The demand for data transmission has been increasing rapidly over the last 20 years, and is still increasing. Data from DARPA (Defence Advanced Research Projects Agency) in the U.S. show data transmission growth rates in the U.S. reaching 20-30% per year in the 1980s, while increasing to 30-40% per year in the early 1990s. Initially this was due to the development of private (mainly corporate) networks, followed by the growth of email and the Internet from the mid-1990s.

By way of comparison, voice traffic across fixed-line networks is increasing at 5-9% annually, with voice cellular traffic increasing at 30-40% per year. The use and introduction of 2.5G and 3G mobile telephone networks is expected to increase the rate data traffic. It is estimated that data traffic exceeded voice traffic across the networks at some time in 2000; exact traffic data is not kept by the telecom carriers.

The growth of the Internet is currently the biggest driver of bandwidth demand. In particular, the increased transmission of multimedia data, such as music and video files, is increasing the volume of data associated with it.

As a result of this continual increase in demand, the operation speed of the equipment used in these communication systems have also been increasing. These include equipment types such as Routers, Switches, Cellular Base Stations, Military Systems, Data/Image Acquisition, Control Systems, and Chip-to-Chip Communications.

Speeds are highest in optical communications, where long haul data transmission has already reached 10Gb/s; and 40Gb/s systems are already under test by carriers such as France Telecom and Deutsche Telecom for implementation in the next two to three years.

Within the optical data communications systems areas, the packages can be split into two types: Photonic Packaging and Electronic Packaging.

Photonic Packaging encompasses all components that have either an optical path through them, or generate/receive optical signals. These are categorized into three types of component:

1) Active e.g. Laser, Photodiodes, modulators
2) Passive e.g. multiplexors/demultiplexors, filters
3) Modules e.g. Transceivers, transponders, amplifiers, switches

A typical example of a high-speed photonic package, a 14-pin butterfly package used for laser packaging, is shown in Figure 1.

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A wide range of different package types is now used for photonic applications ranging from metal "TO can" packages to "optical BGA" constructions.

Several material types are used in the packaging. Important materials for the exterior packaging are Kovar and ceramic, although other materials such as copper-tungsten and copper molybdenum are also used. From discussions with component manufacturers, Kovar is still the preferred material, but BPA expects that both ceramic and metal packaging will be used in the future. Kovar and other metal packaging is likely to remain key for long haul component applications where reliability and durability are key, driving the use of hermetic packaging. Lower cost ceramic packaging will be used in uncooled, high-volume packages used in Metro networks and "last mile," fiber to the home applications.

With the severe downturn in the telecom market, cost reduction is currently the key driver for photonic packaging. Consequently, manufacturers are looking for many solutions that will contribute to reducing the cost. The cost of packaging and assembling a photonic package can be up to 70% of the total package cost. This compares to a more mature industry such as semiconductor packaging where the cost of assembly is typically less than 5% of the total package cost.

Another area of interest in the high-speed packaging area is the emergence of optical BGAs for use on printed circuit boards. Manufacturers are now considering optical links for high-speed interconnection on printed circuit boards for several reasons. These include bandwidth/speed limitations of electrical interconnections, signal integrity/EMI issues, difficulty of routing high numbers of I/O from packages, and skew/timing issues.

Two types of solution are being considered for these high-speed packages. One is the use of direct optical fiber links between packages, which is illustrated in the Figure 2.

This type of package is being investigated by Primarion (CA, USA), which is investigating the issues raised by the continued increase of microprocessor speeds.

BPA expects that microprocessor clock speeds will increase to at least 5-7GHz by 2005, with the key issues by that time being, how data can be transferred fast enough to and from the processor to prevent clock cycles being wasted, and how to dissipate the heat generated by the processor. Using high-speed optical fiber links it may be possible to split processors and associated chip sets into several packages connected by optical fibers. This would spread the heat dissipation requirement across more packages.

The other approach that is being taken is the creation of high-speed packages for use on optical printed circuit boards, and an example of this type of package is shown in Figure 3.

Although there has been some rapid progress over the last several years, BPA believes that the high-speed photonic component industry as a whole is ten to fifteen years behind more mature industries.

This approach requires the use of optical pathways within the printed circuit board, such as embedded optical fibers, or embedded glass or polymer waveguides.

The construction of the package is similar to that for standard BGAs, but with the addition of a VCSEL (Vertical Cavity Surface Emitting Laser) device on the underside of the package. The VCSEL is sealed in place using an optical resin. It is linked using vias to the processing die on the topside of the package, and is used for high-speed interconnection to the optical pathway on the printed circuit board.

Micro lenses are mounted both on the package and on the printed circuit board. These have the effect of spreading the optical signal over a wider area, increasing the alignment tolerance of the package to around 50 microns.

This is important, as it allows the package to be mounted using standard SMT equipment, reducing the cost of the assembly process. Without this high degree of tolerance, much more expensive alignment equipment would be required, which may also entail an additional process step on the production line.

The technology and market for very high-speed packaging is still new, and BPA believes that there is still much development work that needs to be completed before these packages are used in volume production environments. However they do offer many new market opportunities for companies in the materials, equipment and package assembly sectors.

Further details on the technologies discussed above, and others being introduced, can be found in BPA's latest Multi-Client report, "Interconnection and Packaging for High-Speed Systems 2002-2007" which is now available from BPA.