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Transactions of The Japan Institute of Electronics Packaging Vol. 3, No. 1, 2010

[Technical Paper]

Synchrotron Micro-XRF Measurements of Trace Element

Distributions in BGA Type Solders and Solder JointsKazuhiro Nogita*,**, Hideyuki Yasuda**, Christopher M. Gourlay***, Shoichi Suenaga****, Hideaki Tsukamoto*,

Stuart D. Mcdonald*,*****, Akihisa Takeuchi******, Kentaro Uesugi******, and Yoshio Suzuki******

*School of Mechanical and Mining Engineering, The University of Queensland, St. Lucia, Brisbane QLD 4072 Australia

**Department of Adaptive Machine Systems, Osaka University, Suita, Osaka 565-0871 Japan

***Department of Materials, Imperial College, London, SW7 2AZ United Kingdom

****Nihon Superior Co. Ltd., NS Bldg., 1-16-15 Esaka-Cho, Suita City, Osaka 564-0063 Japan

*****CAST-CRC, The University of Queensland, Brisbane QLD 4072 Australia

******Japan Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5198 Japan

(Received August 10, 2010; accepted September 30, 2010)

Abstract

Trace elements are increasingly being incorporated into lead-free solder compositions. This paper analyses the distribu-

tion 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. The

mapping results indicate that Ni is present in the Cu6Sn5 intermetallic reaction layer, and is distributed in a relatively

homogeneous fashion as (Cu,Ni)6Sn5. In alloys containing trace levels of Ge (60 ppm), this element is comparatively con-

centrated within the oxide at the solder surface, and a lower concentration is distributed homogeneously in the solder

matrix and the intermetallic reaction layer. In Sn–Pb alloys, the Pb was found to segregate to the boundaries between

adjacent Cu6Sn5 grains.

Keywords: Lead-free Solder, Intermetallics, Synchrotron Micro-XRF, Trace Elements

1. IntroductionIn microelectronic assembly, the intermetallic layers in

the reaction zone between the solder and substrate are

crucial to the mechanical and electrical integrity of sol-

dered joints. Cu6Sn5 is the most commonly formed inter-

metallic in the reaction zone between the Sn-based solder

and the Cu substrate. Recently, much attention has been

paid to the effects of impurities and alloying additions on

the Cu6Sn5 interfacial reaction layers between the Sn-

based solders and Cu substrates.[1–6] Trace elements are

categorised into two groups:[6] (1) elements that show

marked solubility in the Cu6Sn5 solder joint layer such as

Ni, Sb, Au, In, Co, Pt, Pd, and Zn, and (2) elements that are

not extensively soluble in Cu6Sn5 such as Ag, Fe, Bi, Al, P,

Ti, S, and rare-earth elements. The role of dissolved trace

additions in Cu6Sn5 in determining the intermetallic layer

growth and phase stability has been the subject of a range

of recent studies.[1, 4–6] Cu6Sn5 has at least two crystal

structures in the solid state, with an allotropic transforma-

tion at 186°C.[7, 8] At equilibrium, monoclinic eta’-Cu6Sn5

(C2/c) is the lower-temperature phase while hexagonal

eta-Cu6Sn5 (P63/mmc) is stable at higher temperatures.[7]

In 2008, it was found[9] by TEM that the faceted eutectic

(Cu,Ni)6Sn5 intermetallic containing around 9 at% Ni in

hypo-eutectic Sn–0.7Cu–0.05Ni alloys remains in the hex-

agonal eta phase at room temperature in contrast to binary

(Ni-free) Cu6Sn5 intermetallics which transform into the

low-temperature monoclinic eta’ phase under identical

cooling conditions. Since then, DSC[10, 11] and synchro-

tron powder XRD[10, 12, 13] have shown that Ni additions

suppress the eta to eta’ transformation, and the intermetal-

lic appears to remain as the eta phase at room tempera-

ture. In support of this, first principle calculations have

shown that Ni additions reduce the energy difference

between eta’ and eta at 0 K.[14] It also has been reported

that small amounts of Sb in Cu6(Sn,Sb)5 shift the thermal

41

stability range of high-temperature Cu6Sn5.[15] Further

research is required to understand the role of impurities

on solder joint reliability since impurities influence factors

including the Cu6Sn5 grain size, the mechanical properties

of Cu6Sn5, the thickness and growth of the IMC reaction

layer, Cu6Sn5 phase transformations, and cracking in

Cu6Sn5.[4, 6, 16] This report presents the results of a

microanalysis of the distribution of various trace elements

in solders of several common compositions. In particular,

the distribution of the elements in the Cu6Sn5 phase is

examined in an attempt to understand the relationship

between microstructure, grain size, thickness of the IMC

solder layer, and composition.

A method for elemental mapping using an X-ray fluores-

cence microscope technique (μ-XRF) has been developed

at the synchrotron radiation light source, SPring-8

Japan.[17, 18] In the micro-XRF technique, elemental map-

ping is performed by detecting the characteristic X-rays of

constituent elements. Since the intensity of the X-ray beam

emitted from the undulator source at SPring-8 is extremely

high, the X-ray fluorescence from a substance with a mass

in the order of femtograms on the surface can be detected.

This technique has been utilised to analyse the distribu-

tion of trace elements in the modified eutectic Si phase of

commercial Al–Si alloys,[19, 20] to characterise the distri-

bution of doping elements in a Mg–Ni alloy,[21] and to

analyse the microstructures of lead-free solder Sn–Cu

based alloys.[22] The spatial resolution is below 100 nm,

which is comparable to the current resolution achievable

by FE-SEM/EDX. In the current research, this technique

was used to investigate the distribution of the trace ele-

ments Ni (500 ppm) and Ge (60 ppm) along with the major

elements Sn, Cu, Ag and Pb in the region of the solder-

substrate interface (and accompanying IMC Cu6Sn5 layer)

for a variety of solder compositions.

2. Experimental procedureBall Grid Array (BGA) type soldered samples with Cu

substrates were used in the experiments. Table 1 shows

the chemical composition of the three solder alloys inves-

tigated, the soldering methods used and the solder tem-

peratures. BGA type samples were obtained from 500 μm

diameter solder balls on organic solderability protection

(OSP) Cu boards using a common reflow process with

RMA-type organic halogen-containing flux. This cycle was

conducted twice for each BGA specimen. For micro-XRF

experiments, all samples were embedded in epoxy resin

and polished in cross-section to observe the solder joints.

Figures 1(a) to 1(c) show cross-sectioned optical micro-

graphs of a) Sn–0.7Cu–0.05Ni+Ge, b) Sn–1.2Ag–0.5Cu–

0.05Ni and c) Sn–37Pb. Figure 2 shows a sample SEM

micrograph of a BGA type Sn–0.7Cu–0.05Ni+Ge solder on

a Cu substrate, which has been reported previously.[12]

An EDS point analysis indicates around 4–5 at% Ni is pres-

ent in the Cu6Sn5.

The micro-XRF experiments were performed at undula-

Table 1 Sample details.

Samples Chemical compositions TypeSolder Temp.

(°C)

a Sn–0.7Cu–0.05Ni+Ge BGA < 240

b Sn–1.2Ag–0.5Cu–0.05Ni BGA < 240

c Sn–37Pb BGA < 240

Fig. 1 Optical micrograph of samples, (a) Sn–0.7Cu–0.05Ni+Ge, (b) Sn–1.2Ag–0.5Cu–0.05Ni, and (c) Sn–37Pb.

Nogita et al.: Synchrotron Micro-XRF Measurements of Trace Element Distributions (2/7)

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Transactions of The Japan Institute of Electronics Packaging Vol. 3, No. 1, 2010

tor beamline 47XU at SPring-8, Japan. A schematic dia-

gram of the experimental set-up for micro-XRF is available

elsewhere.[17, 18] The undulator radiation was monochro-

matized at 6.1 and 15 keV by passing through a liquid-

nitrogen-cooled Si 111 double-crystal monochromator. A

Fresnel zone plate (FZP) was used as an X-ray focusing

device to produce a fine probe. The FZP was fabricated

using an electron-beam lithography technique at NTT

Advanced Technology Co. Ltd. A zone structure with a 1

μm thick tantalum is deposited onto a 2 μm thick SiC

membrane. The diameter is 155 μm and the focal length at

the X-ray energy of 15 keV is 187.5 mm. The outermost

zone width is 100 nm. In this setup, the beam size was 180

nm (vertical) × 150 nm (horizontal) and the total flux of the

focused probe was 2 × 109 photons/sec. The focused X-ray

beam was used as the probe. The samples were mounted

on a translation scanning stage with a motion accuracy of

better than 10 nm. The XRF spectra were measured with a

Si drift diode detector (Rontec Xflash D301). The X-ray

path between the sample and the detector was through

helium gas in order to reduce the decrement of intensity

by the absorption of air.

3. Results and DiscussionThe XRF spectra corresponding to the sample of Sn–

0.7Cu–0.05Ni+Ge are shown in Figure 3. The signal inte-

gration time for the acquisition of the XRF spectra was 200

s. Spectra are taken from a region in the middle of the

BGA solder. Sn Lα1 (3.444 keV), Fe Kα1 (6.404 keV), Ni

Kα1 (7.478 keV), Cu Kα1 and Kβ1 (8.047 and 8.905 keV,

respectively), and Ge Kα1 (9.886 keV) are present. The X-

ray energy of Ag Lα1 is 2.984 keV and since Sn is the dom-

inant element of the solder, it is expected to be difficult to

measure trace levels of Ag in this experiment due to the

overlapping with the Lα spectra of Sn. Table 2 shows the

measured X-ray spectra for this experiment.

A μ-XRF elemental map of the Sn, Cu, Ni and Ge at the

soldered-copper interface taken with high resolution scan-

ning is shown in Figure 4(a) (scan pitch: 100 nm, integra-

tion: 0.2 sec) for BGA type Sn–0.7Cu–0.05Ni+Ge solders,

and a map including the solder surface is shown in Figure

4(b) (scan pitch: 200 nm, integration time: 5 sec). In the

elemental mappings, brighter regions indicate a higher

concentration of a specific element. It is clear from the

mapping results that Ni is present in the intermetallic

region, and is distributed in a relatively homogeneous

fashion throughout the IMC phase, Cu6Sn5.

Figure 4(a) shows that only trace levels (60 ppm) of Ge

are present in the bulk solder or in the reaction layer IMC.

However, Figure 4(b) shows that Ge is concentrated to a

degree at the solder ball surface region. This measure-

ment is consistent with past work on Sn–3.0Ag–0.5Cu–

0.05Ge alloy solder balls after a five-time reflow as

analysed using XPS.[23] This result suggests the trace Ge

addition in the solder may influence the oxidation behav-

iour of the solder during melting and solidification but not

the formation and growth of the IMC. At the resolution of

this study, there is no discernable segregation or precipi-

tation of Fe in the bulk solder or the IMC layer.

Fig. 2 SEM micrograph of a cross-sectioned BGA type Sn–0.7Cu–0.05Ni+Ge solder on a Cu substrate.

Fig. 3 XRF spectra of Sn–0.7Cu–0.05Ni+Ge solder.

Table 2 X-ray spectra from Fig. 3.

Element X-ray Energy (keV)

Ag Lα1 2.984

Sn Lα1 3.443

Fe Kα1 6.404

Ni Kα1 7.478

Cu Kα1 8.048

Ge Kα1 9.886

Pb Lα1 10.550

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Figure 5 (scan pitch: 100 nm, integration time: 0.2 sec)

shows the Sn, Cu, Ni and Ag distributions in the solder-

substrate interface region for a BGA-type Sn–1.2Ag–

0.5Cu–0.05Ni solder. The distribution of the 500 ppm Ni is

homogenous in the IMC layer in the reaction zone, as was

found in the Sn–0.7Cu–0.05Ni+Ge samples shown in Fig-

ure 4(a) and 4(b). Ag is expected to have very low solubil-

ity in Cu6Sn5, but trace Ag levels could not be examined

here because the Sn Lα1 and Ag Lα1 elemental peaks

overlap. However, Ag could be detected when present at

high concentrations, such as in Ag3Sn particles. Ag3Sn was

found in the solder matrix (Figure 5), as reported in past

research,[3] but was not present between Cu6Sn5 grains in

the reaction layer. Additionally, the Ni distribution in the

IMC was not discernibly affected by Ag.

Previous work has shown that (Cu,Ni)6Sn5 has a wide

solubility range of Ni[24–27] and around 4–5 at% Ni is mea-

sured in Cu6Sn5 solder joints of BGA-type Sn–0.7Cu–

0.05Ni+Ge on Cu substrate by point analysis using SEM/

EDS.[10, 12] Figures 6(a) and (b) schematically show how

the distribution of Ni in the intermetallic may have signifi-

cant implications for joint integrity. In the presence of a

concentration gradient (a), a situation might arise where

some regions of the IMC have Ni concentrations above the

level required for stabilization of the hexagonal allotrope

and other regions may have concentrations lower than that

required. Temperature cycling can therefore result in a

stress gradient within the IMC layer due to the associated

(a)

(b)

Fig. 4 (a) Micro-XRF mapping of BGA Sn–0.7Cu–0.05Ni+Ge.(b) Micro-XRF mapping of BGA Sn–0.7Cu–0.05Ni+Ge includ-ing the solder surface.

Fig. 5 Micro-XRF mapping of BGA Sn–1.2Ag–0.5Cu–0.05Ni.

Nogita et al.: Synchrotron Micro-XRF Measurements of Trace Element Distributions (4/7)

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Transactions of The Japan Institute of Electronics Packaging Vol. 3, No. 1, 2010

volume changes occurring with the transformation.[12] In

contrast, a homogeneous distribution of Ni in the IMC,

such as that shown in (b), where the Ni concentration

exceeds that required for the transformation will avoid

stress concentrations. The micro-XRF mapping results

shown in Figures 4 and 5 clearly indicate that the scenario

of Figure 6(b) occurs and there is 4–5 at% Ni homoge-

neously distributed throughout the Cu6Sn5.

Previous research has shown that (Cu,Ni)6Sn5 contain-

ing 4–5 at% Ni remains as hexagonal eta at room tempera-

ture under cooling conditions that cause binary Cu6Sn5 to

transform to monoclinic eta’.[9, 14] The influence of

retarding or eliminating this solid-state phase transforma-

tion on the thermomechanical stability of solder joints is a

continuing area of research.[10, 12] Recently Chung et

al.[28] discussed the direct evidence for early stage

Cu6Sn5 and Cu3Sn growth between non-Ni-containing Sn–

Cu solder and Cu substrates based on detailed TEM work.

They claimed that the formation and growth of Cu6Sn5 is

a two-stage process with (1) a first stage occurring during

the ramping up of the temperature when the solder had

not yet become molten, and (2) a second stage where

Cu6Sn5 rapidly covers the entire interface and a thin layer

of Cu3Sn also forms. They concluded that the existence of

Cu-enriched regions indicates that the grain boundary and

phase boundary diffusion of Cu atoms was the dominant

mechanism for the growth of Cu6Sn5 and Cu3Sn in the

early stages of the soldering reactions. Since our Ni-

containing solder shows a homogenous Ni distribution, it

is expected that the Cu and Ni atoms responsible for the

growth of (Cu,Ni)6Sn5 were supplied only from the Sn–

Cu–Ni solder and did not originate from the Cu substrate.

It is suggested that the formation and growth mechanisms

of the intermetallics at the interface of the Ni-containing

Sn–Cu solder and Cu substrates might be different from

that occurring between Ni-free Sn–Cu solders and Cu sub-

strates. Very thin layers of Cu3Sn formation on Cu sub-

strates when using Ni-containing Sn–Cu solder also indi-

cate a difference from Ni-free Sn–Cu solders.

Figures 7(a) and (b) show the Sn, Cu and Pb distribu-

tions in the IMC layer for BGA-type Sn–37Pb solders.

High-magnification mapping of the Pb distribution in

Figure 7(b) (scan pitch: 50 nm, integration time: 0.2 sec)

clearly indicates that Pb precipitates between the Cu6Sn5

IMC grains, but is not present within the Cu6Sn5 grains. As

Fig. 6 Significance of Ni distribution in (Cu,Ni)6Sn5 IMC, (a)inhomogeneous distribution of Ni in Cu6Sn5 (b) homoge-neous distribution of Ni in Cu6Sn5. 4–5 at% Ni is homoge-neously distributed throughout the Cu6Sn5 This level of Nistabilises the hexagonal allotrope of (Cu,Ni)6Sn5 (eta) at roomtemperature, minimising stresses associated with phase trans-formation.

(a)

(b)

Fig. 7 (a) Low magnification micro-XRF mapping of BGASn–37Pb. (b) High magnification micro-XRF mapping of BGASn–37Pb.

45

shown schematically in Figures 8(a) and (b), this distribu-

tion of Pb may be more favorable in regards to the accom-

modation of stress in the IMC layer compared to a situa-

tion where no segregates are present. The solid solubility

of Pb in Cu6Sn5 is very low and it is reasonable that Pb is

found at the grain boundaries of Cu6Sn5. It is reported that

Sn–Ag–Cu solder joints have a higher rate of interfacial

failure than Sn–Pb joints.[29] Soft Pb distributions/segre-

gations between the Cu6Sn5 grains along the Cu6Sn5 grain

boundaries in the reaction layer may inhibit cracking in

the reaction zone.

4. ConclusionsThis research utilised a μ-XRF trace element mapping

technique at the SPring-8 synchrotron radiation facility for

solder joint analysis. Three BGA-types of samples on Cu

substrates were used in the experiments. All samples with

a solder of base composition 500 ppm Ni had a homo-

geneous Ni distribution in the Cu6Sn5 IMC layer. Ge,

when present in trace levels, is distributed homogeneously

in the solder at a very low concentration but is also present

at the solder surface at a higher concentration, where it

contributes to the oxidation resistance of the joint. In the

Pb-containing system, the Pb in the Sn–37Pb solder precip-

itates or is segregated to the Cu6Sn5 IMC grain bounda-

ries. The variations in crystal structure and composition

across the soldered interfaces have practical implications

for the mechanical properties and propensity for damage

accumulation within the IMC layer.

AcknowledgmentsThis research has been conducted under an interna-

tional cooperative research program between the

University of Queensland, Australia and Nihon Superior

Company Ltd., Japan. The authors would like to thank Mr.

J. Read, The University of Queensland, Mr. T. Nishimura,

M. Miyaoka and K. Sweatman of Nihon Superior Co. Ltd.

for the experiments and valuable discussions. The syn-

chrotron radiation experiments were performed at SPring-

8 (Proposal No. 2009A1159/BL No. 47XU). Financial sup-

port from the Australian Synchrotron International

Synchrotron Access Program (Program No. AS_IA091) is

acknowledged. Nogita is funded by a Smart Futures

Fellowship, Queensland Government, Australia.

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