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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W128 • <strong>Chapter</strong> <strong>22</strong> / <strong>Materials</strong> <strong>Selection</strong> <strong>and</strong> <strong>Design</strong> <strong>Considerations</strong><br />
Figure <strong>22</strong>.35<br />
Schematic diagrams<br />
showing (a) the cross<br />
section of an<br />
encapsulated TAB<br />
leadframe package,<br />
<strong>and</strong> (b) how bonding<br />
between the IC chip<br />
<strong>and</strong> a copper finger<br />
is achieved with a<br />
solder bump.<br />
[Adapted from<br />
Electronic <strong>Materials</strong><br />
H<strong>and</strong>book, Vol. 1,<br />
Packaging, C. A.<br />
Dostal (Editor),<br />
ASM International,<br />
1989, pp. 233, 234.]<br />
Solder joint<br />
Passivation<br />
layer<br />
Polymer film Encapsulation Solder bump<br />
Die attachment<br />
Solder bump<br />
IC chip<br />
IC chip<br />
Leadframe plate<br />
or substrate<br />
(b)<br />
(a)<br />
Contact pad<br />
Gold layer<br />
Copper finger<br />
Polymer<br />
film<br />
Leadframe<br />
finger<br />
The copper fingers are extremely narrow <strong>and</strong> positioned close together. Separation<br />
distances of the inner contact leads are on the order of 50 mm,<br />
which is much<br />
smaller than is possible for the stamped leadframe. Furthermore, each die chip contact<br />
pad is microjoined directly to one of these copper fingers, which eliminates the<br />
need for any connecting wires. The copper fingers are very thin, so that, for this direct<br />
bonding to be achieved, the chip pad bonding sites must be raised above the metallized<br />
coating. This is accomplished using “solder bumps,” which are normally layers of gold<br />
(or gold-plated copper) approximately 25 mm<br />
thick. Schematic representations illustrating<br />
this attachment design are presented in Figure <strong>22</strong>.35. The finger contacts are<br />
bonded to these raised bumps by soldering using a thermal-compression bonding tool.<br />
This tape-bonding design is fully automated in that all of the hundred or so microjoints<br />
can be made in a single step, a feature not possible with leadframes that require<br />
multiple wire-bonding operations.<br />
The packaging operation for the TAB leadframe is completed, as with the<br />
stamped leadframe, by encapsulation of the assembly (i.e., tape leadframe <strong>and</strong> its<br />
attached chip) within a fluid polymeric material that subsequently cures to form a<br />
protective shield. Protruding from this package are the copper finger conducting<br />
paths to which external electrical connections are made. Furthermore, excess heat<br />
generated by the chip must be dissipated along these copper fingers since the polymer<br />
tape backing does not provide an effective thermal conduction path because<br />
of its low thermal conductivity.<br />
The ultimate design goal of the IC package is to allow for the proper electrical<br />
operation of the packaged device. As frequencies <strong>and</strong> computing speeds creep<br />
ever higher, the mechanical <strong>and</strong> electrical design considerations of the package design<br />
must become more <strong>and</strong> more integrated. The overall electrical performance of<br />
the package is as important to the end user as the overall reliability.