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 Journal of Alloys and Compounds 485 (2009) 219–224

Contents lists available at ScienceDirect

 Journal of Alloys and Compounds

 j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / j a l l c o m

Microstructural evolution of the Au–20 wt.% Sn solder on the Cu

substrate during reflow

Hsang-mou Chung, Chih-ming Chen∗, Chi-pu Lin, Chia-ju Chen

Department of Chemical Engineering, National Chung-Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan, ROC 

a r t i c l e i n f o

 Article history:

Received 28 April 2009

Received in revised form 2 June 2009Accepted 3 June 2009

Available online 10 June 2009

Keywords:

Intermetallics

Precipitation

Surfaces and interfaces

a b s t r a c t

Reflow reactionsof theAu–20wt.%Sn (Au20Sn)solder on the Cu substrateat 330◦C were conducted and

the microstructure of as-solidified solder joint was examined. After reflow for 1 min, the as-solidified

solder matrix presented a typical eutectic  -(Au,Cu)5Sn +-(Au,Cu)Sn lamellar microstructure and two

phases, -(Au,Cu)5Sn and AuCu, were formed at the solder/Cu interface. The  -(Au,Cu)5Sn phase grew

very irregularly at the interface. Upon increasing the reflow time, part of the fine lamellar microstruc-

ture coarsened in the as-solidified solder matrix, and therefore the solder matrix displayed a composite

microstructure of fine lamellar eutectic and coarse eutectic. In addition, dendritic growth of the   -

(Au,Cu)5Sn phase was observed and the    dendrites were found to grow at a faster rate in the solder

matrix of smaller volume. The microstructural evolution at the Au20Sn/Cu solder joint during reflow,

including irregular growth of  -(Au,Cu)5Sn at the interface, dendritic growth of  -(Au,Cu)5Sn in the sol-

der matrix, anddevelopment of eutectic microstructure, was explained using related phase diagram and

vertical section.© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Replacing the Pb-containing solders with the Pb-free solders in

electronic products has been a global trend due to environmen-

tal and human health concerns for Pb toxicity [1–3]. Many kinds

of Pb-free solder alloys have been proposed; however, there is not

yet a “perfect” solder alloy that can meet all requirements for the

electronicpackagingsocietyand industryto date. Atpresent,a com-

promising method is to apply the specific Pb-free solder with the

desired and beneficial characteristics to the specific packagingpro-

cess. Eutectic Au–20wt.% Sn solder (denoted as Au20Sn hereafter)

is one of the commonly used solders in optoelectronic packaging

[4–7]. This solder alloy has excellent fatigue and creep resistances

and, in particular,can be appliedthrougha fluxless bonding process

[8–10].Cu is a popular choice for the metallization layer of surface fin-

ishes and bonding pads in electronic packaging [1,2]. Therefore,

the Au20Sn/Cu solder joints are often encountered in optoelec-

tronic products. The Au20Sn/Cu solder joints can be fabricated

using thermo-compression [11,12] or typical reflow process [13,14].

When the reflow process is applied, the temperature rises to

above the melting point of the eutectic Au20Sn solder and the

∗ Corresponding author. Tel.: +886 4 22859458; fax: +886 4 22854734.

E-mail address: chencm@nchu.edu.tw (C.-m. Chen).

molten solder will be wet and react with the underlying Cu sub-

strate to accomplish joining. Investigations have been made on

the microstructural evolution and interfacial reaction that occur

in the Au20Sn/Cu solder joint for the reliability evaluation of the

solder joint  [13,14].  Song et al.   [13]  f ound that the microstruc-

ture of the solder matrix was strongly affected by the dissolution

of Cu from the substrate during reflow, especially for the solders

with small volumes. Yoon et al. [14] reported that the as-solidified

Au20Sn solder matrix presented a typical eutectic (-Au5Sn +-

AuSn) lamellar microstructure after reflow. Two phases,  -Au5Sn

and AuCu, were formed at the Au20Sn/Cu interface, where the -

Au5Sn phase contained a significant Cu solubility and presented an

irregular shape.

In this study, microstructural evolution of the Au20Sn/Cu sol-

der joint during reflow was investigated. Irregular growth of the-(Au,Cu)5Sn phase at the solder/Cu interface was also observed as

reported previously [13,14], while a detailed mechanism regarding

the irregular growth was proposed. Other interesting phenom-

ena of microstructural evolution were also observed. Part of the

-(Au,Cu)5Sn phase in the as-solidified solder matrix displayed a

dendritic microstructure after a prolonged reflow time, and the

 dendrites were found to grow at a faster rate in the solder

matrix of smaller volume. Upon increasing the reflow time, the as-

solidified solder matrix gradually changed from a uniform lamellar

microstructure into a composite mixture of fine lamellar eutectic

and coarse eutectic.

0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.jallcom.2009.06.018

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Fig. 1.   Illustration of the fabrication process of the Au20Sn/Cu solder joint.

2. Experimental procedures

Sn and Au shots, both of 99.99% purity, were used to prepare the Au20Sn sol-

der. Proper amounts of Sn and Au shots were weighed and sealed in a vacuum

(2.3×10−2 torr) quartz tube. The quartz tube was put into a furnace at 1150 ◦C for

72h to make the Sn and Au shots melt and mix together. To reach homogeneity of 

the molten alloy, the quartz tube was slightly shaken for several times during heat

treatment. Then the quartz tube was moved out of the furnace and quenched in ice

water.The solidifiedalloy ingot wascut intolots of small pieces anddividedinto two

groups by weight, one was 4.00 (±0.3) mg and the other was 2.11 (±0.26)mg. The

correspondingvolumesof thetwo solder groupswerecalculated tobe 5.4×109 m3

and2.84×108 m3, respectively,using the density data(14.51 g/cm3) ofthe Au20Sn

solder [15]. A Cu plate with a thickness of 254 m was used as the substrate. A 50-

m thick PI (polyimide) film with a central 1000m×1000m open window was

adhered to the Cu plate. The solder piece was cleaned and placed on the uncovered

region (i.e. open window) on the Cu plate. Reflow reactions were performed as the

Cu plate with the solder piece was placed in a furnace at 330◦C for 1–90min. The

experimental procedure was schematically shown in Fig. 1.After the predetermined reflow time, the furnace power was turned off and

the sample was cooled down to room temperature naturally in the furnace. Subse-

quently, the sample was taken out of the furnace and mounted in epoxy resin. The

sample was ground and polished to expose the cross-section of the solder/Cu inter-

face for metallographic examination. Field-emission scanning electron microscope

(FESEM) was used to observe the microstructure of the solder matrix and solder/Cu

interface. Energy dispersive X-ray spectrometer (EDX) wasused forthe composition

analysis.

Fig. 2.   SEM images of the cross-sectional microstructure of the Au20Sn/Cu solder joint after reflow for 1 min, 5 min, and 60 min, where the Au20Sn solders in (a–c) were

4.00 mg and those in (d–f) were 2.11 mg.

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Fig. 3.  Au–Sn binary phase diagram [16].

3. Results and discussion

Fig.2 is the SEMimages showing thecross-sectionalmicrostruc-

ture of the Au20Sn/Cu interfaces after reflow for 1–60 min,

where the Au20Sn solders in   Fig. 2(a–c) were 4.00mg and

those in   Fig. 2(d–f) were 2.11 mg. The Au20Sn solder matrix

presents a two-phase microstructure. EDX analysis revealed that

the compositions of the bright and dark phases in   Fig. 2(c)

were 80.22at.% Au–13.73 at.% Sn–6.05 at.% Cu and 50.99at.%

Au–46.27 at.% Sn–2.74 at.% Cu, respectively. Based on the Au–Sn

binary phasediagramshowninFig. 3 [16], the Au20Snsolder shouldpresent a eutectic (-Au5Sn +-AuSn) microstructure at room tem-

perature. Therefore, it is likely that the bright phase is the -Au5Sn

and the dark phase is the -AuSn, and both of them contain some

Cu solubility. Since Cu is chemically similar to Au, the Cu atoms

are likely to enter into the    and    lattices and substitute for the

Au atoms. These two phases were denoted as -(Au,Cu)5Sn and -

(Au,Cu)Sn here in order to show the Cu participation. Two phases

were formed at the Au20Sn/Cu interface. The composition of the

bright phaseadjacentto thesolder sidewas 67.53at.% Au–13.01at.%

Sn–19.36 at.% Cu,suggesting that it was also the-(Au,Cu)5Sn phase

with a significant Cu solubility. The thin phase next to the Cu side

was identified as the AuCu phase with some Sn solubility based on

the composition analysis result (46.9 at.% Au–6.8 at.% Sn–46.3at.%

Cu) and the Au–Cu binary phase diagram [17]. The above resultswere consistent with the previous study [14].

Threeinterestingphenomenawerefound for themicrostructure

observation of the Au20Sn/Cu solder joint during reflow. The first

one is the irregular growth of the interfacial  -(Au,Cu)5Sn phase.

It grew in the form of pillars. Second, some   pillars could extend

upward and grow into the solder matrix in the form of dendrites

after reflow for 60 min. Fig. 4 is the SEM images of lower magni-

fication showing the whole solder joint for clear observation of 

the dendritic growth. It is clear to see that in the initial reflow no

dendrites were observed in the two types of as-solidified solder

matrixes, but after reflow for 60min   dendrites were formed. In

the solder matrix of 4.00 mg shown in Fig. 4(b), only one    den-

drite was found and it grew as long as 400 m. However, in the

solder matrix of 2.11 mg, as shown in Fig. 4(d), large amounts of 

dendrites were observed to form. Some dendrites had developed

tertiary arms. The third interesting phenomenon was the evolu-

tion of eutectic microstructure in the solder matrix. Two types of 

eutectic microstructures could be found in the solder matrix; one

with fine lamellar microstructure (denoted as A) and the other

with coarse microstructure (denoted as B), as shown in  Fig. 2.

Upon increasing the reflow time, the amount of the fine lamel-

lar microstructure decreased but that of the coarse microstructure

increased.

To further understand the growth behavior of the interfacial -

(Au,Cu)5Sn phase, three additional samples with the same solderweight, 4 mg, were prepared and reflowed at 330◦C for 90min.The

cross-sectional microstructures of these three as-solidified sam-

ples are shown in Fig. 5. The major difference in the reflow process

of these three samples is the cooling condition during solidifica-

tion. The samples in Fig. 5(a–c)were cooled naturally in furnace, by

fan convection, and by ice-quenching, respectively. It can be seen

that the microstructures of the solder matrixes in Fig. 5(a) and (b)

are very similar. Both of them contain dendritic precipitates. How-

ever, no noticeable dendrites were observed in the ice-quenched

samples shown in Fig. 5(c). Ice-quenching is an extremely rapid

cooling process, so it is reasonable that any phase transforma-

tion, for example, dendritic growth, during solidification will not

occur because of insufficient solidification time. It canalso be found

that the interfacial -(Au,Cu)5Sn phase in the ice-quenched samplepresents a relatively uniform layered structure without any pillar

formation, which is different from those in Fig. 5(a) and (b). (The

boundary between the solder and the  -(Au,Cu)5Sn phase is not

clear in Fig. 5(c), so a white line was drawn in order to show this

boundary.) According to the above discussion that phase transfor-

mation during solidification does not occur in the sample cooled

by ice-quenching, the interfacial   -(Au,Cu)5Sn phase in the ice-

quenched sample should also present a uniform layered structure

 just before cooling. In other words, the interfacial -(Au,Cu)5Sn

phase in Fig. 5(a) and (b) should also grow in the form of layered

structure as that in Fig. 5(c) just before cooling because the reflow

processesforthesamplesin Fig.5(a)and(b)werethesameasthatin

Fig.5(c) exceptthecooling condition.Therefore, theirregularpillar-

type structure of the interfacial -(Au,Cu)5Sn phase in Fig. 5(a) and

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Fig. 4.   SEM images of lower magnification showing the whole solder joints in Fig. 2, where (a–c), and (d) refer to Fig. 2(a), (c), (d), and (f), respectively.

(b) was formed from slower solidification during furnace and fan

cooling.

During reflow, the Cu atomsfrom the Cu substrate continuously

dissolve into the molten solder. Since Cu acts chemically like Au,

the Sn composition in the molten solder will decrease with the Cudissolution, and the solder composition will shift toward the Au-

rich side, as depicted by the dotted horizontal arrow in the Au–Sn

binary phase diagram shown in Fig. 3. When the solder composi-

tion enters into the two-phase (L +) region, the primary    phase

precipitates and forms as the layered -(Au,Cu)5Sn phase along the

solder/Cu interface, as seen in Fig. 5(c). When the sample starts to

cool down, more phases will precipitate and grow on the existing

 phase at the solder/Cu interface. Since the composition shift of 

solder simultaneouslycreates a supercooling of T 1 below thecor-

responding equilibrium liquidus (solidification) temperature T 1 of 

the new hypo-eutectic composition, it will provide a driving force

forthecellularor dendriticgrowthof the-(Au,Cu)5Sn phaseduring

solidification.Therefore, pillar-type, or dendritic(with onlyprimary

arm), growth of the -(Au,Cu)5Sn phase was observed, as seen inFig.5(a)and(b).Fig.6shows a SEMimage ofhighermagnification of 

thesolder/Cuinterface in Fig. 5(c). Many smallcellular-(Au,Cu)5Sn

grainswereformedsidebysideonthebig-(Au,Cu)5Sngrains.They

werealsoformed during solidification,but without sufficient solid-

ification time for the ice-quenching case, they cannot grow as large

as the  pillars shown in Fig. 5(a) and (b).

Theabove discussion also explainedthe secondinterestingphe-

nomenon of dendritic growth of the   -(Au,Cu)5Sn phase in the

solder matrix. As mentioned above, the composition shift of sol-

der creates a supercooling of T 1 as shown in Fig. 3. Suppose that

thisis the casefor the solderof 4.00mg.Forthe solderof lessweight

of 2.11 mg, the composition shift will be more significant with the

Cu dissolution the same as the solder of 4.00mg. As a result, the

supercooling for the solder of less weight increases as marked by

T 2, which leads to the formation of large amounts of dendrites,

as shown in Fig. 4(d). It can be found that the liquidus line of the

two-phase (L+ ) region, i.e. ab, in the Au–Sn binary phase diagram

(Fig. 3) has a very steep gradient. An estimation revealed that the

equilibrium liquidus temperature will rise by about 43◦C whentheSn concentration is reduced by 1 wt.%. In other words, when the

Cu dissolution causes 1wt.% reduction of the Sn concentration, the

supercooling will increase by 43 ◦C. This may be the reason why

the dendritic growth is very sensitive to the weight of the Au20Sn

solder.

Theeffect ofthe initial solderweighton thedendriticgrowth can

alsobe understood fromTable1. Since thecontact area between the

solder and Cu substrate was clearly defined bythe open window of 

a PI film, and the area of the open window was the same for the

two types of solder samples, the amount of Cu dissolution into the

two types of solder samples could be assumed approximately the

same. The first column from the left-hand side of  Table 1 shows

various dissolved Cu amount. The other columns show the corre-

sponding compositions of the solders with the dissolved Cu. It canbe found that the composition change of the solder of 2.11 mg is

 Table 1

Change of solder compositions with various amount of dissolved Cu.

Amount of dissolved Cu (mg) Initial solder weight (mg)and corresponding

solder composition with dissolved Cu

4.00 2.11

Au (wt.%) Sn (wt.%) Au (wt.%) Sn (wt.%)

0 80 20 80 20

0.21 81 19 81.8 18.2

0.44 82 18 83.5 16.5

0.71 83 17 85.0 15.0

1.00 84 16 86.4 13.6

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H.-m. Chung et al. / Journal of Alloys and Compounds 485 (2009) 219–224   223

Fig.5. SEMimages ofthe solderjointswith4.00mg solders afterreflow at330◦Cfor

90min. Thesolderjointsin (a–c) were cooled naturally in furnace,by fanconvection,

and by ice-quenching, respectively.

more significant than that of 4 mg with the same amount of dis-

solved Cu.For example, when theSn concentration is reduced from

the initial 20 wt.% to 17 wt.% for the solder of 4 mg with the dis-

solved Cu amount of 0.71mg, the corresponding Sn concentration

of the solder of 2.11 mg is reducedto 15 wt.%, which is 2 wt.% lower.

The2 wt.% reduction, however, cancausethe augmentationof 86◦C

in thesupercooling, which canprovidea higherdrivingforcefor the

dendritic growth.

Thethird interesting phenomenon canbe explained using a ver-

tical section along Au20Sn–Cu (Fig. 7) proposed by Song et al. [13].

The composition in the vertical section is in at.%. The phase A is

a ternary phase with a homogeneity range from 42Au38Cu20Sn

to 47Au33Cu20Sn (in at.%). Consistent with the above discussion

based on the Au–Sn binary phase diagram, the Cu dissolution into

Fig. 6.  SEM image of higher magnification showing the microstructure of the

Au20Sn/Cu interface in Fig. 5(c).

Fig. 7.   Schematic verticalsection along Au20Sn–Cu proposedby Songet al. [13]. The

composition shown in this vertical section is in at.%.

the molten solder causes the precipitation of the primary   phase

as depicted by thehorizontal dotted line in Fig. 7. Assuming that Cu

dissolution reaches the point “m” after a certain period of reflow,

the phases that form during solidification can be represented by

the vertical dotted line and the sequence is L + →L + +→+.

During solidification, the molten solder passes through the L +regionfirst, andthe primary precipitates grow dendritically. Then

the molten solder enters into the L + +  region where both   and   phases solidify simultaneously. Finally, the remaining liq-

uid transforms into eutectic  + mixture when entering into the

+ region. This transformation is a typical eutectic reaction and a

fine lamellar microstructure will be observed, as seen in the region

A in Fig. 2. In addition, it can be found that a  + mixture has

formed prior to the eutectic reaction as L +→ L + +. This ear-

lier transformation can alsoproduce a eutectic+microstructure.

Since both    and   phases will grow continuously during subse-

quent cooling, a coarsemicrostructurewas observed, as seen in the

regionB inFig.2. Therefore, the as-solidified solder matrix presents

a composite microstructure of fine lamellar eutectic and coarse

eutectic.

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4. Conclusions

After a typical reflow time for 1 min, the as-solidified solder

matrix presented a eutectic   -(Au,Cu)5Sn +-(Au,Cu)Sn lamellar

microstructure. Two phases, -(Au,Cu)5Sn and AuCu, were formed

at the solder/Cu interface, where the-(Au,Cu)5Sn phase grew very

irregularly, approximatelyin theform of pillars, butthe AuCu phase

wasnearly layered. Dendriticgrowth of the-(Au,Cu)5Sn phase was

observed in the solder matrixwhen the reflow time was prolonged

to 60 min. This was attributed to the Cu dissolution from the Cu

substrate into themoltensolder matrix. Because Cu behaves chem-

ically like Au, the composition of the solder changed from eutectic

into hypo-eutectic with the dissolved Cu atoms. As a result, the

primary -(Au,Cu)5Sn phase precipitated first and grew dendriti-

cally during solidification. It was also found that the -(Au,Cu)5Sn

dendrites grew at a faster rate as the solder volume was reduced.

Changing the cooling condition of the reflow process from moder-

ate furnace cooling to rapid ice-quenching significantly influenced

themicrostructureof theas-solidified solder joint. No dendriticand

irregulargrowth of the-(Au,Cu)5Sn phase in the solder matrix and

at the interface, respectively, were observed in the ice-quenched

solder joint, indicating that both of them in the furnace-cooled sol-

derjointoccurredduringmoderate solidificationof furnacecooling.

Upon increasing the reflow time, the as-solidified solder matrixgradually changed from a uniform lamellar microstructure into a

compositemixture offinelamellareutecticand coarse eutectic.This

microstructural evolution was also attributable to the Cu dissolu-

tion into the solder matrix and could be explained using related

vertical section.

 Acknowledgements

This work was supported by the National Science Council and

Ministry of Economic Affairs, Taiwan, Republic of China, with Grant

Nos. NSC 96-2218-E-007-012 and 97-EC-17-A-07-S1-097, respec-

tively.

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