What You Need to Know About Moisture-Proof Cable Assemblies

There’s a wide range of service environments that require protection, from total immersion at depth in salt water to condensation in high humidity climates for outdoor applications. Moisture induced failure modes can occur indoors, too, in locales as diverse as hospital operating rooms to commercial kitchens.

Extruded thermoplastic jackets give excellent protection for conductors, but the major issue is at the cable termination.

Connectors can be weatherproof as built, protected by boots or both. Slip-on boots mechanically restrained over connectors may use sealants or O-rings to keep moisture out, but for volume production, the gold standard is overmolding.

Overmolding is an injection process where TPE or TPU elastomers or a number of other materials are shot, over dressed and terminated cable ends. For volume runs, several cables can be overmolded simultaneously.

Overmolded cables and cable assemblies are designed for long life cycles and can be manufactured in straight, right-angle or custom molded back shells for unique circumstances. Overmolding can also offer strain relief and improved pull strength versus conventional cable assembly designs.

However, not every overmolded cable assembly is built the same.

Protecting Against Water Ingress

Cable assembly designers refer to the IP code, or International Protection Marking, IEC standard 60529 as published by the International Electrotechnical Commission (IEC), when looking to rate products for protection against water ingress.

The IP code classifies and rates the level of protection against intrusion of fingers or hands, dust, and water into enclosures or connectors. The IP code also provides more specific information regarding protection and resistance.

The IP code typically consists of two or three digits, each indicating compliance to a specific protection criterion. The third digit is not part of the official IEC standard and so is often omitted.

The first digit of the IP code refers specifically to protection from solid objects or materials. The second digit measures protection from liquids and the third rates protection from mechanical impacts.

If there is no data available to a specific protection rating, the digit is replaced with an “X.” If no protection is offered, the digit reads as “0.” The third digit is the only exception to this rule – rather than being replaced, no third digit will appear.

IP67 and IP68 are the most sought-after ratings for cable assemblies designed to withstand high moisture and humidity.

The IP digit ratings can be summarized as follows:

Cable assembly’s IP ratings are determined through testing the design, from how it’s molded to what materials are used to house the cables themselves.

What Materials to Use When Overmolding Cable Assemblies

Porosity, abrasion, chemical resistance, tensile strength and other factors determine the ideal material for designing cable assemblies. Some for example, withstand moisture well but cut or abrade easily. Others are too expensive for cost-effective volume production.

Different materials used for overmolding include PVC, rubber, Santoprene, ABS, Polycarbonate, Macromelt and others, each with their own strengths and weaknesses.

For applications requiring full immersion, Brian Morissette, Cable Assembly Product Manager at Epec Engineered Technologies, recommends using materials compatible with cable jacket compounds to ensure greater water tightness.

“We can design the overmold to adhere to the cable jacket to make the section water tight. If a jacket compound is PVC, you can use a PVC overmold where it can bond right on the jacket material,” Morissette explained. “It’s like supergluing it onto the cable.”

If a cable assembly is expected to last a number of years out in humid, moist or submerged environments, designers must remember that porosity is a significant issue when choosing a material for cable jackets. It can also be an issue for low voltage DC conductors, as even a little water ingress can play havoc with capacitance and Q factor in coaxial RF applications.

Cable assembly designers can offer advice on what materials may work best for your applications and may suggest materials like PVC and polyurethane to help prevent issues with porosity.

Another common problem that requires consideration when designing for water tightness is that the cable itself may need to be flexible, while the connector requires a more rigid design.

“For situations like this, we have to design a good string relay and make sure the transition point is able to handle both water tightness and any flexing that occurs between the cable and connector,” Morissette said.

When selecting the material for a design, it’s crucial to consider other possible environmental threats.

“If you built an assembly to be able to be exposed to sunlight for extended periods of time and if you put the assembly in something like a gasket, the designer needs to ensure that it will not break down by itself over time; you’ll see rubber dry-rotting,” Morissette explained. “You have to consider all of the materials and components as you’re designing the assembly, so that you don’t pick a material that will cause itself problems through aging, degradation, insertion forces or other issues.”

Manufacturers need to know they can trust the design choices made in the development of their cable assemblies and so the design must also run through rigorous testing to see if it achieves the IP rating, as well as other requirements for the intended application.

Testing Overmolded Cable Assemblies

Ideal water resistance is difficult to achieve, and so designers analyze an application to account for factors like intended life cycle, exposure to humidity, environmental spray and duration submerged in total immersion conditions.

“What we normally do is find as many different ways to test an assembly as possible,” Morissette said.

“One test we’ve performed is the bubble test. One end of the assembly is held outside of a chamber and the other end is either in a beaker full of alcohol or water. We apply a measured amount of pressure on the exposed end and check the other to see if any bubbles come out.”

Other tests Morissette and his team at Epec have performed include submerging an assembly into a chamber filled with water and expose the chamber contents to high pressures, attempting to force water in at any point between the cable, overmold, contacts or holder.

Water doesn’t just corrode and conduct, it supports bacterial growth too. A test published by the International Maritime Organization, titled “Testing of Life-Saving Appliances” measures a cable assembly’s ability to resist the growth of molds.

“In the test, you take a cable or assembly and you expose it to a mold chamber where you spray it with bacteria and mold spores to instigate mold growth under certain humidity for a number of days,” said Morissette. “In one case it was 95 percent humidity and eight or nine assemblies were in these chambers for 28 days.”

Cable assemblies can also be tested for a wide range of temperature settings and other environmental exposures, including human-caused accidents and abuse.

“Essentially, you need to design cable assemblies expecting them to be mishandled,” Morissette said.

Working with Your Cable Assembly Designers

To assure the most efficient design for a cable assembly, the most important step in the design process is for manufacturers to work closely with their customers’ assembly designers. This means sharing as much as information as possible about application expectations, concerns, and costs.

Information worth sharing includes:

• What will the cable assembly connect to?
• How many connect/disconnect cycles do you expect?
• How long will it be in service?
• What will the assembly’s environment be like?
• Who will handle the cable assembly?
• How will the assembly be handled?
• What is the assembly’s desired life expectancy?
• What kind of voltage/current is expected?

“There’s such a pressure today to go for cost savings,” Morissette said. “I don’t think some customers understand that when you take cost out of something, you’ve got to take something out of the project too. I look at it from a CEO’s position, trying to make major decisions, so the more information I can get from a customer, the better.”

To learn more about designing cable assemblies and overmolded cable assemblies, visit www.epectec.com .