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Enhancing Switch Performance: Pushing the Boundaries with Contact Lubricants
December 5, 2012 |Estimated reading time: 5 minutes
Editor's Note: This article originally appeared in the October 2012 issue of SMT Magazine.Introduction to Contact Lubricants
Contact lubricants are specially formulated greases and oils designed to reduce friction and wear and enhance electrical performance of current-carrying metal interfaces, such as those found in switches and connectors. Contact lubricants are typically electrically insulative in thick films, but in ultra-thin films they allow the current to flow. As no metal interfaces are entirely even and smooth, when applied to such surfaces in thin films they fill in all surface imperfections, improving contact and electrical performance as well as prolonging the contact life by reducing hot spots, frottage, and arcing. By filling in air gaps between the contacts, contact lubricants dramatically increase the effective surface area, preventing arcing and the related temperature rise and oxide formation. The lubricants also provide a barrier to airborne contamination and reduce the effects of friction by facilitating smooth movement. In addition, contact lubricants are typically evaluated for their “feel” characteristics, improving the quality of movement of a switch or in simple plastic/plastic contacts, for example. Product Selection
Contact lubricants can extend the life of switches by more than 300%, enhancing performance in a variety of applications and preventing the need for expensive maintenance. How can you ensure correct product selection? Many practical considerations must first be taken into account. In typical switch applications, one of the most common factors to consider is whether or not there are any plastics surrounding the contact area. Plastics compatibility can vary depending on the type of plastic and lubricant used. More sensitive plastics, such as polycarbonate, require a specialist lubricant formulated using synthetic base oils, such as CG60. In the case of less sensitive plastics, such as polyamide, mineral oil-based lubricants may be suitable and in some cases can provide enhanced electrical performance. Electrical performance is evaluated by measuring the mV drop of the contacts/switches over a specified number of cycles. This is usually in excess of 20,000 usage cycles. The performance of various contact lubricants can vary depending on the formulation and intended end use. For example, some materials have a more stable low-mV drop over a large number of cycles, but compromise on their plastics compatibility, as shown in Figure 1. It is important to consider all electrical requirements, material compatibility, and environmental influences when choosing the correct lubricant.
Environmental influences can include temperature changes, humidity exposure, and corrosive atmospheres. Changes in temperature can affect the consistency of the contact lubricant, for example, a lubricant may have a cone penetration value of 320 at 25°C, but when exposed to temperatures of -40°C or lower, this value may significantly reduce meaning that the lubricant stiffens. This change can result in a reduction in performance, particularly in terms of reducing friction. Temperature changes can also result in a difference in compatibility of materials. For example, at higher temperatures the plastics compatibility may be reduced. It is vital that the operating temperature range of the connector or switch is compared to the operating temperature range of the lubricant and “in use” tests are performed.
Going Beyond Simple Switch Applications
Humid environments are common and most contact lubricants will withstand high humidity for prolonged periods. When high humidity is combined with corrosive environments, differences between products can be realised. Electrolube designed a series of tests to illustrate these differences and provide further assistance with correct product selection. These tests include wetting humidity followed by salt mist tests, chlorine resistance tests, and consideration of different contact materials.
The combined humidity and salt mist testing subjected the lubricants to 90% humidity at 35°C for three weeks, followed by one week at 35°C in the salt mist chamber. The protection of copper and steel substrates was visually evaluated after this test. Typically, the synthetic lubricants, such as CG53A, provided the best protection in this environment. In addition to this test, gold and silver contacts protected with various lubricants were also subjected to the salt mist environment. Results further strengthened previous findings, highlighting that some mineral-based lubricants in particular are not suitable for protecting these materials in harsh environments. Finally, as an extreme test, an oxidative environment containing chlorine was created and maintained at 35°C for two months and results again showed that CG53A provided exceptional protection, closely followed by CG60 and CG70. In addition to considering environmental effects, Electrolube has evaluated the mechanical performance of their lubricants by conducting four-ball wear tests. The test consists of loading the grease into specialist equipment that has four balls under a defined rotation and pressure loads are applied. The values taken are the pass value and a wearing value at the pass value. The wearing value is measured in millimeters and quantifies the amount of wear observed on the test balls. The weld point is also determined as the point where the wearing is in excess of 4 mm. These tests clearly illustrated that the fluorinated materials offer superior mechanical protection. Lubricants such as EGF far surpassed standard synthetic and mineral-based lubricants, typically using lithium- and clay-based thickeners. Figure 2 highlights the differences between these materials.
Figure 2: Comparison of pass value and weld points for various lubricant types. Summary
Contact lubricants bring a number of benefits to switch/connector applications in a variety of industries. When choosing the correct lubricant, one must consider plastics compatibility, temperature range, and electrical performance. It is essential that the final application environment is considered in detail as the added stress of corrosive environments or mechanical wear can greatly alter the product selection stage. No matter what the application or the lubricants used previously, testing must take place. Whether it is a change in component material or an alteration to the environment or electrical requirements, many factors can alter the performance of a switch or connector. Jade Bridges is European technical support specialist for Electrolube, offering assistance to customers with product selection, implementation, and after sales support. Her expertise is carried over from her position as R&D manager, where she was responsible for new product development and technical support across an array of chemical products for the electronics industry, including conformal coatings, encapsulation resins, thermal management products, contact lubrication, and electronic cleaning solutions.