Online proceedings - EDA Publishing Association

adhesive material with 30μm thickness illustrated in the Fig.
6(b), the thermal deformation in the PWB is in the shape of
upward concave, so that the PWB deformation could affect
the silicon chip in a compressive way if the adhesive as a
stress-transmission layer. Therefore, the tensile thermal stress
in the membrane could be compensated by the external
compressive stress from the bottom PWB substrate. However,
when the thickness is increased, the buffer layer effect
improves to reduce the transmission of the external stress
from the bottom, so that the compressive stress in the
membrane gradually declines in the 30μm-thickness case of
2025D adhesive.
In Fig.7 (a), as the soft adhesive material demonstrates
buffer layer effect from the beginning, as the temperature
rises, the thermal stress of the chip also increases but there is
little difference between the case of 20μm thickness and
30μm thickness. When the thickness of adhesive is thin, the
thermal stress in the membrane mainly comes from the CTE
mismatch between poly-silicon and silicon substrate in the
range of dozen MPa. As the thickness is over 30μm, thermal
expansion of adhesive directly affects the diaphragm
therefore the stress in the diaphragm increase rapidly. In
addition, from the comparison of thermal deformation
between PWB and membrane in the 3140RTV adhesive
material as indicated in the Fig. 7(b), the deformation of PWB
is in an outward convex shape, therefore, the external stress
from bottom PWB to membrane can be regarded as a tensile
way. So that, we cannot find the stress compensation in a soft
adhesive material.
Stress on membrane (MPa)
25
20
15
10
5
0
-5
LCP Cap 2025D
2025D 20um
2025D 30um
2025D 50um
11-13
May 2011, Aix-en-Provence, France
0
Displacement (mm)
-0.002
-0.004
-0.006
LCP Cap - Adhesive 2025D
2025D-30um-Mem
2025D-30um-PWB
-0.008
0 0.4 0.8 1.2 1.6
Distance (mm)
(b)
Fig. 6: (a) The time history of thermal stress in the membrane as COB
packaging with LCP cap and difference thickness of 2025D adhesive; (b) The
deformation of membrane and PWB at 260°C as COB packaging with LCP cap
and 30 μm thickness of 2025D adhesive.
Stress on membrane (MPa)
30
25
20
15
10
5
0
-5
-10
0 50 100 150 200 250
0.006
0.004
LCP Cap 3140RTV
3140RTV 20um
3140RTV 30um
3140RTV 50um
Time (sec)
(a)
LCP Cap - Adhesive 3140RTV
3140RTV-30um-Mem
3140RTV-30um-PWB
-10
-15
0 50 100 150 200 250
Time (sec)
(a)
Displacement (mm)
0.002
0
-0.002
0 0.4 0.8 1.2 1.6
Distance (mm)
(b)
Fig. 7: (a) The time history of thermal stress in the membrane as COB
packaging with LCP cap and difference thickness of 3140RTV adhesive; (b)
The deformation of membrane and PWB at 260°C as COB packaging with
LCP cap and 30 μm thickness of 3140RTV adhesive.
125