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JUNE 2020 I PCB007 MAGAZINE 57 electric. According to one source [1] , "Studies have shown that copper foil types of varying degrees of roughness have a direct effect upon the insertion loss of a stripline structure. New treatments targeted at conductor insertion loss and surface roughness minimization are being offered by chemical suppliers." Insertion Loss and Surface Finish Handheld devices are a key driver for min- iaturization by circuit designers. Fine lines and spaces are being normalized for many of these applications. In addition, the need for wire bonding focuses attention on nickel gold (ENIG) and nickel palladium gold (ENEPIG). These finishes are beginning to meet limita- tions when it comes to high (10 and 10+) GHz RF signal transmission. The electroless nickel layer is an integral part of the surface of the conductor. There is a transmission loss associ- ated with the skin effect of the electroless nick- el as compared to copper. Newer surface finishes for high-frequency RF transmission are now available. These fin- ishes eliminate or reduce the use of EN. The most common one today is electroless palla- dium immersion gold (EPIG). I covered EPIG in a previous column. Here, the focus is on in- sertion loss. Figure 2 includes a plot of the scattering pa- rameter (S-parameter) as the vertical axis ver- sus the signal Frequency on the horizontal ax- is. With S-parameters, scattering refers to the way in which the traveling currents and volt- ages in a transmission line are affected when they meet a discontinuity caused by the inser- tion of a network into the transmission line. A baseline is first established, and a new plot is then measured with the introduction of the DUT. The difference is the transmission loss or insertion loss and is measured in dB. Figure 2 compares the insertion loss of sur- face finishes with nickel (two variations of EN- EPIG and one for ENIG), reduced nickel (thin nickel ENEPIG), and no nickel (EPIG and IGE- PIG). Thin nickel ENEPIG has as little as 4.0 µins (0.1µm) of electroless nickel. IGEPIG is a variation of EPIG. EPIG uses an immersion palladium catalyst on the copper surface to ini- tiate electroless palladium deposition; IGEPIG uses an immersion gold layer as the catalyst for the deposition of electroless palladium. Figure 2: Comparison of the insertion loss of surface finishes with nickel, reduced nickel, and no nickel.