For many electrical engineers, vacuum impregnation, otherwise known as "sealing", is a mysterious, little known process for sealing porous metals, such as in die cast or powdered metal applications. It is routinely used to seal parts made of such materials, such as transmission housings and power steering pumps, so that they become more impervious to liquids and gases.

However, the connection is rarely made that impregnation sealing is just as effective for electronic assemblies, such as those that include wiring and plastics. All engineers know that plastic components must be protected before assembly to ensure optimum effectiveness, but when it comes time to prepare their parts for shipping to the customer, most turn to more familiar, but less effective methods like O-rings, epoxy, silicone or topcoats. Some engineers are not even aware of the expanding range of applications that modern sealing processes can now serve.

Such oversight occurs simply because too few individuals fully understand either the process or the benefits of sealing. Yet, this lack of information could affect the quality of their parts and ultimately lead to a failing or poorly performing product further down the line. Whether you're a design engineer for a vendor or an installation engineer for an OEM, your reputation as an individual and a company rides on your knowledge of the latest techniques in avoiding field failures. Only then can you deliver a quality product to the customer that can be relied upon every time.

Microscopic voids in electronic components can cause failures in all types of parts, including: pin connectors, switches, wire assemblies, capacitors, plugs, fuel tank power pass-throughs, coolant sensors, encapsulated coils for throttle body fuel injection units and brake actuator assemblies for anti-lock braking systems, components for electronic transmissions and light bulb assemblies.

The vacuum impregnation process can be used for sealing all types of electronic components. Traditionally, it's mainly been used for things you might find under the hood of a vehicle, such as a connector, wire assemblies, wire harnesses. But the drive toward zero fault tolerance has prompted the sealing of parts used in all manner of electronic equipment, even in commercial equipment. This includes computer cables, windings for electric motors and portable wireless applications, where mechanical shock can cause connection failures.

Tiny holes or cracks are inherent in all molded electronic components, especially where the metal pins meet the plastic case. These voids can allow moisture or other corrosive particles to penetrate the plastic, shortening the life of the part. Short of complete failure, a poor electrical connection between unprotected parts can result in a temporary short circuit. Nothing will frustrate a customer more than an intermittent problem that fails in the field, but then miraculously starts to work when examined by a repair technician.

The automotive industry has its own particular need for sealing because specifications often demand that components like fuel pumps, wiring harnesses or encapsulated coils meet leak requirements to protect them from road salt and moisture, leaks from transmission or brake fluid, or gasoline.

Finally, most electronic parts must also be protected from heat, whether from an engine manifold or from the hot temperatures inside today's high-powered Pentium computers. Heat can cause plastic and metal to flux at different temperatures, loosening the joint and reducing effectiveness.

If you seal with an O-ring made of rubber, over time it is going to get dry and brittle, and then crack. Likewise, if someone puts a dollop of silicone around the connectors on the surface, that silicone is exposed to the elements, resulting in expansion, contraction and thermal breakdown. Even a topcoat has a good potential to crack, which opens up a path for elements to get in. All of these methods can leave excess residue on external surfaces, altering the dimensions of the part and impairing conductivity.

In contrast, if a vacuum sealing process is performed correctly you should be able to have two products on your desk and look at them under shop or office lighting, and be unable to tell that one went through the process while the other did not.

Vacuum impregnation is not an outside coating, but instead seals the part internally using a vacuum pressure chamber to impregnate liquid resin into minute cracks. The heart of the process takes place relatively quickly, with a typical impregnation time of only 20 to 25 minutes.

Customers provide the sealing house with parts that are molded, finished and ready for assembly. The parts are placed in batches into baskets and then into a vacuum pressure chamber where vacuum is drawn. This evacuates the chamber and opens fine leak paths within the parts, making the holes receptive to filling.

The chamber is then pressurized, forcing a liquid resin into the components. After passing through several rinses to clear excess resin from external surfaces, the remaining resin inside the part is left to catalyze into a polymer (a result of the presence of free ions and the inhibition of oxygen), leaving behind a hard, yet flexible, permanent seal from within.

At room temperature, the resin will cure in a matter of hours. Alternatively, the resin can be cured at 150F, which speeds up the process to completely dry the product in about one hour.

The seal is made with an anaerobic resin composed of methacrylate monomers, which is formulated differently than that of metal sealants. It is purposely designed to seal at the interface between two dissimilar materials, such as plastic and metal, and endure the differing coefficients of the expansions of these materials. The chemical is non-hazardous and leaves no residue, so it carries no adverse effects on solderability or electrical conductivity.

The resin is also formulated to withstand a wide range of temperatures from -40 C to +150 C. It is effective in plugging holes on the surface as well as for what is known as "through porosity" (cracks and holes that travel all the way through the component wall and will clearly cause leakage). Usually a part like this would be scrapped, but with vacuum impregnation, the part can be properly sealed and used.

Vacuum impregnation permanently seals parts in a way that cannot crack or degrade, with better quality control and higher production rates than other sealing methods. There is no shrinkage or corrosion, and it meets thermal shock requirements.

This adds up to financial savings for OEMs and automobile manufacturers alike by reducing labor costs and the number of faulty parts. Initially, 100 percent of parts should be tested to demonstrate the effectiveness of the process. Soon after, 5 percent or less need to be inspected because of the repeatable effectiveness of the seal. Reputable sealing houses include test results in each shipment to assure quality control.

There are also cost savings because of the decreasing scrap and increasing salvage qualities. Most importantly though, manufacturers profit because the proper seal on an electronic part can prevent the failure of a larger component. For example, the failure of a fuel pump can disable the entire vehicle.

In some traditional methods of sealing, employees must handle each part, sometimes numbering millions a year, to shoot epoxy onto the surface. Mistakes can easily be made: applying too much alters the shape of the part and obstructs proper assembly at the next stage; and applying too little creates a sub-standard seal. Vacuum impregnation, on the other hand, can be done in batches of 500, 1,000 or even 2,000 parts with consistent and controllable results, saving both time and labor.

For pennies a part, vacuum impregnation is more cost-effective than other sealing processes. One OEM of lamp assemblies is switching to vacuum impregnation, cutting sealing costs nearly in half because of time and labor savings.

One hundred percent "First Time Capability" is the standard by which many manufacturers measure success. This means they can warrant that their product will perform flawlessly right from the start. By employing proper sealing processes, manufacturers and suppliers stand a better chance of achieving such objectives.

Given the fact that vacuum impregnation can help manufacturers meet their quality goals, it should no longer be looked upon as a value-added process, but rather a value-guaranteed process. The bottom line is that engineers need to know that it works and that it is cost-effective. Through this process, they can eliminate some of their common headaches at the end of the line.

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