Chapter 22 Materials Selection and Design Considerations

W126 • Chapter 22 / Materials Selection and Design Considerations
The material used to encapsulate the package should:
1. Be electrically insulating
2. Be easily molded to the desired shape around the chip die and its wire leads
3. Be highly impervious to the penetration of moisture and contaminants
4. Be able to form strong adhesive bonds with the chip surface, wires, and other
leadframe components
5. Exhibit mechanical and chemical stability for the expected lifetime of the
package
6. Not require exposure to excessively high temperatures during installation
7. Have a coefficient of thermal expansion similar to those of other package
components to avoid thermal stresses capable of fracturing the wire leads
Figure 22.33 shows a schematic diagram of an encapsulated IC package.
Both ceramic and polymeric materials are used to encapsulate IC packages; of
course each of these material types has its own set of assets and liabilities. Ceramics
are extremely resistant to moisture penetration and are chemically stable and
chemically inert. Glasses are the most commonly utilized ceramic materials. The
principal disadvantage of glass is the requirement that it be heated to moderately
high temperatures to lower its viscosity to the point where it will flow around and
make intimate contact with all of the wires that are microjoined to the chip surface.
Some common glass constituents should be avoided (notably Na 2O and K 2O) since
volatile cation species (Na � and K � ) may be emitted from the molten glass. These
species are notorious in accelerating corrosion reactions, and the ions will degrade
the chip performance.
Polymeric materials are used in the largest volume for packaging encapsulation
because they are not as costly as the ceramics, and they may be produced in a lowviscosity
state at lower temperatures. Epoxies and polyurethanes are commonly
used, with the former being the most common. However, these materials have a
tendency to absorb water and do not form moisture-tight bonds with the lead wires.
Some of these polymers require curing at a temperature on the order of 150�C, and
during cooling to room temperature will shrink more than other package components
to which they are attached. This difference in amounts of contraction can give
rise to mechanical strains of sufficient magnitude to damage the connecting wires
as well as other electronic components. The addition of appropriate fillers (such as
fine silica or alumina particles) to the polymer can alleviate this problem but often
IC chip Encapsulation
Connecting
wire
Encapsulation Leadframe
Figure 22.33 Schematic diagram
showing an encapsulated IC
leadframe package. [Adapted
from Electronic Materials
Handbook, Vol. 1. Packaging,
C. A. Dostal (Editor), ASM
International, 1989, p. 241.]