Selecting Dielectric Substrates for High Speed Multilayer Stackup Design
Selecting dielectric substrates requires balancing dissipation factor, glass weave uniformity, foil roughness, and panel yields to meet high-speed impedance targets.
Immersion Silver Finish
Metallic Coating
Chemical displacement processes apply a thin layer of silver over the copper pads of a circuit board to prevent oxidation. The immersion silver finish creates a flat and solderable surface that is ideal for high speed components and fine-pitch pads. Because the silver is extremely thin, it does not alter the dimensions of the copper features or interfere with the fit of tight tolerance parts.
Chemical Displacement
Production tanks use a precise chemical reaction where copper atoms on the board surface are replaced by silver ions in the solution. An immersion silver finish is typically between five and fifteen microinches thick, which is enough to protect the copper without causing electrical issues. The process is faster and less expensive than gold plating while providing better signal performance than finishes that contain nickel.
Manufacturers must monitor the bath chemistry closely to ensure the silver layer is uniform and free of organic contaminants.
Soldering Interface
Assembly plants favor this finish because it remains perfectly flat, allowing solder paste to be applied evenly across all pads. The immersion silver finish dissolves into the solder during the reflow process, creating a strong bond directly between the copper and the tin. Special packaging is required to protect the boards from sulfur in the air, which can cause the silver to tarnish and lose its solderability over time.
This finish represents a balance between high electrical performance and manufacturing cost.