16 Processing and Reliability of Solder Interconnections …
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16.2.1.2 Pb-Free Solders—“High Ag” Alloys
The time frame during which, the most significant growth has taken place for stacked
packages, also coincided with the consumer electronics industry rapid change over to
Pb-free solutions. Eutectic Sn–Pb solder balls destined for both first- and second-level
interconnections were replaced with Sn–Ag–Cu alloy balls. The predominant Sn–
Ag–Cu alloy used initially in Pb-free PoP and PoPoP components was the 96.5Sn–
3.0Ag–0.5Cu (SAC305) solder. This composition is near eutectic, having T s and
T l that are nearly equivalent at 217 °C. This melting temperature is considerably
higher than that of Sn–Pb solder. Therefore, the SAC305 has a peak reflow process
temperature in the range of 235–245 °C. The obvious concern with the higher process
temperatures is an increased warpage experienced by each individual package.
There are also companion alloys that have slightly different compositions, but with
the SAC305 solder, that are categorized together as the “high Ag” solders: 95.5Sn–
4.0Ag–0.5Cu (SAC405), 95.5Sn3.9Ag0.6Cu (SAC396), and 95.5Sn–3.8Ag-0.7Cu
(SAC387). These solders perform very similarly to the SAC305 alloy. They are only
slightly off the ternary eutectic composition, which adds from one (1) to four (4)
Celsius degrees to their respective liquidus temperatures.
16.2.1.3 Pb-Free Solders—“Low Ag” Alloys
The second set of Pb-free solders are the “low Ag” alloys that were developed,
initially, to improve the resistance of BGA solder joints to drop-shock failure. These
alloys have now experienced use in the stacked package arena for the same reasons.
The basic compositions are 98.3Sn–1.2Ag–0.5Cu (SAC125) and 98.5Sn–1.0Ag–
0.5Cu (SAC105). There has been a rapid proliferation of other quaternary as well
as higher-order alloys with elemental additions such as Ni, Mn, etc. that allow the
assembled component to meet or exceed the JEDEC requirements for drop shock
resistance [5, 6]. For example, Wang, et al. who showed that the SAC105 solder ball
provided better performances than did underfilled, SAC305 interconnections [7]. A
particular drawback of all of the low-Ag alloys, is a higher liquidus temperature, in
some cases, reaching 228 °C. The higher liquidus temperature drives up the peak
reflow temperature in the assembly process, which in turn, increases the risk of
package warpage.
16.2.1.4 Mixed Solder Joints
Mixed solder interconnections are defined as those joints that were formed by the
combination of two different solder compositions, one arising from the solder ball
and different composition derived the (assembly) solder paste. This scenario, which
is referred to as backwards compatibility, is likely to take place with the introduction
of stacked packages into high-reliability applications (military, space, etc.). The PoP
477
16.2.1.2 Pb-Free Solders—“High Ag” Alloys
The time frame during which, the most significant growth has taken place for stacked
packages, also coincided with the consumer electronics industry rapid change over to
Pb-free solutions. Eutectic Sn–Pb solder balls destined for both first- and second-level
interconnections were replaced with Sn–Ag–Cu alloy balls. The predominant Sn–
Ag–Cu alloy used initially in Pb-free PoP and PoPoP components was the 96.5Sn–
3.0Ag–0.5Cu (SAC305) solder. This composition is near eutectic, having T s and
T l that are nearly equivalent at 217 °C. This melting temperature is considerably
higher than that of Sn–Pb solder. Therefore, the SAC305 has a peak reflow process
temperature in the range of 235–245 °C. The obvious concern with the higher process
temperatures is an increased warpage experienced by each individual package.
There are also companion alloys that have slightly different compositions, but with
the SAC305 solder, that are categorized together as the “high Ag” solders: 95.5Sn–
4.0Ag–0.5Cu (SAC405), 95.5Sn3.9Ag0.6Cu (SAC396), and 95.5Sn–3.8Ag-0.7Cu
(SAC387). These solders perform very similarly to the SAC305 alloy. They are only
slightly off the ternary eutectic composition, which adds from one (1) to four (4)
Celsius degrees to their respective liquidus temperatures.
16.2.1.3 Pb-Free Solders—“Low Ag” Alloys
The second set of Pb-free solders are the “low Ag” alloys that were developed,
initially, to improve the resistance of BGA solder joints to drop-shock failure. These
alloys have now experienced use in the stacked package arena for the same reasons.
The basic compositions are 98.3Sn–1.2Ag–0.5Cu (SAC125) and 98.5Sn–1.0Ag–
0.5Cu (SAC105). There has been a rapid proliferation of other quaternary as well
as higher-order alloys with elemental additions such as Ni, Mn, etc. that allow the
assembled component to meet or exceed the JEDEC requirements for drop shock
resistance [5, 6]. For example, Wang, et al. who showed that the SAC105 solder ball
provided better performances than did underfilled, SAC305 interconnections [7]. A
particular drawback of all of the low-Ag alloys, is a higher liquidus temperature, in
some cases, reaching 228 °C. The higher liquidus temperature drives up the peak
reflow temperature in the assembly process, which in turn, increases the risk of
package warpage.
16.2.1.4 Mixed Solder Joints
Mixed solder interconnections are defined as those joints that were formed by the
combination of two different solder compositions, one arising from the solder ball
and different composition derived the (assembly) solder paste. This scenario, which
is referred to as backwards compatibility, is likely to take place with the introduction
of stacked packages into high-reliability applications (military, space, etc.). The PoP