Page 64 - Wire Bonding in Microelectronics

Ultrasonic Bonding Systems and Technologies 43

that require epoxy underfills, flip chips are spaced ~0.5 mm (20 mil)
apart. A reasonable amount of heat can be conducted out through the
chip’s front surface by the solder bumps to the package below.
However, very high heat (generated in the fastest devices) must be
removed from the back of the die (which faces upward). This can
involve elaborate heat-conducting fins/rods and expensive pack-
ages, although heat-sink attachment with silicone grease or polymer
has become a cheaper alternative. The package and the I/O pads
must be designed around the specific intended bumped die, and this
implies very high volume or high costs. Recently, flip chips have
been used on laminate (PCB) substrates with polymer under-filling
to correct the CTE mismatch. These polymer substrates reduce the
package cost, but may further limit the heat dissipation. Other efforts
to reduce the process costs have led to bond-pad bumping with TS
ball bonds (and removing the wire) to the existing peripheral Al
bond pads. Also, conductive polymers or microballs have been used.
To take full advantage of flip-chip technology, it is necessary to rede-
sign existing chips for area-array I/O FC pads, which cannot then be
effectively wire bonded for use in normal packages (although area
array autobonding is being developed). Such redesigning originally
slowed the use of flip-chip technology, although there have long
been programs to redistribute the peripheral bond pads into area-
array format [2-71]. This has poorer heat transfer (through the bumps)
and more crosstalk than a chip originally designed for area-array I/Os,
so it is only an interim step. However, the massive use of portable
devices, such as cell phones, has overcome this problem because of
small size and high frequency requirements. Currently, more high-
performance chips and higher Si density (tiling) are required for such
use, and many more chips are currently designed in true area-array,
flip-chip format. This interconnection method is growing at a faster
rate than wire bonds and eventually (in many years) will presum-
ably overtake them as the preferred interconnection method for many
applications.
Ball Bumped/Stud Bumped Flip Chip
Recently, the use of ball bumped (stud-bumped) flip chip has become
increasingly used. For instance, there are billions/year of SAW filters
interconnected with them and hundreds of millions of ICs. It allows a
ball bonder to attach Au (or Cu) flip-chip bumps to existing wafers or
chips designed for normal wire bonding. The volume has become so
high that almost every manufacturer of autobonders produces a ded-
icated ball bumping machine that can attach tens of thousands of ball
bumps to entire wafers rapidly. These are dices and are ready for flip
chipping (usually with conductive epoxy or other bonding method).
Some of these techniques/applications are discussed and shown in
other parts of the book. A publication describes the techniques for
making these, as well as the economics [2-77].