Page 49 - Chip Scale Review_January February_2023-digital

Figure 11: Fiber pistoning force for various fiber free lengths secured inside the
package of the pigtail fiber ribbons. The solder reflow load case is read along the
right axis. As the length is reduced, a higher pistoning force is measured on the
V-groove because there is less bending to accept the various strains induced by the
load cases.
With the fibers coupled to V-grooves, Figure 11 shows the fiber
pistoning force generated at the interface for loading conditions, where
the forces for the solder reflow condition are plotted along the right
axis, while other conditions are plotted along the left axis. The cold
conditions (room, operating and environmental stress temperatures)
all follow a similar trend—the force decreases with an increase in fiber
length. This is coherent with the buckling phenomenon; once the fiber
undergoes instability and lateral deformation, the axial force reaches a
plateau and does not increase with additional increments in compressive
displacement of its ends. The force required to reach this plateau
increases sharply with decreasing length, which explains the drastic
increase in pistoning force for shorter fibers. For the solder reflow load
case, the force does not vary as much with length as compared to the
other loading conditions—excluding assembly pre-stress for which the
pistoning force is negligible. While it may be desirable to reduce the
fiber free length in order to minimize the footprint of the package, it is
clear that a short fiber presents a greater reliability risk when used in
this architecture. We clearly see the benefit of using a longer fiber free
length inside the package to reduce stress.
In the case of attachment on a stiffener, a similar trend is observed,
where longer fibers have more space to generate the required
displacements to accommodate the strain variations caused by
temperature changes. Another significant trend is that the maximum
and minimum stresses increase in magnitude with increasing bend
angle. Although the bend intuitively introduces a certain level of strain
relief, the bending stress rapidly increases with the angle, because
the bending of the fiber is directly related to the exit angle and offset.
Maximum curvature increases with the exit angle and generally
decreases with length—this indicates that shorter lengths might be
more challenging with regards to stress and temperature variations.
The pistoning forces calculated in the simulations are significantly
smaller when introducing an initial internal bend, showing the ability

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