16 Processing and Reliability of Solder Interconnections …
503
Solder Alloy Fatigue Properties
The TMF properties of the Pb-free solders have been observed to change with
isothermal aging over time. In several studies, the solder joints were aged at relatively
high temperatures (125 °C) and durations of several months [34, 35]. Consequently,
the increased thickness of the IMC layer had a role in the failure mode. Zhao, et al.
documented the effect of aging by SAC305 and SAC105 solders on the TMF of
BGA solder joints, including lower temperatures of 25 °C [36]. Aging times of
twelve months (25 °C) caused approximately a 20% drop in characteristic lifetimes
(2P Weibull). The loss of characteristic lifetime increased to approximately 40%
when aging was performed at 55 °C for the same twelve months.
Two caveats are important when considering the above effects vis-à-vis accelerated aging tests and service or use conditions. First of all, aging at 125 °C for time
durations on the order of months is well outside the traditional qualification and
acceptance testing regiments of high-reliability systems prior to being placed in the
field where temperature cycling will take place. Secondly, it is recommended practice
to subject a PWA to an isothermal aging at 100 °C for 24 h to “stabilize” the solder
microstructure prior to commencing with temperature cycling [24]. Nevertheless, the
findings in Refs. [34–36] are not wholly unexpected when consideration is given to
the point that solders are performing at a high homologous temperature, even when
at room temperature. Changes are surely to take place to the solder microstructure,
including recovery and recrystallization, which in turn, are very likely to affect the
mechanical properties, including TMF.
Temperature Cycling—Effect of Solder Alloy Composition
Vianco and co-works have examined the effects of second-level (bottom) solder
joint composition on the TMF performance of PoP interconnections [8]. The firstlevel (top) interconnections were SAC305. Given the continued use of Sn–Pb solder
in high-reliability electronics, the bottom joints were assembled with either 100%
SAC305 or a mixed metallurgy resulting from the use of Sn–Pb paste and SAC305
solder balls (“Sn–Pb/SAC305”), which is referred to as the backwards compatibility
condition. Metallographic cross sections confirmed full intermixing between the
Sn–Pb and SAC305 solder components. The 2P Weibull failure plot is repeated
in Fig. 16.11 for PWAs that did not include underfill. The characteristic lifetimes, η,
and slope, β, are listed, below:
100% SAC305: η = 2030 ± 90 cycles; β = 7.1 ± 1.4
Sn–Pb/SAC305: η = 2660 ± 90 cycles; β = 8.6 ± 1.9
The mixed solder interconnections experienced a longer characteristic lifetime
than did the 100% SAC305 solder joints, which paves the way for system designers
to use PoP technology in a backwards compatibility scenario. The nearly identical
β values signal similar failure modes in the two solder joint genres. Metallographic
503
Solder Alloy Fatigue Properties
The TMF properties of the Pb-free solders have been observed to change with
isothermal aging over time. In several studies, the solder joints were aged at relatively
high temperatures (125 °C) and durations of several months [34, 35]. Consequently,
the increased thickness of the IMC layer had a role in the failure mode. Zhao, et al.
documented the effect of aging by SAC305 and SAC105 solders on the TMF of
BGA solder joints, including lower temperatures of 25 °C [36]. Aging times of
twelve months (25 °C) caused approximately a 20% drop in characteristic lifetimes
(2P Weibull). The loss of characteristic lifetime increased to approximately 40%
when aging was performed at 55 °C for the same twelve months.
Two caveats are important when considering the above effects vis-à-vis accelerated aging tests and service or use conditions. First of all, aging at 125 °C for time
durations on the order of months is well outside the traditional qualification and
acceptance testing regiments of high-reliability systems prior to being placed in the
field where temperature cycling will take place. Secondly, it is recommended practice
to subject a PWA to an isothermal aging at 100 °C for 24 h to “stabilize” the solder
microstructure prior to commencing with temperature cycling [24]. Nevertheless, the
findings in Refs. [34–36] are not wholly unexpected when consideration is given to
the point that solders are performing at a high homologous temperature, even when
at room temperature. Changes are surely to take place to the solder microstructure,
including recovery and recrystallization, which in turn, are very likely to affect the
mechanical properties, including TMF.
Temperature Cycling—Effect of Solder Alloy Composition
Vianco and co-works have examined the effects of second-level (bottom) solder
joint composition on the TMF performance of PoP interconnections [8]. The firstlevel (top) interconnections were SAC305. Given the continued use of Sn–Pb solder
in high-reliability electronics, the bottom joints were assembled with either 100%
SAC305 or a mixed metallurgy resulting from the use of Sn–Pb paste and SAC305
solder balls (“Sn–Pb/SAC305”), which is referred to as the backwards compatibility
condition. Metallographic cross sections confirmed full intermixing between the
Sn–Pb and SAC305 solder components. The 2P Weibull failure plot is repeated
in Fig. 16.11 for PWAs that did not include underfill. The characteristic lifetimes, η,
and slope, β, are listed, below:
100% SAC305: η = 2030 ± 90 cycles; β = 7.1 ± 1.4
Sn–Pb/SAC305: η = 2660 ± 90 cycles; β = 8.6 ± 1.9
The mixed solder interconnections experienced a longer characteristic lifetime
than did the 100% SAC305 solder joints, which paves the way for system designers
to use PoP technology in a backwards compatibility scenario. The nearly identical
β values signal similar failure modes in the two solder joint genres. Metallographic