Page 70 - MEMS Mechanical Sensors

4.3 Packaging Processes 59

Metallization can be realized on ceramic packages using either screen-printed
thick-film or evaporated/sputtered thin-film technology. The thick-film approach
deposits the metal, or indeed dielectric if required, in the pattern required, but it has
traditionally been limited by poor resolution that yields typical line widths and
spacing of 150 µm. Recent developments in photoimageable inks, however, allow
line widths and spacing of 40 µm and 50 µm, respectively [2]. The thin-film
approach, which involves subsequent lithographic and etching processes, is capa-
ble of even finer line widths and spacing (< 20 µm). The processing involved is
not so straightforward and this approach is better suited to high-density, high-
performance applications.

4.2.2 Plastic Packages
Molded plastic packages were developed in order to reduce the cost of IC packag-
ing. At the center of a plastic package is a leadframe to which the die is attached and
electrical connections are made. The leadframe material is typically a copper alloy,
nickel-iron (the most widely used being alloy 42) or a composite strip (e.g., a copper
clad stainless steel) and the leadframe geometry is obtained by stamping or chemical
milling. The assembly is then encased in a thermoset plastic package using a transfer
molding process. The molding resins used are a mixture of various chemicals. These
have been developed in order to obtain the characteristics required by both the
process and application. These characteristics include viscosity, ease of mold
release, adhesion to leadframe, and low levels of ionic contamination. To prevent
difficulties in packaging and future reliability problems, the component materials
making up a plastic package must be chosen with care to avoid thermal expansion
coefficient (TEC) mismatches, to allow adequate thermal conduction away from the
IC, and to prevent moisture ingress.

4.2.3 Metal Packages
These are often used in military applications, since they offer the highest reliability
characteristics, as well as in RF applications. Electrical connections are made using
a metal feed-through and glass-to-metal seals. They are typically hermetically sealed
by welding, soldering, or brazing a lid over the package, which prevents moisture
ingress and resulting reliability difficulties (see Section 4.3.3). Common metals used
in the construction are Kovar, cold rolled steel, copper, molybdenum, and silicon
carbide reinforced aluminum. Hermetic seals can be formed. Common metal pack-
ages types are shown in Figure 4.1. Figure 4.2 shows a photograph of typical metal,
ceramic, and plastic packages.

4.3 Packaging Processes

Irrespective of the type of package used, the assembly of the packaged device
involves mounting the die, making electrical connections to the terminals provided,
and sealing the assembled package. Several standard processes have been developed
by the IC industry to meet these requirements, and these same processes are com-
mon to many MEMS packaging applications.