Page 12 - ChipScale_Jan-Feb_2020-digital
P. 12
Figure 5: A heat map showing the distribution of the
damage parameter across the package with respect
to solder loading. Red indicates high loading and
green indicates low loading.
will fail, and vice versa. The heat map
in Figure 5 shows the distribution of the
damage parameter across the package:
• The damage is high at the balls Figure 6: Plots showing the driving force of each mode versus its fatigue level. Only one fatigue mode is
located at the package corner. This considered in each simulation.
is because the corners have the combined with a script, which subsequently performance of transitions is evaluated
maximum distance to the package changes the respective fatigue level. using scattering parameters (S-parameters).
center and, therefore, have the Separate simulations are done for each The S-parameter matrix is a frequency-
highest thermal strain mismatch. fatigue mode. dependent matrix that relates reflected and
• The damage is high below the The results of the analysis show that the transmitted EM waves. This description
perimeter of the silicon chip and driving forces of both UBM fatigue and takes into account all EM phenomena
especially below the chip corners. RDL fatigue reduce with increasing levels and interactions inside the simulated 3D
This is because the silicon chip of fatigue (see Figure 6). This is because structure. In particular, the return loss on
is considerably stiffer than the the assembly of package and ball gets board and chip side (S11, S22), and insertion
molding compound and has a lower more compliant with an increased level of loss (S21), are important parameters, which
CTE resulting in high damage fatigue. The reduced forces may eventually need to be characterized and optimized.
loading similar to the level near the no longer be able to drive the UBM and To evaluate the impact of the fatigue on
package corners. RDL fatigue modes when dropping below RF performance, RDL, UBM, and solder
the critical values needed to propagate the ball fatigue modes are evaluated. Simulation
In general, the findings discussed fatigue. Consequently, neither the UBM results show that fatigue modes have
above are in agreement with results from nor the RDL fatigue mode may cause end- negligible impact on levels of return and
cross sections of devices after solder of-life of the assembly for the investigated insertion loss independently from fatigue
joint reliability experiments. To evaluate loading. modes and their extent. Because of different
whether a respective fatigue mode is able The solder fatigue mode shows a surface current distributions in the case of
to cause end-of-life of the assembly or not, different behavior: The driving force does fatigue modes, the electrical length of the
the “driving force” of each fatigue mode is not reduce with increasing fatigue level, transitions may slightly change. In Figure
analyzed as listed in Table 1. but remains constant, and/or increases. The 7, the surface current distribution is shown
Because the magnitude of all three interpretation is that solder fatigue will for two RF transitions. The left side of the
driving forces depends on the geometry and always continue and eventually cause end- figure shows results without any fatigue,
the material parameters of the assembly, of-life of the assembly. and the right side of the figure shows RDL,
simulation is used to analyze how the
magnitudes change with increasing levels RF performance
of fatigue, i.e., opened UBM interface area, Electromagnetic (EM) simulation is
length of RDL, and solder ball crack. As a used to optimize the chip-package-board
result, detailed sub-modeling of a ball at a transitions, to increase system efficiency,
selected location of the package is done and or to evaluate different use cases, which
cannot be easily produced or measured. The
last one is the case for evaluating the RF
performance for different fatigue levels as
described above.
RF characterization and optimization
are performed using the ANSYS HFSS
full-wave EM simulator that solves the Figure 7: Surface current distribution without, and
Table 1: Fatigue modes and their driving force. full system of Maxwell equations. RF with RDL, UBM, and solder ball fatigue.
10 Chip Scale Review January • February • 2020 [ChipScaleReview.com]
10