500
P. Vianco and M. Neilsen
alloys such as the SAC105 (98.5Sn–1.0Ag–0.5Cu) and SAC 125 (98.3Sn–1.2Ag–
0.5Cu) as well as later ternary and quaternary compositions. McCormick and coworkers also compared the TMF performance of the high- and low-Ag, Sn–Ag–
Cu alloys for the temperature range of −40 to 125 °C [30]. Although inferior in
drop shock, their findings confirmed the superior TMF performance of the high-Ag,
SAC305 and SAC405 alloys in temperature cycling versus the low-Ag SAC105 and
SAC125 solders. Other studies have examined the drop-shock behavior of the low-Ag
solders, including test matrices that illustrated the synergistic roles of the underfill
materials [14, 31].
Surface Finish Effects
The second supporting technology that has undergone rapid changes concurrent
with PoP and PoPoP technology is surface finishes. A variety of surface finishes are
available for the substrates (interposers) of the individual packages as well as on
the PCB. The surface finish affects the drop-shock reliability of stacked packages
because the high-strain rate nature of the deformation “forces” the failure path out
of the bulk solder and to the solder/pad interface. Fracture usually occurs in the IMC
layer, the development of which is determined directly by both the surface finish and
the solder alloy [18].
The authors, Y. Ejiri, et al. used a rapid shear test to simulate the loads placed on
solder balls under high strain rate events such as the drop test [32]. They concluded
that the combined effects of IMC thickness, which depends upon the solder composition, surface finish, and aging conditions, as well as the “bulk” solder strength,
had contributing roles in the overall failure behavior of the joints. The results of
these experiments are critical towards deciphering the extent to which the solder
microstructure-strength relationship and the interface physical metallurgy control
the fracture behavior of interconnections in the considerably more complicated
configuration of stacked packages.
Importance of Test Standards
Mechanical shock and vibration effects are also strong functions of the PWA design;
the manner in which the PWA is anchored in the next-assembly; as well as the
use environment. These properties vary considerably between applications, causing
significant challenges when an attempt is made to compare relative performance
attributes. The JEDEC standard for drop shock testing provides for a consistent
test methodology with which to compare data from across different materials sets.
Several authors have examined the effect of location on the test vehicle with the
failure behavior [31, 33]. The authors observed that different failure rates occurred
at different locations on the PWA location—that observation was anticipated because
a higher degree of board bending occurs in the PWA center versus near the support
structures. However, the important finding was that the failure mode also changed
P. Vianco and M. Neilsen
alloys such as the SAC105 (98.5Sn–1.0Ag–0.5Cu) and SAC 125 (98.3Sn–1.2Ag–
0.5Cu) as well as later ternary and quaternary compositions. McCormick and coworkers also compared the TMF performance of the high- and low-Ag, Sn–Ag–
Cu alloys for the temperature range of −40 to 125 °C [30]. Although inferior in
drop shock, their findings confirmed the superior TMF performance of the high-Ag,
SAC305 and SAC405 alloys in temperature cycling versus the low-Ag SAC105 and
SAC125 solders. Other studies have examined the drop-shock behavior of the low-Ag
solders, including test matrices that illustrated the synergistic roles of the underfill
materials [14, 31].
Surface Finish Effects
The second supporting technology that has undergone rapid changes concurrent
with PoP and PoPoP technology is surface finishes. A variety of surface finishes are
available for the substrates (interposers) of the individual packages as well as on
the PCB. The surface finish affects the drop-shock reliability of stacked packages
because the high-strain rate nature of the deformation “forces” the failure path out
of the bulk solder and to the solder/pad interface. Fracture usually occurs in the IMC
layer, the development of which is determined directly by both the surface finish and
the solder alloy [18].
The authors, Y. Ejiri, et al. used a rapid shear test to simulate the loads placed on
solder balls under high strain rate events such as the drop test [32]. They concluded
that the combined effects of IMC thickness, which depends upon the solder composition, surface finish, and aging conditions, as well as the “bulk” solder strength,
had contributing roles in the overall failure behavior of the joints. The results of
these experiments are critical towards deciphering the extent to which the solder
microstructure-strength relationship and the interface physical metallurgy control
the fracture behavior of interconnections in the considerably more complicated
configuration of stacked packages.
Importance of Test Standards
Mechanical shock and vibration effects are also strong functions of the PWA design;
the manner in which the PWA is anchored in the next-assembly; as well as the
use environment. These properties vary considerably between applications, causing
significant challenges when an attempt is made to compare relative performance
attributes. The JEDEC standard for drop shock testing provides for a consistent
test methodology with which to compare data from across different materials sets.
Several authors have examined the effect of location on the test vehicle with the
failure behavior [31, 33]. The authors observed that different failure rates occurred
at different locations on the PWA location—that observation was anticipated because
a higher degree of board bending occurs in the PWA center versus near the support
structures. However, the important finding was that the failure mode also changed