Phase Diagrams for Lead-Free Solder Alloys

Overview
Phase Diagrams
Phase Diagrams for Lead-Free
Solder Alloys
Ursula R. Kattner
Author’s Note: The identification of any commercial product or
trade name does not imply endorsement or recommendation by the
National Institute of Standards and Technology.
The need for new, improved solder
alloys and a better understanding of
reactions during the soldering process
grows steadily as the need for smaller
and more reliable electronic products
increases. Information obtained from
phase equilibria data and thermodynamic calculations has proven to be
an important tool in the design and
understanding of new lead-free solder
alloys. A wide range of candidate
alloys can be rapidly evaluated for
proper freezing ranges, susceptibility
to contamination effects, and reactions
with substrate materials before the
expensive process of preparing and
testing candidate alloys is initiated.
INTRODUCTION
Intense international competition
makes it necessary for microelectronics
and supporting industries to design
and produce smaller, more functional,
and reliable electronic products more
economically. At the same time, there is
growing global interest in eliminating
toxic elements, such as lead, from
electronic products. In recent years,
substantial efforts were made to develop
lead-free solders that are suitable
substitutes for classic tin-lead eutectic
solders. Simultaneously, there is a
growing need for solders that can
be used for applications with more
demanding service conditions such as
in automotive, avionic, military, or
oil-exploration industries. For these
purposes, it is necessary to evaluate the
properties of candidate solder alloys
that are related to manufacturing and
reliability. Of particular interest are
properties such as freezing range
(liquidus and solidus), the effects from
2002 December • JOM
possible contamination from other
solder materials such as the formation
of low-melting eutectics, and reactions
with various substrates. A recent study
Knowledge of the
phase equilibria
of solder-alloy and
solder/substrate
systems provides the
basic roadmap for
the initial selection
of candidate solders
and contributes to
the understanding
of solder wetting
and spreading.
of high-temperature lead-free candidate
solder alloys1 showed that the only
lead-free alloys that fulfilled the initial
selection criteria (toxicity, cost, oxidation resistance, solidus temperature,
Figure 1. The calculated phase diagram
of the Sn-Pb system.16
and flux compatibility) were tin-based.
Common alloying additions are lowmelting metals, such as bismuth,
antimony, and indium, or metals forming
a eutectic reaction with (Sn), such as
silver and copper. (The elemental symbol
in parenthesis is used to distinguish
the disordered solid solution based on
this element from the pure element.)
Substrate materials may consist of
copper, copper that has been coated
or plated with tin-lead or tin-bismuth
solders, nickel-tin, nickel-gold, or
nickel-platinum alloys.
Knowledge of the phase equilibria
of solder/alloy and solder/substrate
systems provides the basic roadmap for
the initial selection of candidate solders
and contributes to the understanding
of solder wetting and spreading. Phase
equilibria data provide not only information about the liquidus and solidus
temperatures of a candidate solder alloy,
but also information about possible
intermetallic phase formation, either
within the solder during solidification
or in reaction with the substrate material by a combination of isothermal
solidification and solid-state reaction.
Reaction of substrate material that
has been pre-tinned with a tin-lead or
tin-bismuth alloy with a solder of a
different composition may result
in the formation of a low-melting
higher-component eutectic. In this
case, the multi-component phase
diagram can be used to evaluate
the possible effects resulting from such
a contamination.
Traditionally, phase diagrams are
determined from thermal analysis,
examination of microstructure, and other
experimental methods. However, the
experimental determination of phase
diagrams is a time-consuming, costly
task since the number of possible systems
45
THE CALPHAD METHOD
Figure 2. The calculated phase diagram
of the Sn-Bi system.19
increases drastically as the number of
elements increases. For example,
increasing the number of elements under
consideration from six to seven increases
the number of binary systems from 15 to
21, the number of ternary systems from
20 to 35, and the number of quaternary
systems from 15 to 35. Experimental
information for the entire phase diagram
is available for most of the binary systems
that are of interest for solders, but
experimental information becomes
increasingly sparse as the number of
constituent elements increases (i.e., for
ternary, quaternary, and highercomponent systems). It has been shown
that thermodynamic calculation of phase
equilibria with the CALPHAD method2,3
is extremely useful for obtaining
quantitative information about these
higher-component systems.
The thermodynamic descriptions that
are used with the CALPHAD method
can also be used to obtain data of other
properties that are important for understanding the wetting behavior of a molten
solder, such as surface tension and
viscosity.4,5 In addition, Lee et al.6 used
the calculation of metastable equilibria
and the driving forces for phase formation to predict the phase that forms first
at a solder/substrate interface. Furthermore, thermodynamic calculation also
provides information that is needed for
the simulation of kinetic processes. For
example, tie-line information (i.e., the
compositions of two phases in equilibrium) and thermodynamic factors for
the calculation of diffusion coefficients
are necessary for the simulation of
diffusion processes.7
46
The CALPHAD method uses a series
of models to describe the concentration,
temperature dependence, and, if necessary, pressure dependence of the Gibbs
energy functions of each individual
phase in a system.2,3 Commercial and
public domain software packages are
available for the calculation of phase
equilibria from these Gibbs energy
functions.2,3 One of these, ThermoCalc,8 was used for the calculation of
the phase diagrams shown in Figures
1 to 10. The most common models for
the description of the concentration
dependence are the regular solution
model for disordered solution phases
and the sublattice model for ordered
compounds. The choice of the sublattice
model description for solid phases
depends on their crystal structure.9 If
the homogeneity range of a phase is
narrow, this phase can also be described
as a stoichiometric phase.
The usual strategy for the assessment
of a multi-component system is to
first derive thermodynamic model
descriptions that are consistent with the
experimental data of the binary systems.
These descriptions are then used
together with a standard thermodynamic
extrapolation method to calculate ternary
and higher-order systems. If indicated
by experimental data, it is possible to
add ternary interaction terms to the
thermodynamic models to obtain a
more accurate calculation of the ternary
system. This strategy is usually followed
until the constituent systems of a
higher-order system have been assessed.
Experience has shown that no or very
minor corrections are necessary for a
reasonable prediction of quaternary and
higher-component systems.
The quality of the results that are
obtained from calculations with the
CALPHAD method depends not only
on the quality of the thermodynamic
models but also on the quality of the
available experimental information that
was used to derive the model parameters
of the individual phases. The quality of
the extrapolation of a ternary system
from the three constituent binary systems
depends on the magnitude of possible
ternary interactions and the occurrence
of ternary intermetallic compounds. For
most binary systems that are relevant
for solder, the magnitude of the binary
excess Gibbs energies is fairly small,10
indicating that ternary interactions
may not be significant. Therefore, the
accuracy of the calculated constituent
binary systems is of crucial importance
for obtaining quality calculations
of ternary and higher-component
systems.11
The efforts in the development of
thermodynamic descriptions for systems
that are relevant to solders have resulted
in several thermodynamic databases
that are either available in the public
domain4,12 or are commercial.5
ACCURACY REQUIREMENT
The knowledge of temperatures and
compositions of the invariant equilibria
and regimes of primary phase solidification is of significant importance for
the design of new solder alloys and
for the specification of manufacturing
tolerances of these solders. The liquidus
temperature changes little with composition in many tin-based systems when
(Sn) forms as a primary phase, while the
temperature dependence on composition
can be relatively large for other phases,
especially for intermetallic compounds.
If the liquidus is steep, composition
fluctuations in the solder alloy can
cause the solder to have a significantly
higher liquidus temperature than for the
nominal composition.
The invariant temperatures of solder
alloys are usually known with fairly high
accuracy while there is usually a larger
variation in the reported composition
of the liquid phase at the invariant
temperature. This is, in part, due to the
Figure 3. The calculated phase diagram
of the Sn-In system.19
JOM • December 2002
Figure 4. The calculated phase diagram
of the Sn-Sb system.22
fact that temperatures between 50°C and
300°C can be easily measured with high
accuracy. However, the interpretation
of the microstructure of an alloy with
near-eutectic composition is not always
a straightforward task. The phase
observation may be misleading due
to a phenomenon caused by a skewed
coupled zone.13 In this case, the fast
growth kinetics of the unfaceted phase,
(Sn) in most solder alloys, may lead
to the formation of dendrites of this
phase even on the other side of the
eutectic composition where the faceted,
frequently intermetallic phase, should
form primary dendrites. This phenomenon may result in an overestimation
of the extent of the primary solidification area of the unfaceted phase. The
interpretation of the microstructure in
higher-component alloys can be even
more difficult. Moon et al.11 found
that the remaining liquid phase had
a tendency for supercooling after
the initial formation of the primary
intermetallic phase in tin-rich alloys of
the Sn-Ag-Cu system. The sequence of
solidification of this supercooled liquid
consisted of dendritic (Sn), coupled
formation of non-faceted (Sn), and a
faceted intermetallic phase (Ag3Sn or
Cu6Sn5, depending on alloy composition), followed by the ternary eutectic
reaction. This solidification behavior
resulted in microstructures with a
smaller amount of eutectic structure
than is expected from the equilibrium
phase diagram.
The fact that the slope of the liquidus
in tin-based systems with intermetallic
phases is usually very steep may lead
2002 December • JOM
to insufficiently accurate calculation of
a system. Moon et al.11 showed that,
although the agreement between the
calculated and measured liquidus data
for the binary Ag-Sn and Cu-Sn system
is satisfactory, it was not possible to use
the descriptions of these binary systems
to obtain an accurate fit of the ternary
tin-rich part of the Ag-Cu-Sn system. It is
known that the results of the calculation
of higher component systems indicate
whether further refinements of the
thermodynamic description of a binary
system are needed.14
BINARY SYSTEMS
A summary of the evaluated phase
diagrams and available assessments
of binary and ternary systems in the
literature has been compiled.12 Most
binary phase diagrams that are of interest
for solder alloys are fairly simple
and most of the phase boundaries are
well established. Since the binary tinbased systems are key systems for the
evaluation of candidate solder alloys,
important features of these systems will
be briefly discussed. The elements that
are most often considered as alloying
elements to tin for lead-free solders are
Ag, Bi, Cu, In, and Sb. Although its
susceptibility to corrosion is a concern,
zinc is occasionally considered as an
alloying element for solders because
the temperature of the binary Sn-Zn
eutectic is similar to that of the Sn-Pb
eutectic. The elements gold and nickel
are of interest since they are frequently
used in substrate materials. Although
the present goal is to eliminate lead from
solder alloys because of its toxicity, the
knowledge of this system is still relevant
for either the study of the mechanisms
that take place during soldering or
the effects that may be caused in new
lead-free solders that are applied to
components that were pre-tinned with
classical Sn-Pb solder. The transformation from the high-temperature form,
βSn, to the low-temperature form, αSn
(Tβ α = 13°C), can be neglected since
it does not affect the equilibria with
other phases. Also, the transformation,
which is kinetically inhibited, rarely
occurs in solders.15
Sn-Pb
The phase diagram of this system
is well established and a number of
thermodynamic assessments are available for this simple eutectic system.
At 183°C, the liquid with xSn = 0.619
decomposes into the two terminal solid
solutions, (Pb) and (Sn). (All compositions are given in weight fraction
unless otherwise noted.) In addition
to the stable phase equilibria data, the
thermodynamic assessments of Fecht
et al.16 and Ohtani et al.17 also used
data for metastable equilibria. The
major differences between the two
assessments are that Ohtani et al. also
considered the pressure dependence
of the phases while Fecht et al. emphasized an accurate reproduction of the
experimental quantities at atmospheric
pressure. The phase diagram obtained
from the assessment of Fecht et al. is
shown in Figure 1.
Sn-Bi
Most parts of the phase diagram of
this simple eutectic system are well
established. At 138°C, the liquid with
xSn = 0.43 decomposes into the two
terminal solid solutions, (Bi) and (Sn).
However, the solubility limit of tin in
(Bi) is not reliably known although
a lower value is preferred.18 Lee et
al.19 accepted this lower value for the
(Bi) homogeneity range and also used
experimental data of the Sn-Bi-In system
for the refinement of the description of
the Sn-Bi system. The phase diagram
obtained from this assessment is shown
in Figure 2.
Sn-In
In addition to liquid and the two
terminal solution phases (In) and (Sn),
Figure 5. The calculated phase diagram
of the Sn-Ag system.24
47
established. The two available thermodynamic assessments10,24 give very similar
results. Moon et al.11 pointed out that in
both assessments, the liquidus for
primary Ag3Sn formation needs further
refinement. The phase diagram obtained
from the assessment of Oh et al.24 is
shown in Figure 5.
Sn-Zn
Figure 6. The calculated phase diagram
of the Sn-Zn system.25
this system has two intermediate phases,
β and γ. Both intermediate phases form
through a peritectic reaction from the
liquid and one of the terminal solution
phases. At a temperature of 120°C, the
liquid with xSn = 0.491 decomposes into
the two intermediate phases. The
evaluation by Okamoto20 concludes that
most of the boundaries of the solid phases
need to be better established for
concentrations with xSn ≥ 0.75. The two
thermodynamic assessments19,21 available are based on the evaluation by
Okamoto. The phase diagram obtained
from the assessment of Lee et al.19 is
shown in Figure 3.
Sn-Sb
In addition to liquid and the two
terminal solution phases, (Sb) and (Sn),
this system has two intermediate phases,
SbSn (β) and Sb3Sn2. Both intermediate
phases, as well as (Sn), form by peritectic
reactions. The peritectic reaction forming
(Sn) occurs at a temperature of 250°C.
The two available thermodynamic
assessments17,22 are based on the experimental work of Predel and Schwermann.23 The phase diagram obtained
from the assessment of Jönsson and
Ågren22 is shown in Figure 4.
Sn-Ag
In addition to liquid and the two
terminal solution phases, (Ag) and (Sn),
this system has two intermediate phases,
(ζAg) and Ag3Sn. Both intermediate
phases form by peritectic reactions. The
eutectic reaction in which liquid with
xSn = 0.965 decomposes into Ag3Sn and
(Sn) at a temperature of 221°C is well
48
The phase diagram of this system is
relatively well established and a number
of thermodynamic assessments are
available for this simple eutectic system.
At 198.5°C, the liquid decomposes into
the two terminal solid solutions, (Zn)
and (Sn). However, the composition
reported for the liquid phase at the
eutectic temperature varies between xSn
= 0.906 and xSn = 0.921. The phase
diagram obtained from the assessment
of Lee25 is shown in Figure 7.
Sn-Cu
In addition to liquid and the two
terminal solution phases, (Cu) and (Sn),
this system has seven intermediate
phases, β, γ, Cu41Sn11 (δ), Cu10Sn3 (ζ),
Cu3Sn (γ), and Cu6Sn5/Cu6Sn5’ (η/η1,
high- and low-temperature forms). All
of the intermediate phases form by
peritectic or peritectoid reactions. All of
the copper-rich intermediate phases
decompose in eutectoid reactions at
temperatures above 350°C and, therefore, only the Cu3Sn and Cu6Sn5/Cu6Sn5´
phases are of interest for solder applications. The temperature of 227°C for the
eutectic reaction, where liquid decomposes into Cu6Sn5 and (Sn), is well
established. However, various evaluations of this system disagree on the exact
composition of the liquid phase, either
xSn = 0.91 or xSn = 0.93. Moon et al.11
showed that the composition of xSn =
0.91 is consistent with the eutectic
temperature and the slope of the liquidus
for primary (Sn) formation. The two
available thermodynamic assessments26,27 give very similar results. Moon
et al.11 pointed out that the liquidus for
primary Cu6Sn5 formation needs further
refinement. The phase diagram obtained
from the assessment of Shim et al.26 is
shown in Figure 7.
Sn-Au
In addition to liquid and the two
terminal solution phases, (Au) and (Sn),
this system has six intermediate phases,
Au10Sn (β), (ζAu), Au5Sn (ζ´), AuSn (δ),
AuSn2 (γ), and AuSn4 (η). Except for the
ζ´ phase, which forms peritectoidally,
and the δ (AuSn) phase, which forms
congruently, the other intermediate
phases form by peritectic reactions. The
phase diagram obtained from the
thermodynamic assessment by Chevalier28 is consistent with the phase diagram
that was accepted by Okamoto and
Massalski. 29 Since then, the phase
equilibria involving the Au10Sn, (ζAu),
and Au 5Sn phases were revised, 30
extending their stability ranges to lower
temperatures. However, only the AuSn,
AuSn2, and AuSn4 phases are of interest
for solder applications. The tin-rich
eutectic occurs at a temperature of 217°C
and a liquid composition of xSn = 0.90
(xSn = 0.918 calculated). The tin-rich
AuSn4 phase decomposes eutectoidally
at a temperature above room temperature, according to the diagram of
Okamoto and Massalski. Although the
exact temperature is not known, the
calculated decomposition temperature
is 104°C. The phase diagram obtained
from the assessment by Chevalier28 is
shown in Figure 8.
Sn-Ni
In addition to liquid and the two
terminal solution phases, (Ni) and (Sn),
this system has five intermediate phases:
Ni 3Sn (high- and low-temperature
forms), Ni 3 Sn 2 (high- and lowtemperature forms), and Ni3Sn4. The
high-temperature forms of Ni3Sn and
Ni3Sn2 form congruently from the liquid
phase and Ni3Sn4 forms by a peritectic
Figure 7. The calculated phase diagram
of the Sn-Cu system.26
JOM • December 2002
Figure 8. The calculated phase diagram
of the Sn-Au system.28
reaction. The tin-rich eutectic occurs at
a temperature of 231.2°C and a liquid
composition of xSn = 0.998. The only
available thermodynamic assessment of
this system was carried out by Ghosh;31
the calculated phase diagram is shown
in Figure 9.
TERNARY AND HIGHER
COMPONENT SYSTEMS
Critical evaluations of experimental
data are available for ternary Ag, Al, and
Au systems,32 and Cu systems.33 Also,
Villars et al.34 compiled a summary of
the available experimental data for most
ternary systems. Many solder alloys are
included in these references and,
therefore, only systems that are most
relevant for solder applications will be
discussed here.
Sn-Ag-Bi
The phase diagram of this system was
experimentally determined by Hassam
et al.35 No ternary phases have been
reported, and the solid phases have fairly
small ternary homogeneity ranges. The
ternary eutectic is reported to occur at
138.4°C where the liquid with xAg =
0.010 and xBi = 0.563 decomposes into
the two terminal solid solutions, (Bi)
and (Sn), and the binary Ag-Sn phase,
Ag3Sn. The experimental phase diagram
is in good agreement with the one
predicted by Kattner and Boettinger10
using thermodynamic extrapolation of
the descriptions of the binary systems.
The tin-rich part of the liquidus projection obtained from this calculation is
shown in Figure 10a. Ohtani et al.36
used the available experimental data
2002 December • JOM
for a refinement of the description
of the ternary systems. However, the
differences between the results for the
tin-rich part of the system from the two
calculations are not significant.
liquid phase. However, it should be
noted that the intermediate phase,
Sb2Sn3, of the binary Sn-Sb system
was not detected in this experimental
work.
Sn-Ag-Cu
Sn-Bi-Cu
The phase diagram of this system was
experimentally determined by Gebhardt
and Petzow.37 No ternary phases have
been reported and the solid phases
have fairly small ternary homogeneity
ranges. The invariant reaction in the
tin-rich corner was first reported to
be non-eutectic. More recent work38,39
showed that this reaction is eutectic
with a temperature of 217 ± 0.2°C,
and the liquid decomposes into (Sn)
and the binary intermediate compounds
Ag3Sn and Cu6Sn5. However, there was
disagreement on the composition of the
liquid phase at the eutectic temperature.
The experimental work of Moon et al.11
confirmed the composition of the liquid
phase as xAg = 0.035 and xCu = 0.009
and the temperature as 217.2°C. The
calculated tin-rich part of the liquidus
projection is shown in Figure 10b.
No experimental phase diagram data
are available for this system. It can
be expected that the thermodynamic
extrapolation of the description of the
binary systems gives a fairly accurate
prediction of the ternary phase diagram,
as was the case for the Sn-Ag-Bi system.
The liquidus projection obtained from
the binary descriptions19,26,41 is shown
in Figure 10c. The predicted tin-rich
eutectic occurs at a temperature of
138.8°C and the composition of the
eutectic liquid phase is xBi = 0.428 and
xCu = 0.0004.
Sn-Ag-In
Few experimental phase diagram data
are available for this system. Korhonen
and Kivilahti21 used six ternary alloys
that were annealed at 250°C to gain
information about phase boundary
locations and differential scanning
calorimetry (DSC) to investigate the
melting/solidification behavior of the
alloys. The temperature of the ternary
eutectic where the liquid decomposes
into Ag2In and the intermediate Sn-In
phases β and γ was reported to be
about 113°C.
Sn-Cu-Ni
Extensive phase diagram data are
available for the Cu- and Cu,Ni-rich
part of this system.34 The experimental
data show that the binary Sn-Cu and
Sn-Ni phases have extended ternary
homogeneity ranges. A series of ternary
phases has been reported but it is not
clear whether these phases are true
ternary phases or extended ternary
homogeneity ranges of the binary
phases. No experimental data are
available for tin concentrations larger
than an atomic fraction of 0.6. Gupta et
al.42 proposed that a transition (type II)
reaction, L + Ni3Sn4 Cu6Sn5 + (Sn),
occurs in the tin-rich corner. However,
without experimental evidence this is
rather speculative. A thermodynamic
Sn-Ag-Sb
A series of 21 alloys was studied with
differential thermal analysis (DTA) and
electron microprobe analysis (EMPA)
and used for the construction of a
liquidus surface.40 The intermediate
phases of the Ag-Sn ((ζAg) and Ag3Sn)
and Ag-Sb systems ((ζAg) and Ag3Sb)
were found to form continuous homogeneity ranges in the ternary system.
The tin-rich invariant reaction was
reported as a transition (type II)
reaction, L + SbSn Ag 3(Sb,Sn) +
(Sn), at 234.8°C with xAg = 0.05 and
xSb = 0.06 as the composition of the
Figure 9. The calculated phase diagram
of the Sn-Ni system.31
49
a
b
c
Figure 10. The composition regimes (shaded) with suitable
freezing ranges (<35°C) for candidate solder alloys in the
systems (a) Sn-Ag-Bi,10 (b) Sn-Ag-Cu,11 and (c) Sn-Bi-Cu.12
extrapolation of the description of the
binary systems is not likely to give
a reliable prediction for the ternary
system since the extrapolation cannot
predict the ternary homogeneity of
the solid phases. However, once the
parameters for the description of the
liquid and solid phases in the ternary
system have been established utilizing
the available experimental data, the
calculation should reliably predict
the liquidus for the tin-rich corner of
the system.
APPLICATION EXAMPLES
Freezing range evaluation is the most
basic application of phase diagram
information for the selection of new
candidate solder alloys. The maximum
acceptable component temperature
during assembly depends on the device,
package, and PCB material being used,
establishing the maximum temperature
for a candidate solder. The processing
temperature of a solder should be at
least 10°C to 20°C above its liquidus
temperature. The combination of these
criteria limits the maximum liquidus
temperature for most solders to
225°C43 and 260°C for high-temperature,
fatigue-resistant solders. 44 Solders
for high-temperature applications
are required to perform at operating
temperatures up to 160°C. This requirement results in a minimum solidus
temperature of 208°C if the operating
temperature is set to be 90% or less of
the absolute melting temperature of
the solder. 1 At the same time, the
solidus temperature should not be
much more than 30°C lower than
50
its liquidus temperature in order to
avoid manufacturing problems, such as
defective joints caused by vibrations
during cooling from the soldering
temperature.43 Alloy compositions that
fulfill the freezing range criterion are
shown as shaded areas for the Sn-Ag-Bi,
Sn-Ag-Cu, and Sn-Bi-Cu systems in
Figure 10. However, it should be noted
that in systems that show noticeable
changes in the maximum solubility of
the alloying element in (Sn) during
cooling, the amount of liquid phase
during cooling can be larger than
predicted by the equilibrium diagram.2
Support of experimental design
through the calculation of phase equilibria can reduce the number of experiments
that are necessary for the determination of a higher-component system.
Moon et al.11 presented an example
where experimental and computational
methods were used to complement each
other in the determination of the tinrich part of the Sn-Ag-Cu system. The
phase equilibria information was used
to understand which signals are likely to
be observed during the thermal analysis
so that special attention could be paid
to the sensitivity of the measurement
technique. This strategy resulted in a
highly accurate determination of the
eutectic temperature and composition
of the liquid phase, as well as two
temperature-concentration sections.
The experimental results were then
used to refine the thermodynamic
description.
Analysis of contamination effects
is of great importance in avoiding
detrimental effects such as the formation of low-melting eutectics. Moon
et al.45 showed that even though the
contamination level of a Sn-Bi solder
with lead was not high enough to show
the formation of the equilibrium ternary
eutectic, which occurs at 95°C, the
effects of non-equilibrium solidification occurred at fairly low levels of
contamination. It was also shown that the
experimental observation was in accord
with predictions obtained assuming
the worst case of microsegregation,
the so-called Scheil path. Moon et al.
performed Scheil calculations for a
range of Sn-Bi solders with a very low
level (< 1%) of lead contamination
(Figure 11). They found that a 0.1% lead
Figure 11. The effect
of lead contamination on the lowest
possible freezing
temperature predicted from Scheil
path calculations for
Sn-Bi solders.45
JOM • December 2002
contamination promotes the formation
of ternary eutectic in Sn-Bi solder with
xBi ≥ 0.005, although, according to the
equilibrium diagram, eutectic formation
should only occur for concentrations
larger than xBi = 0.113 and xPb = 0.007.
Kattner and Handwerker9 applied the
Scheil analysis to a series of solder
alloys and found that the combination of
bismuth and lead was especially detrimental to the final freezing temperature
of solders.
Reaction with substrate materials is
a strong function of processing and
services temperatures and depends
on substrate metallization and solder
composition. It is highly desirable to
control these interfacial reactions to
optimize joint properties. For example, it
was shown that the kind of intermediate
phase formation on nickel substrates is
very sensitive to the copper concentration in Sn-Cu46 and Sn-Ag-Cu47 solders.
The formation of the Ni3Sn4 phase was
only observed for fairly low copper
concentrations in the solder and the
interface was dominated by the formation of Cu6Sn5. The observations were
rationalized by diffusion path analysis.
The prediction of other properties is
another advantage of the thermodynamic
description of phases, since quantities
such as the partial excess Gibbs energies
are readily available. For example, Lee
and Lee4 used the correlation between
the partial excess Gibbs energies and the
surface tension to calculate the surface
tension of binary solder liquids. In
addition to the prediction of the surface
tension, Ohnuma et al.5 used thermodynamic properties to predict the
activation energy of viscosity. Both
of these properties are important for
understanding the wetting behavior of
a molten solder.
CONCLUSION
Phase diagram information provides
basic information for the design and
understanding of solder alloys and
is not limited to the application of
equilibrium processes. The calculation
of phase equilibria is a powerful tool
for developing new solders since a
2002 December • JOM
wide range of potential alloys can be
relatively rapidly evaluated. The power
of the calculation is that information
can be provided for higher-component
systems using thermodynamic extrapolation methods where no or little
experimental information is available. In
addition, the thermodynamic quantities
provide the key to the prediction of other
solder properties, such as surface tension
and viscosity. The coupling of phase
diagram calculation and experimental
determination of phase equilibria
provides useful information for interpreting experimental observations and the
experimental results can be used to
further improve the thermodynamic
description of the system.
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Ursula R. Kattner is a physical scientist with the
Materials Science and Engineering Laboratory
at the National Institute of Standards and
Technology.
For more information, contact Ursula R. Kattner,
National Institute of Standards and Technology,
Materials Science and Engineering Laboratory, 100
Bureau Drive, Stop 8555, Gaithersburg, Maryland
20899; e-mail [email protected].
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