Microelectronics Packaging:Hermetic Packages

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Hermetic Packages

The package environment has two sources: the ambient environment in which the package is placed and the internal environment that results from adsorbed/absorbed moisture and residual ionics from incomplete or inadequate rinsing. In hermetic packages, the packaged cavity is sealed such that external chemical species are prevented from entering into it. However, a finite rate of leakage occurs through diffusion and permeation. Water can dissolve salts and other polar molecules to form an electrolyte, which together with the metal conductors and the potential difference between them, can create leakage paths as well as corrosion problems. Moisture is contributed mainly by the sealing ambient, the absorbed and dissolved water from the sealing materials, lid and the substrate, and the leakage of external moisture through the seal. The permeability to moisture of glasses, ceramics, and metals, is orders of magnitude lower than that of any plastic material. Hence, the hermetic packages are those made of metals, ceramics, and glasses. The common feature of hermetic packages is using a lid or a cap to seal in the semiconductor device mounted on a suitable substrate. The leads entering the package are also hermetically sealed. For micro- wave circuits, seals to prevent the entry of undesirable liquids and gases as well as to provide EMI protection are required. The type of seal depends on the packaging of the electronics within and the environmental requirements.

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