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80 DESIGN007 MAGAZINE I SEPTEMBER 2019 Article by John Coonrod ROGERS CORPORATION Abstract Many final plated finishes are used in the PCB industry, each with its own influence on insertion loss. The impact of an applied fin- ish on insertion loss generally depends on fre- quency, circuit thickness, and design configu- ration. This article will evaluate the effects of final plated finishes on the insertion loss of two popular high-frequency circuit design config- urations: microstrip transmission-line circuits and grounded coplanar-waveguide (GCPW) transmission-line circuits. Data will be presented for loss versus fre- quency for six plated finishes commonly used in the PCB industry, and opinions will be of- fered as to why the loss behavior differs for the different plated finishes and for the vari- ous circuit configurations. Because the inser- tion loss of high-frequency circuits also de- pends on substrate thickness, circuits fabri- cated on substrates with various thicknesses will be evaluated to analyze the effects of sub- strate thickness on insertion loss using various plated thicknesses. This article will also explore many aspects of the final plated finishes on PCB performance. The nickel thickness in electroless nickel im- mersion 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 thick- nesses of immersion tin. The effects of plat- ed finish on PCB performance can vary wide- ly 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 transmit- ter. Details on insertion loss can be found in a previous IPC paper [1] , although a simple re- view of insertion loss might be helpful before examining the data on PCB final plated finish- es. The total insertion loss (α T ) is comprised of four loss components: α T = α C + α D + α R + α L Insertion Loss Performance Differences Due to Plated Finish & Circuit Structure

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