3.1 Finite Element Modeling and Stress Analysis. In order
to obtain the accurate damage model, thermal cycling test data
from different combinations of component types and thermal pro-
files are needed. Besides thermal cycling test data from this study,
another previous NCMS work, reported in the literature, is se-
lected and their test data are included into this study ?9?. Similar
fleXBGA144 packages with different die size from ours were em-
ployed in their tests and the corresponding FE models are devel-
oped with minimal modifications. More importantly, test data un-
der three different thermal profiles are available so that the effect
of the thermal profile can be addressed in the obtained durability
models. All five combinations of component type and thermal
profiles are summarized in Table 8 and in Fig. 17. It is seen that
there are three component types, fleXBGA1, TABGA, fleXBGA2,
subjected to four different thermal cycle loading with temperature
range varying from 100°C to 180°C ?mean temperature from
35°C to 50°C?, ramp rate from 6°C/m in. to 90°C/m in., and
also dwell time from 5 to 15 min.
The packages modeled include a 144-joint fleXBGA package
and a 96-joint TABGA package on FR4 board. There are two
models for fleXBGA144 packages with different die size. One has
Fig. 15 Optical micrographs of PBGA352 solder joints 〓SAC ball/SnPb paste…
Fig. 16 Overall approach for thermomechanical durability
analysis
Table 8 Summary of temperature profiles
Fig. 17 Schematic of thermal profile
Journal of Electronic Packaging DECEMBER 2005, Vol. 127 / 423
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the die size shown in Table 1 and the other one has a die size of
6.4 mm?6.4 mm?0.27 mm. Other dimensions are the same for
two fleXBGA144 packages. They are labeled as fleXBGA1 and
fleXBGA2, respectively. The FR4 PWB length is much larger
than the component and only a relevant part of the substrate is
modeled in the analysis. The distance between the centers of two
adjacent components is used as the length of the PWB in the FE
models. Dimension of solder joints is obtained by averaging mea-
surement of many solder joints after cross sectioning.
Nonlinear viscoplastic finite element analysis is conducted with
ANSYS 7.1. Three-dimensional quarter models are built up for fleX-
BGA144 and TABGA96 packages in the FE analysis due to the
symmetry in geometry. An example of FE model of fleXBGA144
package are shown in Fig. 18. Solid element 185 is used in all FE
models.