They include treatment of the materials, cleaning of the materials, and controlling outgassing from the packaging materials. MEMS have moving parts and when melted plastic flows around them, they generally don't work anymore. Conventional hermetic packaging is not a practical solution. While hermetic sealing is frequently used in military and aerospace applications, where parts in a harsh environment require very high reliability and cost is no object, it is too expensive for the price goal MEMS manufacturers are trying to achieve. Cronos relay puts a heating coil under an actuator fastened at both ends.
Heat expands the beam driving the actuator forward, closing the contacts.
Many developers are considering a capping alternative in which the die becomes part of the package, as opposed to conventional hermetic sealing where the die is put into a separate package. Next, a cap, fabricated from the same material as the semiconductor substrate, would be placed over the MEMS devices and bonded to the substrate with organic adhesives, glass paste, ultrasonic welding, or another conventional bonding technique. Once the MEMS is protected, conventional assembly processes could be used to dice the wafer and wire-bond the parts, and the entire assembly could be placed into a plastic molded package. Those are very competitively priced devices in a mature market. MEMS will serve applications where the form factor won't allow a conventional device or where a large number of relays are needed in one area.