A Comprehensive Study of Surface Finishes for High Frequency/High Speed Applications

Abstract

The introduction of 5G/6G created growing demand for faster rates of data transfer and operation at higher frequencies, pushing signals to travel towards the outer edges of conductors. Therefore, the surface finish applied over the copper circuitry is now gaining more attention. It has been previously shown that the low conductivity and magnetic properties of the Electroless Nickel (EN) layer negatively affect electrical signals as they travel along the conductor outer surfaces, leading to insertion losses. Subsequent studies show that reducing the EN thickness can offset some of the insertion losses observed, leading to a reduction in insertion losses. In more recent times “Nickel Free’ finishes, such as EPIG (Electroless Palladium - Immersion Gold, no EN) have been promoted as solutions to improving signal losses at higher frequency. Exactly how much high frequency performance these new “nickel-free” finishes by the user has not yet been fully quantified, and what are the considerations or trade-off’s for solderjoint reliability with smaller diameter solder spheres when assembled using these newer surface finishes? Comprehensive studies of signal loss, solderjoint reliability testing (ball shear, drop shock, electromigration) and other critical to quality performance data were undertaken for standard ENEPIG, Thinner EN ENEPIG’s, EPIG (no EN) as well Immersion Silver, OSP and a new Ag/Au surface finish. All surface finishes were compared across the performance requirement matrix and a Quality Function Deployment was constructed to produce a data driven tool to align surface finish performance against design needs, allowing for optimal surface finish selection for each application.