Detecting the presence of leaks
One of the ways by which the integrity of operations within a chip can be maintained is by mandating certain environmental conditions. Factors such as humidity, dust, and the presence or absence of trace gases can all have a beneficial or harmful effect on operations. This requires the circuitry to be placed in airtight packages. We've already seen how we sometimes need to open or decapsulate packages in order to access their contents for testing.
Before decapsulation, we may need to test whether or not the package is doing a good job of keeping dust and gases out. This requires us to proceed in a way that doesn't destroy it. There are many such techniques and some of them are refinements of tests that we conduct in our everyday lives. For example, we place an inflated bike tube under water and if we see bubbles escaping we know there's a leak. The bubbles also conveniently show us where the leak originates from.
Indeed, this simple technique is used with some modifications to help us detect large leaks in integrated circuit packages. Water is replaced with a fluorocarbon compound for accuracy.
In order to detect extremely fine leaks however, we use helium due to its unique properties of being able to penetrate even the smallest cracks. The tests need to be conducted carefully since the helium can get trapped in pockets on the surface and get released during the detection phase leading to a false conclusion of a crack. But if one really does exist, then we detect the tiny amounts of helium that escape using a mass spectrometer which ionizes the helium atoms, passes them through a magnetic field and collects them as they emerge. The quantity of the ions and the rate at which they escape can be used as a sort of electrical current which is used to determine the extent of the leak.
Fine and Gross leak testing is just one way in which leak testing is performed. Other methods exist as well and we'll cover them in some later articles.