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Book Excerpt: The Printed Circuit Designer’s Guide to... DFM Essentials, Ch. 1
October 25, 2024 | I-Connect007Estimated reading time: 1 minute
Excerpt from: The Printed Circuit Designer’s Guide to... DFM Essentials
By Anaya Vardya, American Standard Circuits / ASC Sunstone Circuits
Chapter 1: Materials
Standard Multilayer Materials
Most PCBs are manufactured using three basic materials: glass-reinforced epoxy laminate, prepreg, and copper foil. PCBs are constructed from three basic material types: copper foil, prepreg, and cores.
Copper foil: Sheets of copper foil are incorporated into the outer layer of the PCB, laminating it onto the prepreg to create the outer layers. Outer layers are generally constructed using ½-ounce copper or thicker depending on the design requirements. Internal layers are constructed with copper that is specified on the fabrication print. Half-ounce copper foil is commonly used for signal layers; 1 ounce for plane layers, and 2 ounce or greater for power planes where there is a high DC current. Other thicknesses may be used based on the design requirements.
Prepreg: This is a semi-cured glass resin material. The resin used for the FR-4 type materials is epoxy-based. There is no copper attached to this material.
Core: This is fully cured glass-resin material with copper laminated to both sides. This is typically used for internal layers. It is occasionally used for outer layers, but that is not a preferred construction method. A core is constructed from either one sheet of prepreg (single ply) or two or more sheets of prepreg, and two layers of copper foil. Single ply is considered the preferred core construction and has better dimensional stability.
High-frequency (RF/Microwave) Materials
High-frequency designs (1 GHz and up) require materials with closely controlled dielectric constants and dissipation factors. The FR-4 materials normally used for PCBs don’t have the desired controlled characteristics. The substrate materials used for high-frequency applications were originally based upon PTFE resin formulations that have the desired properties, i.e. dielectric constant (Dk) controlled to +/- 0.04 and dissipation factor (Df) to 0.0004. These values may vary somewhat depending on the material type and supplier. Today, there are a number of materials on the market that do not contain PTFE resin but still have controlled values that can be used for high-frequency applications.
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