Design of Epoxy Encapsulating Compounds for Highly Reliable Surface Mounting Semiconductor Devices
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71 Mat. Res. Soc. Symp. Proc. Vol. 390 0 1995 Materials Research Society
CRACK MECHANISM AND REQUIREMENTS FOR MOLDING COMPOUNDS
Crack mechanism The first type of crack is that caused by thermal stress. The stress is induced by a mismatching of the coefficient of thermal expansion between the compound (15-20ppm/*C) and the silicon chip (3ppmf C). The thermal stress is expressed by the following equation [1, 2]:
a=j a (T)E(T)dT
(1)
where E(T ) and a(T) are the elastic modulus and the thermal expansion coefficient (TEC) of molding compound, respectively. Fig.1 shows the proposed mechanism of crack under the reflow soldering process. The surface mounting devices are exposed to a soldering temperature of more than 215'C .When they are soldered, the moisture absorbed in the packages is rapidly vaporized. The vapor pressure causes a delamination in the interface between the encapsulating compound and the silicon chip, and then deformation of the encapsulant. Above the glass transition temperature (Tg), the flexural strength of the encapsulating compounds rapidly decreases. When the stress in the package generated by the water vapor pressure is larger than the flexural strength, a package crack will occur. The stress is dependent on the moisture absorption and structural factors of the package, such as package thickness and die size [1, 2]. 80
80
oj 60
60
-o
Lu
":40
40 C'
-D 0
. 20
20 iaC
Cs
Lo 0.2 0.4 0.6 0.8 Volume fraction of fillers
Fig.2. Filler content dependence of calculated thermal stress parameter.
New types
Conventionaltypes
(El)
(H3) O
(E3)
(H1)
4 (E2)
(O,0: Experimental value)
Reflow solder crack mechanism
Fig.1.
-1-&.0T-l-
(E4)OG
?GG
(H5)
9H3
Hb-q
(H2)o OG (E5) H4C, GQ~q OG OG3 GO 2
HGO41-(1-12 &H c H OG 3
0
(H4)r
H
C
Fig.3. Chemical structure of epoxy resin
G
OHG4
C)CH30 H(E7) -&O8G CH 3
r
72
(E6) G
H[LJý
Requirements for preventing cracks There are several requirements of molding compounds to prevent package cracks. Table I lists the requirements and methods for molding compounds. Molding compounds consist of epoxy resin and filler such as silica. The most moisture absorbing components is the epoxy resin which contains hydroxyl groups formed during curing. To ensure low moisture absorption, a low concentration of hydroxyl groups, hydrophobic structures, and decreased fraction of epoxy resin matrix are required. The adhesive strength between the compounds and the silicon chip/lead frame should be improved in order to prevent delamination and entry of moisture through the interface. To reduce the thermal stress, there are two ways: lowering the TEC and decreasing the elastic modulus. Increasing filler contents is the most effective measure for lowering the TEC, but high loading of filler leads to a larger elastic modulus and higher viscosity at molding. Fig.2 shows the filler content dependence of the TEC and elastic modulus. The solid lines represent the results obtained with Ishai's equation for the TEC [6] and Fahmy's equation for elastic modulus [7]. With
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