In todays high-tech, electronic world, one component you may not think about when considering lubrication is the electrical connector. Fact is, lubricating the critical electrical connections in modern vehicles and equipment can mean the difference between long-term reliable operation and early failure.
Electrical connectors are critical to the reliable operation of all modern machinery and vehicles. Todays equipment contains countless electromechanical devices with numerous sensors, switches and connectors to provide convenience and add entertainment features. Electrical connectors impact the operating life, performance, and quality of almost every product we use today. Unfortunately, every year many of these products fail because of faulty connectors. Most connector failures can be reduced or eliminated simply by applying the correct lubricant to the contacts. Connector lubricants protect against corrosion, oxidation and wear to ensure long, trouble-free operation. They also reduce insertion force, helping improve production efficiency and reducing the potential for worker injury.
The Connector Explosion
While connector failures are an issue in every industry, the proliferation of electrical sensors and components is most apparent in the automotive industry. An average car, for example, can have more than 400 connectors, ranging from simple two-way connectors up to 120-way connectors with 3,000 individual terminals. While the vast majority of connectors and terminals operate reliably for their expected lifetimes, each ultimately represents a potential point of failure.
Studies show that nearly 30 percent of signal and accessory circuit failures and more than 50 percent of power circuit failures in cars can be attributed to connectors. The same holds true for other markets and applications. It has been said that the Apollo space program experienced more connector failures than any other technical problem.
The number of electronic components in the average car has increased dramatically over the past decade and will likely double with the development of Electric Hybrid Vehicle (EHV) technology and the continued integration of convenience items into cars. All these components require electrical connections. To speed assembly, terminals are ganged in large connectors that are assembled once and hopefully forgotten.
The automotive environment offers severe challenges for even the highest quality connector. In the engine compartment, connectors must survive thermal shock (rapid heating and cooling) as well as corrosive gasses, fuel, saltwater and dirt. Power mirrors, door locks and other external systems must resist water and even detergent from car washes. Inside the passenger compartment, temperatures can soar when the car sits in the sun and can drop well below freezing in cold climates.
Reducing Fretting and Injury
While auto designers try to place connectors away from moisture and water sources, connectors remain susceptible to failure from a condition called fretting corrosion. This type of corrosion is mechanical wear caused by low-amplitude vibration that produces micromotion between the connector mating surfaces. The movement results from vibration from the engine, drivetrain, suspension, fans, small motors and thermal shock. This motion eventually wears away the surface and allows an oxide layer to form on the contact.
The oxide creates an insulating layer between the contacts that causes an open circuit, increasing contact resistance across the terminal. This makes the connector act like a resistor and consume power (heat up) rather than passing it through to the operating device. Coating the contacts with an anti-fretting lubricant reduces mechanical wear, provides an oxygen barrier, and helps keep oxide debris away from the contact area.
An additional challenge faced by connector manufacturers and automakers is the ergonomics of the connector design; specifically, the force required to mate connectors. Previously, electrical connectors in vehicles had mating forces over 130 Newtons (30 pounds), and repetitive mating resulted in increased worker complaints. As a result, SAE/USCAR-2, Revision 3 reduced the maximum allowable insertion force to 75 N (16 lb) to avoid workplace injuries. A contact lubricant can keep insertion forces below allowable limits.
Lubricants in Action
A connector lubricant protects against corrosion in two ways. First, it seals out the environment to prevent wear particulates from oxidizing and forming an insulating layer on the contact. Electrical contacts touch only at asperity points (see figure, at right). While oxidation does not occur where peaks touch, it does erode the sides of the peaks, eventually causing a contact break.
The second function of a contact lubricant is to act as a shock absorber to reduce micromotion and prevent fretting corrosion.
Contacts made from tin are more susceptible to fretting corrosion because they oxidize more readily, whereas gold-plated contacts do not oxidize. This does not mean that gold-plated contacts do not benefit from lubrication. Gold is typically used in micron-thick layers on a copper or nickel base metal. Under micromotion the gold layer soon wears away to expose the underlying substrate, which is more susceptible to corrosion.
Contact lubricants reduce gold metal wear during mating and separation, and protect against substrate corrosion. Also, thin gold plating is porous, and a lubricant film seals the pores to prevent substrate oxidation and the eventual formation of an insulating layer, which increases contact resistance. By sealing these microscopic pores, a contact lubricant also enables manufacturers to apply thinner plating, thereby reducing costs.
Selecting a Connector Lube
A number of factors must be considered when selecting connector lubricants. First, the lubricant should not be electrically conductive, to prevent a short circuit or isolative breakdown should it migrate between connector cavities. In addition, the lubricant must be compatible with all housing plastics. Finally, thermal stability has become a critical factor today, especially in automotive applications where engine compartment temperatures consistently reach 125 to 200 degrees C.
Connector lubricants are formulated from a variety of chemicals:
Polyalphaolefin (PAO) is most common and provides good thermal protection at temperatures up to 135 degrees C.
Alkylated Naphthalene is typically used in combination with PAO to increase thermal stability. Advantages include good compatibility and thermal stability up to 175 degrees C.
Phenyl Ethers are typically used on gold contacts. They provide high film strength to prevent galling of gold when the contact is made. Polyphenyl Ether (PPE) is the most expensive option while Alkylated Diphenyl Ether (ADE) is more reasonably priced. Thermal stability is 260 degrees C for PPE and 200 degrees C for ADE.
Perfluoropolyether (PFPE) is used in high-temperature applications and protects at temperatures up to 250 degrees C. It provides good insertion force reduction and excellent compatibility.
The most important selection factors to consider include:
Temperature Range. Below 135 degrees C use PAO; 135 to 200 degrees C use Phenyl Ethers; above 200 degrees C use PFPE.
Elastomer Compatibility. It is virtually impossible to make all-inclusive compatibility recommendations without actually testing a lubricant/material combination. This is because a single elastomer family can have as many as 100 possible formulations, each with different compatibility issues.
Insertion Force Reduction. PFPE oils thickened with low-friction solids provide the greatest reduction.
Cost. While only a fraction of a gram is used for electrical connectors, PAO provides the lowest cost where it can be used. However, for a few cents more a PFPE lubricant can be used which will provide the best overall protection in the most extreme environments.
Connector lubricants are available in three forms:
Oils are typically used only during production, where they are atomized and sprayed onto terminals as the parts are stamped.
Greases are applied both in production and as a field fix to correct a problem. Greases can be applied both before and after a connection is made, and they can contain a variety of additives to solve specific problems. They are injected into the female connector on the production line to prevent fretting wear and corrosion and to reduce insertion forces.
Dispersions consist of a lubricant dispersed in a solvent to make application easier. They are applied during production by spraying or dipping, which leaves a thin lubricant film on the contact. Dispersions can be costly to apply in production environments because evaporation must be limited to control dispersion concentration. Environmental issues should also be considered.
Future Connections
With the development of EHV technologies and the expanded use of electromechanical devices in equipment and vehicles, electrical connectors will be exposed to higher temperatures and higher current. This will require contacts to be made from higher-quality substrates like silver and thicker gold plating. Each of these requires specially formulated contact lubricants.
Dispersions will also come under scrutiny in the future because of increasing regulatory pressure on solvents. New solvent-less systems and precise piezoelectric jetting systems could replace solvent-based dispersions in the near future.
Whatever the future holds for electrical contact design, the sheer number of connectors and their importance will dictate care in selecting the correct electrical contact lubricant.