System in a Package (SIP) : The 3-D Package

System in a Package (SIP)

The term “System in a Package” or SIP refers to a semiconductor device that incorporates multiple chips that make up a complete electronic system into a single package. Electronic devices like mobile phones conventionally consist of several individually packaged IC's handling different functions, e.g. , logic circuits for information processing, memory for storing information, and I/O circuits for information exchange with the outside world. In a System-in-a-Package, all of these individual chips are assembled into a single package, allowing tremendous space savings and significant down-sizing of electronic gadgets.

SIP must not be confused with SOC, or System-on-a-Chip, which is a complete electronic system built on a single chip. SOC's suffer from long development time and high development costs, mainly because it is difficult to make an entire system of differently functioning circuit blocks work on a single chip. SIP technology, on the other hand, simply takes several readily available chips and put them together in a single package.

The predecessor of the SIP is the multichip module (MCM) of the early 1990's, wherein several specialized chips are also assembled in a single ceramic package as a system solution using traditional assembly processes. Some people consider the SIP and the MCM as still the same thing, but most people prefer to give SIP its own distinct identity because of its mass-production nature and use of cutting edge assembly technologies. For instance, the chips in an MCM are mounted on the same plane (the cavity substrate), whereas SIP employs die stacking as its natural configuration.

Figure 1. Example of an MCM, the predecessor of the SIP

The ability to take existing chips to come up with a totally new system in a single package has one clear advantage: it drastically reduces development time and risk to bring new products to the market more quickly. With SIP technology, vendors are able to cram multiple flash devices, SRAMs, DRAMs, microcontrollers, ASICs, DSPs, and passive components into very thin packages that can fit into sleeker, more stylish, and yet more complex electronic gadgets.

Aside from shorter time-to-market, SIP manufacturing reduces its over-all assembly and test costs, since only one package will be assembled and tested to come up with the system. Better electrical performance is also achieved because of the shorter interconnections within the SIP. SIP's also simplify the process of assembling the final application module by requiring simpler PCB lay-outs, since the complex interconnections required by the system have already been taken care of inside the SIP.

The challenge in SIP manufacturing lies in the assembly process itself. Touted as the next-level multi-chip module (MCM) assembly technology, it requires the ability to assemble and interconnect several die not only horizontally (wherein die are placed side by side), but vertically as well (wherein several die are placed on top of each other).

Mounting die on top of each other and interconnecting them is known as die stacking, a new technology that is harnessed extensively in state-of-the-art SIP manufacturing. This extensive use of stacked die configuration is the reason why SIP is also known as the 3-D package.

One challenge posed by die stacking is the need to keep the stack thermally and mechanically stable on the substrate, while allowing good interconnection among the die, and keeping the package as thin as possible in doing so. Needless to say, package thickness largely depends on the number of die that are vertically stacked inside. For instance, current technology would generally require a 1.4-mm chip scale package (CSP) to accommodate a six-die stack whereas a four-die stack can fit within a 1.2-mm CSP.

For more details about die stacking, please see the article: Die Stacking.

Flip chip bonding is also used in SIP interconnection, either on its own or as a complement to wirebonding. Flip chip configuration may be applied either to the upper die or the lower ones, depending on the intent of the design. Flip chipping a bottom die directly onto the substrate enables that die to operate at a high speed. On the other hand, flip chipping a top die eliminates the use of long wires for connection to the substrate.

Figure 2. Example of a 3-die SIP configuration employing

both wirebonding and flip chip bonding

Heat dissipation is another challenge in the development of SIP's. Taking chips off-the-shelf and using them in SIP's isn't always easy from the thermal point of view, since these chips were designed to dissipate heat through their own packages. Crowding them all together inside a SIP can accumulate enough heat to be of major concern in the field. Thermal management is therefore an important ingredient of any SIP development process.

SIP manufacturing not only offers assembly challenges, but test challenges as well. SIP's combine microelectromechanical systems, optoelectronic devices, various sensors, linear and digital circuits, etc., which were built on a different wafer fab process technologies and therefore have varying excitation requirements. Add to this the fact that each of these system blocks require special test methods of its own. A test solution to meet the various test resources and methods required by a complex SIP can turn out to be expensive.

For these test issues, some quarters propose a cost-effective solution in the form of an open-architecture automated test equipment (OA-ATE) that allows semiconductor manufacturers to specify their own test resource and instrumentation requirements. 'Specialization' of test capability nonetheless require some standardized vital elements: 1) an industry-standard bus structure; 2) compatibility with industry-standard data formats; 3) browser technology to access and control resources; 4) a modular hardware and software structure to enable reconfigurability; and 5) partitioned test supported by ATE and EDA tools.

Successful implementation of SIP manufacturing brings in many advantages that are important to the semiconductor industry of the future: shorter time-to-market, lower cost, flexibility, smaller size, etc. To get there, however, requires a monumental engineering effort to address all technical obstacles along the way.

IC Packaging; IC Manufacturing

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