Page 238 - An Introduction to Microelectromechanical Systems Engineering

CHAPTER 8
Packaging and Reliability Considerations

for MEMS

“Reality has surpassed fantasy. We’re like kids in a candy store.”
—Art Thompson, tactical activity lead of the NASA Mars Exploration
Rovers mission after landing on Mars, January 2004.

Packaging is the process, industry, and methods of “packing” microelectromechani-
cal components and systems inside a protective housing. Combining engineering
and manufacturing technologies, it converts a micromachined structure or system
into a useful assembly that can safely and reliably interact with its surroundings.
The definition is broad because each application is unique in its packaging require-
ments. In the integrated circuit industry, electronic packaging must provide reliable
dense interconnections to the multitude of high-frequency electrical signals, as well
as extract excessive heat from the chips. By contrast, MEMS packaging must
account for a far more complex and diverse set of parameters. It must first protect
the micromachined parts in broad-ranging environments; it must also provide inter-
connects to electrical signals and, in most cases, access to and interaction with the
external environment. For example, the packaging of a pressure sensor must ensure
that the sensing device is in intimate contact with the pressurized medium yet pro-
tected from exposure to any harmful substances in this medium. Moreover, packag-
ing of valves must provide both electrical and fluid interconnects, and packaging of
lasers must allow for optical fibers. As a consequence of these diverse requirements,
standards for MEMS packaging lack, and designs often remain proprietary to com-
panies. Invariably, the difficulty and failure in adopting standards implies that pack-
aging will remain engineering-resource intensive and thus will continue to carry
rather high fixed costs.
Packaging is a necessary “evil.” Its relatively large dimensions tend to dilute the
small-size advantage of MEMS. It is also expensive: the cost of packaging tends to
be significantly larger than the cost of the actual micromachined components. It is
not unusual that the packaging content is responsible for 75% to 95% of the overall
cost of a microelectromechanical component or system. These factors, prevalent in
the early days of electronic integrated circuits, contributed towards large-scale inte-
gration in that industry in order to minimize the impact of packaging on overall
cost, size, and performance. High-density packaging methods, such as surface
mount technologies (SMT), are today at the core of advancements in electronic
packaging. By contrast, the evolution of MEMS packaging is slow and centers
largely on borrowing from the integrated circuit and other industries in an effort to
benefit from the existing vast body of knowledge. Whether sophisticated packaging

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