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Many different final plated finishes are used in the PCB industry, each with its own influence on insertion loss. The impact of an applied finish on insertion loss generally depends on frequency, circuit thickness, and design configuration. This article will evaluate the effects of final plated finishes on the insertion loss of two popular high-frequency circuit design configurations: microstrip transmission-line circuits and grounded coplanar-waveguide (GCPW) transmission-line circuits.
Data will be presented for loss versus frequency for six different plated finishes commonly used in the PCB industry, and opinions will be offered as to why the loss behavior differs for the different plated finishes and for the different circuit configurations. Because the insertion loss of high-frequency circuits also depends on substrate thickness, circuits fabricated on substrates with different thicknesses will be evaluated to analyze the effects of substrate thickness on insertion loss using different plated thicknesses.
This article will also explore many different aspects of the final plated finishes on PCB performance. The nickel thickness in electroless nickel immersion gold (ENIG) finishes normally has some variations; data will show the effects of these variations on the RF performance of a PCB. Immersion tin is often used to minimize thickness variations and analysis will show the effects on RF performance for different thicknesses of immersion tin. The effects of plated finish on PCB performance can vary widely over frequency, and those effects will be shown for a wide range of frequencies from 1 to 100 GHz.
Insertion Loss Overview
The insertion loss of a high-frequency PCB circuit can decrease the usable signal levels of a system, whether in a receiver or a transmitter. Details on insertion loss can be found in a previous IPC paper, although a simple review of insertion loss might be helpful before examining the data on PCB final plated finishes. The total insertion loss is comprised of four loss components.
To read the rest of this article, which appeared in the September 2019 issue of Design007 Magazine, click here.