The growing number of high-performance ICs--with greater functional
complexity, higher integration, and improved per- formance--continues
to create a higher standard for IC packaging. In response, advanced interconnect
technologies, such as flex-based circuits and tape ball-grid arrays (TBGAs)
have stepped up to the plate. With significantly improved electrical and
thermal performance over older IC interconnect methods, TBGAs, like other
enhanced BGA packages, are becoming ever-more popular.
Recent tests have demonstrated the TBGA's strengths with respect to
integration levels, defect levels, joint reliability, and thermal and
electrical performance. While the results are highly encouraging, migrating
to a new format is never a cut-and-dry decision, as a number of factors
must be taken into account. Chief among these is overall cost, mainly
affected by the format's compatibility with the existing manufacturing
infrastructure. Due to its compatibility with surface-mount-construction
techniques, the TBGA may have the edge over competing packaged options.
Why BGA?
For IC interconnect packages, plastic packages with gull-wing leads--particularly
small-outline IC (SOIC) and quad flat pack (QFP)--represent the majority
of surface-mount-technology (SMT) compatible first-level packages in use
today. In the near future, they will continue to be the packages of choice
for first-level IC packages. However there are newer alternatives that
offer designers an option, and one of those is the BGA.
The reasons for the rising popularity of BGAs are simple: They offer
higher reliability, a smaller form factor, improved electrical and thermal
performance, and more. According to the Worldwide IC Packaging Market
publication, the relatively new BGA will grow by more than a factor
of 10, from 0.319-billion packages in 1996 to 3.269 billion in 2001.
Within the BGA family, there are three alternatives: plastic BGA (PBGA),
ceramic BGA (CBGA), and TBGA (see the figure). Defined as
any BGA package which uses flex circuitry as the substrate, the TBGA delivers
many of the advantages of its cousins, and is expected to be a major player
within the rapidly growing BGA product family.
TBGA (also called a flex-circuit-based BGA) can include larger high-lead-count
packages, as well as small, chip-scale packages (CSPs). The superior wiring
density of flex circuitry endows the TBGA with all the advantages of regular
BGAs, and then some. With capability rapidly approaching 25-µm lines
and spaces, a ball-array pattern that would normally require two, or even
four layers of circuit board to route can now be accomplished on a single
layer of flex circuitry. Consequently, the form factor and cost/performance
ratio can be considerably more attractive than other packages.
A die can be interconnected to a flex circuit through any of the three
conventional methods: wire bonding, thermal-compression bonding, or flip-chip
attachment. Fine-pitch flex offers obvious advantages when interconnecting
with the latter two methods, while offering improved wire-bonding capabilities.
Wire-bond pads on the flex can be positioned closer together, and therefore,
moved closer to the die itself. As a result, the required length of wire
can be minimized, which offers a reduction in assembly cost and an improvement
in electrical performance.
TBGA Formats
TBGAs can be classified into two main categories:
Cavity down. Here, solder balls fan out away from the edge of
the die, and a heat spreader is used for high-power dissipation. The cavity-down
format is an excellent solution for higher-I/O applications (above 200)
requiring thermal dissipation of over 3 W. Applications for cavity-down
formats include higher-end digital signal processors, network routers,
microprocessors, microcontrollers, programmable logic, and a variety of
application-specific ICs.
Cavity up. In the cavity-up format, solder balls can fan in under
the die, and in some cases actually become a CSP, or near-CSP package.
Cavity-up products are ideally suited for applications requiring a smaller
form factor. This would include packaged die for cell phones, pagers,
video cameras, digital cameras, and handheld devices.
TBGAs will displace the other more widely-used, gull-wing lead packages
in many applications within two to five years, mainly because of:
Increasing lead counts. As lead counts continue to grow, the
reliability of the package will become increasingly important, especially
as typical IC lead counts surpass the 208 I/O mark.
Faster devices. As devices become faster, they will require higher
levels of thermal and electrical performance. Here again, TBGA holds an
advantage, not only compared to gull-wing packages, but also compared
to plastic packaging, including PBGA.
Mobile electronics and the demand for space. The explosion of
small mobile electronics will increase the demand for more functionality
in a small form factor. The CSP- or near-CSP-style flex-based BGAs have
a form factor, significantly smaller than SOIC, with a higher I/O density
than PBGAs.
TBGA Implementation
When evaluating new technological solutions such as TBGA, it is important
for the designer to examine total applied costs. If new packaging technologies
require significant investments in manufacturing infrastructure, they
likely will not be accepted by designers. Cost-effective solutions must
include compatibility with the existing infrastructure, both at the board
level and at the IC packaging-assembly operation level.
Compatibility with SMT assembly techniques allows high-performance,
wire-bond TBGAs to meet the applied-cost challenges because minimal new
infrastructure investment is required. Because approximately 97% of die
are currently wire bonded, a vast infrastructure for wire bonding is already
in place.. TBGA carriers can be supplied in strip format similar to a
leadframe or PBGA. This format allows assemblers to easily use the existing
infrastructure for die attach, wire bonding, overmold or encapsulation,
and ball attach. Compatibility is furthered by the fact that circuits
for cavity-up applications are typically connected to a carrier, enabling
the package to be used in the most cost-efficient assembly operations
without significant additional costs for manufacturing infrastructure.
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