Cavity packages

The first example is a technology originally patented by S. Suzuki et al., Hitachi Ltd., Japan [49]. Their solution for absolute pressure sensors, as well as for relative pressure sensors, is shown in a cross-sectional view in Figure 5.1.15. The piezoresistive silicon sensor element is anodically bonded to a thick glass part that constitutes the vacuum reference volume in the absolute pressure sensor or that contains a hole as a pressure inlet port for the relative pressure sensor. The pressure sensor die is typically housed in a cavity package with pressure inlet ports as part of the body. The surface of the sensor element is usually protected with a gel or other flexible material for corrosion resistance. 

Adhesives in both paste and film form have been used to attach and seal ceramic or metal lids to cavity packages that may also be ceramic or metal. These packages cannot be considered hermetic in the sense that seam-sealed, welded, or soldered fids are, but they are adequate for most commercial and consumer electronics where extreme moisture-temperature environments are not encountered. Packages sealed with Epo-Tek H77 epoxy adhesive are reported to provide near-hermetic sealing of hybrid microcircuits, integrated-circuit devices, and MEM packages withstand 100 hours of 95% relative humidity."... 

The prime function of adhesives is to mechanically attach or bond devices, components, heat sinks, wire, connectors, and other parts onto a circuit board or an interconnect substrate. Adhesives are also used as pastes or films to attach lids in sealing cavity packages and as dielectric films in fabricating multilayer interconnect substrates. The most important consideration in obtaining a reliable adhesive bond is the ability of the adhesive to flow and wet the surfaces. For a reliable bond, strong adhesion to both surfaces and strong cohesion within the adhesive are necessary. 

Figure 5.29 Cross-section of cavity package with epoxy-attached lid. Source Epoxy Technology.

Unlike the package s exterior regions, the electric fields within the die cavity are too large to be altered significantly by minor changes in the structure. In this case, encapsulants with relatively high field strength (>15 MV/m) are needed for the... 

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