Page 263 - An Introduction to Microelectromechanical Systems Engineering

242 Packaging and Reliability Considerations for MEMS

Encapsulation gel
Premolded plastic cap

Bond wire Adhesive/epoxy

Metal lead frame

Adhesive die attach Premolded plastic body
Pressure sensing die
Figure 8.15 Illustration of a premolded plastic package [24]. Adapting it to pressure sensors
involves incorporating fluid ports in the premolded plastic housing and the cap.

5mm

Figure 8.16 Photograph of the NovaSensor NPP-301, a premolded plastic, surface mount
(SOIC-type) and absolute pressure sensor. (Courtesy of: GE NovaSensor of Fremont, California.)

pressure (~ 6 MPa) into the mold cavity before it is allowed to cool. The plastic
material is frequently an epoxy. Novolac epoxies are preferred because of their
improved resistance to heat. The temperature cycle gives rise to severe thermal
stresses due to the mismatch in coefficients of thermal expansion between the plas-
tic, the lead frame, and the die. These stresses may damage the die or cause localized
delamination of the plastic. The material properties of the plastic, and especially its
coefficient of thermal expansion, are carefully adjusted by the introduction of addi-
tives to the epoxy. Fillers such as glass, silica, or alumina powder make up 65% to
70% of the weight of the final product and help tailor its coefficient of thermal
expansion as well as its thermal conductivity. In addition, mold release agents (e.g.,
synthetic or natural wax) are introduced to promote releasing the plastic part from
the mold. Flame-retardant materials, typically brominated epoxy or antimony triox-
ide, are also added to meet industry flammability standards. Carbon and other
organic dyes give the plastic its all-too-common black appearance, which is neces-
sary for laser marking.