Electronic Design

Tape BGAs Satisfy High-End IC Demands

As signal density rises, designers turn to technologies such as tape ball-grid arrays to fend off the laws of physics.

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.

The latest generation of TBGA packages stand out in several areas:

Lower defect levels. Studies have found lower defect levels for all BGA variations as compared to QFPs. The relatively coarse pitches associated with the BGA package (1.27 and 1.5 mm) allow for routine solder-paste deposition and placing of the component. Many BGA packages can be placed up to a half pad off center, and will self-align upon reflow. As a result, card assembly defect levels are under 2 ppm, compared to 48 ppm for 0.5-mm pitch QFPs. This applies to both cavity-up and cavity-down formats.

Solder-ball reliability. Some TBGAs improve reliability by using a sloped sidewall on the via. The sloping sidewalls capture the solder ball when it is placed prior to reflow. In addition, this gradual slope allows the solder ball to maintain a low-stress attachment, free from the sharp impingement angles typical of a soldermask-defined attachment pad. These sharp impingement angles can provide locations for crack propagation on the solder ball. The gradual slope provided by the etched sidewall has demonstrably higher sheer values for the solder ball. This applies to both cavity-up and cavity-down formats.

While reliability is improved, smaller, faster, and hotter-running ICs are pushing the packaging industry to place more emphasis on thermal and electrical performance. Whatever the standard for reliability is today, a package that only meets last year's performance requirements will not be viable for long. The latest cavity-down TBGA packages have shown significant advantages in thermal and electrical performance during tests. These packages go well beyond the typical electrical and thermal performance of QFP packages.

Thermal cycling test results. Results of board-level thermal cycling tests on one particular TBGA show increased board-level reliability. The boards were cycled between -55° and 125°C at a rate of 3 cycles/hr. (a five-minute transition period, with five minutes of dwell time). One of the key requirements of an IC package is to have acceptable reliability on the circuit board as the device undergoes temperature excursions. A common approach to ensuring such reliability is to perform accelerated temperature cycling through a range from -40° to 125° C. For many applications, the requirement is to survive 1000 cycles of this testing without any solder joint failures. However, exposure to certain environmental conditions, such as the engine of an automobile, requires even better reliability. One particular TBGA package has been shown to survive over 5000 cycles before first failure.

In addition, the characteristic life of the Weibull distribution (point at which 63% of the devices failed) was 6240 cycles. Thus the TBGA package demonstrates excellent board-level reliability. This is primarily because the copper stiffener in the package has a matched expansion coefficient to that of the pc board on which the package is mounted.

For the largest 600-µm ball pads, the characteristic life was calculated at 6239 cycles, and the slope was determined to be 18.2. This compared favorably to a similarly sized 360-I/O PBGA with a characteristic life of 3500 cycles and slope of 5.8. Though this applies only to a cavity-down TBGA format, the figures give a good idea of a TBGA's capabilities.

Thermal performance. The thermal performance of a cavity-down TBGA is also encouraging, due in large part to the die being attached directly to a thermally conductive copper stiffener. The performance is enhanced by the thin adhesive layer (1 to 2 mils) between the circuit and the stiffener, which allows a great deal of heat to be dissipated from the stiffener, through the solder balls, and into the circuit board. This is significant, as it allows all the solder balls to act as vias, as opposed to only those under the die--as is the case for standard PBGAs. The result is that up to 90% of the heat is dissipated through the board. This is particularly important in low-airflow applications, such as a laptop computer, where heat buildup can be a serious problem.

Electrical-performance tests on a cavity-down TBGA product show:

Reduced self inductance. The finer-pitch circuits of the TBGA allow the bond pads to be positioned closer to the die. This can reduce the self-inductance of the wire bond by almost 4 nH, allowing wire bonding to keep up with higher-speed demands.

Electrical advantages of stiffener. The close proximity of the circuitry to the metal stiffener in the patented cavity-down TBGA, gives inherent electrical performance superiority when compared to plastic packages such as QFP, SOIC, and PBGA.

Positioning the circuit side of the flex toward the stiffener also provides an electrical advantage. It has been determined that the close proximity of the traces to the metal stiffener (approximately 1-mil spacing) makes this stiffener an excellent floating reference plane, thus reducing signal crosstalk between parallel traces. In addition, a process has been developed to make an electrical connection to the stiffener, thereby providing a low-inductance ground path for high-speed devices.

CSP or cavity-up, flex-based BGAs are very close to reality, with BGA fan-in construction techniques enabling a variety of CSP packaging alternatives. In most CSP designs, interconnects from the die pads are "fanned-in" to area-array connections (typically solder balls or metallurgical bumps) underneath the device. The high wiring density and fine via etching capability make CSP applications a natural extension of the flex-circuit technology.

Flex circuits themselves have a very-fine-pitch capability. Present capabilities are approaching 50-µm pitch, while most pc boards are limited to 150-µm pitch or greater. Finer pitch enables routing of more balls with a single-metal layer flex circuit, where a pc board may require two or more layers, and not be as cost-effective. Also, fine pitch enables 0.8- and 0.5-mm ball-pitch designs on a one-metal-layer circuit.

In addition, these fine feature dimensions make it possible to position the wire-bond pads closer together, and closer to the die itself. This allows for a shorter wire-bond length and reduces the self-inductance of the wire. These shorter wire lengths also reduce the chance for wire sweep.

Another advantage of the fine-pitch traces relates to die shrink. As IC technology migrates from a circuit-trace width of 0.5 to 0.18 µm, there is a strong tendency to pack everything tighter and reduce the size of the die and, therefore its cost. In some packages, the wire length is already at a maximum, and shrinking the die further would stretch the wire beyond its limits. On flex-based packages, the wire-bond pads can be moved in closer, eliminating the wire-length problem, and allowing the die to shrink much further.

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