High-reliability plastic aims to replace ceramic in military packages
Light weight and low cost suit improved plastic for all but most crucial
packaging needs
DAVID LOCKE
American Microsystems, Inc.
Pocatello, ID
IC developers who need to meet stringent reliability requirements have
traditionally chosen ceramic packages. But shrinking military budgets and
increasing competition within the commercial aerospace industry are forcing
both government and industry to look closely at the new, high-reliability
plastic packages.
Plastic IC packages have several key advantages when compared to ceramic
packages. They are lighter, smaller, and less expensive. And–because plastic
is less brittle than ceramic–they can withstand physical shocks and G-forces.
Moreover, process improvements are overcoming the traditional temperature and
hermeticity problems of the material.
Plastic advantages
Cost is the most significant advantage of plastic IC packages over ceramic. The
cost-effectiveness of plastic packages is due to the lower cost of the raw
materials, the simplicity of the manufacturing process, and the high volume of
plastic packages produced at one time.
In general, a plastic package weighs about two-thirds as much as an equivalent
ceramic package. Although small compared to most of the products that will use
the ICs, the actual weight difference can be significant in commercial
aerospace and military applications. Every gram shaved from a commercial
jetliner results in significant fuel savings gathered over the plane's
lifetime.
The weight savings achieved by today's plastic packages increase as pin counts
go up. This can be important in systems with size constraints, such as laptop
computers and communication devices like cellular phones. These applications
don't typically encounter the stringent reliability standards that require
ceramic packages. However, in devices such as military ordnance–including
cruise missiles and smart-bombs–the tradeoff between size and reliability may
be critical.
Manufacturing the plastic package is a simple one-step process: the plastic is
molded around the die and lead frames (see Fig. 1). The lead frames belonging
to different ICs are connected to one another in long strips, allowing for tens
to hundreds of ICs to be packaged at once, with a resultant savings from the
economies of scale.
By comparison, a ceramic package requires more expensive materials and a more
complex manufacturing process. Why ceramic costs more is explained in the
accompanying box. Figure 2 shows that the difference in cost between plastic
and ceramic packages increases exponentially as pin counts rise.
Since the plastic package manufacturing process is standard, whole families of
off-the-shelf lead frames exist that can be used with different ICs. For,
example, a standard 28 x 28-mm package can be used with several off-the-shelf
lead frames, depending on the pin count desired.
Besides size and manufacturing advantages, plastic packages can also better
withstand physical shocks and G-forces than ceramics. This is because the
plastic package is molded around the die and wires, creating a solid, immovable
unit. By comparison, in ceramic packages both the die and the wires that attach
to the die are free, rather than being molded to the ceramic. Thus, physical
shocks may cause them to short together. The plastic's superior ability to
withstand physical shocks could be important on a battlefield, where equipment
requiring a high-reliability design is likely to suffer ballistic shocks as
well.
Plastic drawbacks
With all the advantages of plastic packages, one may ask why ceramic packages
are used at all. Historically, there are three basic reasons: ceramic
withstands a greater number of repeated temperature fluctuations than plastic;
ceramic provides a greater level of hermeticity; and ceramic remains functional
at higher power levels and temperatures.
Fortunately, recent developments in high-reliability plastics have eliminated
the first two concerns. The third concern, high-temperature operations, applies
to only a few IC applications.
Recent process developments have resulted in new, high-reliability packages
that meet the physical demands of most commercial aerospace and military
equipment. The plastic is created by coating the silicon die with a polyimide
film before the die is molded to the plastic. This polyimide fills in the hills
and valleys on the surface of the die, creating a planar surface on which the
plastic can expand and contract without damage. The polyimide itself has a
thermal coefficient of expansion close enough to silicon that it poses no
threat.
Eventually, this polyimide coating breaks down after several thousand
temperature cycles. But while a ceramic package can withstand more temperature
cycles than a high-reliability plastic, such added reliability extends the IC's
lifetime far past the lifetime of any conceivable system. It adds little value
to the IC.
Hermeticity
Unlike ceramic packages that are hermetically sealed to resist moisture,
plastic packages have traditionally experienced hermeticity problems. However,
improvements in the plastic packaging process have eliminated hermeticity as a
concern.
At American Microsystems, Inc. (AMI), testing results demonstrate the useful
life of AMI's plastic encapsulated microcircuits (PEMs) is at least 50 years in
environments as extreme as 104 degreesF and at 85% humidity–way beyond the
necessary life of most circuits.
At AMI, such high-reliability IC packages have been tested and found able to
withstand 1,000 temperature cycles over a -65 degrees to +150 degreesC,
simulating 20 years of active life of a commercial airliner. The price of the
high-reliability process is small, about $10 to $30 to coat a wafer containing
as many as 1,000 individual ICs. This is insignificant compared to the total
cost savings that can be gained through switching to plastic packages, a
savings that increases as the total pin count goes up.
AMI is now developing plastic IC packages for a major DoD ordnance program.
Using high-reliability plastic packages for this IC will save the DoD more than
$10 million. Additional use of high-reliability plastic packages in these
systems will save over $50 million.
The proliferation of high-reliability plastic packages in military/aerospace
applications should also get a boost from the government's Qualified
Manufacturers Listing (QML) program. Under QML, a supplier is validated to use
its existing quality system and best commercial practices that meet or exceed
military requirements. AMI is setting up its high-reliability plastic process
according to QML guidelines and expects to be in accordance with QML by early
1995. As a result, the inherent cost savings associated with plastic packages
will be enhanced by a military specification program that lowers the cost of
product qualification.
BOX:
Why ceramic costs more
That eramic packages cost more than plastic ones can be traced to more
expensive materials and a more complex manufacturing process. A ceramic package
has at least three layers: a die-attach pad at the bottom; a die-attach cavity
in the middle with a pattern printed in tungsten connecting the package pins to
the cavity; and a cavity and a seal ring on top, on which an expensive
nickel-gold alloy lid is attached (see diagram).
The manufacturing process for ceramic packages requires multiple steps, as
shown in the table. The manufacturer starts with a paste known as “green
ceramic.” Expensive tooling equipment individually punches each of the three or
more ceramic layers out of this paste. The layers are joined and the package is
fired at well over 1,000 degreesC–a process lasting as long as several days.
After firing, the die is placed inside the package, the wires are bonded to the
die, and the lid is attached to the top of the ceramic. The complexity of the
ceramic package's manufacture demands that each package must be produced
independently of one another.
CAPTIONS:
Opening shot: (color slide.)
High-reliability plastic packages meet the reliability requirements of
MIL-STD-883, at a potential cost savings of 25% to 50% over ceramic packages.
The ARINC-629 terminal controller IC in the bottom right resides next to the
larger ceramic packaged version.
Fig. 1. Manufacturing the plastic package involves molding plastic around the
die and lead frames. Strips of lead frames allow many ICs to be packaged at
once, reducing production costs.
Fig. 2. The cost difference between plastic and ceramic packages increases
exponentially as pin counts go up.
Box diagram caption:
Ceramic packages have at least three layers: a die-attach pad, a die-attach
cavity with tungsten interconnect pattern from the package pins to the cavity,
and a layer with a cavity and a seal ring.
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