dc.description.abstract | The flip chip technology has been the dominating packaging solution for
high performance chips and will remain so in the foreseeable future due to its
shorter electrical connection length between the chip and substrate. As the chip
complexity increases, the I/O density on each chip also increases. To
accommodate the continuing rise of the I/O density, the diameter of the flip chip
solder joints must shrink. At present, the diameter of a solder joint is about 100
μm, and it will be reduced to 50 μm soon. It means that the average current
density in such a 50 μm joint is about 103 A/cm2 when a 0.02 A current is
applied. Electromigration in flip-chip solder joints has become a serious
reliability concern when the current density reaches the 103 A/cm2 level, which
is about two orders of magnitude smaller than that in Al and Cu interconnects.
The reason for this lower threshold current density to cause electromigration in
solders has been pointed out to be the combination of several factors in the
“critical product” of electromigration, including the higher resistivity, the
smaller Young’s modulus, and the larger effective charge of solders. This lower
threshold makes electromigration in solders now one of the major reliability
threats to microelectronic devices.
This investigation studies how electron flow distribution and vacancy
concentration gradient affect the diffusion of solder atoms in a flip-chip solder
joint under current stress. The migration of materials was traced by monitoring
the positions of 21 Pb grains of the eutectic PbSn solder joint. Experimental
results indicate that the displacements of the Pb grains were not uniform along
in the electron flow direction. Additionally, certain Pb grains exhibited lateral
displacements. The non-uniform material migration is attributable to the combined effect of electromigration and the vacancy concentration gradient,
which was caused by electromigration.
The combined effects of electromigration and thermomigration on material
migration were also examined in this study. When the direction of electron flow
is the same with temperature gradient, more solder atoms migrate. When the
direction of electron flow is opposite with temperature gradient, less solder
atoms migrate. Considering the effect of thermomigration in solder bump, the
displacements of the Pb grains were measured, and the DZ* value of Sn in
eutectic SnPb solder estimated to be -3.4×10-10 cm2/s. The calculated Z* value is
about -34.
This study also reported that the solder joints failed by local melting of
PbSn eutectic solder bump. The local melting occurred due to a sequence of
events induced by the microstructure changes of the flip chip solder joint. The
formation of a depression in current crowding region of solder joint induced a
local electrical resistance increased. The rising local resistance resulted in a
larger Joule heating, which, in turn, raised the local temperature. When the local
temperature rose above the eutectic temperature of the PbSn solder, the solder
joint melted and consequently failed. This result also shows that several points
need to be considered when we face the issues of electromigration on reliability
of flip chip solder joints. Firstly, the geometry of flip solder joints should be
designed to avoid the formation of current crowding region in solder bump.
Secondly, in order to resist the microstructure change, the higher mechanical
intensity solder need be chose. Thirdly, increasing heat dissipation of solder joint
under current stressing or choosing the solder which has higher melting point in
order to prevent the melting phenomenon occurred. | en_US |