Many applications call for the use of hermetic seals, or seals that are waterproof and airtight. Hermetic seals prevent ingress of dust and water, limiting contamination and biological growths, such as bacteria or mold. They also prevent deterioration of electronic components due to moisture contamination.
In this article, we will discuss the use cases, design requirements, and manufacturing of hermetic seals, as well as how titanium and titanium metal injection molding (TiMIM) are ideal for creating hermetically sealed components.
The biomedical field uses hermetic seals to prevent damage to sensitive devices, as well as to reduce the likelihood of components being rejected by the human body. While other industries also benefit from hermetic seals, biomedical devices are particularly difficult to install, meaning it is especially important to extend the life of these devices. Long-lasting, safe devices ultimately mean faster recovery times, improved bio-acceptance, and a happier, healthier, and more comfortable patient.
The electronic components industry also requires hermetic seals. Integrated circuits, wiring assemblies, connectors, and other components are packaged in ways to prevent the ingress of moisture. Besides the direct manufacture of electronic components, this need for hermetic protection from ingress extends deeply into the aerospace, automotive, and defense industries.
In some cases, ingress protection may be accomplished by simply “potting” the assembly or component in an epoxy or another material that can used to envelop the component and exclude moisture. More complicated assemblies or systems that require more robust packaging are often encased in metal packaging. For implantable packaging, the preferred material is titanium because of its exceptional biocompatibility.
Most commonly, commercially pure titanium (CP-Ti) is used for these enclosures. This is because titanium is much easier to stamp or deep draw than titanium alloys. Typically, the package may consist of one or more stamped components assembled and welded together with a feedthrough, which provides the electrical connections to the device.
In these cases, the integrity of the seal is a concern wherever the package has been welded, or where electrical connection is made from the exterior to the interior of the package. Certain design geometries are difficult to weld, such as very small components. Also, some components, such as glass-to-metal seals, cannot be manufactured by welding, yet the two materials must fit together as a hermetic seal.
In order for an assembly to be hermetic, the metal components are typically welded together. These assemblies are small and complex and are typically laser welded with no filler material. This requires that the assembly be a precise fit, so dimensional control on these types of components is critical to provide for successful welding and consequently a hermetic seal.
Furthermore, a hermetic seal must have low porosity. Consider a gasket — if the gasket is full of small channels and holes due to porosity, it will leak. The same is true with a hermetic seal. Any manufacturing process used to make hermetically sealed components must be able to produce parts with no connected porosity.
Depending on the application, the materials used in hermetically sealed components may need to have matching coefficients of thermal expansion. This is particularly important in cyclic applications, as the constant expanding and contracting of materials relative to each other will cause the seal to fail. In glass-to-metal seals, mismatched thermal expansion can lead to cracking in the glass. Thermal properties must be carefully examined, particularly in applications where components are heated and cooled rapidly, cyclically heated, or heated to high temperatures.
There are several common manufacturing processes for hermetically sealed components. Traditionally, laser welding has been used to join metal parts, and ultrasonic welding has been used for plastic parts. These techniques are good for sealing assemblies but do not work for mixed materials, such as glass-to-metal or ceramic-to-metal seals.
Potting compounds, such as epoxies, can be used to make hermetic seals and are commonly used in the electronics industry. Components are sealed from moisture infiltration by an epoxy or other resin, but they still have metal electrical contacts. While this works great in the automotive industry for sensors and in the microelectronics industry for components, this method is not biocompatible and is not used in the medical implant industry.
Glass-to-metal seals are constructed by placing wire leads in a housing and then securing the leads by melting a glass seal around them. The liquid glass bonds easily to the metal. Careful consideration of thermal processing is necessary, as some metals will expand much more rapidly than some glasses. The pitch of the wiring, the differences in expansion, and the maximum temperature of normal use must be considered in these applications. Titanium is a popular choice for wiring, as it can withstand the high-temperature glass process and has a similar coefficient of thermal expansion to many glass compositions. Furthermore, glass and titanium are both corrosion resistant; contaminants may not react with the glass, but could attack a less corrosion-resistant metal.
A novel way to manufacture small, hermetically capable components is with titanium metal injection molding (TiMIM). Shapes are pressed from thermoplastic material into net-shape components. From there, hot isostatic pressing (HIPing) and post-process finishing can be used to create a hermetic-seal-ready part.
While hermetic seals can be made from many titanium materials, CP-Ti is the most common material in electronics packaging. Titanium materials are biocompatible and resistant to corrosion, meeting all health and safety standards for internal use. Unlike steels, even stainless steels, titanium’s oxide layer protects the component, making it less reactive with the human body. Stainless steel alloys form a protective oxide layer, like titanium, but the oxide layer is potentially hazardous to the patient if used for biomedical applications. Ultimately, CP-Ti or Ti-6Al-4V are better choices for medical devices than stainless steel alloys.
The corrosion resistance of titanium is perhaps the most important characteristic for use in electronics packaging. In biomedical applications, this corrosion resistance translates to biocompatibility, and in aerospace or defense applications, it translates to the long-term stability of the components and assemblies.
TiMIM is a manufacturing technique that forces a thermoplastic compound, made from a titanium powder and a binding agent, into a mold. Once the mold has been filled, the binder is removed and the titanium is bonded in a sintering process. The TiMIM process is particularly useful for the high-volume production of small complex parts.
The molds used in TiMIM allow for net-shape fabrication. They are carefully designed to optimize the thermoplastic compound flow to fill the mold entirely, as well as to manage the heat distribution during sintering to minimize distortion. Titanium is very compatible with the glass used for these seals, and the slightly rougher texture of a sintered surface helps create a strong bond by adding a mechanical interlocking element to the surface. The end result is better adherence of the glass to the titanium substrate. Unlike traditional hermetic seals, the slightly rougher surface is an advantage, as the glass fills in the valleys and improves mechanical strength.
TiMIM can produce very tiny components weighing only a few milligrams, which is exceptionally difficult to accomplish using other manufacturing techniques. Components can be manufactured to extremely tight dimensional tolerances, within +/-0.1-0.2%. This makes hermetic sealing possible and reduces the variation from part to part.
Design geometries are virtually unlimited with TiMIM. The small sizes, tight tolerances, and ability to force the thermoplastic compound into the mold under pressure mean thin-walled sections and other hard-to-manufacture geometries are possible. While mold design and fabrication are the longest part of the TiMIM production timeline, the flexibility in design possibilities make it a preferred option for biomedical devices.
When compared to investment casting, TiMIM is much denser (99.9%), meaning fewer pores and less risk of a defect due to the discovery of a pore during machining or finishing. Since the surface of investment cast components are much rougher than those of TiMIM parts, more secondary operations are needed to achieve the required finish.
TiMIM results in an affordable hermetic seal and greater design flexibility for high volume production versus other manufacturing techniques.
Related: Titanium MIM Products: 4 of the Best Use Cases for MIM
Praxis has over 15 years of experience in TiMIM for the medical device industry. We are capable of producing parts as small as 20 mg, yet fully dense (i.e. with zero porosity). From mold design of complex geometries, to medium-to-high volume production (> 10,000 parts/year), Praxis can develop a TiMIM solution for precision biomedical implants.
Furthermore, because we only work with titanium and our production lines are specialized for titanium alloys, there is little chance of contamination. In powder metallurgy, a small contaminant in a commonly shared die, feed funnel, mold, or other processing equipment can lead to bio-incompatibility and rejection of the medical device. With Praxis’ composition-dedicated production lines, the possibility of contamination is greatly reduced.
Attesting to our quality, we certify our TiMIM products to ASTM F2885 or ASTM F2989.
For more information on our TiMIM medical implant capabilities, contact us today.
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