3.4. Mechanical strength and fracture behavior of Cu/SAC305/Cu
joints
Fig. 6 shows the yield and shear strength of the Cu/SAC305/Cu
samples treated at varied USV time. Significant enhancement on
the yield and shear strength of the solder joints was observed after
the ultrasonic treatment. However, for the effect of USV time, no
significant variation was observed on the shear strength of the
ultrasonic-treated samples, but the yield strength decreased gradually
with increasing USV time.
To further confirm the influence of USV time on the mechanical
strength of the Cu/SAC305/Cu solder joints, the fractured surface at
the top Cu substrate after shear testing was observed under SEM
and the composition ofthe fractured surface was identified by XRD.
As seen in Fig. 7a–c and g–i, macrovoids were found in every sample
but the size of macrovoids were apparently smaller after the ultrasonic
treatment. Furthermore, all the samples behaved in a ductile
manner under shear stress as the parabolic dimples were found
on the topography of the fractured surfaces, as shown in Fig. 7d–f
and j–l. Greater number of dimples were observed in the samples
treated with USV time of 1 s–4 s. The topography of the dimples
surface in the sample treated with 6 s of USV time was seemingly
the roughest than the other counterparts.
Fig. 8 shows the XRD pattern of the fractured surface at the top
Cu substrate of every sample. As compared to the control sample,
higher intensity of -Sn (peaks located at 2 angles of 32◦,
62.5◦ and 79.5◦), Cu (peaks at 50.4◦ and 74.1◦), Ag3Sn (peaks at
37.6◦ and 39.6◦) and Cu6Sn5 (peaks at 43.3◦ and 74.6◦) phases were
found in the ultrasonic-treated samples. However, the variation on
the intensity of these phases was not significant with the effect of
increasing USV time.
The results obtained above revealed the presence of macrovoids
in the solder matrix region of every Cu/SAC305/Cu sample. As
reported in the study of Ruifen et al. (2014), the macrovoids
were formed by the evaporation of flux from the solder paste,
which was trapped in the molten solder during reflow soldering.
Upon shear testing, these macrovoids acted as the crack initiation
site and propagated within the solder matrix, possibly along
the boundaries between the -Sn phase and the eutectic phase.
This speculation is supported by the XRD analysis, in which the
intensity of Cu, -Sn, Ag3Sn and Cu6Sn5 was found to be higher
in the ultrasonic-treated samples. This is because the refinement
of solder matrix after the ultrasonic treatment has increased the
boundary area between the -Sn phase and the eutectic phase.
It is well known that USV would induce degassing action in a
liquid medium and the trapped flux gases were removed from the molten solder, contributing to the marked improvement on
the yield and shear strength of all the ultrasonic-treated samples.
The decreasing trend exhibited on the yield strength of the
ultrasonic treated samples at USV time above 1 s is correlated to
the degree of refinement of the -Sn phase. Finer grain size contributed
to higher yield strength, as reported from the study of Deng