Page 262 - An Introduction to Microelectromechanical Systems Engineering

Types of Packaging Solutions 241

Die with first level silicon packaging
Plastic molding compound
Bond wire

Lead

Paddle

Metal lead frame

Figure 8.14 Schematic showing a sectional view of a post-molded plastic package. The die is first
mounted on a center platform (the paddle) and wires bonded to adjacent electrical leads. The
paddle and the leads form a metal lead frame, over which the plastic is molded. A MEMS die
should include a first level of packaging (e.g., a bonded silicon cap) as protection against the harsh
effects of the molding process. This particular illustration is of a plastic quad-flat pack (QFP) with
electrical leads along its entire outer periphery.

in situations where the risk of damaging the die is high or if openings through the
plastic are necessary (e.g., for pressure or flow sensors). However, it tends to be
more expensive than post molding.
The metal lead frame in either approach is an etched or stamped metal sheet
consisting of a central platform (paddle) and metal leads supported by an outer
frame. The leads provide electrical connectivity and emanate from the paddle in the
shape of a fan. The metal is typically a copper alloy or Alloy-42 (Ni Fe ); the latter
42 58
has a coefficient of thermal expansion 4.3 × 10 −6 per degree Celsius that matches
that of silicon.
In postmolded plastic packaging, the lead frame is spot-plated with gold or sil-
ver on the paddle and the lead tips to improve wire bonding. The die is then attached
with adhesive or eutectic solder. Wires are bonded between the die and the lead tips.
Plastic molding encapsulates the die and lead frame assembly but leaves the outer
edges of the leads exposed. These leads are later plated with tin or tin-lead to
improve wetting during soldering to printed circuit boards. Finally, the outer frame
is broken off and the leads are formed into a final S-shape (see Figure 8.14).
The sequence of process steps differs for premolded plastic packages. First, a
plastic body is molded onto a metal lead frame. The molded thermosetting plastic
polymer encapsulates the entire lead frame with the exception of the paddle and the
outer edges of the leads. Deflashing of the package removes any undesirable or
residual plastic on the die bonding areas. The molded body may contain ports or
openings that may be later used to admit a fluid (e.g., for pressure or flow sensing).
The lead frame is spot-plated with gold or silver to improve wire bonding and
soldering. At this point, the die is attached and wire bonded to the lead frame. A
protective encapsulant, such as RTV or silicone gel, is then dispensed over the die
and wire bonds. Finally, a premolded plastic cap is attached using an adhesive or
ultrasonic welding. If necessary, the cap itself may also contain a fluid access port
(Figures 8.15 and 8.16).
The molding process is a harsh process involving mixing the component for the
thermosetting plastic at approximately 175ºC, then flowing it under relatively high