http://support.spring8.or.jp/Report_JSR/PDF_JSR_26B/2014B1620.pdf

2014B1620

BL20XU

In-situ Observation of Cu6Sn5 Growth in Sn-Cu Solder Alloys
M.A.A. Mohd Salleha, H. Yasudab, A. Sugiyamac, T. Nagirad, K. Uesugie, S. D. McDonalda, K. NogitaaaNihon Superior Centre for the Manufacture of Electronic Materials (NS CMEM), School of Mechanicaland Mining Engineering, The University of Queensland,b
Department of Materials Science and Engineering, Kyoto University,c
Department of Mechanical Engineering for Transportation, Osaka Sangyo University,dDepartment of Adaptive Machine Systems, Osaka University,e
The Japan Synchrotron Radiation Research InstituteReal time solidification observation of Sn-0.7wt%Cu and Sn-0.7wt%Cu-0.05wt%Ni solder pastes on Cu
substrates were achieved using a synchrotron X-ray imaging technique. It was found that, upon wetting, therapid dissolution of Cu adjacent to the solid–liquid interface was followed by near-instantaneous interfacialintermetallic compound (IMC) formation and later by the independent nucleation and growth of Cu6Sn5intermetallics in the solder alloys. The size of the independent Cu6Sn5 intermetallics is dependent on thepresence of Ni. Large needle-like Cu6Sn5 intermetallics grow in Sn-0.7wt%Cu alloys while very small
flake-like Cu6Sn5 intermetallics are found in Sn-0.7wt%Cu-0.05wt%Ni. The results provide direct
experimental evidence of the effect of 500ppm Ni on the formation and growth behaviour of Cu6Sn5intermetallics in solder alloys.
Keywords: In-situ X-ray imaging, soldering, solidification
Background and aim:
Recently, there has been a dramatic increase in research activity in the field of lead-free soldering,particularly for applications in high density microelectronic packaging, namely, Three Dimensional
Integrated Circuits (3D ICs). Due to the inherent toxicity of lead (Pb), environmental regulations around theworld have been targeted at eliminating the use of Pb-bearing solders in electronic assemblies. The
European Union (EU) has issued a directive that has effectively banned the use of lead-containing solders
in such equipment placed on the market after 1st July 2006. This has prompted the development of “Pb-free”
solders, and has greatly enhanced the research activities in this field. However, alternative (lead-free) solder
systems are associated with uncertainties relating to reliability and this has limited the use of these alloys insome products. As such there are opportunities for the development of high performance lead-free solders
that are actively being pursued in the international research arena.
Reported effects of small additions of Ni in Sn-0.7wt%Cu solder alloys have included better fluidity[1][2], alterations to the eutectic composition and promotion of a near eutectic Sn-Cu6Sn5 microstructure[3],stabilisation of the hexagonal high temperature phase of Cu6Sn5 [4][5], and suppression of cracking in
Cu6Sn5 solder joints formed between Sn-0.7wt%Cu solders and Cu substrates [6]. This report focuses on
the use of real-time solidification observations using synchrotron techniques to further the understanding of
the growth behaviour of Cu6Sn5 intermetallics with and without Ni in Sn-0.7wt%Cu alloys under simulated
reflow soldering conditions.
Experimental:
Two solder pastes, Sn-0.7wt%Cu and Sn-0.7wt%Cu-0.05wt%Ni (SN100C solder), and polycrystalline
Cu substrates were used for this experiment.
The experiments were performed at BL20XU beamline in the SPring-8 synchrotron using an in situ
X-ray real-time solidification observation setup developed from previous research [7][8]. Figure 1 shows(a) Synchrotron X-ray real time in-situ observation experimental setup, (b) sample position, (c) the sample
cell setup. With a high degree of coherence, absorption contrast and phase contrast are observed ontransmitted images with enhanced boundaries. The collected signals are then converted into a digital formatof 2000 x 2000 pixels. This area represents a 1 mm x 1 mm area on the BL20XU experiments (giving aresolution of 0.5 μm per pixel). A planar undulator was used as a light source and the radiation was
monochromatized with Si double-crystal monochromators. An exposure time of 1 s per frame was used tocapture the images. To mimic the process of reflow soldering, a furnace with graphite heating elements― 63 ―

where heat is transferred through radiation in an
enclosed sample chamber was used. Both
Sn-0.7wt%Cu and Sn-0.7wt%Cu-0.05wt%Ni
solder pastes and the Cu sheet were used at 100
m thickness. The sample was then placed in asample cell with an observation window area of
10 x 10 mm2 with a vent for flux outgassing
createdusinga100μm
thicknesspolytetrafluoroethylene sheet placed between
two SiO2 plates. Samples were set to be heatedfrom room temperature to approximately 250 °C
at 0.17 °C/s and cooled down at 0.33 °C/s. Thereis a temperature difference between the actual
temperature of the sample and the thermocouple
reading due to the positioning of thethermocouple. The sample temperature wascalculated by calibrating with the relative
difference between the furnace temperature at theobservation of melting to the melting point of
Sn–0.7 wt.% Cu as determined by differential
scanning calorimetry. The temperature profile for
the BL20XU beamline experiment in Figure 2
shows the difference between the thermocouple
reading and the actual temperatures.

Fig.1 ． (a) Synchrotron X-ray real time in-situ
observation experimental setup, (b) sample
position, (c) sample cell setup.

Results and discussion:
Figure 3 shows real-time imaging observationsof reactions between Sn-0.7wt%Cu solder paste
and the Cu substrate interface at experimental
times of (a) 293 sec, (b) 305 sec, (c) 441 sec and(d)571
sec,andFigure
4of
Sn-0.7wt%Cu-0.05wt%Ni solder paste and theCu substrate interface at experimental times of(a) 197 sec, (b) 202 sec, (c) 324 sec and (d) 574
sec.
Fig.2．SPring-8 BL20XU beamline soldering
Figure 3a and 4a show the solder paste (withtemperature profile.
an average solder grain size of around 40 μm
with flux media) starting to melt.
From Figure 3b and 4b, it can be seen that at the moment the molten solder contacts the solid Cu an
instant planar (within the resolution of the imaging) intermetallic layer of significant thickness forms, while
molten solder flows onto the Cu substrate. Subsequently, the intermetallics layer develops a scalloped
interface and the rapid formation of interfacial solder voids is observed.
Based on Figure 3c and 3d, it is observed that large needle-like Cu6Sn5 intermetallics over 100 μm in
length tend to nucleate and subsequently rapidly grow during cooling in the Sn-0.7wt%Cu solder. Theselarge intermetallics tend to form in the solder matrix far from the solder/substrate interface since Sn and Cu
are consumed at the interface to form the interfacial IMC in the early stages of soldering. On the other hand,
as shown in Figure 4c and 4d, small flake-like Cu6Sn5 intermetallics on a scale of a few tens of μm in
length are observed in the presence of 0.05wt%Ni.
Conclusions and future works:
From real-time soldering observations using a synchrotron radiation facility it is clear Ni additions affectSn-0.7wt%Cu solder in terms of the formation and growth of Cu6Sn5 intermetallics. Without Ni, large
Cu6Sn5 intermetallics grow to around 100 μm in length, while with 500 ppm Ni, their growth is effectively
suppressed to a length of around a few tens of micrometers.
Acknowledgments:
Synchrotron in-situ solidification observations were performed at the Japan Synchrotron Radiation― 64 ―

Research Institute (JASRI) on BL20XU of the SPring-8 synchrotron, under Proposal No. 2014B1620. We
acknowledge travel funding provided by the Australian Synchrotron International Synchrotron Access
Program (AS/IA143/9218) managed by the Australian Synchrotron and funded by the Australiangovernment.

Fig.3．In situ real-time imaging observationsof reactions between Sn-0.7wt%Cu
solder paste and the Cu substrateinterface at experimental times of (a)293 sec, (b) 305 sec, (c) 441 sec and(d) 571 sec.

Fig.4．In situ real-time imaging observations ofreactions between Sn-0.7wt%Cu-0.05wt%Ni
solder paste and the Cu substrate interface at
experimental times of (a) 197 sec, (b) 202
sec, (c) 324 sec and (d) 574 sec.

References:[1] Ventura, T. et al. Solidification characteristics of lead-free solder Sn-0.7Cu-0.06Ni. in 5th Decennial
International Conference on Solidification Processing. 2007. Sheffield, UK.

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[2] Ventura, T. et al., Journal of Electronic Materials, 37 (1), 2008.
[3] Nogita, K. et al., Materials Transactions, 46 (11), 2005.
[4] Nogita, K. and T. Nishimura, Scripta Materialia, 59 (2), 2008.
[5] Nogita, K., Intermetallics, 18, 2010.
[6] Nogita, K., C.M. Gourlay, and T. Nishimura, JOM, 61 (6), 2009.
[7] Salleh, M.A.A.M. et al., Scripta Materialia, 100, 2015.
[8] Salleh, M.A.A.M. et al., Applied Mechanics and Materials, 754-755, 2015.

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