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

Properties of LCP
Not only are LCPs highly impervious to moisture, but as a result of the tightly packed crystalline nature, the interstices allow very little absorption of moisture or other gasses. Consequently, out-gassing, which is a problem for many polymers, is reduced to insignificant levels. Further, since LCP is merely heated to become fluid, there is no need to use solvated LCP, which eliminates another major source of outgassing. 





Fig 2: Scanning electron micrographs of metal traces of LCP circuits made on a) a laminate substrate and b) a vapor-metallized LCP film (from “Liquid Crystal Polymers – a flex circuit substrate option,” Rui Yang, Advanced Packaging, March 2002).

LCPs are thermoplastic so that there is a intermediate temperature such that the LCP is made fluid without break down of the crystal structure. They typically melt at about 2800 C  and are thermally stable to 3500 C. The coefficient of thermal expansion (CTE) is very low, and highly anisotropic, being lowest in the direction of molecular alignment. The actual bulk value of the CTE can therefore be controlled to some extent by either controlling the degree of orientation, or by laminating layers with orthogonal orientations. This is a desirable feature, since it means that the CTE can be matched to
that of the substrate material, thus significantly reducing stress associated with thermal cycling.

LCPs also exhibit very little creep. This means that microscopic features produced by molding, embossing or other such processes will retain their sharp edges and dimensional stability.  Complex packaging designs are therefore possible, in which finely detailed features can be defined to locate, align and secure the various optical and opto-electronic components. Complex structures have been written in LCP films using lithography as shown in Part I..

LCPs have a low dielectric constant and loss factor from 1 kHz to 45 GHz. For instance copper clad Biac LCP, sold by W. L. Gore for flex circuit applications, has a dielectric constant of 3.0 and a loss tangent of 0.003 from 3 to 45 GHz.

What is less well known is that LCPs also have better radiation resistance than almost all other class of polymers. Table 2 compares the gamma and UV radiation hardness of several classes of polymers. LCPs have excellent radiation resistance. Acrylates and fluoropolymers also have good radiation hardness. However, acrylates are very susceptible to moisture while the moisture barrier properties of fluoropolymers are a factor of 30 worse than LCPs. 

Table 2: Radiation hardness comparison of polymer classes
                                                                               

Base Resin	Gamma Radiation	UV Radiation
Nylon 6/6	F	P
PET	G	F
PMMA	G	G
PVDF	G	G
LCP	G	G

Ratings:
E= Excellent; G= Good; F= Fair; P= Poor  (Data from RTP Imagineering Plastics)

It is expected that this will lead to significantly lower packaging costs in the future.

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