ORIGINS OF HERMETICITY
Hermeticity has lately become an elusive concept in cavity micropackaging, with a definition as hard to pin down as the Greek god whose name it bears. “Air-tight” hermetic sealing may be traced back at least to the experiments of Robert Boyle in the 1660’s. Hermetic cavity micropackages are a bit more recent, developed since the 1950’s. Their original purpose was to protect semiconductor devices from excessive moisture. The driving need was high-reliability devices for the then-dominant military and aerospace applications. These require decades-long operating lifetimes, while subjected to uncontrolled environmental extremes, such as bullets. Fortunately, the developers were not constrained by commercial budgets.
The concept was simple and the implementation satisfactory. Large, heavy, expensive packages were made from metal or dense ceramic, materials having low moisture permeability. Packages were sealed in a controlled atmosphere, typically by brazing or soldering a metal lid onto the package. Package and seal integrity were 100% tested by a variety of procedures described in military standards.
This happy melding of Military, Metallurgy and Manufacturing has served its intended purpose well for many years. Indeed, some military and aerospace electronics equipment still in use today may include hermetic packages older than the operators. Meanwhile, the fourth M, Money, led manufacturers to less costly non-hermetic packaging solutions, such as the ubiquitous molded plastic package, for budget-sensitive applications.
HERMETICITY: THE TESTS THAT FAILED
The past decade exposed some holes in hermetic cavity leak tests. In the mid-nineties, the most used method for cavity leak testing failed when leak-testing large-volume cavities. Large cavities, such as those for microwave transmit/receive modules, have too great an internal volume to be tested by the pressurized helium leak test described in MIL-STD-883, Method 1014. The minute amount of helium penetrating the cavity through a small leak becomes highly diluted by the large cavity volume. The resulting low concentration of helium, upon leaking back out of the cavity into the leak detector, can be below the detector limit, giving a false assurance of hermeticity. [1, 2]
More recently, a mirror image of this large volume problem was found to occur in very small volume cavities. The quantity of helium that can be contained in a nano-liter cavity is so small that, within minutes of its removal from the pressure chamber, not enough helium remains in a leaking cavity to trigger the detector. 
A third leak test limit results from the push towards sustaining high cavity vacuums over long periods of use (30 years). Analogous to the large-cavity case, leak rates too large to meet this goal can result from holes too small to provide detectable helium. 
ALTERNATIVE HERMETIC UNIVERSES
New leak test methods, which avoid the above limitations, are being explored, and some will succeed. However, the cookie-concept of establishing cavity hermeticity by merely leak testing is crumbling under the combined weight of new micropackaging technologies and growing market needs.
Twenty-first century micropackaging is far more complex and varied than that of 1950. The package content now may not be silicon; if silicon, it may not be a microcircuit. The structures may be mechanical, electrical, optical, or even a blend of all three. Potential cavity contaminants may include hydrogen, carbon dioxide, organic residues, chemically-reactive gases – or plain old water vapor. Contaminants may originate from leaks, or from permeability, outgassing, surface desorption, or similar time-dependent processes.
In today’s throw-away society, product life expectancies for consumer electronic products are shrinking towards less than five-year lifetimes. Neither the hermeticity nor the cost required for a thirty-year lifetime suits today’s consumer markets for cell phones, digital cameras, computers, pagers. There, a briefer life amid milder surroundings opens the competition to lower cost packaging adequate for the intended purpose. Cavity packages now comprise a broader spectrum, ranging from non-hermetic, quasi-hermetic, near hermetic, hermetic, to ultra-hermetic.
QUASI? WHO’S QUASI?
The quasi-hermetic package offers hermeticity over a limited period. “Quasi” is a well-chosen term for this approach, since the word defines something “having some, but not all of the features of” the original. 
Package materials are still the traditional moisture-impermeable metal, ceramic, or glass, but the lid is sealed with an organic adhesive. This seal is permeable to moisture, but it comprises only a small fraction of the internal cavity area, and the seal can be designed to minimize permeability.
The first quasi-hermetic cavity package was introduced, as a potential cost reduction, in 1980.  Currently, quasi-hermetic sensor packages consisting of adhesive-sealed glass are in high volume consumer products such as digital cameras.  Recent computer modeling of a specific quasi-hermetic MEMS package showed potential operating lifetimes of two to five years, dependent upon duty cycle. 
“Near-hermetic” describes a recent cavity packaging development with potentially wide application. The package is comprised of liquid-crystal polymer (LCP), a newly perfected organic packaging material, with permeability close to that of glass. LCP permits near-hermetic cavity packages to offer the low cost and weight of plastic, with simple shaping and sealing methods. These near-hermetic packages pass the present hermeticity leak test, and provide acceptable cavity hermeticity for periods suitable for most commercial applications. [9, 10]
OUR HERMETIC FUTURE
The archaic concept of cavity hermeticity needs a radical makeover. Hermeticity is no longer an absolute term. It has mutated to a relative and flexible term, describing varying degrees of cavity protection from specified gases for specified times.
Perhaps our twenty-first century cavity package specifications should be called “hygienic,” rather than hermetic. The word hygienic means “promoting good health and long life.” Hygienic cavity package specifications will anticipate the threats to long-term stability in a particular application. The specifications will describe an appropriate level of protection to meet performance, cost, and reliability goals over the design lifetime. The hygienic specifications may detail the allowable cavity atmosphere, both from internal and through-package contributions. They must detail suitable tests for assuring quality and reliability.
A horde of standards-committee members and specification writers should now scramble to elevate this twenty-first century standard!
Technical information and papers provided by the following are gratefully acknowledged. Any errors or misinterpretations here are entirely my own, unless I find plausible deniability.
Piet De Moor, IMEC (Belgium)
Brian Farrell, Foster-Miller, Inc.
Tom Marinis, Draper Laboratories
Bruce Romanesko, Johns Hopkins University Applied Physics Laboratory
1. B. M. Romanesko and K. J. Ely, “Leak Rate Measurement Comparisons,” Proceedings 45th IEEE Electronic Components and Technology Conference, Las Vegas, Nevada, May 21-25, 1995, p.315.
2. B. M. Romanesko and K. J. Ely, “Helium Fine Leak Testing of Large Packages – Method 1014 Shortcomings,” Proceedings of the Sixth International Workshop on Moisture in Microelectronics, October 15 – 17, 1996, in Moisture in Microelectronics VI, M. Schen and B. Moore (eds), NISTIR 5960, 1997.
3. A. Jourdain, et. al., “Investigation of the hermeticity of BCB-sealed cavities for housing RF MEMS Devices,” MEMS 2002 IEEE International Conference, pp.677-80.
4. T. F. Marinis & J.W. Soucy, “Vacuum packaging of MEMS inertial sensors,” Proc. IMAPS 2003 International Symposium, Boston, MA, Nov. 18-20, 2003, pp 386-391.
5. Random House Dictionary of the English Language, 2nd Ed., Random House, New York, 1987.
6. I. Memis, “Quasi-Hermetic Seal for IC Modules,” 30th Electronic Components Conference, 1980, pp 121 – 127. Cited in Microelectronics Packaging Handbook, R. Tummala & E. Rymaszewski (eds) Van Nostrand Reinhold, New York, 1989, page 254.
7. ShellCase Ltd., www.ShellCase.com
8. J. Jacobs and J. Malone, “Suitability of an Epoxy Seal for a MEMS Package,” Proc. IMAPS International Symposium, Boston, MA, September, 2000, page 586.
9. B. Farrell et al., “The liquid crystal polymer packaging solution,” Proc. IMAPS 2003 International Symposium, Boston, MA, Nov. 18-20, 2003, pp 18-23.
10. J. Roman, “Near Hermetic Air Cavity Packaging for MEMS,” IMAPS 5th Topical Workshop on MEMS, Related Microsystems, and Nano-packaging, Boston, MA, Nov. 20, 2003.