(This article is sponsored by The Boston Group)

Introduction
The commercial market for optical telecommunication components has reached $5 billion for 2003. About 60% to 80% of the manufacturing cost of these components resides in fiber pigtailing and packaging (see Figure).

A major portion of packaging cost is associated with active alignment and attachment of chip to chip and single mode fibers to chips. Active alignment involves lighting up input fibers and active components and maximizing light throughput before attachment. This is labor and capital intensive. The approach the industry has taken to reduce packaging costs is to automate the active alignment process using machine vision and stepper motors. This is a stopgap measure that is still capital intensive, brings no innovation to packaging and fails to address chip-to-chip packaging. By contrast, in the microprocessor industry when the number of leads in a single package grew rapidly, wire bonding was replaced by innovations such as surface mount technology, flip-chip and solder bump technology.



Fig. 1 Packaging as a fraction of total manufacture cost of an optical modulator.

Linden Photonics is developing low cost, hermetic packaging using liquid crystal polymers.

Hermetic Packaging
Hermetic packages are used for electronic & opto-electronic components and micro electro-mechanical systems (MEMS) to protect the internal components from potentially corrosive or otherwise damaging elements such as water vapor and oxygen. Typically hermetic packages are formed using materials such as metals, ceramics or glass, since these are highly impervious to corrosive vapors or gasses. Input and output ports pose the greatest problem, since these represent a breach of the external casing. For electrical ports, where electrical insulation is required, the conductors are generally fed through glass insulating beads which are then soldered to the metal cover. In the case of optical ports, where glass fibers must enter the package, the fibers must first be metalized, and then sealed by soldering. Both these operations are time consuming and costly, and add significantly to the complexity of the overall packaging.

A generally accepted test of the hermeticity of a package is the Telecordia damp heat test, in which the device is required to survive 1000 hrs at a temperature of 850 C. and  85%  relative humidity. Polymers are used extensively in the electronics industry for less demanding packaging applications, but most common polymers do not exhibit the necessary barrier properties required for hermetic packaging.
 
Recently, a new class of polymeric materials, liquid crystal polymers (LCP), have been shown to exhibit excellent moisture and oxygen barrier properties. LCPs are polymers that contain rigid crystalline molecular chains interconnected by more flexible segments. When LCPs flow in the molten or solvated state the rigid molecular chains tend to align mutually like logs in a river. It is the rigidity of the molecules combined with the tight packing that gives rise to the high strength and low permeability of these materials. LCPs also exhibit many other properties, physical, electrical and chemical, that make them very attractive candidates for packaging. These properties and other details will be discussed in the next issue..

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