Vacuum resin impregnation is a cost-effective method for eliminating corrosion problems in electronic connectors and associated wiring harnesses.
BY GARY STEVENS
Electronic connectors can be found everywhere, from automobiles to airplanes. Consequently, the effects of their failure can range from the inconvenient to the tragic. But the incidence of such failures can be reduced by better methods of sealing connectors against the conditions that lead to corrosion.
Due to their very nature, connectors are the most vulnerable points in a circuit. Corrosion, from either oxidation or galvanization, reduces current-carrying capacity and results in intermittent, and eventually permanent, circuit failure. In harsh environments, the major cause of connector failure is galvanic corrosion, a process in which dissimilar metals give up or collect electrons in the presence of an electrolyte (usually water).
Although many connectors are designed for use in harsh environments, too often, their service lives are limited by corrosion due to gaps and other leak paths-even microscopic porosity-in the wires, insulation, plastic housing, and pins. Before complete failure, a corroding connector causing its circuit to fail intermittently can lead to downtime and maintenance expenses as frustrated mechanics search out the source of the problem.
Temperature cycling causes the dissimilar materials in connectors to expand and contract at different rates. Exacerbated by this and other stressful conditions, such as vibration or repeated bending, leak paths allow penetration by the main culprit-moisture. In some cases, leak paths even function as wicks, drawing in moisture.
 FIGURE 1. Leak paths of even microscopic porosity allow penetration by moisture, causing corrosion that shortens the life of an electronic connector. |
Harsh environments where connectors are commonly used include automobile engine compartments; military, aircraft, and aerospace equipment; outdoor devices; and a wide range of industrial facilities. Lubricants and coolants used to keep automated assembly lines running can attack plastic insulating materials, as can the steam and caustic chemicals regularly used to wash down certain food-processing equipment. Also, in marine applications, such as shipping and offshore oil rigs, corrosion is further accelerated by the effects of sea salt.
Minimizing methods
The two basic ways to minimize corrosion are plating and sealing.
The plating of an electronic connector’s contacts with a layer of tin has been a common practice for many years, especially where copper and aluminum are both present. Tin coating not only reduces electron transfer, but also lowers resistance and prevents bare copper discoloration. Aluminum contacts, if not plated, are often coated with an oxide-inhibiting compound; however, in the long run, neither practice is an effective defense against harsh environments.
Sealing, if done properly, closes off the leak paths that moisture and oxygen can follow. Sometimes, sealing is done in addition to plating. Silicone-based sealants are effective in many applications, as are epoxy-based “potting” compounds. Both varieties are typically applied to connectors by hand; but this can be a relatively expensive solution to the corrosion problem. In some cases, hand application of sealants may make the final assembly of end products more difficult and time-consuming, since the sealants are often not uniformly applied. Further, in the case of certain automated assembly machines, connectors that don’t easily slide together can shut down an entire assembly line.
But a recent advancement in connector sealants, methacrylate polymer resin, provides maximum protection for connectors in harsh environments.
Impregnation solution
Instead of being painted or brushed onto individual connectors one-by-one, these nonconductive resins can uniformly and economically seal a large batch of connectors, assemblies, and wiring harnesses all at once through a process known as vacuum impregnation.
Vacuum impregnation technology has been employed for decades as a way to completely seal off leaks resulting from porosity in metal castings and powdered-metal parts. The resins, either thermoset or anaerobic, are introduced as a liquid into all voids in a casting, usually via vacuum and pressure. The material is then washed and cured, completely sealing porosity and leaving the part leak-free, even under pressure.
Sealing out trouble
Four common methods of impregnation are dry vacuum and pressure, internal pressure, wet vacuum and pressure, and wet vacuum only.
The leak paths and porosity in connector components are easily filled using the wet vacuum-only approach. Via this method, connectors, assemblies, or entire wiring harnesses are housed in a vacuum chamber and placed in a bath of low-viscosity impregnating resin. Air is entirely evacuated from the chamber; all air in the connectors, wires, etc., rises to the surface of the bath and is likewise removed from the chamber. Once the vacuum is drawn, the return-to-normal atmospheric pressure is enough to drive the resin into all leak paths.
At this point, the product is water-washed prior to curing, to prevent the sealant from interfering with conductivity. Next, the resin is cured in a hot bath, typically at about 140° F.
 FIGURE 3. After resin impregnation, a water bath removes excess resin from the outer surfaces of the connectors. |
Though not necessary for the curing process, ions from copper, aluminum, or iron inside connectors can, in fact, function as a catalyst to the process. These metals give up electrons to the resin as if it were a more cathodic metal. Notably, anaerobic resin has proven particularly effective for sealing connectors, since it does not require air to cure.
Yet another benefit of methacrylate polymer resins is their potential for flexibility. When necessary for an anticipated application, the resin can be formulated to cure to a flexible state.
Once cured, the resin is irreversibly cross-linked and will not reliquify. The cured part will withstand temperatures up to 350° F and will resist corrosion from such substances as solvents, Freon, steam, oil, gasoline, glycols, and printing inks.
After the impregnation process, you can perform a simple air-pressure test to prove the connector assembly is thoroughly sealed against ambient moisture and salts that could otherwise cause corrosion and product failure.
 FIGURE 4. Continuing the impregnation process, hot water cures the resin. |
The durability of connectors sealed in this manner can be especially helpful in installations with prolonged conditions of thermal cycling, vibration, and moisture changes, such as those encountered in many military/aerospace applications. In tanks and other land vehicles, vibration and frequent temperature changes call for rugged, tightly sealed connections. Humidity, rain, and washdowns supply plenty of moisture, which is, of course, more abundant in ships, boats, and amphibious vehicles.
The U.S. Air Force Research Laboratory has concluded that corrosion at the junctions of connectors carrying signals is one of the most frequent causes of intermittent failure and other erratic performance in aircraftelectrical systems.
High-altitude pressure drops can draw trapped air out of a connector, possibly opening new pathways for corrosion-causing moisture. Moisture can then quickly enter such pathways in wet weather or during washdown/de-icing operations. Condensation or water vapor in humid air could be forced into the pathways as the aircraft descends and air pressure rises; however, a connector totally sealed using vacuum impregnation has no air pockets, since these have been filled with resin.
 FIGURE 5. Typical connectors sealed against moisture, to prevent corrosion. |
Vacuum resin impregnation is the most cost-effective method to completely eliminate corrosion problems in electronic connectors and associated wiring harnesses. In addition to filling visible gaps between the parts of a connector, the process prevents unseen internal galvanic corrosion problems by filling microscopic gaps and porosity throughout the connector and associated wiring-an outcome impossible to achieve using hand-applied sealants.
This added reliability, together with high, repeatable sealing percentages and the low-cost benefits of bulk processing, make the vacuum impregnation process an economical solution for all connectors subjected to mild environments, and a must for connectors destined for use in harsh environments.
GARY STEVENS is director of sales and marketing for IMPCO, Inc. (www.impco-inc.com).
Bad chemistry
Key to the chemistry of galvanic corrosion is the tendency of dissimilar metals to give up or collect electrons in the presence of an electrolyte, which is usually water.
In the electromotive series of elements shown in the table, the farther apart the names of two metals appear on the list, the greater the potential for corrosion. In contact with moisture, anodic metals sacrifice themselves to more cathodic metals. Aluminum, for example, gives up electrons to copper, and it is not unusual to find both of these conductive metals in a connector. - GS
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