A method for protective packaging of multichip modules and related assemblies of microelectronic circuitry involves coating the assemblies with composite organic/inorganic layers only 1 to 2 mils (0.025 to 0.05 mm) thick. The method is suitable for a variety of advanced packages of microelectronic circuitry, including "chip-on-flex" circuitry, "smart" cards, flip-chips, flip-flips (flip-chips assembled onto ball-grid-array substrates), and such three-dimensional assemblies as stacked memory arrays.
Older methods for protective packaging of electronic circuitry include the following:
- Conventional hermetic sealing in metal or ceramic. This method provides effective protection. However, conventional hermetic enclosures add considerable weight and are relatively bulky [0.15 to 0.25 in. (4 to 6 mm) deep]; thus, conventional hermetic sealing defeats advances in miniaturization.
- Encapsulation in epoxy. Epoxy encapsulants can be applied to depths about one-fourth of those of conventional hermetic packages, but even at these depths, they add unacceptable amounts of weight and bulk. Epoxies are too rigid for use on the new generation of flexible multichip modules. Epoxies are also susceptible to penetration by moisture; in other words, they do not necessarily protect the packaged circuitry against moisture.
- Vapor deposition of a thin film of parylene (a thermoplastic polymer made from para-xylene). Such a film is susceptible to penetration by moisture and to thermal oxidation at temperatures greater than approximately 120 °C.
In the present method, one coats the assembled circuitry with a thin film of parylene, followed by a thin film of silicon oxide or silicon nitride (see figure). Both the organic (parylene) and inorganic (silicon-based) films are deposited at relatively low temperatures (between 25 and 100 °C). The outer inorganic film acts as a barrier to moisture and protects the underlying organic film against oxidation at temperatures up to 200 °C or even somewhat higher. The thin composite organic/inorganic film thus affords almost as much protection as does heavier, bulkier conventional hermetic packaging. The cost of depositing the composite organic/inorganic film is a small fraction of the cost of conventional hermetic packaging.
Although two-layer coats of the type described above are viable, the inorganic outer layers can be broken by mechanical handling. Therefore, it can be desirable to deposit a third (intermediate) layer for protection against mechanical stress (the third layer also provides additional protection against oxidation). For example, one can deposit a base organic layer of Parylene C (a commercial type of parylene), followed by a second organic layer of Cyclotene [or equivalent poly(benzocyclobutene)], followed by an outer inorganic layer.
In the three-layer case described above, one must take special care to cure the second organic layer according to the manufacturer's specifications, to (1) avoid compromising the base organic layer, (2) ensure a full cure to make the second organic layer relatively invulnerable to oxidation, and (3) ensure a smooth, hard second organic layer, over which the final inorganic layer can act as an effective barrier against oxygen. The basic concept can be extended to four or more layers, provided that due consideration is given to adhesion and compatibility between layers. The concept can also be extended to include other materials: For example, other inorganic coating materials that have been considered but not yet evaluated include silicon carbide, silicon oxynitride, and zirconium oxide.
This work was done by Frederick Pool and James Licari of Caltech for NASA's Jet Propulsion Laboratory. NPO-20304