Choosing the Best Copper Clad Laminate : A Practical Guide

Building a PCB – or any electrical connection – involves a combination of conductors and insulators to ensure controlled current flow. Instances where light cords or phone charger cables become exposed due to flexure illustrate the importance of a robust insulating layer. Circuit boards operate similarly, employing alternating layers of dielectric material and conductor, typically copper foil. The fundamental material in this process, known as a copper-clad laminate or PCB core, plays a crucial role. Manufacturers can manipulate the properties of the dielectric and conductor layers to optimize performance, making copper-clad laminates a critical component in maximizing PCB efficiency.

Manufacturers have the ability to tailor the properties of copper-clad laminates to meet specific design requirements. Parameters such as dielectric constant, loss tangent, and peel strength significantly impact the electrical performance and reliability of the PCB. By understanding and optimizing these key parameters, designers can achieve optimal signal integrity, thermal management, and mechanical stability in their PCB designs. In this article, we will explore the significance of copper-clad laminates and their pivotal role in enhancing PCB performance.

Copper Clad Laminate Variables

1. Glass Weave: The pattern and tightness of the weave can affect the dielectric constant and loss tangent of the laminate.
2. Epoxy Resin: The resin properties impact the overall mechanical and electrical characteristics of the laminate.
3. Copper Foil: The type, thickness, and surface treatment of the copper foil influence its adhesion, impedance, and mechanical properties.
4. Rolls/Thickness: The thickness of the laminate affects the overall board thickness and mechanical strength.
5. Glass Transition Temperature: The temperature at which the epoxy resin transitions from a hard, glassy state to a soft, rubbery state.
6. Dielectric Constant: The ability of the material to store electrical energy in an electric field.
7. Loss Tangent: The ratio of the imaginary part of the dielectric constant to its real part, representing the material’s dissipation of energy.
8. Weight: The weight of the laminate affects the overall weight of the PCB.
9. Foil Type: Different types of copper foil have varying mechanical and electrical properties.
10. Peel Strength: The ability of the laminate to resist delamination during processing and use.

Understanding the Components of a Copper-Clad Laminate

Copper clad laminate is the essential building block of the PCB during the lamination process. The laminate consists of two discrete pieces: the copper foil, which forms the signal or plane layers, and the (commonly) glass-fiber weave and epoxy resin prepreg. These provide the board’s fundamental conductor/insulator relationship: the former carries the electrical signals, while the latter offers electrical isolation to prevent undue influence to or from each conductive layer. While all basic laminate adheres to this format, the implementation can vary wildly depending on the material needs of the board. The stackup design – the material layer-by-layer arrangement of the board – can use single- or double-sided copper-clad laminate (in combination with foil-less prepreg) to create a board structure that meets electrical and mechanical requirements.

Glass Weave and Its Impact on Signal Integrity

Starting first with the glass fiber weave that forms the structure of the core’s prepreg, designers can specify the weave ply and tightness. Both characteristics affect the average dielectric constant encountered by signals’ transmission paths. As signals travel over the dielectric, they spend varying amounts of time over the fiberglass weave or the weave gaps (which contain only resin) – the difference in dielectric constant between the two materials can be significant enough on long enough signal pathways to impact synchronization. While it’s entirely possible the sum of the timing differences “average out” to a negligible amount, there’s also the chance of maximum difference where two time-sensitive lines experience the extremes of glass weave dielectric and epoxy resin dielectric.

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 blind 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.

Copper Foil Production Processes

Functionally, the prepreg only covers half of the laminate. Although seemingly less complex, copper foil manufacturing variations can drastically change its performance. What appears to be a flat planar surface without magnification, copper foil under magnification can show a variety of rough surfaces that influence its ability to grip the substrate and affect the impedance for high-frequency signals. Different processing techniques can cut down the copper teeth and offer distinct capabilities to the PCB:

• Electrodeposited: Manufacturers attach copper using an anode/cathode system connected to a copper solution and a rotating titanium drum. The electric field strength (DC power into the titanium drum) controls the deposition rate, with thicker copper requiring longer processing time. It possesses excellent tensile strength for applications with considerable mechanical requirements but has copper tooth formation.
• Rolled-annealed: A series of rollers mechanically reduce the thickness of the foil below recrystallization temperatures for strain-hardening. It significantly reduces surface imperfections, which makes it a fantastic choice for high-frequency boards.
• Reverse Treated: A roughening step improves the adhesion between the foil and prepreg for better peel strength and, therefore, better resistance against mechanical delamination. By far, the most common foil choice for “standard” PCB applications due to its improved etch precision and miniaturization alongside lower-profile copper teeth.
• Double Treated: Both sides of the foil undergo roughening to improve adhesion, allowing manufacturers to bypass a black oxide step during fabrication.

Conclusion

Copper clad laminate (CCL) is an indispensable material in PCB manufacturing, influencing the electrical, thermal, and mechanical performance of the final product. By understanding the various types, properties, and applications of CCL, PCB designers and manufacturers can make informed decisions to optimize their designs and ensure the reliability and longevity of their electronic assemblies. This comprehensive guide serves as a valuable resource for professionals seeking to deepen their knowledge and enhance their expertise in the field of PCB fabrication.

By staying updated with the latest advancements in CCL technology and adhering to best practices in material selection and manufacturing processes, the PCB industry can continue to innovate and meet the evolving demands of modern electronics. Whether designing consumer gadgets, automotive systems, or aerospace applications, the choice of CCL plays a pivotal role in achieving superior performance and reliability.