Managing Controlled Impedance: A Guideline

Managing Controlled Impedance: A Guideline

Manufacturing controlled impedance printed circuit boards (PCBs) requires meticulous attention to detail, particularly when it comes to specifying the dielectric board thickness. Here are key guidelines to follow for proper thickness specification.

Dielectric Material and Thickness Selection

Selecting the appropriate dielectric material and thickness is crucial to meet your impedance and frequency requirements. For high-frequency PCBs, materials like Rogers RO4350B are commonly used and come in standard thicknesses such as 0.254 mm, 0.508 mm, 0.762 mm, and 1.524 mm. These thicknesses facilitate impedance control by allowing precise stack-up design.

Dielectric Thickness Precision

Precisely defining the dielectric thickness between signal layers and the reference ground or power plane is vital. Typical values for maintaining 50-ohm impedance often lie within the 0.1–0.2 mm range but may vary with material and design.

Manufacturing Tolerances and Minimum Dielectric Thickness

Consider manufacturing tolerances and standard fabrication limits, including minimum dielectric thickness. According to IPC standards (IPC 6012 revision F), the minimum dielectric thickness after lamination can be as low as 2.56 mils (~0.065 mm) for Class 2 and Class 3 boards to support fine features like microvias. Maintaining thickness tolerances is essential to avoid impedance variation.

Collaboration with PCB Manufacturers

Work closely with your PCB manufacturer to specify final effective dielectric thicknesses after lamination and ensure their ability to maintain tight control on layer thickness and material Dk (dielectric constant). Consistency in Dk and thickness directly translates to impedance stability, crucial for RF and high-speed digital designs.

Impedance Calculations and Simulations

Use impedance calculators and simulation tools to correlate stack-up thickness, dielectric constant, and copper weight to meet your target impedance. After production, verify impedance using time-domain reflectometry (TDR) to confirm the effectiveness of the thickness specification.

Other Considerations

• It is recommended not to use more than three different types of prepregs in a stackup, and the dielectric thickness of each prepreg layer should be less than 10 mils.
• When a trace is defined as a controlled impedance trace, the impedance matters more than the size of the copper feature for PCB manufacturers.
• For IPC Class 3 boards design, different requirements include CAF and etch-back in addition to controlled impedance.
• For HDI designs, the thickness of the dielectric is controlled by the aspect ratio of the microvia.
• The manufacturer's cross-section technician measures dielectric thicknesses depending on the trace location, either inner or outer layer.
• When it is a differential pair, the spacing between the two traces is measured to understand if the projected impedance aligns with the recorded impedance.
• Upon failure, a cross-section of the impedance coupon is taken to investigate deviations from calculated impedance to recorded impedance.
• Some customers choose to go with a core construction over a foil construction to take away the variation in dielectric height.

In summary, specifying dielectric board thickness involves selecting appropriate standard material thicknesses aligned with your design requirements, adhering to industry standard minimum thicknesses and tolerances, and validating with manufacturer capabilities and impedance simulations to ensure controlled impedance performance.

1. To achieve controlled impedance in high-frequency printed circuit boards (PCBs), rely on an impedance calculator and technology to determine the optimal stack-up design and control the dielectric thickness.
2. Throughout the manufacturing process of controlled impedance PCBs, ensure a close collaboration with the manufacturer to specify precise dielectric thicknesses after lamination and maintain consistency in dielectric constant and thickness for stable impedance, particularly important for RF and high-speed digital designs.

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