Page 69 - MEMS Mechanical Sensors

58 Mechanical Sensor Packaging

4.2 Standard IC Packages

From a cost point of view, it would certainly be advantageous if the mechanical
sensor die could simply be mounted in one of the many standard IC packages
available. These can be grouped into three types: ceramic, plastic, and metal. The
functional requirements of microelectronics packages are to enclose the IC in a
protective shell, to provide electrical connection from the IC to circuit board,
and to enable adequate heat transfer. Key considerations in the design of an
IC package are reliability (affected by packaging stresses and moisture ingress),
heat flow, ease and cost of manufacture, and electrical characteristics such as
lead resistance, capacitance, and inductance. For further information refer to
Tammala et al. [1].

4.2.1 Ceramic Packages
Ceramic materials have been used to make a wide range of package types and,
although more expensive than their plastic counterpart, possess an unrivaled range
of electrical, thermal, and mechanical properties. Ceramics packages can be her-
metically sealed and can be made very small with large numbers of reliable electrical
interconnects. A wide variety of ceramic packages have been developed, including
basic dual in-line packages (DIPs), chip carriers, flat packs, and multilayer packages.
Such packages are used in high-performance applications where the increased cost
can be justified. The most common ceramic materials used are alumina (Al O ), alu-
2 3
mina/glass mixtures, aluminum nitride (AlN), beryllium oxide (BeO), and silicon
carbide (SiC).
Two approaches are used in the fabrication of ceramic packages. The first
approach uses a mixture of ceramic and binders, which are molded into shape using
a dry pressing process, and then sintered to form the finished component. A ceramic
package is formed by sandwiching a metal leadframe between two such dry pressed
ceramic components (the base and the lid). The three-layer package is held together
hermetically by glass frit reflowed at temperatures between 400°C and 460°C.
These pressed ceramic packages are lower in cost that the laminated multilayer
package, but their simple construction limits the number of possible electrical fea-
tures and interconnects. DIP packages fabricated in this manner are commonly
known as CerDIPs.
The second approach is based upon a multilayer ceramic (MLC) structure.
These are made from layers of unfired (green state) ceramics metallized with screen-
printed tungsten patterns, which are then fired under pressure at high temperature
(~1,600°C). Exposed metal features are electroplated with nickel and gold. Metal
components, such as the contact pins, are attached using a copper-silver alloy braze.
The laminated structure allows the package designer to incorporate electrical fea-
tures into the package itself. Such MLC packages can be used for individual die or
for mounting multiple die, known as multichip modules (MCMs). This approach
can improve systems performance and can reduce the number of interconnects
required at the circuit board level to a workable amount. Multilayer packages can
now be produced with as many as 70 layers. MCMs can be used to package MEMS
devices, and this is discussed further in Section 4.4.