Lead-Free Solder Research Summary Tin Pest in Sn-0.5 wt ...

Lead-Free Solder Research Summary

Tin Pest in Sn-0.5 wt.% Cu
Lead-Free Solder

Yoshiharu Kariya, Naomi Williams, Colin Gagg, and William Plumbridge

INTRODUCTION

Over the past several decades, lead-tin solders have been preferred for interconnect- Tin pest (the product of the allotropic
ing and packaging electronic components. However, increased environmental and transformation of b-tin into a-tin at tem-
health concerns regarding the toxicity of lead have stimulated research and develop- peratures below 286 K) has been observed in
ment of alternative, lead-free solders.1 Since the early 1990s, considerable effort has a Sn-0.5 wt.% Cu solder alloy. Some 40
been focused on finding suitable alternatives to lead-containing solders.2 percent of the specimen surface was trans-
formed into gray tin after aging at 255K for
Lead-free solders have to satisfy varied requirements within the electronics 1.5 years, and after 1.8 years, the proportion
industry, ranging from safety, economy, material availability, and manufacturabil- increased to about 70 percent. The degree of
ity to long-term reliability. Long-term reliability is a key issue for lead-free imple- transformation in work-hardened areas is
mentation. Electronic packages are used to control a myriad of operational and much higher than in other areas, suggesting
safety functions in everyday life. With the advent of surface-mount technology, the residual stress might provide an additional
solder joint has become an integral structural member of the board-track-joint-lead- driving force for the transformation into a-
chip assembly. tin. The allotropic change results in a 26
percent increase in volume, and cracks are
During their service lifetime, electronic components are exposed to a wide range of initiated to accommodate the changes in vol-
environmental and temperature variation (e.g., 223–398 K) that could diminish the ume. Results indicate that tin pest could lead
reliability of the solder joint. A temperature change within the range 223 K to 398 K, to total disintegration of micro-electronic
for example, causes strain in electronic assemblies. That strain, which is concentrated solder joints. The tin-copper eutectic system
within the solder joints, results from a difference in thermal expansion coefficients may become a prominent lead-free solder,
between each component that constitutes the joint. To ensure reliability of joints in an and tin pest could have major ramifications
electronic device, replacement lead-free solders will require mechanical characteris- on service lifetime of electronic assemblies.
tics (fatigue strength, creep resistance, rigidity, etc.) capable of withstanding the range
of temperature, associated strains, and changing environment found in service. Recent
efforts toward developing lead-free solders have concentrated on mechanical reliabil-
ity features such as thermal fatigue and creep.3 Corrosion and ion-migration have also
been considered in reliability evaluation.

An additional problem with lead-free solders might be the allotropic transformation
of b-tin (bct) into a-tin (diamond cubic). This transformation product, termed tin pest,
occurs at temperatures below 286 K.4 The allotropic change is accompanied by a 26
percent increase in volume, which could have serious repercussions when considering
a solder joint lifetime. Given the dilute nature of some lead-free solder alloys, and
because the major element of lead-free solder is tin, tin pest may be a potential problem
as the joints could be exposed to temperatures below transformation points during
their service lifetime. However, the spontaneous appearance of gray tin is rare in
conventional solders, and tin pest has generally been ignored in electronics industries.
Recent work has revealed that tin pest can occur in tin-based, lead-free solder alloys.5
Before any widespread implementation of lead-free solder alloys can commence, a
fundamental understanding of its allotropic transformation behavior will be required.
In this paper, the conditions surrounding the appearance of tin pest in Sn-0.5 wt.% Cu
solder alloy are explored more fully.

EXPERIMENTAL DETAILS

The chemical composition of the as-received Sn-0.5 wt.% Cu solder alloy is shown

in Table I. As-received ingots were cut into smaller pieces and heated in ceramic

crucibles to about 20 K above the melting point of the alloy, and then the molten alloy

was cast into an aluminum mold pre-

heated to 513 K. The mold, with the Table I. Chemical Composition of Sn-0.5 wt.%Cu*
ingot, was immediately quenched into
water, and the specimens were then aged Sn Pb Ag Cu As Bi Fe Al Sb Zn Others
0.001 <0.003
at 255 K for two years in vinyl bags. Bal. 0.014 0.008 0.55 <0.001 <0.001 0.001 <0.0002 0.012

Specimens were sectioned by elec- * Unit is mass%

tric discharge machining. Grinding

was accomplished with three grades of SiC papers (800, 1,200, and 2,500 grit), and

then mechanically polished with two grades of diamond paste

(3 mm and 1 mm). Microstructural observation was undertaken on a scanning
electron microscope (SEM: JEOL 8250) using an accelerating voltage of 15 kV.

Optical microscopy was undertaken on a Reichert-Jung MEF3 microscope. En-

ergy dispersive x-ray analysis (EDX: Kevex Quantum series) and electron back-

scatter diffraction analysis (EBSD: HKL technology) were used to determine the

elemental composition of the phases and to confirm crystal structure.

2001 June • JOM 39

Aged for 1.5 years

As cast

Aged for 1.5 years

25 mm

Figure 1. As-cast microstructure of Sn-0.5 wt.%
Cu solder alloy.

Aged for 1.8 years

20 mm 10 mm

ab
Figure 2. The transformation of b-tin (white tin) into a-tin (gray tin) occurring in Sn-0.5 wt.% Cu
solder alloy, aged at 255 K for 1.5 years and 1.8 years.

Table II. Physical Data of White Tin and Gray Tin4

White Tin (b) Gray Tin (a)

Crystal Structure Body-centered tetragonal Diamond cubic
Lattice Parameter
(a axis) 5.831 Å 6.489 Å
(c axis) 3.181 Å
Density 7.29 g/cm3 at 288 K 5.77 g/cm3 at 286 K
Electrical Resistivity 11 mW (273 K) 300 mW (273 K)
Coefficient of Thermal Expansion 20 ppm/K 4.7 ppm/K

a-Sn a-Sn

b-Sn b-Sn

a 500 mm b 25 mm

Figure 3. SEM micrographs of the surface of grip section (end of specimen) aged at 255 K for
seven months.

RESULTS AND DISCUSSION

Initial Microstructure of Specimen

The as-cast microstructure of 0.5 wt% tin copper solder alloy is shown in Figure 1. The
microstructure of the alloy consists of eutectic regions of fine dispersed spherical Cu6Sn5
intermetallic compounds within a b-Sn matrix and large b-Sn globules. The grain size
of b-Sn globules region is approximately 10 mm. The specimen was rapidly cooled, so
that the size of Cu6Sn5 intermetallic compound is in the order of sub-micrometers.

Appearance of Transformed Specimen

Figure 2 shows the transformation of b-tin (white tin) into a-tin (gray tin) occurring
in Sn-0.5 wt.% Cu solder alloy, aged at 255 K for 1.5 years and 1.8 years. Some
40 percent of the specimen surface was transformed into a-tin (having a wart-like
appearance) after 1.5 years, and this proportion had increased to about 70 percent
after 1.8 years.

Some physical property data of white tin and gray tin are listed in Table II.4 The
crystal structure of white tin is body-centered tetragonal, while the structure of gray
tin is diamond cubic. The volume of gray tin is 26 percent higher than that of white tin.
Therefore, the transformation into gray tin is accompanied by a significant increase in
volume. Furthermore, gray tin is fairly brittle. In order to accommodate the changes
in volume, cracks are initiated, as seen in Figure 2.

40 JOM • June 2001

Nucleation and Growth a a-Sn

The transformation into gray tin occurs by nucleation and growth. Tin exhibits a b-Sn
supercooling phenomenon, so the formation of tin pest will require a long incubation
time and will need some additional driving force for transformation to occur over a 25 mm
short time frame. Generally, the transformation into gray tin is preceded by an b
incubation period in the order of one year at 255 K, during which time no sign of the
transformation will be apparent.6 In the Sn-0.5 wt.% Cu solder alloy, the gray tin 25 mm
becomes evident on the surface of the grip section (the specimen head) after exposure
for about seven months at 255 K. Figure 3 shows the early stage of tin pest on the surface c
of the sample head. The light contrast region represents the transformed structure,
which is about 0.7 mm. Within the transformed area, many new grains are visible, a-Sn
along with a localized volume expansion associated with the transformation. EDX
analysis of this area indicated the absence of segregation in the specimen. Initiation of b-Sn
gray tin in the gauge section is much slower than that of the head section. The surface
of the grip section was almost totally transformed into gray tin, whereas only about 30 a-Sn A
percent of the gauge section was transformed after aging for 1.5 years. The specimen
used in this study was originally designed for mechanical testing, and the grip section B
had been machined. The grip ends are presumed to contain residual stresses due to
work hardening of the structure. This suggests that residual stress may accelerate, or 2 mm
provide an additional driving force, for transformation to gray tin.
Figure. 4 Optical and SEM images of the
In contrast, the growth kinetics of gray tin are faster than those for nucleation. Once cross section of transformed specimen aged
a wart of the gray tin appears in white tin, its growth rate is high compared with at 255 K for 1.5 years.
incubation. Almost a year at 255 K is required for appearance of gray tin, though 70
percent of the surface of the specimen is covered by gray tin after exposure for 1.8 years References
at 255 K. Generally, the maximum rate of transformation will occur at about 233 K.7
Because the volume of solder in a real joint is small (usually less than 1 mm), the joint 1. D. Napp, SAMPE Journal, 32 (1996), p. 59.
might disintegrate in a short period once the tin pest has initiated. 2. B. Trumble, IEEE Spectrum, 35 (1998), p. 55.
3. For example, Lead Free Solder Project, Final Report, NCMS
The transformation into gray tin initiates at the surface, and then expands mainly on Report 0401RE96 (Ann Arbor, MI: National Center for Manu-
the surface, as can be seen in Figure 2. Figure 4 shows the micrographs of the cross- facturing Sciences, August 1997).
section at the grip of sample after exposure for 1.5 years at 255 K. It can be seen that 4. The Properties of Ti, Publ. 218 (Uxbtridge, U.K.: Tin Re-
transformation has started to penetrate into the material bulk from surface nucleation. search Institute, 1954).
There was no evidence of nucleation within the bulk. EBSD was carried out on the cross 5. Y. Kariya, C.R. Gagg, and W.J. Plumbridge, Soldering and
section to confirm the crystal structure of tin. Grey tin was not identified within the Surface Mount Technology, in print.
bulk interior. The nucleation of gray tin in the interior of bulk white tin and growth 6. W.G. Burgers and L.J. Groen, Faraday Soc. Discussions, 23
from an internal nucleus is unlikely, as the volume increase associated with the (1957), p. 183.
transformation will be constrained by the bulk. 7. H.E. Hedges, Tin and its Alloys (London: Edward Arnold
Ltd., 1960), p. 51.
Tin Pest in Other Solder Alloys 8. G.V. Ratnor and R.W. Smith, Proc. Roy. Soc., 224 (1958),
p. 101.
The transformation into gray tin is very sensitive to the alloying element or Yoshiharu Kariya, Naomi Williams, Colin Gagg, and
impurities.8 The effect of various alloy additions in retarding transformation from William Plumbridge are with the Materials Engineering
white tin into gray tin is shown in Table III.7 In general, soluble impurities (such as lead, Department, The Open University, in the U.K.
bismuth, and antimony) suppress the transformation into gray tin. Therefore, tin pest For more information, contact Y. Kariya, The
is not observed in conventional lead-tin solders even when the alloy is placed at low Open University, Materials Engineering Depart-
temperatures for a long time. No visible transformation was observed in Sn-37Pb, even ment, Walton Hall, Milton Keynes, MK7 6AA,
though the ingot was aged at 255 K for more than two years. United Kingdom; 011-44-1908-652-630; fax 011-
44-1908-655-120; e-mail [email protected].
In contrast, the inhibiting effect of insoluble elements, such as copper, on transfor-
mation is less than those of soluble impurities. Therefore, it seems that tin pest can be
observed in Sn-0.5 wt.% Cu solder alloy mentioned above. As silver is also an insoluble
element, tin pest might be expected in Sn-3.5Ag alloys, and the authors have observed
it on the strain-hardened surface of a Sn-3.5Ag ingot. The ingot had been thermally
aged at 255 K for 1.8 years, in a similar manner to that of the Sn-0.5 wt.% Cu solder alloy.
Surface growth of gray tin in the tin-silver alloy was small when compared to that of
the tin-copper alloy. Details of allotropic transformation behavior in Sn-3.5Ag and
other lead-free solder alloys are underway, and will be reported in the near future.

Table III. The Effect of Alloy Additions on Retarding
the Transformation from b-tin into a-tin6

Material (Inoculated with 0.1mg a-Sn) Time for the Formation of a Visible Wart

99.998% pure Sn 2 hours
Sn+0.024mass%Pb 12 days
Sn+0.48mass%Pb 29 days
Sn+50mass%Pb 190 days
Sn+0.1mass%Sb 14 hours
Sn+0.5mass%Sb No wart visible after 5 years
Sn+0.3mass%Bi 330 days
Sn+0.1mass%Cu 4 hours
Sn+0.5mass%Cu 7 hours

2001 June • JOM 41