Electronic Failure Analysis of Hermetic Packages
Consumer electronics can experience various failure mechanisms, including environmental factors, abuse, and packaging. High-reliability electronics, especially in aerospace applications, are housed in hermetic containers made of ceramic and metal, while integrated circuits are packaged using plastic or epoxy-based mold compounds. These hermetic packages present unique challenges for conducting electronic failure analysis, as specific failure mechanisms and tests are only applicable to hermetic packaging. To prevent impurities from entering the cavity, hermetic containers are typically sealed in a dry, neutral atmosphere. However, these seals don’t always function perfectly and can allow gases to flow into the hollow.
To test hermetic packaging for potential leaks, radioactive tracers and inert gases like helium or fluorocarbons can be used for gross and fine leak testing. The device is placed in a pressured chamber and left to “soak” for a period, then subjected to a detecting mechanism to identify large leaks. As the tracer leaves the cavity, a stream of bubbles emerges from the leak’s location.
Even after the device has been completely welded shut, problems could still exist inside the cavity. The integrated circuit may be sealed inside the hermetic chamber along with small metallic specks, chips of substrate material accidentally broken off during packing, or other materials. These particles are frequently just one bump or jostling away from shorting pins and bouncing off the glassy surface of the semiconductor die, which would result in catastrophic failure.
A particle-induced noise detection (PIND) system is the most popular method of determining the presence of such particles for electronics failure investigation. To jar the particle free from any crevice it may have wedged itself in, the system comprises a platform that can provide brief, acute shocks or prolonged vibrations to a sample and a sensitive transducer (akin to a microphone) that can pick up even the faintest sounds. The problematic component is put on the PIND tester, where it is subjected to a series of shocks and vibrations that shake the particle and cause it to bounce off the metal cage’s walls.
The transducer detects and amplifies the sound produced by each impact, and the result is displayed as an oscilloscope trace and played on a speaker for the analyst’s enjoyment (since particles are often possessed of an impeccable sense of rhythm). It goes without saying that an analyst must be able to isolate and identify the particle in order to ascertain where in the process it was introduced. Just confirming that a particle is present inside the cavity is not sufficient. In order to achieve this, a hole is pierced in the device lid and taped shut. When the particle can no longer be detected after vibrating the device in the PIND tester, it has most likely bounced onto the tape by that time. The particle is then examined when the tape has been taken off.
While PIND and leak testing are two techniques for identifying flaws in hermetic packaging, they are by no means the only analytical techniques available. The greatest strength of an analyst is versatility. Even though the tests described here are specifically designed to find flaws in packages that have been hermetically sealed, there are many other methods in the analytical toolbox that can be modified to find these kinds of failures with just as much success.
