Page 261 - An Introduction to Microelectromechanical Systems Engineering

240 Packaging and Reliability Considerations for MEMS

Packaging solutions for harsh environments, namely those found in heavy
industries and aerospace, can be complex and costly. The custom requirements of
the application, coupled with the lack of high-volume market demand, have turned
packaging for harsh environments into a niche art. One particularly interesting
design is the metal packaging of media-isolated pressure sensors for operation in
heavy-industrial environments. The design immerses the silicon pressure sensor
within an oil-filled stainless steel cavity that is sealed with a thin stainless steel
diaphragm. The silicon pressure sensor measures pressure transmitted via the steel
diaphragm and through the oil. The robust steel package offers hermetic protection
of the sensing die and the wire bonds against adverse environmental conditions
(see Figure 8.13).
Each stainless steel package is individually machined to produce a cavity. The
die is attached to a standard header with glass-fired pins and wire bonded. This
header is resistance welded to the stainless-steel package. Arc welding of a stainless-
steel diaphragm seals the top side of the assembly. Oil filling of the cavity occurs
through a small port at the bottom that is later plugged and sealed by welding a ball.

Molded Plastic Packaging
Unlike metal or ceramic packages, molded plastic packages are not hermetic. Yet
they dominate in the packaging of integrated circuits because they are cost-effective
solutions (costing on average a few pennies or less per electrical pin). Advances in
plastic packaging have further improved reliability to high levels. Today’s failure
rates in plastic-packaged logic and linear integrated circuits are less than one failure
in every ten billion hours of operation [23].
There are two general approaches to plastic packaging: post molding and pre-
molding (see Figure 8.14). In the first approach, the plastic housing is molded after
the die is attached to a lead frame (a supporting metal sheet). The process subjects
the die and the wire bonds to the harsh molding environment. In premolding, the die
is attached to a lead frame over which plastic was previously molded. It is attractive

Steel diaphragm
Silicone oil

Weld joint

Wire bond
Silicon die

Steel housing

Sealed fill port
Glass fired pins
(a) (b)
Figure 8.13 (a) Photograph, and (b) cross-sectional schematic of a pressure sensor mounted
inside an oil-filled, stainless-steel package. Pressure is transmitted via the stainless-steel diaphragm
and through the oil to the silicon sensor. (Courtesy of: GE NovaSensor of Fremont, California.)