Acquired Electronics360

RF & Microwave

Tackling the Unique Requirements of Hermetic Packaging for Harsh Environments

24 May 2018

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For electronic warfare systems, implantable medical devices, down-hole oil logging tools, and other applications that require components to operate reliably in harsh and mission-critical environments, reliable hermetic packaging and components provide a critical defense against failures that can quickly lead to catastrophic results. To respond efficiently to those reliability challenges, manufacturers should look to a comprehensive approach to hermetic packaging — one able to combine the optimal blend of techniques and technologies required to meet each application’s unique requirements.

Hermetic Packaging Considerations:

  • Every mission-critical application needs reliable connector options. For harsh environments, reliability depends on the availability of rugged hermetic seals, regardless of whether the connector is a standard or custom configuration.
  • To secure sensitive electronics within a hermetic package, engineers need flexible attachment options for solder and preform geometries. Selecting the optimum alloy and shape for these preforms requires a careful understanding of the associated physical and chemical requirements.
  • Applications increasingly need diverse thermal management solutions. High temperatures can degrade device performance and reliability. Manufacturers need to apply manufacturing methods and materials able to protect the system from temperature stress.
  • Manufacturers may need getters to mitigate damage from outgassing in many designs. Inside a sealed package, the release of moisture, hydrogen or other volatile gases commonly associated with electronic devices can cause serious performance and/or reliability issues in systems operating in harsh environments. Placed within the package, getters can absorb these materials before they impact the packaged electronic systems.
  • Finally, manufacturers need robust package lidding and sealing options able to maintain a hermetic seal in the face of mechanical, chemical or thermal stress found in the target application.

To address these challenges, manufacturers should consider a diverse range materials and methods best able to meet each application's unique requirements.

The Package

Multiple materials are available to address specific packaging priorities including:

  • Kovar. Kovar is common in the industry. It is an iron-nickel alloy that is heavy, somewhat expensive and has low thermal conductivity. It does however offer a coefficient of thermal expansion (CTE) similar to hard glass -- it expands and contracts at about the same rate and is often matched with borosilicate glass.
  • Aluminum. Aluminum is lightweight and possesses high rates of thermal expansion, low density and low strength, and offers good thermal conductivity.
  • Titanium. Titanium is strong and light with a low CTE but poor thermal conductivity and is typically required for implantable packages.
  • Titanium composite. A titanium composite exhibits all the good characteristics of titanium. It can be used with integrated copper-molybdenum (CuMo) heatsinks for greatly improved thermal conductivity.

As you can see, each of these materials offers certain advantages and your package manufacturer should have experience working with the material that will best help meet design requirements.

The Connectors

Connectors have two fundamental requirements:

1) to carry current/signals in and out of the package; and

2) to ensure a hermetic environment inside the package.

To ensure that their connectors can accomplish these two tasks, manufacturers need to assess methods for proper sealing, pin/body composition and attachment that are most effective for their particular application.

For DC connectors, manufacturers have two sealing options:

  • Glass to metal sealing. This option is common in the industry and typical in applications that seal Kovar pins into a Kovar connector body. This sealing option requires a connector shell material that can withstand the melting temperature of glass and is typically found in applications requiring lower current-carrying capabilities. In addition, a glass seal can crack resulting in a hermetic failure.
  • Ceramic to metal sealing. This type of sealing cannot crack and is proven in a range of applications, from down-hole oil tools to implantable devices. Lower sealing temperatures mean this approach can benefit from the use of copper alloy pins, commonly using beryllium-copper, which is about 2.8 times more conductive than Kovar.

For the connector body material choice is driven by the housing material, so, manufacturers should consider their options:

  • Kovar housings. Use of a Kovar housing in general dictates the need for a Kovar connector body, which is heavy and expensive.
  • Aluminum or titanium housings. These housings can accommodate composite Al/stainless-steel or Ti/stainless-steel connector bodies made from the Hermetic Solutions Group's explosively welded metals.

Additionally, component designers should consider the following connector attachment alternatives:

  • Solder integration. This attachment approach is common but can become unreliable due to solder fatigue after repeated thermal cycling.
  • Laser weld. This approach creates a strong metal-to-metal interface with a low heat-affected zone. Furthermore, laser welded connectors are reworkable: They can be easily removed and replaced even when a package is fully populated.

Internal Packaging Components

Along with package materials and connectors, package application requirements can drive the need for one or more of the following components:

  • Getters. Engineers need to determine if there are electronic components within the package that will outgas and potentially cause damage over time. If so, they may want to consider getter solutions that are able to trap moisture, oxygen, hydrogen, carbon monoxide, organic compounds or multiple contaminants.
  • Thermal management. For packaging electronic circuits sensitive to excessive heat build-up, manufacturers may want to consider adding heat sinks, thermal carriers and thermal tabs made with advanced thermal management materials, such as molybdenum/copper (MoCu), copper/molybdenum copper (CMC) and tungsten/copper (WCu).
  • Solder/braze pre-forms. Designers need to consider how their electronics will attach to the package, what material options are available and which choice is best for their application. Solder and braze preforms can be custom built to support specific requirements and simplify electronics integration.
  • Lids. Design engineers need to consider how they will lid their package, and whether they will use laser weld, seam seal or solder in the process. In addition, they will need to account for applications that require a window or an integrated getter. For the lid itself, manufacturers will need to consider whether they need a stepped lid or a stamped lid. Lidding may be the last stage in a packaging process, but it should be considered early in the design stage. Manufacturers should work with suppliers who have the range of open tooling designs on hand needed to address specific lid requirements, who are able to produce new lid designs as needed, and who are able to complete a manufactured lid design within days of receiving a customer’s instructions.

Services

Along with materials and components, the ability to reliably manufacture a package design requires multiple capabilities including:

  • Lidding. Organizations without lidding capabilities in house will need outside expertise in seam sealing or laser welding. If these services are available as part of an overall manufacturing process, designers will not need to manage another vendor.
  • Laser services. Manufacturers need to consider any requirements for special laser marking/engraving or features that require laser drilling. Manufacturers can simplify their supply chain and gain consistent results by looking to vendors who can offer these types of services as part of a complete manufacturing process or through separate a la carte offerings.
  • ISO Class 7 cleanroom services. Manufacturers need to consider the value a seamless step from package/connector manufacture to final electronics integration would provide to their manufacturing process. For implantable medical device applications in particular, final packaging and sterilization services will also be required. Manufacturers can choose to manage yet another vendor for this process or consider selecting a company that can provide these services as part of a comprehensive manufacturing solution.

Conclusion

Design engineers should investigate the complete range of available options for developing a full-featured electronic package. Rather than work with multiple vendors to employ individual options, they should consider simplifying their supply chain by selecting a company like Hermetic Solutions Group — the only single company that can provide a comprehensive solution. Contact them today.



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