Technology - Optoelectronic Packaging/LCPs: Part II
(This article is sponsored by The Boston Group)(Amaresh Mahapatra has a M.Sc. from IIT, Kharagpur, and Ph.D. in Physics from Syracuse University. He founded Ramar Corporation which was acquired by JDS-Uniphase in 1999. He is President and Founder of Linden Photonics, Inc. and can be reached at 978.392.7985. )
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
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
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
Table 2: Radiation hardness comparison of polymer classes
|Base Resin||Gamma Radiation|
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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