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X-Ray–Based Fabrication 5-13
TABLE 5.4 Properties of PMMA (nom. at 20°C)
E – bulk modulus 3.0 GPa
G – shear modulus 1.7 GPa
ν – Poisson’s ratio 0.40
–5
α – linear thermal coefficient of expansion 7(10) /°C
Water absorption (by wt. %)
(during immersion)
– 24 hrs. 0.2%
– 7 days 0.5%
– 21 days 0.8%
– 48 days 1.1%
Glass transition temperature 105°C
σ – tensile strength 70 Mpa
Y
δ – density 1.19g/cc
κ – thermal conductivity 0.193 W/m
K
Heat capacity 1.42 J/g
K
n – refractive index at 365 nm, 1014nm 1.514, 1.483
Abbe number 58.0
ε – dielectric constant
r
– 60 Hz 3.5
– 1kHz 3.0
– 1MHz 2.6
– 30GHz 2.57
4 8 12 16 20
0.5 Plate
thickness (µm)
0.4
Plate buckling strain (%) 0.3
0.2
0.1
0
0 100 200 300 400 500
PMMA plate height (µm)
FIGURE 5.13 Buckling strain of PMMA plates typical of DXRL-defined vertical geometry with bottom side attached to
a substrate and top side free as a function of plate height for various plate thicknesses (for plate length plate height).
for PMMA. Many of these parameters are also dependent on the molecular weight or degree of cross-
linking, as well as on common additives for enhanced UV absorption and stabilization. The large thermal
coefficient of expansion leads to significant concerns of geometry distortion and strain-induced buckling.
Similar effects also can occur with water absorption. For example, in a PMMA sheet bonded to a silicon
substrate, a 10°C temperature change leads to a 0.11% strain in the PMMA. This situation typically man-
ifests itself via buckling of PMMA plates defined by three built-in edge conditions at the substrate interface
and side edges and a free edge condition at the top edge. The solution for this plate-buckling problem
may be used to generate design curves that indicate mechanically limiting regions of geometry and
thereby establish minimum PMMA design rules [Christenson, 1995a]. One such graph is provided in
Figure 5.13. The net effect is that this behavior constitutes the major restriction on aspect-ratio and
© 2006 by Taylor & Francis Group, LLC
