318
S. Lee
As previously explained, the diffusion mechanism of Cu and Ni atoms occurs
by a schematic illustrated in Fig. 10.62, where the IMC layer acts as a diffusion
barrier [54], which can be illustrated by longer blue arrows and thinner red arrows in
Fig. 10.62a and green arrows in Fig. 10.62b. Therefore, EM-induced failure was not
observed for reflow-joints (Fig. 10.58) during the EM-aging test period that the TCB
joint failed at as the diffusion of Cu and Ni atoms was highly blocked and impeded
by a thick layer of IMC scallops, which left the solder joints intact at both interfaces
and integral within the SAC 305 matrix. Sn–Cu compounds were verified to have
better EM resistance than eutectic SAC solders [79].
EM-induced damages would result from the correlations of the growth of voids,
increased current density, Joule heating and temperatures, which is explained by a
positive feedback loop (Fig. 10.63) [76]. An initial high current density causes void
growth and cross-sectional degradation, increasing the local current density that
causes a rise in temperature owing to local Joule heating. This local Joule heating
can be expressed as power, P, stated in Ohm’s law in Eq. (10.2). Local Joule heating
occurs when the current, I, passes through a solder joint producing heat. The heat
accelerates diffusion, which further stimulates the void growth.
P = I
2
∗ R
(10.2)
Figure 10.64 shows the temperatures on the top surfaces of silicon dies of the
reflow and TCB flip-chip packages. In addition to the temperature of the preheated
hot plate at 120 °C, the temperature of both the silicon dies increased with time,
but TCB joints exhibited the higher temperature increase than the reflow joints.
Consequently, TCB joints failed upon EM-aging test as the package was observed
with an open circuit, while the temperature of the reflow joints reached a plateau and
was maintained. It was also observed from a similar EM-aging test that the initial
resistance value for TCB solder joints is higher than that for reflow solder joints (in the
Supporting Information). Also, the resistance profile in the Supporting Information
also correlates to the temperature profile depicted in Fig. 10.64. The greater increase
in temperature of the TCB joint than that of the reflow joint indicates that the growth
of voids within TCB joint takes place more severely during the EM test, which
was caused by the positive feedback, i.e. the increased local current density, then
the joule heating and the temperature rise, resulting back in the growth of voids
(Fig. 10.63). Besides, the positive temperature coefficient of resistance [80] of metal
(solder joints) could further increase the joule heating, although this effect is not
predominantly governing the temperature increase.
Therefore, this higher power generates local heating on the TCB-processed solder
joints, which is known as Joule Heating effect, that increases the temperature of the
solder joints. This self-heating phenomenon was observed in TCB flip-chip test
coupons under EM-aging test. The degree of self-heating between the two processed
flip-chips was up to twice as higher for the TCB flip-chip packages (Fig. 10.64).
This Joule Heating effect also generates temperature gradients, which fuels TM.
Elevated temperatures cause an increase in the average speeds of atomic movements.
Atoms in hotter regions have a greater probability of dislocation than in colder
S. Lee
As previously explained, the diffusion mechanism of Cu and Ni atoms occurs
by a schematic illustrated in Fig. 10.62, where the IMC layer acts as a diffusion
barrier [54], which can be illustrated by longer blue arrows and thinner red arrows in
Fig. 10.62a and green arrows in Fig. 10.62b. Therefore, EM-induced failure was not
observed for reflow-joints (Fig. 10.58) during the EM-aging test period that the TCB
joint failed at as the diffusion of Cu and Ni atoms was highly blocked and impeded
by a thick layer of IMC scallops, which left the solder joints intact at both interfaces
and integral within the SAC 305 matrix. Sn–Cu compounds were verified to have
better EM resistance than eutectic SAC solders [79].
EM-induced damages would result from the correlations of the growth of voids,
increased current density, Joule heating and temperatures, which is explained by a
positive feedback loop (Fig. 10.63) [76]. An initial high current density causes void
growth and cross-sectional degradation, increasing the local current density that
causes a rise in temperature owing to local Joule heating. This local Joule heating
can be expressed as power, P, stated in Ohm’s law in Eq. (10.2). Local Joule heating
occurs when the current, I, passes through a solder joint producing heat. The heat
accelerates diffusion, which further stimulates the void growth.
P = I
2
∗ R
(10.2)
Figure 10.64 shows the temperatures on the top surfaces of silicon dies of the
reflow and TCB flip-chip packages. In addition to the temperature of the preheated
hot plate at 120 °C, the temperature of both the silicon dies increased with time,
but TCB joints exhibited the higher temperature increase than the reflow joints.
Consequently, TCB joints failed upon EM-aging test as the package was observed
with an open circuit, while the temperature of the reflow joints reached a plateau and
was maintained. It was also observed from a similar EM-aging test that the initial
resistance value for TCB solder joints is higher than that for reflow solder joints (in the
Supporting Information). Also, the resistance profile in the Supporting Information
also correlates to the temperature profile depicted in Fig. 10.64. The greater increase
in temperature of the TCB joint than that of the reflow joint indicates that the growth
of voids within TCB joint takes place more severely during the EM test, which
was caused by the positive feedback, i.e. the increased local current density, then
the joule heating and the temperature rise, resulting back in the growth of voids
(Fig. 10.63). Besides, the positive temperature coefficient of resistance [80] of metal
(solder joints) could further increase the joule heating, although this effect is not
predominantly governing the temperature increase.
Therefore, this higher power generates local heating on the TCB-processed solder
joints, which is known as Joule Heating effect, that increases the temperature of the
solder joints. This self-heating phenomenon was observed in TCB flip-chip test
coupons under EM-aging test. The degree of self-heating between the two processed
flip-chips was up to twice as higher for the TCB flip-chip packages (Fig. 10.64).
This Joule Heating effect also generates temperature gradients, which fuels TM.
Elevated temperatures cause an increase in the average speeds of atomic movements.
Atoms in hotter regions have a greater probability of dislocation than in colder