Hermeticity Testing

Hermeticity Testing is a failure analysis technique performed to detect ambient atmosphere leakage paths into the cavity of a hermetic package.  Leakage in this context refers to the free movement of moisture and gases to and from the package cavity through openings that an otherwise perfect hermetic seal wouldn't have.  The amount of leakage determines the magnitude of the hermeticity failure of the package.

Loss of package hermeticity can result in internal corrosion and parametric shifts due to moisture effects. It is therefore necessary to detect hermeticity failures so that affected materials may be properly quarantined and the root cause of the problem properly addressed.

Hermeticity testing has two major categories: fine leak testing and gross leak testing. As their names imply, fine leak testing checks for package damage or defects that result in very small leakage.  On the other hand, gross leak testing checks for large package damage or defects that result in gross package leakage.

The methods used to conduct fine leak and gross leak testing are very different from each other.  As such, one can not substitute for the other nor can either stand alone.  In fact, a unit that passes gross leak testing may fail fine leak testing while a unit that passes fine leak testing can fail gross leak testing. Thus, hermeticity testing can not be considered complete unless both fine leak and gross leak testing have been done.

Fine Leak Testing

Among existing fine leak testing methods, the Helium tracer gas method is the most popular and widely-used technique.  The He tracer gas method consists of the following sequence of steps:  1) a vacuum cycle to remove any trapped gases or moisture within the package cavity and/or surface nooks and crannies; 2) a soak of the package in He atmosphere under pressure to drive He atoms into all accessible spaces in the package; and 3) precise measurement of He leakage rate under vacuum.

Gross Leak Testing

The Fluorocarbon leak test is the most commonly-used gross leak testing technique in the industry.  It consists of the following test sequence:  1) a vacuum cycle to remove any trapped gases or moisture within the package cavity and/or surface nooks and crannies; 2) a soak of the package in a fluorocarbon liquid under pressure; and 3) visual observation of the package while it is immersed under a heated clear fluorocarbon liquid for signs of bubble emission, which indicates gross leak failure.

Dye Penetrant Test

Aside from the hermeticity testing methods discussed above, the Dye Penetrant Test is another age-old FA technique useful in analyzing hermeticity failures. It is performed to identify leakage paths within packages that exhibit hermetic failures.

The Dye Penetrant Test consists of the following steps: 1) immersion of the sample in a fluorescent dye such as Zyglo; 2) pressurization of the dye solution in a pressure chamber to about 90 psig for about 8 hours; 3) rinsing of the sample in acetone followed by air drying; 4) examination of the sample under an ultraviolet lamp, which will make traces of Zyglo visible.

The analyst must thoroughly inspect the package to identify spots where Zyglo is leaking out of the sample.  After all the leakage points have been identified,  the analyst may delid the sample to further check where the Zyglo entered the package in order to isolate the point of hermeticity failure.

Examples of What Hermeticity Testing Can Detect: 1) cerdip seal glass cracks or holes; 2) multilayer package seal cracks or holes; 3) inadequate metal can welding; 4) header glass eyelet cracks; 5) microcracks in hermetic packages; 6) incomplete sealing in hermetic packages; 7) non-wetting issues in combo lid solder seals

See Also:  Failure Analysis;  All FA Techniques;  Optical Inspection;  SEM/TEM;

FA Lab Equipment;  Basic FA Flows;  Package Failures;  Die Failures