High sulfur environments can play havoc on film chip resistors. In standard film resistors, the internal terminal is made of silver (Ag) or silver palladium (AgPd) material, and is only protected by the epoxy passivation layer. Unfortunately, this layer is susceptible to the ingress of sulfur-based gasses. These gasses can diffuse and attack the silver in the Ag/AgPd internal terminal. Depending on factors such as temperature, humidity and even air flow in the environment, sulfur gasses cause the corrosion process to begin with the formation of silver sulfide. This leads to disastrous results where the silver sulfide can crack the resistor’s ceramic package, causing it to go open circuit. And, once the non-conductive silver sulfide has formed along the length of the terminal, it tends to gradually increase with resistance. So, as the conductive path is reduced, it eventually results in another open circuit condition causing the failure of the resistor.
To put this into perspective, systems such as a server farm exposed to higher sulfur contamination on a factory floor or a data system located in a region of the world that has a higher percentage of potentially corrosive gasses including biological and volcanic gasses, can experience catastrophic failure from corrosion. Beyond gaseous conditions, sulfur can be found in connectors, oils, lubricants, gaskets, rubber sealings, molded coatings, etc., that can also contribute to the formation of silver sulfide, which starts the corrosion process in sensitive electronic components like a film resistor. It is no wonder that there are now recommendations on corrosivity levels for IT centers to stem the harmful effects harsh environments can have on sensitive equipment.
Bourns has engineered a solution to this problem. We have designed the plating termination layers of our sulfur-resistant thin film resistors with a special structure in the area between the protective layers and the terminations. Our design covers the silver (Ag) terminals protecting them against sulfurous gases. Using a specialized Palladium (Pd) protection layer that covers the silver (Ag) terminals eliminates the ingress of the sulfur gasses removing the chance for corrosion and formation of silver sulfide.
We tested a standard film resistor compared to our sulfur-resistant film resistor. The test was performed using a 2400 cm3 glass desiccator vessel with 40 to 60 grams of powdered sulfur that was heated to an elevated temperature of 105 ˚C to accelerate the effects of the sulfurization. The resistors were then measured at regular intervals to verify resistance. Under these test conditions, the test showed our sulfur-resistant device had about 20 times greater operating lifetime than the standard device.
To learn more about how Bourns® sulfur-resistant film resistors help reduce or eliminate corrosion- related hard failures for improved system reliability and performance, please see our informative Advantages of Sulfur-Resistant Film Resistors white paper.