Advanced phenyl ether lubricants offer properties similar to high-performance polyphenyl ethers, without the high cost.
By Sibtain Hamid
Like hundreds of other decisions that electronics assemblers make, the selection of a lubricant for electronic connectors is based on considerations of cost versus benefits. Applying a lubricant to connectors during the assembly process involves the cost of the lubricant itself, as well as the costs in equipment and time to apply the lubricant. The benefit is that applying a lubricant (rather than shipping product with unlubricated connectors) may significantly reduce field failures related to the connectors.
The two enemies of connectors are wear and corrosion, which work together to block electrical conduction across the connector, or to make conduction intermittent. Corrosion is promoted by the accumulation of airborne contaminants and humidity on the surfaces of the connector. Wear may result from mating and unmating, or simply from prolonged micromotion over a range of a few microns. Production engineers generally aim for a level of lubrication that will prevent both wear and corrosion during the anticipated life of the system.
The highest level of connector lubrication protection is provided by a class of lubricants called polyphenyl ethers (PPEs). Approximately a half-dozen performance characteristics are important for evaluating connector lubricants, and PPEs excel in all of these areas. Applied to a connector, a PPE lubricant will remain where it is applied. It evaporates only after four or five decades-far longer than any other lubricant, and far longer than the anticipated lifetimes of most products. During that time, the PPE will protect the connector surfaces from wear and corrosion, and will even inactivate any contaminant particles that reach the connector.
Polyphenyl ethers, however, are too expensive for the vast majority of electronic products. While PPEs are widely used in military, aerospace, and medical systems, they are not used widely in consumer products where reliability demands are less rigorous and where costs must be kept low.
Recently, a low-cost lubricant has been formulated that makes PPE-like performance available for lower-cost products. The connectors in most electronics systems, it was realized, do not require all of the high-level characteristics of expensive PPEs, but only a few of the most critical characteristics.
The newly formulated lubricant is an advanced phenyl ether (APE) lubricant, related to PPEs but less expensive to manufacture. As a result, the APE lubricant has about the same cost as commonly used silicone-based connector lubricants. It excels in one of the most significant characteristics-its ability to remain where it is applied. The most common weakness of currently available connector lubricants, particularly of silicone-based lubricants, is their tendency to migrate away from the connector surfaces and onto other surfaces.
Surface tension
Whether a lubricant migrates or not depends on its surface tension. Some lubricants are designed to have very low surface tension, much like a penetrating oil that migrates into the thinnest crevices. Connector lubricants, on the other hand, need fairly high surface tension. The newly formulated APE lubricant has a surface tension of 32 dynes/cm, not as high as the surface tension of PPEs, but high enough to prevent the lubricant from migrating off of the connector and onto adjacent surfaces.
The advantage of the higher surface tension can be demonstrated by putting a drop of a conventional connector lubricant, such as a hydrocarbon- or silicone-based lubricant, beside a drop of the new lubricant on a suitable surface, and then tilting the surface by 10° or so. Surface tension holds the APE lubricant in place, while the adjacent lubricant slowly flows downward, just as it would “follow the metal” in an electronic connector (see Fig.1).
PCB compatibility
A lubricant that has migrated from the connector is no longer protecting the electrical conductivity of the connector. In addition, it may cause a failure by a different route: silicone-based connector lubricants, for example, may interfere electrically in printed-circuit-board (PCB) materials. The problem occurs when the lubricant “follows the metal” from the connector, winds up on the board, and causes damage to the epoxy that is holding the fiberglass elements of the board together. Because most boards are tightly engineered in the first place, it does not take long for this chemical action to find a track or a lead and cause electrical failure.
Because APE lubricant is closely related to PPEs, which are essentially non-reactive, the APE lubricant would not interact chemically with the surface of the PCB, even if it was somehow applied there. It thus has two critical properties not previously associated with low-cost connector lubricants: it stays where it has been applied, without migrating, and it cannot chemically attack the board surface. Specifically, it has shown to be non-reactive in the presence of numerous elastomers and other materials, including Buna N, Viton, nylon, polycarbonate, and high-density polyethylene.
 FIGURE 2. The performance characteristics of the APE lubricant are most similar to those of the desirable silicon-based phenyl lubricants. |
Some of the performance characteristics of the APE lubricant are similar to those of existing lubricants (see Fig. 2). It has performed well in anti-wear tests (see Fig. 3), and in low-temperature use, an area where its performance exceeds that of current lubricants (see Fig. 4).
 FIGURE 3. In wear tests, silicone-based lubricants typically fail at 250 to 500 lbs, and have scar diameters of 0.8 to 0.9 mm. |
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 FIGURE 4. The APE lubricant performs consistently over a wide range of low temperatures. |
The new APE lubricant has undergone extensive production testing, including testing in automotive electronics assembly, where essentially no failures in long-term reliability were detected. In this testing, the lubricant demonstrated that it fulfills all of the requirements for a connector lubricant, and in particular showed that it does not migrate off of connectors onto other surfaces.
How to lubricate
In general, the lubricant can be applied in three possible ways-by dipping, spraying, or brushing. Dipping involves diluting the lubricant with a solvent, while the other two methods do not require dilution. Those experienced in production tend to favor brushing as the most efficient and least costly application method.
Some electronic connectors do not really require lubrication, especially if the appliance has a relatively brief anticipated lifetime, or if the perceived reliability is not high. Connectors in these applications may consist of tin in contact with tin, which is adequate for the limited demands that will be placed on them.
When reliability expectations are higher, the appliance has a longer anticipated lifetime, or the connector cannot easily be reached for servicing, some form of lubricant becomes necessary. Connectors in these applications are typically coated with a thin plating of gold, palladium, or perhaps silver. While the plating will fend off corrosion for a while, it is not usually a complete answer. Gold, for example, is a reasonably good lubricant, but when there is relative motion between two gold surfaces, the molecules in this very soft metal tend to migrate on to one of the two surfaces, leaving the other surface exposed.
To prevent the exposure of the tin beneath the gold, many assemblers apply a fluid lubricant to the gold plating. The fluid lubricant separates and protects the individual gold layers, which in turn protect the base material of the connector.
Test performance
Two of the many laboratory tests carried out on the new lubricant are worth discussing because they shed light on its performance. The first is the fretting test, a standard test for connectors. A 1-mm-diameter metal pin, coated with a 60/40 tin-lead alloy 3.8-μm thick, is positioned vertically. Its tip makes contact with a flat plate measuring 25 × 25 mm that is also coated with a tin-lead alloy.
During testing, the pin moves back and forth over a 50-μm-long path 60 times per second. The very short path-0.002"-is designed to simulate micromotion that damages connectors in the field.
The fretting test is first conducted with no lubricant. When the pin damages the lead-tin surface of the plate, corrosion begins to occur very quickly. The output of the test measures the change in electrical resistance between the pin and the plate. Without a lubricant, corrosion is sometimes detected less than 60 seconds after the test begins.
The new lubricant was used in the fretting test with a 50-gram load on the pin, and again with a 200-gram load. With the 50-gram load, the test was stopped after 1,000,000 cycles. Contact resistance had increased modestly over 1,000,000 cycles from 0.005 Ω to 0.015 Ω. There was no damage to the pin, and no evidence of corrosion. With a load of 200 grams, essentially identical results were obtained after 500,000 cycles.
The second test is designed to reveal the performance of a lubricant at high temperatures, and to detect evaporation of the lubricant. The lubricant is applied to a metal plate and held at a constant high temperature for a pre-determined time. In this case, the temperature was 180°C and the time was 500 continuous hours.
At the conclusion of the test, less than 3% of the lubricant had evaporated. No sludge formed during the test, even though high temperatures tend to accelerate the chemical reactions that produce sludge. High temperatures also promote polymerization, but the inherent thermal stability of the lubricant resulted in a rate of polymerization after 500 hours of less than 0.15%.
SIBTAIN HAMID is technology manager at Santovac Fluids, 8 Governor Drive, St. Charles, MO 63301. Tel: (636) 723-0240; E-mail: shamid@santovac.com.
