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.