The Anisotropy of Dielectric Constant (Ε
r
)
in TLY-5A Material by Bereskin
By Seth J Normyle
Process Engineer
Petersburgh, NY – Tel: 800-833-1805 Fax: 518-658-3988
Europe – Tel: +353-44-95600 Fax: +353-44-44369
Asia – Tel: +82-31-704-1858 Fax: +82-31-704-1857
www.taconic-add.com
www.taconic.co.kr
Background
The general definition of anisotropy is variation in a given material property with respect to
orientation. In the case of woven fiberglass composite laminates such as TLY-5A, Ε
r
anisotropy specifically refers to the differences in dielectric constant in the X and Y plane of
the laminate compared with the Z axis. For a laminate material the degree of anisotropy is
defined as the ratio of the average Ε
r
in the X and Y axis to the Ε
r
in the Z axis.
Degree of anisotropy (Ε
r
) = Average Ε
r
(x, y) / Ε
r
(z)
In printed circuit boards, the degree of anisotropy has relevance in cases where electric field
lines are oriented in perpendicular to the Z axis. For fields in the Z plane, Ε
r
anisotropy may
also impact field fringing effects, effecting equivalent lengths of transmission lines.
Method
To evaluate Ε
r
in the X and Y planes of Y-5A material a thick test laminate was constructed by
sequential lamination from a Y-5A-1870 building block using 1080-74% glass as a bond ply.
The thick laminate was pressed in a laboratory scale high temperature/pressure press in
keeping with standard Taconic lamination conditions. The final laminate had a thickness of
1.3”. The panel was then cut on a panel saw, slicing a strip approximately 60 mils thick from
both the length and width of this thick laminate, to obtain test coupons in both the x (fill yarn
direction) and y (warp yarn direction).
These pieces were cut to the appropriate size and tested by the Bereskin method (Modified IPC
TM-650 2.5.5.5.1). Briefly, two samples of identical thickness are placed in the test fixture
under pressure with a standardized copper strip sandwiched in between to create an imbedded
stripline resonator. A signal is propagated through the z axis of the sample and the resonant
frequency is found. Using the resonant frequency Ε
r
is derived from the equation:
Ε
r
= C / (2.54*F
0
*Leq)
2
Where
C
= speed of light,
F
0
= resonant frequency and
Leq = conductor length including field fringing.
Dissipation Factor (DF) is derived by observing the 3dBm or half power points around the
resonant frequency. This procedure is repeated at each harmonic to yield data for Ε
r
and DF
across a frequency bandwidth from ~ 2 - 24 GHz.