http://support.spring8.or.jp/Report_JSR/PDF_JSR_24B/2012B1440.pdf

2012B1440

BL37XU

Trace Element Distributions in Sn-Cu and Sn-Zn Lead-free Solder Joints
Guang Zenga, Kazuhiro Nogitaa,b, Hideyuki Yasudac, Stuart D. McDonalda, Yasuko TeradadaNihon Superior Centre for the Manufacture of Electronic Materials (NS CMEM),b
School of Mechanical and Mining Engineering, The University of Queensland,c
Department of Adaptive Machine Systems, Osaka University,
dJapan Synchrotron Radiation Research Institute, SPring-8

Trace element additions are increasingly being incorporated into lead-free solder compositions. Thispaper analyses the distribution of trace elements in solder joints when commercial purity Sn-based alloys
are soldered onto Cu substrates. Analysis techniques include µ-XRF (X-ray fluorescence) mapping
performed at the SPring-8 synchrotron radiation facility.
Keywords:

Lead-free solders, Cu6Sn5, micro-XRF

1. Introduction
Recently, there has been an increase in research activity in the field of lead-free soldering. This is due
to legislative changes driven by the rapid increase in the volume of scrapped electrical and electronicequipment finding its way into landfill sites causing growing concerns about lead in the solders (used in theassembly of this equipment) leaching into the groundwater. This concern has prompted the European Union(EU) to issue a directive that has effectively banned the use of lead-containing solders in such equipment
placed on the market after 1st July 2006. As a result alternative (lead-free) solder systems (e.g. Sn-Ag-Cu,Sn-Cu, Sn-Ag, Sn-Bi, Sn-Zn) have been developed but uncertainty about reliability has limited theirapplication. The foreshadowed removal by 2014 of the exemption from the requirements of EU Direction
on the Restriction of Hazardous Substances in Electrical and Electronic Equipment (RoHS) for lead in highmelting point solders has prompted a search for lead-free alloys that could take their place. Accordingly
this has increased the demand for suitable micro-electronics and high-temperature lead-free solders,
particularly for transport, power device and medical applications.
This report presents the results of microanalysis on the distribution of various trace elements in soldersof several common compositions. In particular the distribution of the elements Zn and Ni in the Cu6Sn5
phase is examined in an attempt to understand the relationship between microstructure, grain size, thicknessof the IMC (intermetallic compound) solder layer and composition, then investigate the effects of Zn andNi on the formation and growth of interfacial intermetallic layer of lead-free solder joints.
2. Experimental Procedure
The solder joints are formed with interface reaction between polycrystalline Cu and a variety oflead-free Sn-0.7Cu-0.1Zn, Sn-0.7Cu-0.05Ni and Sn-0.7Cu-0.1Zn-0.05Ni solders. The samples wereprepared by dipping the Cu plates (C1220P) of 10 mm×30 mm×0.3 mm with a common flux (Flux B) into
the molten solder at a temperature of 270℃ for 60 s. Samples were polished down to ~80 μm, alongcross-sectional direction to the solder interface, using conventional grinding and polishing techniques forsample preparation.
The experiment has been carried out at an undulator beamline 37XU of SPring-8[1]. The undulator
radiation X-rays are monochromatized with a Si 111 double-crystal monochromator. The rotated-inclined
geometry is used to manage high heat-load from the undulator radiation. The pin-post crystal is used as thefirst crystal and cooled by water directly, while the second crystal is cooled indirectly. The focusing optics
is located at 57 m from the light source while the vertical source is determined by the electron beam in theundulator. The horizontal source size is defined by a XY slit placed at 34 m from the light source[1]. The
diffraction limited spot size determined by the numerical aperture is 0.19 mm (V)×0.15 mm (H) at 30 keV.
The effective length of each mirror is about 90% of full length, because the platinum coating shows
relatively thin thickness at the either marginal area of mirrors. In the focusing experiment, the illuminating
beam size is determined by a 4-jaws slit in front of the mirror, and only the center area of the mirror (90mm for each mirror) is illuminated[1]. The micro-focusing optics is adopted in a scanning X-ray

microprobe system. The sample of polished thin sections of solder joints is mounted on an XY stage, and afluorescent X-ray take off angle of 10° is usually used for the XRF experiments. An energy dispersive
detector, Si(Li) solid state detector, was used to measure the fluorescence signal from the sample, andplaced perpendicular to the incident beams to minimize the X-rays scattered from the sample. The data wasprocessed and analysed using Igor 6.22A software.
3. Results and Discussion
Firstly in order to accurately measure
the distribution of trace alloying elements,
the energy dispersive spectrum of μ-XRF
need to be decomposed to eliminate theinfluence of strong peaks contributed fromthe major/ultra-high concentration elementsin the sample. For instance, as illustrated inFigure 1, the peaks of CuKα and CuKβ havegreat influence on the ZnKα and NiKαpeaks. The correction procedure is asfollows,
1. Determine the intensity of backgroundoftheEDS(Energy
Dispersive
pectoroscopy) spectrum, assumed to beconstant,proportional to the intensity of CuKα andFigure 1．μ-XRF spectrum from interfacial IMC layer
also independent of channels/energy.
(Position A in Figure 2) of2. Use Gaussian functions to fit eachSn-0.7Cu-0.1Zn-0.05Ni/Cu solder joint
peak of the EDS spectrum. The parametersof the Gaussian functions are refined andobtained to minimise the differential
between the modelling and measuredspectrum.
3. Apply the parameters obtained in Step
2 to the dataset of μ-XRF mapping, tomodify
theintensityofeachelement/channel in mapping (to eliminate
the interference contributed by strong peaksof other elements). The corrected μ-XRF
maps are then obtained and plotted.
4. Verify the modified mapping results byother spectrums of different locations tomake sure the corrected intensity ofelements have realistic values.
As shown in Figure 2, the interface ofsolder joint can be divided into three parts:
Cu substrate, interfacial layer and soldermatrix. Cu, Sn, Ni and Zn elements werepresent in the interfacial layer. It is widelyconfirmed that the interfacial phase of Sn/Cu
Figure 2．μ-XRF mapping of Sn-0.7Cu-0.1Zn-0.05Ni/Cu
couple is Cu6Sn5[2]. In this experiment, ittaken by high magnification scan of Sn, Cu, Ni
can be clearly seen that Ni and Zn elementsand Zn (Scan pitch 200nm, Exposure time: 0.1both have a strong concentration in thes, X-ray:10keV).
interfacial Cu6Sn5 IMC layer. But it worth tonote that Ni is even more strongly concentrated in the IMC layer than Zn, despite Ni being present at only
0.05 wt.% compared to Zn at 0.1 wt.% in Sn-0.7Cu based alloys. Zn is also distributed within the Sn-richsolder matrix, with a lower concentration than the interfacial IMCs. Above all, both Ni and Zn are
relatively homogenously distributed in the Cu6Sn IMC layer of the Sn-0.7Cu-0.1Zn-0.05Ni/Cu solder joint.
Based on this experiment, it can be concluded that trace amounts of Ni and Zn have a strong effect onthe interfacial IMCs layer of Sn-0.7Cu/Cu solder joints. As discovered in our previous studies[3-5], alloyed
Ni and Zn stabilise the Cu6Sn5 intermetallic to keep the high temperature hexagonal structure over therange from -80 to 240℃, inhibiting the polymorphic phase transformation of Cu6Sn5. Besides thestabilisation effect, Ni and Zn also have effects on the interfacial reactions of Sn-0.7Cu solder and Cusubstrates during soldering and subsequent heating treatment/service conditions, which is also very
important to the reliability of solder joints[2]. This experiment reveals for the first time, the concentration
effect and homogenously distribution of Ni and Zn within the interfacial IMC phase in
Sn-0.7Cu-0.1Zn-0.05Ni/Cu solder joints.
4. Conclusions
This research utilised a µ-XRF trace element mapping technique at the SPring-8 synchrotron radiation
facility for the purpose of solder joint analysis. All samples with a solder of base composition 500ppm Niand 1,000ppm Zn have homogeneous Ni and Zn distributions in the Cu6Sn5 IMC layer. The variations incrystallography and composition across soldered interfaces have practical implications for mechanicalproperties and the propensity for damage accumulation within the IMC layer.
References[1] Y. Terada, et al., Nuclear Instruments and Methods in Physics Research Section A: Accelerators,
Spectrometers, Detectors and Associated Equipment, 616 270-272(2010).
[2] T. Laurila, et al., Materials Science and Engineering: R: Reports, 68 1-38 (2010).
[3] K. Nogita and T. Nishimura, Scripta Materialia, 59 191-194 (2008).
[4] K. Nogita, et al., Journal of Alloys and Compounds, 489 415-420(2010).
[5] G. Zeng, et al., Journal of Materials Research, 27 2609-2614(2012).