Investigating the Impact of Final Finishes on the Insertion Loss in As Received and After Aging

Abstract

The task of the final finish on the printed circuit board is to protect the copper surface and at the same time keep it active for the subsequent assembly steps. In high frequency applications, additionally the impact on the signal loss becomes of interest. Typically, the choice of the final finish is defined by the target application. The demands on solderability and bondability need to be considered and the acceptable price level needs to be defined. For the use in high frequency applications the signal integrity becomes of interest and adds on the reliability criteria such as solder joint reliability or corrosion resistance. In this study various surface finishes were tested towards their performance on insertion loss at high frequency. In contrast to earlier studies the frequency range was investigated up to 150 GHz and the impact of thermal or humid aging of the final finish was evaluated. Overall, seven different finishes have been selected including EPAG (Electroless palladium/Autocatalytic gold), I-Sn (Immersion tin), I-Ag (immersion silver), DIG (direct immersion gold), Au/Pd/Au (Gold/Palladium/Gold), ENIG (Electroless Nickel/Immersion Gold) with high and medium phosphorous Nickel and ENEPIG (Electroless Nickel/Electroless Palladium/Immersion Gold). The paper compiles the data for insertion loss versus frequency for the different final finishes at layer thicknesses as commonly used in the PCB industry. The results in the insertion loss performance are correlated to other properties of the different final finishes such as reliability and assembly performance to identify the best candidates for high frequency applications.