Chapter 22 Materials Selection and Design Considerations
Chapter 22 Materials Selection and Design Considerations
Chapter 22 Materials Selection and Design Considerations
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W126 • <strong>Chapter</strong> <strong>22</strong> / <strong>Materials</strong> <strong>Selection</strong> <strong>and</strong> <strong>Design</strong> <strong>Considerations</strong><br />
The material used to encapsulate the package should:<br />
1. Be electrically insulating<br />
2. Be easily molded to the desired shape around the chip die <strong>and</strong> its wire leads<br />
3. Be highly impervious to the penetration of moisture <strong>and</strong> contaminants<br />
4. Be able to form strong adhesive bonds with the chip surface, wires, <strong>and</strong> other<br />
leadframe components<br />
5. Exhibit mechanical <strong>and</strong> chemical stability for the expected lifetime of the<br />
package<br />
6. Not require exposure to excessively high temperatures during installation<br />
7. Have a coefficient of thermal expansion similar to those of other package<br />
components to avoid thermal stresses capable of fracturing the wire leads<br />
Figure <strong>22</strong>.33 shows a schematic diagram of an encapsulated IC package.<br />
Both ceramic <strong>and</strong> polymeric materials are used to encapsulate IC packages; of<br />
course each of these material types has its own set of assets <strong>and</strong> liabilities. Ceramics<br />
are extremely resistant to moisture penetration <strong>and</strong> are chemically stable <strong>and</strong><br />
chemically inert. Glasses are the most commonly utilized ceramic materials. The<br />
principal disadvantage of glass is the requirement that it be heated to moderately<br />
high temperatures to lower its viscosity to the point where it will flow around <strong>and</strong><br />
make intimate contact with all of the wires that are microjoined to the chip surface.<br />
Some common glass constituents should be avoided (notably Na 2O <strong>and</strong> K 2O) since<br />
volatile cation species (Na � <strong>and</strong> K � ) may be emitted from the molten glass. These<br />
species are notorious in accelerating corrosion reactions, <strong>and</strong> the ions will degrade<br />
the chip performance.<br />
Polymeric materials are used in the largest volume for packaging encapsulation<br />
because they are not as costly as the ceramics, <strong>and</strong> they may be produced in a lowviscosity<br />
state at lower temperatures. Epoxies <strong>and</strong> polyurethanes are commonly<br />
used, with the former being the most common. However, these materials have a<br />
tendency to absorb water <strong>and</strong> do not form moisture-tight bonds with the lead wires.<br />
Some of these polymers require curing at a temperature on the order of 150�C, <strong>and</strong><br />
during cooling to room temperature will shrink more than other package components<br />
to which they are attached. This difference in amounts of contraction can give<br />
rise to mechanical strains of sufficient magnitude to damage the connecting wires<br />
as well as other electronic components. The addition of appropriate fillers (such as<br />
fine silica or alumina particles) to the polymer can alleviate this problem but often<br />
IC chip Encapsulation<br />
Connecting<br />
wire<br />
Encapsulation Leadframe<br />
Figure <strong>22</strong>.33 Schematic diagram<br />
showing an encapsulated IC<br />
leadframe package. [Adapted<br />
from Electronic <strong>Materials</strong><br />
H<strong>and</strong>book, Vol. 1. Packaging,<br />
C. A. Dostal (Editor), ASM<br />
International, 1989, p. 241.]