Hermeticity of electronics packages: How do you prove an ultra-low leak rate?

Earlier this year, I attended an excellent IMAPS-UK semiconductor packaging workshop. I had the pleasure of learning from Prof. Anne Vanhoestenberghe of King’s College London as she explained requirements for reliability of active implantable devices. Eleven years on from its publication, I was surprised and proud to have my hermeticity book referenced and recommended by her. I’ve been working on a few projects in this field lately and it seems, hermeticity issues are still leaking through the cracks (pun intended, I’m not even sorry ??). The subject area is vast enough for a PhD (trust me, I have the thesis to prove it). This blog is intended to provide a brief summary of hermeticity of electronics packages. Whether you are designing and manufacturing a new device or looking to select the right device for your application, hermeticity is an important consideration when it comes to reliability. By the end of this blog, I hope you will feel like you have a better grasp of theory and are informed enough to avoid any pitfalls when it comes to proving the leak rate of your microelectronics or micro-electro-mechanical systems (MEMS) package.

Here are my top tips for avoiding the frustration of hermeticity test problems:

1.????? Determine your package cavity volume based on performance and manufacturing requirements.

2.????? Identify the environmental needs of your device early in the design process. Do you require a vacuum or moisture protection or both?

3.????? How long do you need the device to be reliable for? What is the required lifetime?

4.????? With this information, calculate the maximum permissible leak rate that your device requires to operate reliably over its lifetime.

5.????? If comfortably greater than 10-12 atm.cm3.s-1, you may be able to leak test using established external test methods.

6.????? If under 10-12 atm.cm3.s-1, you may need to consider an internal test structure. Time to invest some more effort in hermeticity and plan how you will prove the package leak rate.

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Why do we need hermetic packages?

As electronic devices in general have evolved over the years, they have become smarter, smaller, more sensitive, more powerful, more integrated and the industry have developed some mind-blowing techniques to manufacture these devices reliably. In fact, so reliably that they are now used in very demanding applications such as space and the human body. In the applications that we are most used to as engineers, we are generally concerned about keeping our precious devices safe from anything leaking into our active structures and causing performance issues through corrosion-migration, stiction and other failure mechanisms. Implantable medical device designers must consider all of this along with risk mitigation to prevent the device harming the patient. This introduces a need for a new set of bio-compatible materials which may or may not be fully hermetic. Packaging technology for medical devices is an exceptionally important and interesting research and development area that is gaining pace at present. More to come on this subject at a later date.

Going back to non-implantable device types, most microelectronics and MEMS rely on a controlled, constant environment in which to operate. ?Therefore, to deal with these demanding new applications, designers have had to consider packaging at a much earlier stage. Most often, designers aim to seal the device in a hermetic package at a known pressure. When designers are dealing with all the different requirements of the system, this is often as much thought as hermeticity gets during the design stage. We are all familiar with design for manufacture and design for test concepts that ensure our designs are aligned with manufacturing capabilities and come the end of the process, the performance of the device can be easily tested. ?However, after decisions on the packaging method, materials and internal cavity environment have been made, the package is not often considered as something we need to test. In fact, hermeticity is rarely thought about until:

1.????? Prototype or early-stage production testing shows a potential moisture or environmentally driven failure mechanism. When this happens, failure analysis can help us to determine likely root-causes. With experimentation we can prove it out and find a solution.

2.????? You need to write specifications for suppliers or customers. Often this is the first time we think about what the leak rate needs to be and how we can measure it.

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What makes a package hermetic? ?

A perfectly hermetic package prevents anything leaking in or out of it. Easy.

The trouble is - reality is never perfect. We really ought to be asking how hermetic the package needs to be? Not quite so easy.

Hermeticity theory can seem overwhelming with complex equations and multitude of potential leak path types. However, if we break it down, it’s not so bad. Let’s work through it.

In this blog, we will consider cavity packages only. Overmoulded components also require hermeticity characterisation but is perhaps a subject for another blog.

The first thing we need is the internal cavity volume of the package. Spoiler alert, if the cavity volume is less than 0.001cm3, this is going to get tricky particularly if hermeticity testing is an afterthought.

Next, we need to think about the operation of the part. Does is require a specific operating pressure? For example, RF-MEMS often require a vacuum environment for reliable and sensitive operation, but others simply require a steady operating environment that is moisture free. Moisture is by far the biggest enemy of electronics systems when it comes to issues associated with hermeticity.

If a vacuum is required, we need to work out a pressure specification for operation i.e. the maximum and minimum pressures at which acceptable performance is maintained. This can be simple if the most dominant factor to assure accurate performance is the vacuum level. However, if levels of moisture are of concern, we have to consider the external operating environment (pressure, relative humidity, temperature) and potentially carry out some experiments to establish a critical moisture level that impacts performance of your device. We are looking to establish the level of moisture that can be present in the cavity without impacting performance. When we know this, we can work back to an equivalent pressure increase in the cavity that would result in a performance change based on moisture content.

We then need to have a think about the lifetime of the part – how long after manufacture do you need the part to reliable? Don’t forget to include additional product manufacturing steps and expected storage time as well as the actual product lifetime.

With this information we can use a simple calculation to work out how hermetic your package needs to be.

Where L = leak rate in atm.cm3.s-1, ΔP is the maximum pressure rise that your device can handle (i.e maximum pressure – original sealing pressure) in atm, V is the cavity volume in cm3 and t is the required lifetime of the device after cavity sealing in seconds.

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How can fluid leak into cavities?

Now we have an acceptable leak rate, we need to start thinking about how we are going to measure it. This must be driven by the types of leak that may be present. The types of leak that may exist depend on the materials, design & manufacturing steps and quality control.

Design and Manufacturing

Assuming for the moment that we are designing a “hermetic” package, sealing methods such as thermo-compression bonding, eutectic bonding or anodic bonding will be used. Optimisation of this process including alignment is key to achieving a bond between the substrate and cap. When this process has been optimised, the only way that gas can move from outside to inside the package is through an unintentional leak path. This could be in the form of a crack or delamination that produces a small pathway, or leak channel, through which gas can move from one side of the package to the other.

Of course, leaks channels can occur in a package whether it is metallic or contains glass, epoxy or other materials. Most external hermeticity test methods are designed to quantify the rate of gas leaking through a leak channel.

When there is a need for a known cavity gas and pressure, it is also important to consider the device fabrication processes used. Many deposition and etching steps necessary to produce features require the use of supporting gasses and chemistry. Remnant process materials require careful removal procedures to ensure that any absorbed or adsorbed species are released from the device materials prior to bonding. For example, if argon is used during a sputtering step, it is possible that the sputtered material “holds” argon within its structure. If not degassed, this entrapped argon can be released into a vacuum cavity over time. This is referred to as outgassing. External hermeticity test methods are not able to detect outgassing.

Materials

The materials used to create microelectronics and MEMS devices typically fall into one or combination of three categories: metals, glasses and epoxies. As explained earlier, there is no such thing as a perfectly hermetic package and that is at least partly because gasses permeate through all materials when there is a pressure gradient (i.e. a low pressure at one side of the material and a high pressure at the other). The good news for us is that is takes centuries for gasses to permeate through metals so we can, for the sake of argument, consider metals impermeable. Glasses such as borosilicate are often used as a device capping material. Depending on the specifics of the glass selected, gasses will permeate through glass in anything from a few weeks to a few decades. Of course, this is well known, and materials used in manufacture of “hermetic” devices will use glasses with the lowest permeation rates. With the correct material specification and thickness, glass capped devices can also be considered hermetic. Some applications require the use of materials such as epoxies. Epoxies by their nature are not hermetic. Gasses can permeate through epoxies in a matter of minutes. Whilst epoxies can be designed to reduce permeation rates, they are inherently porous materials. That being said, there is a place for epoxy packaging and coatings which can be very effective moisture barriers. We just need to be careful when attempting to quantify the leak rate of such packages.

Traditional external hermeticity test methods are not designed to measure permeation through materials. When we use traditional test methods to assess packages containing permeable materials, we must be aware that we are attempting to measure the leak rate through any leak channels present and risk ambiguity resulting from permeation. ?Specialist adaptation to traditional testing methods have made it possible to identify leaks as a result of permeation where required. However, most often, material characterisation and selection negates the need to measure permeation rates (unless epoxy condition is called into question).

In summary, there are three types of leak that we need to be able to quantify:

1.????? Permeation – movement of fluid through the internal structure of materials. This occurs through all materials. However, we can consider metals to be impermeable due to the time it will take fluid to move through a metal. When the correct glass has been specified, glass can also be considered impermeable. Generally speaking, material specification of epoxies and similar materials allows us to quantify the permeation rate of gas into the cavity through any permeable materials. We generally do not need to test the permeation rate of packages unless we are using a new material in a new application. Or, we are carrying out a failure investigation and, for example, are questioning whether the epoxy has been fully cured.

2.????? Leak channels – movement of fluid through cracks, breaks or delamination between layers. Can occur in any package type through any material particularly at interfaces. Depending on cavity volumes, can be measured using conventional leak test methods such as helium leak testing or Kr-85 combined with a gross leak test.

3.????? Outgassing – release of entrapped gasses from internal materials. Outgassing can occur from any material. Can be controlled by appropriate post-processing but cannot be measured by external testing methods. Residual gas analysis (RGA) can be used to quantify the amount of outgassing. This technique also identifies the outgassing species which can be used to find the process step responsible. Outgassing may be unavoidable. If so, getter materials can be used to absorb known species over the device lifetime. ?

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How to measure leak rates?

This is where the more complex equations come in, but I’ll spare you from those. If you want to see them, you’ll find them all in my thesis or book. Let’s keep it simple here – we don’t need the maths to understand the problem.

To measure the leak rate of an electronics package, there are two main test techniques available: helium leak detection and Kr-85. There are differences in these techniques when it comes to detection methods (helium is detected by mass spectrometry whereas Kr-85 is detected by x-ray scintillation) but essentially the process is similar. Let’s take helium leak detection as an example:

1.????? The parts are pressurised in Helium (He).

2.????? The pressure should be high enough to allow the test to be conducted in a reasonable timeframe but not so high that the package is subject to excessive stress.

3.????? The parts are then moved from the pressurisation chamber to the mass spectrometer.

4.????? The amount of helium leaking out of the package is quantified and a leak rate determined.

5.????? The time between helium pressurisation and measurement of the helium leaking out is referred to as the dwell time.

6.????? The dwell time should be kept as low as possible to prevent all the helium leaking out of the package before it can be measured.

7.????? To ensure large leaks aren’t missed, a gross leak test is also conducted. This could be a fluorocarbon test or similar.

8.????? Leak detectors that combine pressurisation and leak detection are now commercially available (for example HSHLD by ORS). The means that there is no dwell time which enables lower cavity volumes to be successfully assessed. Improvements have also reduced the minimum detectable leak rate that can be measured reliably to 10-12 atm.cm3.s-1.

Let me reiterate the simple equation given earlier that allows you to calculate the leak rate your package needs to achieve to operate successfully over its intended lifetime:

Where L = leak rate in atm.cm3.s-1, ΔP is the maximum pressure rise that your device can handle (i.e maximum pressure – original sealing pressure) in atm, V is the cavity volume in cm3 and t is the required lifetime of the device after cavity sealing in seconds.

Looking at this expression even without the exact requirements of your package, we can surmise that the lower the cavity volume and the greater the lifetime; the lower your required leak rate will be. Even without tight cavity pressure requirements, devices with a low cavity volume (1x10-3 cm3 or less) and a moderate expected lifetime of 5 years, have a leak rate that is below the minimum detectable by standard methods.?

And here lies the hermeticity frustration! You have the design, materials, tools and techniques to create a package with an ultra-low leak rate (<10-12 atm.cm3.s-1) but there are no test methods that can detect such a low leak rate.

Summary

So, how do you prove an ultra-low leak rate?

There is no getting away from it. This requires serious thought and action right at the start of the design process. To successfully prove that your package is hermetic enough to provide the ultra-low leak rate that you and your customers require, in-situ test structures are essential. These can be specially designed or, if you have an oscillator or resonator as part of your device, the quality factor may be sensitive enough to monitor internal cavity pressure. Combined with external pressurisation to accelerate any potential leak, in-situ test structures provide the best opportunity to successfully prove ultra-low leak rates.