Cause analysis and countermeasures for the blackening of the gold-plated layer of the coaxial connector

The gold-plated layer has low contact resistance, good corrosion resistance and wear resistance, and is widely used in precision instruments, electromechanical components, integrated circuits, printed circuit boards and other fields . The gold-plated coaxial connector exported to Japan by the author’s company can pass the 48h neutral salt spray (NSS) test, but when assembled as a cable and transported to a Japanese customer, there is oxidation and blackening, and the defect rate is as high as 90%. This article analyzes the reasons for this and proposes solutions.

01 Features of coaxial connectors

The gold-plated part of the connector is a brass base, and the thickness of the gold-plated layer is 0.25μm. The main production process of the connecting wire is: stripping the outer sheath of the cable → bending the braid of the cable → riveting the connector PIN needle and the inner conductor of the cable → assembling the connector → riveting the connector tail tube → injection molding PVC (polyvinyl chloride) outside Die→wrap the PVC film to protect→put it into the outer box. The appearance of the product is shown in Figure 2.

02 Analysis of the reasons for blackening

1 The blackening caused by the defects of the gold-plated layer itself. The morphology of the black-plated gold layer was observed with the JSM-7001F scanning electron microscope, and it was found that there were scratches, cracks and pinholes in the gold-plated layer at the corresponding part (see Figure 3), so It is speculated that the blackening of the gold-plated layer may be related to these defects. Therefore, in accordance with GJB548B-2005 “Test Methods and Devices for Microelectronic Devices”, a 48-hour NSS test was carried out on the gold-plated coaxial connector. But the results show that the gold layer can maintain the original bright golden yellow (see Figure 4). This shows that the blackening of the gold layer has nothing to do with the above-mentioned defects, that is, there is no problem with the gold plating process itself.

2 Blackening caused by packaging materials Carefully observe the blackened samples and find that there is condensation between the PVC film and the covered connector.

Because the thermal conductivity of the brass connector is about 120 W/(m.K), the thermal conductivity of the non-metallic PVC film is about 0.14 W/(m.K). The connector heats up faster than PVC, and the water vapor generated after the product is heated will condense on the connector. This batch of products was shipped in August. It is the high temperature season. The temperature in the container can reach 70 ℃ or even higher, and it takes about 2 months to ship to Japan. The PVC film will be thermally decomposed under this high temperature condition for a long time to produce HCl. The hydrochloric acid solution formed after the HCl is dissolved in the water on the connector surface will corrode the gold plating layer. From the above analysis, it can be known that isolating the PVC film from the connector can avoid oxidation of the gold plating of the connector. Therefore, when the product is packaged with PVC film, PE (polyethylene) film is added to cover the connector separately, and the high temperature aging test is carried out together with the connector product that only uses the PVC film (according to GB/T 7141-2008 “Plastic Thermal Aging” “Test Method”), the test conditions are: temperature (100 ± 3) °C, relative humidity 68% ~ 92%, time 72 h. As shown in Figure 5, the gold-plated layer of the connector coated with PE film did not change color after the aging test, and the gold-plated layer of the connector covered with PVC film only showed blackening. Therefore, it is feasible to increase the PE film to cover the connector. In fact, the products packaged in this way were shipped to Japanese customers, and no blackening of the gold-plated layer was found.

03 Conclusion

The HCl produced by the thermal decomposition of the PVC film in a long-term high temperature environment will corrode the gold-plated layer of the connector. After the PE film is added to cover the connector, the problem of blackening of the gold-plated layer is solved. At the same time, in the production process of the finished product, after the PVC outer mold is injected, the product can be packaged after it has cooled to room temperature to prevent similar problems.