Chapter 22 Materials Selection and Design Considerations

W128 • Chapter 22 / Materials Selection and Design Considerations
Figure 22.35
Schematic diagrams
showing (a) the cross
section of an
encapsulated TAB
leadframe package,
and (b) how bonding
between the IC chip
and a copper finger
is achieved with a
solder bump.
[Adapted from
Electronic Materials
Handbook, Vol. 1,
Packaging, C. A.
Dostal (Editor),
ASM International,
1989, pp. 233, 234.]
Solder joint
Passivation
layer
Polymer film Encapsulation Solder bump
Die attachment
Solder bump
IC chip
IC chip
Leadframe plate
or substrate
(b)
(a)
Contact pad
Gold layer
Copper finger
Polymer
film
Leadframe
finger
The copper fingers are extremely narrow and positioned close together. Separation
distances of the inner contact leads are on the order of 50 mm,
which is much
smaller than is possible for the stamped leadframe. Furthermore, each die chip contact
pad is microjoined directly to one of these copper fingers, which eliminates the
need for any connecting wires. The copper fingers are very thin, so that, for this direct
bonding to be achieved, the chip pad bonding sites must be raised above the metallized
coating. This is accomplished using “solder bumps,” which are normally layers of gold
(or gold-plated copper) approximately 25 mm
thick. Schematic representations illustrating
this attachment design are presented in Figure 22.35. The finger contacts are
bonded to these raised bumps by soldering using a thermal-compression bonding tool.
This tape-bonding design is fully automated in that all of the hundred or so microjoints
can be made in a single step, a feature not possible with leadframes that require
multiple wire-bonding operations.
The packaging operation for the TAB leadframe is completed, as with the
stamped leadframe, by encapsulation of the assembly (i.e., tape leadframe and its
attached chip) within a fluid polymeric material that subsequently cures to form a
protective shield. Protruding from this package are the copper finger conducting
paths to which external electrical connections are made. Furthermore, excess heat
generated by the chip must be dissipated along these copper fingers since the polymer
tape backing does not provide an effective thermal conduction path because
of its low thermal conductivity.
The ultimate design goal of the IC package is to allow for the proper electrical
operation of the packaged device. As frequencies and computing speeds creep
ever higher, the mechanical and electrical design considerations of the package design
must become more and more integrated. The overall electrical performance of
the package is as important to the end user as the overall reliability.