Electronic Design

The Devil Is In The Details—Bulkhead Wiring Penetration Considerations

It’s often the little things in life that seem to cause the biggest problems, and it’s no different in design engineering. That small detail, that one piece of the puzzle that was overlooked or underappreciated, can cause the largest project to grind to a halt, resulting in missed deadlines, expensive rush charges and contractual delay fines.

From an overlooked stress concentration in the hull plating on a Liberty ship to the aero-elastic flutter of the original Tacoma Narrows Bridge, miscalculating small details has led to very public disasters. Similarly, small errors have led to the failure of innumerable projects and untold amounts of wasted money. The lesson? The same attention and design diligence that goes into planning for large design challenges is needed in smaller projects, as well.

An area where personal experience has shown this to be the case is in the seemingly innocuous area of pressure or vacuum bulkhead wiring penetrations. Over the course of their careers, design and test engineers in many fields will likely encounter a pressure or vacuum penetration design challenge. In applications such as vacuum systems, manufacturing equipment, process monitoring, fluid-filled devices, and pressurized equipment, penetrations are needed to get power, electrical signal or optical feeds into and/or out of sealed enclosures.

Over the years, it has been our experience that missteps often occur when engineers:

  • Underestimate the importance of the feedthrough in their final design or product;
  • Overestimate their ability to build something in-house;
  • Overestimate the ease of finding the right feedthrough solution;
  • Underestimate the lead time that will be involved when they find a vendor

When these missteps occur, designers can end up with delays, expenses and embarrassment that a little preplanning could have eliminated.

There are some situations in which the application is straightforward, and an off-the-shelf feedthrough may suffice. But there are other design, material or envelope considerations that require a customized solution. For example, if an existing bulkhead already has NPT penetrations, a matching feedthrough should be sourced to eliminate drilling and rethreading the penetration.

Often, a design may require more wires than can fit in an off-the-shelf feedthrough. Rather than adding another penetration or enlarging the existing penetration (if either option is even feasible), a feedthrough can be designed to precisely match the customer’s conductor requirements within the existing envelope.

Following are some examples all design engineers can learn from—applications in which either judicious preplanning or custom feedthrough design kept a project on track.

With the increase in space exploration and commercial space ventures, space simulation testing has become a necessity for many companies. Testing typically requires multiple penetrations for power and signal cabling in vacuum chambers of all sizes where hermetic sealing is essential. For years, companies have been getting by with connector-to-connector or connector-to-solder cup feedthroughs that don’t provide complete, wired, and connectorized solutions, therefore requiring excessive time to install and adding potentially problematic connections. Often, twisted and shielded wire pairs are needed to eliminate electromagnetic noise in the data and control signals passing through the feedthroughs. As there are no off-the-shelf feedthroughs for twisted shielded pairs available, a custom solution was required to enable the needed testing without forcing a redesign of the vacuum chamber.

The application of flywheels as energy storing devices has been around for centuries, but interesting new applications of this age-old technology have been integrated into our power grid on both large and small scales, including UPS power backup systems to bridge the time between a power outage and backup generator activation, leveling the power in the grid, and flywheel farms for storing energy from solar and wind installations for use in off-peak hours.

The use of magnetic bearings and vacuums to maximize the efficiency of these flywheel systems has greatly improved their utility, but also made them dependent on wire feedthroughs to provide a means of delivering the power and signal feeds required for these precision systems to run safely without loss of vacuum. With flywheels weighing upwards of 100 pounds and running at 15,000 RPM, the need for constant monitoring is essential and provided by use of numerous Hall-effect sensors and their corresponding wire feedthroughs.

The challenge can be in the quantity of the leads required to get through a small space, because the larger the hole in the chamber, the greater the risk for leakage. Smaller, high-density feedthroughs are better than multiple hermetic connectors. Then there are the issues with mating a hermetic connector in an environment that is submerged, often the case with flywheel applications because the flywheel chambers are sometimes encased in some sort of heat transfer fluid.

Further, there is the risk of using welding to hermetically seal the feedthrough to the chamber wall—oftentimes process control issues in welding create unanticipated temperatures that damage electrical feedthroughs.

An interesting example of the potential impact of a feedthrough on final product design can be seen in the case of mass spectrometers, which require high voltage (up to 30kV) to be carried into a vacuum chamber. In one instance, the only off-the-shelf solution was approximately 1.5” in diameter and anywhere from 5-10” long on either side of the chamber wall. Given that the equipment was being designed as a bench-top unit, this would have tripled the overall size of the unit, and the space needed to accommodate the feedthrough would have possibly prevented the unit from even functioning to the required specifications.

Starting from scratch, a custom solution was created that was only 5/8” in diameter and 1” on either side, satisfying the envelope requirements and including the pre-installed O-ring and snap ring. In addition, the feedthrough was QC prequalified for continuity and vacuum sealing so that the feedthroughs could be supplied directly to manufacturing for installation without having to go through the internal QC process. This value-added service increased the efficiency of the production process significantly.

With intense price pressures placing a premium on component and manufacturing costs, the challenge of replacing a substandard solution for a feedthrough in the compressors used in residential air conditioning units was extreme. A manufacturer was experiencing failures in the glass-to-metal feedthroughs used for power after the unit became encased in ice and snow with subsequent melting.

The freeze/thaw cycle induced a potentially dangerous “venting” problem in which the leaking part would sometimes “launch” a pin or series of pins out of the compressor, followed by a flammable mixture of oil and gases. A hermetic feedthrough can’t vent in the traditional sense, and it also allows for the immediate integration of additional components inside of the compressor to achieve much higher levels of efficiency.

On the component side, a custom solution was developed that eliminates this failure point with a better-designed feedthrough constructed with the proper materials. On the cost side, the solution is not only competitive on a per-piece basis, it adds considerable value by reducing assembly time with the pre-stripped, pre-tinned, and labeled wires and connectors for easy installation, plus it can be supplied as an integrated part of existing components, further simplifying the supply chain and enhancing value to the customer.

The attention to detail needed for semiconductor manufacturing at the micro level is impressive, but the macro level of manufacturing equipment and processing can’t be overlooked. With intense focus on the end product, and an incredibly competitive market, any efficiency or reliability to be gained in the manufacturing process can pay big dividends.

In the case of one new piece of manufacturing equipment, the overall design led to the need for a feedthrough that had a 90° bend with less than a ½” radius. Another design required a feedthrough with an oval housing. Both of these design requirements eliminated the possibility of finding an off-the-shelf solution. Instead, they necessitated a one-off housing design to satisfy the requirements without redesigning a significant portion of the unit. Unless a supplier specializes in designing and manufacturing customized feedthroughs, these sorts of solutions often entail high tooling and setup costs and long lead times. It’s important to shop around for a solution that not only meets your project’s design needs, but also satisfies your cost and delivery expectations.

With the introduction of the RoHS standards, electronics manufacturers need to have ovens capable of melting the compliant solder in a nitrogen purged atmosphere and at a higher temperature than traditional solders. One manufacturer of these reflow ovens was seeing both excessive nitrogen consumption and unacceptably high power usage in its first production run.

Upon investigation, the issue was traced to a wire penetration in the oven wall that had been sealed in-house with RTV silicone during manufacturing. Nitrogen was found to be escaping through the wire windings in the penetration, carrying away large amounts of heat and nitrogen. A customized feedthrough developed to hermetically seal the penetration ensured that subsequent versions of the oven performed to specifications.

Samples of feedthroughs are shown in Fig. 1, Fig. 2, Fig. 3, and Fig. 4.

Whoever coined the phrase “Don’t sweat the small stuff,” was more than likely not an engineer. Because, when you get right down to it, everything engineers do is ultimately defined by the success of “the small stuff” that comprises the larger design projects they tackle.

Although the examples provided here are based on personal experience with feedthroughs, the concept is universally applicable. No matter how small, or seemingly insignificant the detail, failure to incorporate the best solution for any given application can result in production delays, cost-overruns, frustration, and, in the worst-case scenario, catastrophic failure.

Most vendors of components and subassemblies will be happy to provide input during the design stage to help you avoid such issues. If your supplier is unable to provide a solution that meets your needs, your budget and your delivery timetable, you may want to consider working with one who is. Chances are they’re well aware of how important the small stuff really is.

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