Solvents and lubricants, in the right compositions and concentrations, can even make up for manufacturing shortcomings.
BY GERALD KINGSBURY
From the start, connector designers are faced with a dilemma of contradictory requirements.
There is a need to combine different materials, dimensions and, above all, maximum contact area and contact force between mating pairs to achieve long-term functional excellence with minimal wear and corrosion. But if excessive contact force between mating pairs is designed in, wear is increased, while good protection from corrosion is achieved by reducing exposure to atmospheric contamination.
What harmful conditions do contacting connector surfaces meet? After manufacture and during transportation (which can be thousands of miles) prior to assembly, connectors can meet-in different concentrations and levels of exposure-heat and cold, humidity, salt, sulphur, smoke, industrial pollution, and other chemical contamination in the form of airborne effluent from production processes.
All of these are tarnish-promoting conditions for contacts, even before assembly, and they can lead to an increase in contact resistance or millivolt drop before they start their operating life. So, they need to be protected as soon after manufacture as possible, preferably as part of the manufacturing process.
During their operational life, connector contacts are subjected to many additional harmful conditions:
- Abrasion, caused by the force of initial and subsequent insertion and withdrawal, can be responsible for removing thin gold and other plating layers, exposing the more corrosion-susceptible substrate metal.
- Differential thermal expansion between mating pairs leads to slippage and wear.
- Environmental oxidation, sulphation, and other corrosive conditions.
- Vibration caused by equipment resonance frequencies and other movement-induced vibration; e.g., with automotive connectors or mobile consumer electronic devices.
- Frettage wear, or mechanical wear caused by the small, repetitive motion of contacting surfaces, even when they appear to be motionless. Gold-plated connector pairs assembled in new equipment are known to experience frettage wear, even during transport, when the equipment is brand new and suitably packaged. During transport or transmit, varying temperatures result in micromotion between contacting surfaces.
- Frettage corrosion, which results from frettage wear combined with corrosive conditions, comes in many different types and actually has many different causes depending on the metal/alloys used for the contact materials. Tin is particularly susceptible, as it is naturally soft and ductile. But when oxidized, tin oxide is hard and abrasive. Particles of hard tin oxide infiltrate the contacting surfaces, gradually increasing the contact resistance. Under electrical load, this corrosion is accelerated unless the contact surfaces are suitably protected.
The large range of different-style connectors designed for different applications all share, to different degrees, the aforementioned problems. There are a number of different ways in which cleaning solvents, aqueous and non-aqueous, contact/connector lubricants, and thin-films of protective fluoropolymers can help in optimizing the function, design, and cost of almost every connector.
Keep them clean
Experience has shown that the cleaning of connectors, circuit boards, and other electronic assemblies is vital to ensure long-term reliability and performance. Contaminants that must be removed are metal salts formed during plating processes; process lubricants and greases, including plastics mould-release agents; fluxes and other soldering residues; and airborne contaminants.
The process of cleaning has three elements: cleaning medium, equipment type, and drying system.
- The cleaning medium can be aqueous, semi-aqueous, or an evaporating solvent, and can be flammable or non-flammable.
- The equipment type may be ultrasonic, vapor phase, or other cavitation/agitation-inducing systems for aqueous or solvent-based media. Solvent-based systems require specific equipment to minimize solvent loss and volatile organic compound (VOC) emissions during operation for flammable solvents (to minimize fire risk), and for non-flammable solvents (which can be expensive) to reduce evaporation losses.
- The intent of the drying system is to leave no residue. With aqueous systems, connectors normally can be air-dried, provided that suitable metal oxidation inhibitors are present in the cleaning medium. With more complex connectors, however, a hybrid aqueous/fluorocarbon drying system is effective for removing all aqueous residues. The very low surface energy of the fluorocarbon solvent (10 to 15 dynes/cm or lower) displaces any water and waterborne contaminants into a residue-removal chamber.
With solvent-based systems, fluorosolvents are normally used with ultrasonic equipment. Although more widely used than other methods, they also are more expensive to operate because the process uses more fluorosolvent. Also, fluorosolvents or their azeotropes are not always as efficient as aqueous-based systems at removing water-soluble contaminants.
Once the cleaning process for the connectors and connector assemblies has been carried out, they are ready for the next stage.
Protecting the connectors
It is possible to use contact lubricants to enhance performance and to enable poorly designed connectors to function to specification. Contact lubricants are available in different forms, including neat oil or fluid, greases, gels, or dispersions in various solvents.
The best contact lubricant for one connector is not necessarily best for another. The type and degree of protection depends on the connector design, its subsequent operating conditions, its specification, and the conditions met during transit and transportation before assembly. Most connector contacts will oxidize or tarnish within a short time after manufacture, so it is important to apply the protective layer as soon as possible after the plating or manufacturing process is completed.
Connector lubricants can be divided into several general types, including formulated oils, greases, gels, and thickened oils with silica or microcrystalline wax-all are available as dispersions diluted in various solvents.
The base oils used for these lubricants can be subdivided into several classes and should be selected for use depending on the type of connector and the specified environmental conditions.
For the majority of tin/lead, brass, copper, and other metal-alloy connectors, synthetic hyrdrocarbons and esters have traditionally provided reliability and good frettage wear and corrosion protection over a wide range and in most common environments.
Perfluoropolyethers (PFPE), because of their affinity to gold, are the base contact oil of choice for gold, gold-plated and other noble metal contacts. They also combat extremely aggressive chemical environments at very high temperatures. But PFPEs need to be combined with various additives to improve, in particular, their frettage wear and corrosion protection, and to reduce their tendency to spread. They have extremely low surface energy (less than 20 dynes/cm) and additive thickeners are needed to prevent them from spreading.
Polyphenylethers (PPEs) are also extremely stable at high temperatures, but solidify below 0° C. They have been used successfully for many years, particularly on gold-plated and other noble metal-plated connector contacts. PPEs’ frettage wear and corrosion protection are very good, particularly when used with microcrystalline wax.
A lifetime of protection
Care must also be taken to select a contact lubricant that conforms to the connector specification. Main parameters to be considered are plastics compatibility, operating temperature range, environmental conditions, and the millivolt-drop tolerance over the connector life. Reduction in insertion/withdrawal force, especially for larger multipin connectors, should also be considered.
Most connectors are best served and protected for their operating life using contact greases, connector greases, or connector gels. All these options are formulated oils that have been thickened using different types of gelling agent.
A grease structure helps to keep the oil and desirable additives in place, and also acts as an environmental barrier and dust shield, especially for large connectors. While this electrically insulating gel/grease structure can sometimes be the cause of an increase in contact resistance or millivolt drop, this need not happen. Using a good contact grease can reduce the millivolt drop between mating pairs of connectors or contact assemblies, especially when different components are made in different locations. This can often result in the millivolt drop being reduced to a level where these pairs can pass a particular specification that they would otherwise fail.
A contact/connector lubricant can make a connector function correctly in a number of ways, compensating for an unsatisfactory design or for a fault during manufacture. Examples of such flaws that may benefit from compensation in the form of a lubricant include contact resistance that is too high, wear debris when wear is too excessive, plating that is too thin or of poor quality, inadequate contact forces, unsatisfactory contact forces during thermal cycling, and excessive insertion/withdrawal forces during assembly.
The application of contact/connector greases depends on the connector type and the volume of connectors to be treated.
Normally, pneumatic equipment applies contact/connector greases during production to the right places in the correct quantity. This activity greatly reduces connector contact forces and abrasion, and assists the insertion/withdrawal process. Contact/connector greases can also be applied manually, using brushes or other suitable, small pneumatic equipment.
Solvent dispersions and solutions of connector lubricant as low as 0.2% and as high as 50% are available. These leave different thicknesses of lubricant after the solvent has evaporated. Solvent dispersions are to be used only when a thinner lubricant film is required, can be applied using spraying or dipping equipment, or by using a brush, precision bottles, aerosis, or by manual application via pressured containers. Contact/connector greases, whether in dispersion or neat, usually contain UV tracers to ease inspection. They should also be biodegradable.
In-transit assistance
Before assembly, connectors need only to be protected with a thin film of contact lubricant-usually, a solvent-diluted contact oil or a dispersion of contact grease. Care must be taken in selecting the correct contact lubricant that meets the connector specification requirements detailed earlier.
Among the most common methods of application is dipping the connectors in a 0.2% to 2% solution/dispersion of contact lubricant in either isopropanol (IPA)-which is flammable but inexpensive-or in a fluorinated non-flammable solvent, such as an azeotropic blend of either hydrofluoroether (HFE) or of decafluoropentane (HFA). These fluorosolvents are expensive, but if suitable recovery equipment is used in production where evaporation losses are minimized, fluorosolvent losses of less than 0.5% per week should be expected. VOC emissions are also minimal.
For certain low-force, small connectors that are prone to dust attraction, fluorocoatings in thin-film (TFC) protect extremely well. TFCs are fluoropolymers that dry to a coating thickness of less than 1 μm and have been found to protect and give long-term reliability to certain designs of telecommunications connectors for which contact lubricant is considered unsuitable. When the connector mating pairs are connected, the TFC coating is broken through and pushed aside. The non-contacting parts of the connector remain protected, and because of the TFC coating’s very low surface energy, the coating slowly flows back over any exposed areas, restoring protection.
TFC is also useful for preventing the spread of oils as well as eliminating the need for masking. These fluoropolymer coatings, soluble only in fluorosolvents, are also applied to connectors and printed wiring boards as diluted solutions in fluorosolvents-usually at a concentration of 2% or less.
This type of application provides a dry coating thickness of 1 μm or less (using the equipment described earlier), making these fluoropolymer coatings economical in production while virtually eliminating VOC emission as well as fluorosolvent evaporative losses.
The various electrochemicals described in this article, particularly contact/connector lubricants, can be of great benefit to connector designers. Consider them useful tools in the quest to achieve ongoing improvement.
GERALD KINGSBURY is an executive with UK-based Electrolube (www.electrolube.com).
