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Types of Packaging Solutions 233

telecommunications industry. Solder is a common material to hermetically seal the
feed-through tube. The fiber plastic jacket is first stripped over a short distance,
exposing the glass core of the fiber, which is then metallized with layers of nickel and
gold. The fiber is subsequently inserted into the feed through, and solder is dispensed
through a tiny opening in the tube [see Figure 8.7(b)]. Occasionally, an intermediate
metal ferrule is used between the fiber and the feed through [16]. Indium-based
solders, such as In 97%/Ag 3% or In 80%/Pb 15%/Ag 5%, are common, as they
offer good wetting and low melting temperatures (≤ 150ºC) to minimize the risk of
damage to the fiber. Because they are soft alloys, they exhibit little stresses at the fiber
surface.

Types of Packaging Solutions

In its basic form, a package is a protective housing with an enclosure to hold one or
multiple dice forming a complete microelectromechanical device or system. The
package provides where necessary electrical, optical, and fluid connectivity between
the dice and the external world.
In some cases, it is advantageous to provide a first level of packaging (chip- or
die-level encapsulation) to the micromechanical structures and components [17].
This is particularly of interest in applications where the surfaces of the microstruc-
tures need not be in direct exposure to liquids or gases. A top silicon cap attached, for
example, by silicon fusion bonding can maintain a hermetic seal and hold a vacuum
while protecting the sensitive microstructures from damage during saw and assem-
bly. A top cap also allows the use of plastic molding, ubiquitous in low-cost packag-
ing solutions. In this method, molten plastic flows under high pressure, filling the
inner cavity of a mold and encapsulating a metal lead frame. The die or capped
microstructure rests upon this frame. For example, a crystalline silicon cap protects
the sensing elements of the accelerometer from VTI Technologies (see Chapter 4)
during molding of the plastic package over the die. Fixed to ground potential, the cap
also becomes an effective shield against electromagnetic interference [18].
There are three general categories of widely adopted packaging approaches in
MEMS. They are ceramic, metal, and plastic, each with their own merits and limita-
tions (see Table 8.4). For instance, plastic is a low-cost, oftentimes a small-size (sur-
face mount) solution, but it is inadequate for harsh environments. The asking price
for a plastic packaged pressure or acceleration sensor is frequently below $5. By
contrast, a similar sensor packaged in a hermetic metal housing may cost well over
$30. It is not surprising that packaging is what frequently determines economic
competitiveness.

Ceramic Packaging
Ceramics are hard and brittle materials made by shaping a nonmetallic mineral,
then firing at a high temperature for densification. The vast majority of ceramics are
electrical insulators and often are good thermal conductors (see Table 8.5). Ease of
shaping along with reliability and attractive material properties (e.g., electrical
insulator, hermetic sealing) have made ceramics a mainstay in electronic packaging.
They are widely used in multichip modules (MCM) [19] and advanced electronic