The dielectric constant of materials has taken on an increasingly larger role in manufacturing, with tighter tolerances on controlled impedance transmission lines and greater losses at high-speed rise/fall times. While the dielectric constant is a material property, fabrication methods and the shape of laminates, sub-products, and other board materials can lead to a significant deviation in the measured dielectric constant over a random trace walk. Because the PCB dielectric constant is one of the most important aspects of stackup design with modern electronic layout trends, designers will want to consider the totality of effects when selecting laminates.
| Ranking Laminates by Dielectric Constant | ||
|---|---|---|
| Laminate Material | Approximate dielectric constant | Relative dielectric constant |
| FR4 | 4.0~4.8 | Mediocre |
| High-frequency Hydrocarbon | 2.6~6.2 | Good~Poor |
| Filled PTFE | 2.6~2.9 | Great |
| PTFE | ~2 | Excellent |
Performance and the PCB Dielectric Constant
In its simplest terms, the dielectric constant describes the charge storage ability of a material. Materials with higher dielectric constants are better able to store charge than materials with lower dielectric constants; at the extremes, a theoretical zero dielectric material would function as a perfect dielectric, whereas an infinite dielectric material would become a perfect conductor. For PCB design and manufacturing, the dielectric constant of materials becomes a significant performance factor, especially at high speeds. In the stackup, the dielectric layers that separate the copper layers function as insulators while increasing the polarizable charge between the layers. This behavior is identical to what occurs inside a capacitor, where alternating conductors and insulation increase the amount of energy storage at a given voltage.
The simple concept of polarizability has a vast impact on electronics: the relative permittivity (synonymous with the dielectric constant) influences how fast signals propagate through the medium and energy lost during transmission compared to a vacuum. A material’s relative permittivity is a frequency-dependent complex value representing both a real and imaginary component, with the former component representing the conductivity and the latter representing the field propagation loss (i.e., energy absorption). When the real component of the relative permittivity is much larger than the imaginary component, it is a poor conductor but an excellent dielectric. On the other hand, when the imaginary component of the relative permittivity is much larger than the real component, it is a poor dielectric but an excellent conductor. Materials with real and imaginary components of relative permittivity that are approximately equal are unsuitable as dielectrics or conductors.
How Physical Shape Affects Dielectric Constants
The construction of the epoxy resin-impregnated fiberglass weave will also affect the dielectric constant due to its inhomogenous construction. FR4s and similar materials use a fiberglass weave for rigidity during lamination and curing. However, the difference in dielectric constant seen by a signal running over the weave and the epoxy resin “gap” can become significant over sufficiently long transmission lines. The dielectric constant seen by a signal traversing a long pathway on the board can vary significantly enough that length-matched signals no longer reach their destinations with an acceptable level of skew. Higher-quality laminate materials attempt to reduce this disparity with a thicker weave that replaces more of the epoxy resin gap, ensuring that randomly routed signals experience a more consistent dielectric constant during transmission.
Laminate manufacturers provide designers with various weave options to account for the weave effect (as it’s known). To be clear, line skew arising from the differences in the weave and resin relative dielectric can be trivial when the line transmission speeds are not appreciably fast. The most common/popular 106 and 1080 weave patterns are sufficient for the stated design intent in these cases and are economical to boot. Thicker weave dimensions and tighter-knit weaves will experience less random divergence (from a random routing perspective) between the relative dielectric constants yet add to the bottom line of a fabrication – for smaller production runs (i.e., prototyping, limited NPIs, etc.), the total cost increase may be reasonable.
A consequence of a tighter weave is a reduction in resin %, as the “gaps” in the weave, which contained resin, exchange for additional glass fibers. This resin reduction has a marked change in how the board reacts during processing techniques, with the end product exhibiting different mechanical and material properties, especially when considering isotropic (direction-dependent) effects along the weave lines. In addition to the more extreme manufacturing outcomes, resin loss corresponds to a decrease in the electrical properties (relative dielectric constant, loss tangent), as the resin is more insulative than the weave.
Your Contract Manufacturer’s Constants? Quality and Reliability
The PCB dielectric constant is one of the most important factors to consider when selecting laminate materials for board production, as it is ultimately responsible for the maximum transmission speed through the medium and energy lost during transmission. While the knee-jerk solution would be to select the laminate with the lowest dielectric constant every time, designers must weigh other material property tradeoffs, such as cost and manufacturability. For designers needing stackup help before they take the manufacturing plunge, VSE is here. Here at VSE, we’re a team of engineers committed to building electronics for our customers. We’ve been realizing life-changing and life-saving devices for over forty years with our manufacturing partners.
