<p><a>Low reflow
temperature solder interconnect technology based on Sn-Bi alloys is currently
being considered as an alternative for Sn-Ag-Cu solder alloys to form solder
interconnects at significantly lower melting temperatures than required for
Sn-Ag-Cu alloys. </a></p>
<p>A new low temperature
interconnect technology based on Sn-Bi alloys is being considered for attaching
Sn-Ag-Cu (SAC) solder BGAs to circuit boards at temperatures significantly
lower than for homogeneous SAC joints. Microstructure development studies of
reflow and annealing, including Bi diffusion and precipitation, are important
in understanding mechanical reliability and failures paths in the resulting
heterogeneous joints. Experiments in several SAC-SnBi geometries revealed that
Bi concentration profiles deviate from local equilibrium expected from the
phase diagram, with much higher local concentrations and lower volume fractions
of liquid than expected during short-time high temperature anneals in the
two-phase region. As annealing time increased and Sn grain coarsening occurred,
the compositions and fractions revert to the phase diagram, suggesting an
“anti-Scheil” effect. A Bi interface segregation model based on Bi segregation
at Sn grain boundaries was developed to explain the Bi distribution
characteristics in Sn during two-phase annealing process. </p>
<p>Besides hybrid joints,
microstructural evolution after reflow and aging, especially of intermetallic
compound (IMC) growth at solder/pad surface finish interfaces in homogeneous
SnBi LTS joints, is also important to understanding fatigue life and crack
paths in the solder joints. This study describes intermetallic growth in
homogeneous solder joints of Sn-Bi eutectic alloy and Sn-Bi-Ag alloys formed
with electroless nickel-immersion gold (ENIG) and Cu-organic surface protection
(Cu-OSP) surface finishes. Experimental observations revealed that, during
solid state annealing following reflow, the 50nm Au from the ENIG surface
finish catalyzed rapid (Au,Ni)Sn<sub>4</sub> intermetallic growth at the
Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to
significant solder joint embrittlement during fatigue testing. Further study
found that the growth rate of (Au,Ni)Sn<sub>4</sub> intermetallic could be
reduced by In and Sb alloying of SnBi solders and is totally eliminated with Cu
addition. Fatigue testing revealed Au embrittlement is always present in solder
joints without Cu, even with In and Sb additions due to (Au,Ni)Sn<sub>4</sub>
formation. The fatigue reliability of Cu-containing alloys is better on ENIG
due to the formation of (Ni,Cu,Au)<sub>6</sub>Sn<sub>5</sub>
at the solder-surface finish interface instead of (Au,Ni)Sn<sub>4</sub>.</p>
<p>With the development of SnBi LTSs,
a new generation alloy called HRL1 stands out for its outstanding reliability
during thermal cycling and drop shock testing. This study focused on
microstructure evolution in SnBi eutectic, SnBiAg eutectic and HRL1 solders
(MacDermid Alpha) homogeneous joints and hybrid joints with SAC305 formed with ENIG
and Cu-OSP surface finishes. Experimental results revealed that with more
microalloying elements, HRL1 has significantly refined microstructure and
slower Sn grain growth rate during solid-state aging compared with SnBi and
SnBiAg eutectic alloys. Intermetallic compound growth study showed that during
solid state annealing following reflow, the (50nm) Au from the ENIG finish
catalyzed rapid (Au,Ni)Sn<sub>4</sub> intermetallic growth at the Ni-solder
interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to
significant solder joint embrittlement during creep and fatigue loading.
However, (Au,Ni)Sn<sub>4</sub> growth and gold embrittlement was completely
eliminated for HRL1 due to Cu additions in it, and HRL1 has significantly
better fatigue reliability than SnBi and SnBiAg eutectic alloys on both OSP and
ENIG surface finishes.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15074385 |
| Date | 29 July 2021 |
| Creators | Yaohui Fan (11203503) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/TIN-BISMUTH_LOW_TEMPERATURE_SOLDER_SYSTEMS_-DEVELOPMENT_AND_FUNDAMENTAL_UNDERSTANDING/15074385 |
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