The supplier's datasheet should be used when accurate construction and performance data are required.
Why the Glass-Fabric Designation Is Not Enough
This is one of the most important points in stack-up design. When a stack-up calls for 1080 prepreg, it is not rigorous to assume that 1080 always corresponds to a fixed thickness such as 75 μm. The original glass-fabric thickness is not the same as the final thickness of the laminated prepreg.
During lamination, the resin flows. The final dielectric thickness is affected by multiple variables, including:
- Glass-fabric type
- Resin Content (RC)
- Copper thickness
- Copper coverage and distribution
- Lamination temperature
- Lamination pressure
- Resin flow characteristics
- Laminate material system
Consequently, the same glass-fabric type can produce different final dielectric thicknesses when used with different RC values. PCB material tables generally list the glass-fabric construction, RC, and post-lamination thickness as separate parameters. These values must be considered together when defining a manufacturable stack-up.
Why Are Different Glass-Fabric Types Combined?
In an actual PCB stack-up, it is common to see combinations such as 1080 + 2116, 7628 + 1080, or two layers of 1080. The reason is that stack-up design is not simply a matter of making every dielectric layer as thin as possible.
Engineers must balance total board thickness, impedance, lamination behavior, mechanical strength, reliability, signal integrity, and cost.
For example, an outer-layer high-speed signal may require precise impedance control. A smaller distance between the signal layer and its reference plane can make it easier to achieve the required impedance with the available trace geometry. A finer glass fabric can help form a thinner dielectric layer in this situation.
In other regions of the stack-up, a thicker dielectric layer or greater mechanical strength may be required. A thicker glass fabric such as 7628 may then be more suitable. As a result, practical stack-ups can combine 1080, 2116, 3313, and 7628 rather than using a single glass-fabric type throughout the board.
A Practical Summary for PCB Designers
For an initial understanding, the common designations can be remembered as follows:
- 7628: Coarse and thick
- 2116: Intermediate construction
- 3313: Fine and thin
- 1080: Finer and thinner
- 1078: Fine-weave category with its own material specification
- 1506: Special glass-fabric type that cannot be judged by numerical size
For high-speed PCB design, however, the glass-fabric designation is only one part of the engineering picture. Electrical performance depends on the combined effect of glass-fabric type, RC, post-lamination thickness, Dk, Df, stack-up structure, and trace geometry.
Understanding this relationship helps prevent a common stack-up error: treating a glass-fabric code as if it were a guaranteed dielectric thickness or a complete material specification. The code identifies the fabric construction, but the final PCB behavior is determined by the entire material and lamination system.
