Hermetic Packaging for Highly Space-Constrained Applications

Hermetic packaging is required for many electronic components, such as crystals, MEMS inertial sensors and resonators, lithium ion batteries, relays, and various types of chemical sensors. In addition, integrated circuits and multichip modules, which are used in critical military applications, require hermetic packaging to insure stable, reliable operation over many years in a wide range of environmental conditions. Traditionally, devices are hermetically packaged in co-fired ceramic chip carriers with braze attached covers, or machined Kovar boxes with glass sealed feed-throughs. The volume of devices packaged with these technologies typically exceeds that of the functional components, by a factor of three or four.

We have been successful in hermetically packaging small, low I/O components by using an electroformed can that is soldered to a metalized film of liquid crystal polymer. Cans with volumes as small as five cubic millimeters, and with walls 50 micron thick, have been fabricated in pure copper, copper with a finish of gold over nickel, and pure gold metals. The cans are manufactured in an array format, with as many as 36 cans per sheet, to facilitate batch assembly and sealing processes. The liquid crystal polymer covers are also fabricated in an array format, which aligns with that of the cans. The film is metallized on both sides and patterned in such a way as to minimize the area of exposed polymer, while maximizing the distance that gases much diffuse through the polymer to enter the package. Both solder paste and performs have been used to tin the covers before sealing. Reflow is accomplished by aligning the covers and cans, and clamping in place with a glass plate and then rapidly heating the assembly with a flash lamp.

Calculations predict leakage rates in these packages that are comparable to those of conventional hermetic packages. These calculations have been validated using standard gross and fine leak tests as well as aging tests on packages containing lithium metal. We believe that this technology can be adapted to larger devices with higher I/O, such as hybrid circuits and multichip modules.