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difference used for cycling also has a big
impact on TCoB. A ball’s size, as well as
the composition of the balls are of great
importance for increasing the TCoB
robustness. By adjusting the package
thickness, only a minor impact is seen.
On the other hand, choosing the wrong
RF board-sheet or -stack, may have one
of the biggest impacts on reducing the
TCoB reliability. The type of RF laminate
used with the hybrid PCB is one of the
major factors determining the solder
joint lifetime [3,4]. For setting up the
application system, attention must be
paid to the TCoB differences of a free
test board compared to a PCB within a
real system housing. Solder mask defined Figure 2: Overview of typical TCoB characteristics.
(SMD) pads on board versus non-solder
mask defined (NSMD) pads typically
yield lower numbers for TCoB cycling.
Finally, yet importantly, introducing
UBM shows a major advantage compared
to solutions without a UBM layer. For all
possible measures, the consistency of the
fatigue modes always has to be studied
and verified.
For analyzing UBM fatigue, the loading
at the liner interface is investigated
using thermomechanical finite element
simulation. The highest tensile normal
loading is at low temperature, which is
reasonable because the solder material
of the balls creeps less with lower
temperature allowing for higher elastic
stresses at lower temperature. On account
of the tilt of the solder balls, the tensile Figure 3: UBM loading analysis plot showing a view of the normal loading at the liner interfaces across the
normal stresses are located at the side package at -40°C. Tensile stresses (in red) are located at the side of the interface oriented towards the perimeter
of the interface oriented towards the of the package. At the right of the plot is a close-up view of a single interface (at a different scale).
perimeter of the package (Figure 3).
This finding correlates with experimental
results from delayering (Figure 1).
For understanding RDL fatigue, the
RDL layer is included in the simulation
model and the first principal stress in the
copper is investigated. As seen with the
UBM fatigue, the highest RDL stress
loading occurs at low temperature and
at the sides of the tear drops oriented
towards the perimeter of the package
(see Figure 4). Furthermore, the stress
on the RDL is higher for wider RDL
connections. This finding correlates with
experimental results (Figure 1).
Solde r fat ig ue is a n a ly z e d by
calculating the accumulated strain
increment. This describes the amount
of inelastic work done on the solder
material per cycle (referred to as “damage
parameter”). The higher the damage Figure 4: RDL loading analysis plot. The left side of the plot shows the first principal stress in the RDL layer for a
parameter, the earlier the solder balls part of the package at -40°C. Tensile stresses (in red) are located at the side of the RDL layer oriented towards the
perimeter of the package. The right side of the figure shows a close-up view of a single tear drop (at a different scale).

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