Corrosion resistance of surface finishes for high reliability devices

As air pollution increasing due to the development of industry, corrosion becomes a serious issue to the electronic products. Sulfur is one of the hazardous element in the polluted gas. Creep corrosion occurs on the electronic components caused by high sulfur content environment and high moisture. The corrosion affects the reliability of Printed Circuit Boards (PCBs) in consumer products, automobile electronic devices and air pollutant control sensor. Silver is commonly used in high-frequency devices as surface finish. It is especially keen to be corroded in high sulfur content ambient. In addition, electroless nickel immersion gold (ENIG) and immersion tin (ImSn) are widely adopted as the surface finishes for the consumer products. These coatings are also vulnerable to the hazardous gas that contains sulfur. For high reliability devices or automotive electronics, it is critical to develop new surface finish that can effectively inhibit corrosion gas. This paper proposes new coating layers that include cobalt and tin to enhance the corrosion resistance. This study compared different surface finishes, ENIG, ImSn, and electroless cobalt (EC) on the automotive PCBs. Since electroplated copper is beneath the surface finishes, bare copper without any surface finish was used as the base line for comparison. Corrosion tests were conducted in the corrosion chamber with a humidity of 100 %. All samples were exposed to sulfur dioxide (SO₂) gas at 80°C for different duration of times. Creep corrosion would cause the formation of corrosion products that migrated on the sample surface. Surface morphologies were analyzed by scanning electron microscopy (SEM). The composition of corrosion products was analyzed by energy dispersive spectrometer (EDS). Copper signals could be detected when the surface finishes failed to resist the corrosion. The corrosion products were characterized by X-ray diffraction (XRD), and their chemical bonds were investigated by electron spectroscopy for chemical analysis (ESCA). More corrosion products were grown on bare copper, ENIG and EC than on ImSn. Hence, ImSn demonstrated high propensity to resist corrosion. However, both EDS and ESCA results showed that copper signals were detected only in the corrosion products on ImSn, ENIG, and bare copper but not on EC. In addition, the crystal structure of EC remained amorphous after corrosion test. It indicated that EC could be an effective diffusion barrier for copper diffusion. The results showed that the combination of ImSn and EC are potential candidates as new surface finish to inhibit both the growth of corrosion products and the interaction between Cu and SO₂. The design could be a promising surface finish for high reliability devices.