Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
chip and improve the reliability of cap bonding.
30
Max stress on membrane (MPa)
20
10
LCP Cap
2025D
3140RTV
DA6501
Fig. 4: The deformation and stress distribution of 3D model with 20um-thick
adhesive at 260°C, all the deformations are magnified by 10 times. (a) and (b)
are LCP cap with different adhesives 2025D and 3140RTV, respectively. (c) and
(d) are Ni cap with different adhesives 2025D and 3140RTV, respectively.
(b) Influence of adhesive material and thickness
As in the COB packaging, only adhesive material was used
to attach the chip and the PWB together, therefore, the
thickness and material properties of the adhesive material
play a critical role to reduce the thermal stress in the
membrane during reflow process. The maximum stress on the
membrane at 260 ℃ for different adhesive material and
thickness in the cases of LCP cap and nickel cap were shown
in the Fig. 5(a) and 5(b), respectively. As the results, no
matter packaged under LCP cap or nickel cap, the difference
of thermal stress in the membrane is small. Moreover, the
softer adhesive material (3140RTV and DA6501) shows a
rising trend of maximum thermal stress in the membrane
when the thickness increases owing to the thicker adhesive
leads to larger thermal expansion in the adhesive layer, which
induces larger stress in the membrane. In addition, the stress
values in the cases with softer adhesive were higher than the
values in the cases with harder adhesive (2025D). This can be
attributed to the CTE of 2025D is only about half value of
3140RTV or DA6501. Another interesting phenomenon in
the Fig. 5 is that when the thickness of 2025D adhesive ranges
between 20μm and 30μm, its stress first declined before it
increased, showing a different trend from the other two soft
adhesive materials. This is due to the high elasticity of 2025D
adhesive material; therefore, when the thickness of 2025D
adhesive is thin the thermal deformation of PWB could affect
to the silicon chip. However, when the thickness of 2025D
adhesive material is thicker above 30μm the thermal
deformation of PWB is less influence to the silicon chip due
to the stress relaxation and energy absorption by adhesive
material, so called the buffer layer effect. In contrast, the
buffer layer effect starts to play at the very beginning on an
adhesive material with lower Young’s modulus, so the silicon
chip is not strongly affected by the PWB deformation at the
bottom for a soft adhesive case.
Max stress on memebrane (MPa)
0
30
20
10
0
20 30 40 50 60
Adhesive thickness (um)
(a)
Metal Cap
2025D
3140RTV
DA6501
20 30 40 50 60
Adhesive thickness (um)
(b)
Fig. 5: The thermal stresses in the diaphragm for various cases with different
adhesive material and different thickness, (a) Material of cap is LCP and (b)
Material of cap is nickel.
(c) Discussion on buffer layer effect
As indicated in the Fig. 5, a different phenomenon between
soft and hard adhesive when the thickness blew 30μm. Hence,
the elasticity of adhesive material has significant influence on
the thermal stress in the membrane. When a hard adhesive
material (2025D) with 20μm to 30μm in thickness, the
inadequate thickness fails to bring obvious buffer layer effect
so that the thermal stress of the PWB at the bottom goes
upwards. Fig.6 (a) shows how the membrane stress changes
as the heating temperature described in Fig. 3. The membrane
stress increases when the 2025D adhesive material is
thickened. In the stress history of the 20μm-thickness case,
the stress increased with temperature at beginning in a tensile
range, but the stress turned to compressive value as the
temperature went to 260℃. The similar trend can be observed
in the case of 30μm thickness, but the compressive stress is
smaller. As the results, the thermal stress in the membrane
with 2025D adhesive material declined first as the thickness
is between 20 to 30μm. From the comparison of thermal
deformations between PWB and membrane for 2025D
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