Creep corrosion on high reliability printed circuit boards

Abstract

Due to the increase of air pollution in recent years, the sulfur content increases in the atmosphere. Creep corrosion occurs on the electronic components caused by high sulfur (S) environment and high moisture. This phenomenon enhanced the failure time for the Printed Circuit Boards (PCBs) in the consumer products, automotive engine and air pollutant control sensor. One of the coating materials is immersion silver (Ag), but Ag quickly degrades and induces failure caused by corrosion. It is crucial to develop a new coating of surface finishes on the PCBs that can effectively prevent the occurrence of creep corrosion in the atmosphere and enhance the reliability of the devices. This paper proposes cobalt (Co) as a barrier layer to prevent corrosion in harsh environment. In this work, PCBs for automobile devices were coated with cobalt layers. Different surface finishes are plated with various layers, such as electroless cobalt (EC). Take electroless cobalt (EC) as an example. The EC layer was deposited in the water bath with a pH value of 9.6. The average thickness of the films was about 7, 10 and 14 nanometer with deposition time of one, two and three hours. The average thickness of the film was about 15 μιη after 3 hours of deposition. The surface morphology is flat and shine after deposition. EDS analysis showed that phosphorus (P) is about 7.2 wt% and Co is about 86.84 wt% in the surface finish. Cross cut adhesion test with 3M scotch 600 tape was used to verify the adhesion of the Co film on the substrate with a dimension of 10 mm × 10 mm square, 10 lines in each direction. The results showed that zero percent of the films were removed and suggested a very coercive adhesion. Surface roughness was analyzed by atomic force microscope (AFM). Before deposition of the EC layer, the root-mean-square (RMS) roughness of the substrate is about 114.09 nm. After deposition process, the RMS of our work is about 52.69 nm. It suggests that the EC process can modify the substrate surface morphology. Corrosion tests were performed in a chamber with high humidity. The relative humility was set at the range from 75% to 95% at a temperature at 80 oC. Dramatic surface corrosion can be observed without the coatings of Co. After the deposition of Co, the corrosion was inhibited and shows good corrosion resistance.