THE EFFECTS OF Bi AND AGING ON THE MICROSTRUCTURE AND
MECHANICAL PROPERTIES OF Sn-RICH ALLOYS
André Delhaise and Doug Perovic
University of Toronto
Toronto, ON, Canada
[email protected]
Polina Snugovsky, Ph.D.
Celestica, Inc.
Toronto, ON, Canada
ABSTRACT
This paper examines the effects of Bi on the
microstructure and hardness of Sn-Bi and Sn-Cu-Bi
alloys subjected to ageing treatments at room and
elevated temperatures. One main concern with SAC
alloys that has led to research of Bi-containing alloys is
the degradation of mechanical and thermomechanical
properties due to the coarsening of microstructure
during aging, and in earlier studies, the inclusion of Bi
in the alloy results in a uniformity of microstructure and
an increase in alloy hardness. The goal of this paper and
ongoing research is to investigate whether these trends
hold for binary alloys, and to understand what
mechanisms are responsible for these effects.
Four alloys – Sn-1Bi, Sn-5Bi, Sn-0.7Cu-1Bi, and Sn0.7Cu-5Bi - were aged at room temperature for 10 days
or 28 days. Two of these, Sn-1Bi and Sn-5Bi, were
aged at 100oC for 7 days, and were all cooled in air.
The microstructure of the samples after solidification
and aging were compared using Scanning Electron
Microscopy (SEM). Alloy hardness after solidification
and aging was measured using a Rockwell hardness
tester HR15X with a ¼” Carbide ball indenter. In alloys
containing Bi precipitates, these particles became more
uniformly distributed with aging. Hardness was
observed to not undergo any significant changes after
aging, which differs significantly from SAC alloys.
Keywords: Bismuth, Aging, Microstructure, Hardness
INTRODUCTION
With the phasing out of lead-containing alloys in the
electronics manufacturing industry as a result of
legislation such as the Restriction of Hazardous
Substances (RoHS), lead-free solders such as SAC305
(Sn-3.0Ag-0.5Cu) have become the principal joining
materials in electronic devices. The main concern with
lead is toxicity and while this issue is addressed by
utilizing lead-free solders instead, several new
problems arise. As a result, SAC alloys are less wellsuited as replacements, and has led to the continuing
development of new lead-free alloys:
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
Melting temperature – SAC alloys have a
significantly higher melting point. This has led
to the need for use of high Tg board materials,
which are more susceptible to failure modes
such as pad cratering.
Cost – With the removal of lead and the
addition of silver, material cost increases. With
a higher melting temperature, the cost of
manufacturing processes increases.
Whiskers – Pb tends to suppress whisker
formation; with the removal of lead, whiskers
can grow to lengths that may cause short
circuit failures.
Reduced reliability – Silver and copper tends
to form intermetallic compounds (IMCs) with
Sn such as Ag3Sn and Cu6Sn5 – these reduce
the toughness of the alloy and subsequently
performance in accelerated reliability tests
such as drop and thermal cycling.
Degradation of Microstructure during aging –
Numerous studies1,2,3,4 have shown that the
microstructure of SAC coarsens and
recrystallizes over time (Figure 1), which leads
to a decay in mechanical properties (Figure 2).
These concerns have led to an increased interest in
developing new lead-free solder alloys which remedy
the above issues. Bismuth (Bi) has shown to be a
promising alloying element – the Sn-Bi binary phase
diagram (Figure 3) indicates that Bi should reduce the
melting point, which would decrease manufacturing
costs, allow for the use of standard Tg board materials
and reduce the likelihood of thermal damage to
assemblies. Furthermore, the phase diagram suggests
that Bi does not form any IMCs with Sn – these phases
tend to be brittle and can reduce reliability. Rather, Bi
will exist in solid solution with tin (fully dissolved) and
possibly, depending on Bi content and temperature, also
as a secondary precipitate phase with the tin matrix. It
is therefore likely that solid-solution strengthening and
precipitation hardening by this secondary Bi phase
increase the strength of the alloy. Bi has also been
shown in some preliminary studies to inhibit the growth
of tin whiskers in a similar mechanism as Pb6.
Figure 2: Evolution in mechanical properties of
SAC305 after aging. Hardness after aging at 100C3
(top); Ultimate tensile strength after room
temperature aging3 (bottom).
Figure 1: Evolution of Microstructure of SAC305,
after aging at 125C1
Several studies have investigated the effects of bismuth
on the microstructure and mechanical properties of
aged lead-free alloys1,7,8,9,10,11. In all of these studies, it
was shown that Bi-containing alloys are more resistant
to mechanical degradation caused by aging. In one joint
UofT/Celestica study in 201410, seven Bi-containing
alloys were subjected to aging at 100oC for either 25
hours or 100 hours. The microstructure of these alloys
underwent substantial changes – deviating from a
typical dendritic as-cast microstructure with IMCs and
Bi situated in the interdendritic regions, to a more
uniform microstructure with these secondary phases
distributed equally throughout (Figure 4). This
corresponded to a marked increase in alloy hardness
Figure 3: Sn-Bi phase diagram5.
(Figure 5), indicating that Bi could potentially be used
to age- harden Pb-free solder alloys.
These
dendritic as-cast
This study is associated with the Refined
Manufacturing Acceleration Process (ReMAP)
‘Materials’ research area, specifically the ‘M3 – Aging’
project. It follows a similar methodology to this past
UofT/Celestica study, however the main alloy system
under investigation is the binary Sn-Bi system. The
effects of aging on alloy microstructure and hardness
are examined in this study.
EXPERIMENTAL APPROACH
Alloys
The binary Sn-Bi system is the primary family of alloys
under consideration in this study. This is considerably
different from the main ReMAP M3 project, which
focuses on more practical alloys (Cu and Ag additions
improve wetting properties, for example). Selecting the
binary system allows for direct analysis of the effects of
Bi on the properties of the alloy, without any potentially
influential effects by alloying elements such as Cu
and/or Ag. Bi content was chosen to be similar to the
more practical alloys under consideration by the main
M3 ReMAP project (less than 7wt%). Higher Bi
content has been shown to have a negligible influence
on alloy properties, and may produce an undesirable
microstructure. For example, at high Bi content, the
encapsulation of Sn grains by Bi precipitates severely
weakens grain boundaries (Figure 6). The alloys
considered in this study to this point contain 1, 3, 5, or
7 wt% Bi.
Figure 4: Evolution of the microstructure of
"Orchid" (Sn-2.0Ag-7.0Bi) after aging at 100C for
300h10. As cast microstructure (top); aged
microstructure (bottom).
Figure 6: Sn-10Bi aged at 100C for 3 days, showing
distribution of Bi at grain boundaries.
Figure 5: Evolution of Hardness of Bi-containing
alloys after aging at 100C10.
Cu-containing alloys are also being studied to examine
the effects of a second alloying element on
microstructure and mechanical properties. The family
of alloys chosen is the Sn-Cu-Bi system, where Cu
content is kept constant at 0.7wt% and Bi content is
varied. Bi content was chosen to directly correlate with
the selected compositions for Sn-Bi – 1, 3, 5, or 7wt%
Bi.
Sample Preparation
Samples were roughly 10g in size to allow for a
uniform temperature gradient during the aging process.
These were prepared using two master alloys – one
containing 7wt% Bi, the other containing no Bi. Pieces
of each were weighed to yield the desired compositions,
and were melted in an alumina crucible on a laboratory
hot plate. These were then cooled for seven minutes on
an iron slab at room temperature. For each alloy/aging
condition, 4 samples were prepared – two for
microstructure analysis, the other two for hardness
testing.
Aging Profiles
Alloys were left in either as-cast conditions, or
subjected to aging at room temperature or elevated
temperature. Room temperature aging involves leaving
the alloys in the laboratory at room temperature
(roughly 22oC). In this paper, only results from 10 and
28 days are shown. Elevated temperature aging was
conducted at 100oC for 7 days.
moisture after the final polish. Samples prepared in this
manner were free of moisture during SEM analysis.
This step is especially important for as-cast
microstructure.
The samples were then analyzed using SEM (Hitachi
SU-3500).
Hardness
Sample surfaces required for hardness testing were
prepared using the two coarsest grinding papers only.
Samples then underwent Rockwell hardness testing
using a 1/4” carbide ball indenter (superficial, 15X
scale). 9 readings were taken from each sample – the
highest and lowest measurements on each sample were
excluded, and the remaining measurements were
averaged. Figure 7 shows an image of a typical
indentation using this testing method.
Table 1 below gives a breakdown of the results shown
in this paper for each alloy. The main alloys being
considered are Sn-1Bi, Sn-5Bi, Sn-0.7Cu-1Bi, and Sn0.7Cu-5Bi; the as-cast hardness for the remaining alloys
was included to provide a fuller representation of how
Bi content affects hardness.
Table 1: Summary of Analyses on Alloys
Alloy
Microstructure Hardness
Sn
No
As cast only
Sn-1Bi
All
All
Sn-3Bi
No
As cast only
Sn-5Bi
All
All
Sn-7Bi
No
As cast only
Sn-0.7Cu
No
As cast only
Sn-0.7Cu-1Bi As cast, Room As cast, Room
Temperature
Temperature
Sn-0.7Cu-3Bi No
As cast only
Sn-0.7Cu-5Bi As cast, Room As cast, Room
Temperature
Temperature
Sn-0.7Cu-7Bi No
As cast only
Microstructure
To prepare the samples for Scanning Electron
Microscopy (SEM), a series of progressively finer
silicon carbide (SiC) papers and 6µm diamond paste
were used. Final polishing was done using colloidal
silica. To ensure the observed microstructure was truly
representative of the bulk alloy microstructure, sample
preparation was done immediately preceding SEM
inspection. Samples were placed in vacuum for at least
twenty minutes in order to draw out all residual
Figure 7: Indentation of 1/4" ball on Sn-0.7Cu-1Bi
sample.
RESULTS
As-cast
As-cast microstructure was analyzed for Sn-1Bi and
Sn-5Bi, as well as their copper-containing counterparts
(Figure 8). As the solid solubility of Bi in Sn at room
temperature is roughly 2wt%, Bi for the most part is not
visible in the Sn-1Bi and Sn-0.7Cu-1Bi alloys, however
small clusters are observable in places. More Bi
precipitates are observable in the copper-containing
alloy, as the presence of copper reduces the solid
solubility of Bi in Sn. The copper-containing alloy also
contains numerous Cu6Sn5 intermetallics. These are
also located in the interdendritic spaces as they form
from the last remaining liquid. The samples containing
5wt% Bi predictably show significantly more
precipitation of Bi. Bi tends to form in large clusters
and the microstructure in general is quite non-uniform.
Hardness testing of the as-cast alloys revealed that
increasing bismuth content increases hardness, for both
the binary and ternary alloys (Figure 9), and that solidsolution strengthening and precipitation hardening are
the likely mechanisms behind these changes. In close
agreement with a previous study9, it was seen that at
around 5wt% Bi, hardness starts to plateau, indicating
that optimal reliability can be achieved with bismuth
content at or around this level. Copper has a
significantly larger effect on alloy hardness at lower Bi
content – at higher Bi content the copper containing
alloy has nearly identical hardness as the corresponding
binary alloy. Pure Sn could not be measured using this
hardness scale as it was too soft.
Figure 9: As-cast hardness for Sn-xBi and Sn-0.7CuxBi alloys.
Room Temperature
The four alloys investigated after room temperature
aging did not demonstrate any appreciable changes in
microstructure after both 10 and 28 days of aging, with
the exception of the Sn-5Bi alloy (Figure 10).
The Sn-1Bi alloy microstructure remained nearly
identical owing to the single phase present. In the Sn0.7Cu-1Bi alloy, intermetallic particle size did not
change significantly, and the overall microstructure
indicates that the dendritic structure of tin remained
intact after aging. In the alloys containing 5% Bi, Bi
precipitates remained clustered, however these clusters
grew larger and precipitates became more variable in
size. These changes were more significant for the
binary alloy.
The hardness of the four alloys investigated did not
change considerably with aging time at room
temperature (Figure 11). This is a distinctly different
result that what was observed for SAC3, in which a
pronounced decay in strength was observed after just
five days.
Figure 8 (opposite): From top to bottom, as-cast
microstructure of Sn-1Bi, Sn-0.7Cu-1Bi, Sn-5Bi,, and
Sn-0.7Cu-5Bi.
Figure 11: Hardness of select Sn-xBi and Sn-0.7CuxBi alloys after room temperature aging.
High Temperature
Only two alloys were aged at 100oC to this point in the
study – Sn-1Bi and Sn-5Bi. The microstructure of the
former remained largely the same as the as-cast after
aging. In the latter, the microstructure changed
significantly – the Bi particles became more uniformly
distributed throughout the microstructure (Figure 12).
The solvus temperature (at which all precipitates fully
dissolve into the matrix upon heating) of Sn-5Bi is
approximately 60oC. Therefore, heating the sample to
100oC allows for all Bi to enter solid solution. In
addition, as diffusion occurs more rapidly at higher
temperatures, Bi can distribute itself more equally
throughout the matrix, and precipitation is more
uniform upon cooling to room temperature. In the 1Bi
alloy, aging at a high temperature ensured all
precipitates fully dissolved in the Sn matrix, and hence
no Bi precipitates were visible, unlike in the as-cast and
room temperature-aged samples.
Alloy hardness does not change considerably with time
at 100oC (Figure 13). This is comparable to several
earlier studies1,7,10, which showed that the strength of
bismuth-containing alloys is more stable after hightemperature ageing.
DISCUSSION
It was seen that the addition of copper to the alloy has a
larger effect on hardness when Bi content is low, and
has very little effect at higher (~5wt%) levels of Bi in
the alloy. This is likely the result of competing
mechanisms of Bi solid solution strengthening, Bi
precipitation, and Cu6Sn5 IMCs.
The reason precipitates were visible in the as-cast Sn1Bi alloy, despite the phase diagram indicating
otherwise, is because the phase diagram assumes a very
slow cooling
Figure 10 (opposite): From top to bottom,
microstructure after room temperature aging: Sn5Bi after 10d and 28d, Sn-0.7Cu-5Bi after 10d
and 28d.
rate – far slower than what occurs in reality. As a result,
as Sn solidifies in dendrites, the interdendritic material
(last remaining liquid) can become supersaturated with
Bi. As the last of the liquid solidifies, Bi can be forced
out of solid solution into precipitate form if the local Bi
content is high enough.
It was observed that the hardness of these Bi-containing
alloys does not behave in the same manner as that of
SAC after aging. This is in agreement with earlier
studies1,7,10, which show that hardness is not degraded.
It is therefore evident that Bi, in precipitate and/or solid
solution form, contributes to the stabilization of
properties.
Figure 12: Microstructure after aging at 100C for 7d.
Sn-1Bi (top); Sn-5Bi (bottom).
Figure 13: Hardness of select Sn-xBi alloys after
aging at 100C.
The dendritic tin structure, as well as IMC distribution
and size, did not undergo any noticeable changes after
aging. The only aspect of the microstructure that
showed any observable changes were the bismuth
precipitates in the alloys containing 5wt% Bi. The
clusters in general tended to grow larger and average
precipitate size decreased. In the samples aged at
elevated temperature, the Bi precipitates also spread
more evenly throughout the microstructure, due to
accelerated diffusion at higher temperature, and that all
Bi entered solid solution during the aging process. The
likely reason the microstructure did not become fully
uniform is because not enough time had elapsed.
Longer elevated temperature aging tests will be
performed to determine this. Elevated temperature tests
will also be performed on copper-containing alloys to
discern whether the Sn or IMCs undergo change in
these conditions.
CONCLUSIONS
Several Sn-Bi and Sn-0.7Cu-Bi alloys were subjected to
aging at either 100oC for 7 days, or at room temperature
for 10d or 28d. The as-cast microstructure was made up
of Sn dendrites with dissolved Bi, and depending on the
Bi/Cu content, clusters of Bi precipitates and Cu6Sn5
intermetallics situated in the interdendritic regions. For
both alloy systems, hardness was seen to increase with
Bi content (likely as a result of solid-solution
strengthening and precipitation hardening) up to
approximately 5wt% Bi, after which it plateaus. When
the same amount of copper is added to alloys with
varying Bi content, the copper increases hardness to a
greater extent at low Bi content, and the coppercontaining alloy has nearly identical hardness as the
binary alloy at higher Bi content. This is believed to be
the result of the strengthening effects of Bi and Cu6Sn5
competing with one another.
Aging the alloys at room temperature caused few
changes in the dendritic structure of the Sn or size and
distribution of the IMCs, however the Bi precipitate
clusters in general tended to grow larger and
precipitates became finer. This was especially
noticeable in the Sn-5Bi alloy. Aging at 100oC did not
result in any observable changes to the microstructure
of Sn-1Bi, however the microstructure of Sn-5Bi
became noticeably more uniform.
Hardness was not significantly changed with these
aging treatments, a significant difference from SAC
alloys. This confirmed results from earlier papers which
showed a stabilization of the mechanical properties of
Bi-containing alloys after aging.
FUTURE WORK
The results in this paper only show two time points for
room temperature aging, however aging times up to six
months will be examined in future, up to six months. It
is hoped that these results will give a clearer indication
of the effects of room temperature aging for Sn-Bi and
Sn-Cu-Bi alloys.
Further elevated temperature aging experiments are
planned, involving more alloys, time points and
temperatures. Temperatures will be selected based on
the solvus temperature of each alloy selected. Aging
above the solvus yields similar microstructures to those
shown in this paper – very uniform due to the
dissolution of Bi and evenly distributed precipitates
upon cooling. Aging below the solvus leads to particle
coarsening through a mechanism known as Ostwald
ripening. Smaller particles tend to dissolve and diffuse
towards the larger particles – leading to a non-uniform
particle size upon cooling. It is anticipated that very
different properties will be observed when comparing
alloys aged above and below the solvus.
While it is fairly straightforward to determine what the
effects of aging are on the microstructure and
mechanical properties of these alloys, it is another
challenge altogether to characterize the mechanisms
behind these changes. Transmission Electron
Microscopy (TEM) is a technique which will allow for
examination of these alloys at very high resolutions and
visualize these mechanisms – for example Bi in solid
solution will impart strain on the Sn lattice, which can
be observed using strain mapping in the TEM.
Finally, while hardness is a quick, easy method to
gauge a material’s strength, it does not give a complete
picture of deformation and failure mechanisms. More
advanced mechanical testing methods such as
nanoindentation (to determine creep properties, similar
to an earlier study12), impact testing (to gauge
toughness) and fatigue will be considered.
ACKNOWLEDGMENTS
The authors would like to thank the Department of
Materials Science & Engineering at the University of
Toronto, as well as ReMAP, for funding.
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