Microelectronics Reliability xxx (2015) xxx–xxx

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Microelectronics Reliability

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The effect of iron and bismuth addition on the microstructural, mechanical,and thermal properties of Sn–1Ag–0.5Cu solder alloy

M.H. Mahdavifard a, M.F.M. Sabri a,⁎, D.A. Shnawah a, S.M. Said b, I.A. Badruddin a, S. Rozali a

a Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

⁎ Corresponding author.E-mail address: faizul@um.edu.my (M.F.M. Sabri).

http://dx.doi.org/10.1016/j.microrel.2015.06.1340026-2714/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: M.H. Mahdavifardproperties of Sn–1Ag–0.5Cu solder alloy, Mi

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Article history:Received 25 May 2015Accepted 29 June 2015Available online xxxx

Keywords:Sn–Ag–Cu solderBi and Fe additivesMicrostructural propertiesMechanical propertiesThermal properties

This work investigates the effect of Fe and Bi addition, 0.05 wt.% Fe and 1wt.% or 2 wt.% Bi, on themicrostructural,mechanical, and thermal properties of the low silver Sn–1Ag–0.5Cu (SAC105) solder alloy. Adding Bi and Fe toSAC105 increased ultimate tensile strength (UTS) and yield strength and decreased the total elongation which isrelated to solid-solution and precipitation strengthening effects by Bi in the Sn-rich phase. While 0.05 wt.% Femade few FeSn2 in the solder bulk which does not have considerable effect on mechanical properties. Scanningelectron microscopy (FESEM) and energy dispersive X-ray (EDX) showed that Bi strengthen solder by scatteringin the bulk of SAC105-Fe solder alloy, thereby increased β-Sn and degenerated Cu6Sn5 and Ag3Sn into a chain-like arrangement. Surface fracture of SAC–Fe–Bi solder alloys showed brittle fracture because Bi prevented β-Snto deform and therefore Bi decreased its ductility. Finally, Bi significantly reduces the melting point andundercooling.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The eutectic Sn–Pb solder has been a dominant alloy for wave-soldering printed circuit boards and other electronic products. Pbtoxicity has propelled the search for a Pb-free replacement. Neareutectic Sn–Ag–Cu lead-free solders, such as Sn-4 wt.% Ag-0.5 wt.%Cu (SAC405) or Sn-3 wt.% Ag-0.5 wt.% Cu (SAC305), suggested as apromising replacements for Sn–Pb solder alloy because of their lowmelting temperature, and favorable thermal–mechanical fatigueproperties. However, because of the rigidity of the high-Ag-contentSn–Ag–Cu (SAC) alloys, portable electronic products that containthese high-Ag–SAC solder joints are more prone to failure due todrop and high impact applications. Moreover, the high Ag contentin Sn–Ag–Cu alloys results in a relatively high cost for these solderalloys. Low-Ag-content Sn–Ag–Cu alloys such as Sn-1 wt.% Ag–0.5 wt.% Cu (SAC105) have been considered as a solution to boththe cost and poor drop impact reliability issues. Reducing the Agcontent of Sn–Ag–Cu alloy increases its bulk compliance and plasticenergy dissipation ability, identified as key factors for improvingdrop resistance [1]. Also, the application of SAC solder alloy is limitedby the coarse precipitation of the Cu6Sn5 phase [2] and also a highermelting temperature and lower strength [3]. Some alloying elements,

, et al., The effect of iron andcroelectronics Reliability (201

such as Ni [4], Co [5], Ce [6], Fe [7], Ag [8], Sb [9] and Zn [10] havebeen added into SAC alloys to refine the microstructure, and improvethe wettability and mechanical properties. The net result of minoralloying addition is to (1) change the bulk alloy microstructure andmechanical properties, and/or (2) control the interfacial intermetalliclayer(s) [3].

Previous research on Sn–Ag–Cu showed that doping Fe in-creased its yield strength and UTS, and decreased elongation [11].On the other hand, Fe improved drop impact reliability and alsostabilized the mechanical properties of solder with aging [12].Furthermore, Fe is able to suppress formation of Kirkendall voids[13].

The Bi addition significantly, linearly increased the UTS of Bi-containing lead-free solders by solid solution hardening mechanism,referring to the microstructure analysis. By increasing Bi addition,the melting temperatures could be dropped further; consequently,the solder would become “strong and brittle” [14].

In the present study, Fe and Bi were added together to the low Agsolder, SAC105, to investigate the effect of these two elements on thebulk alloy microstructure, tensile and thermal properties. On thebasis of the previous works, 0.05 wt.% Fe added to SAC105 becauseD.A. Shnawah et al. [15] showed that by increasing Fe more than0.1 wt.% it makes large FeSn2 intermetallic in bulk of solder whichdeteriorate mechanical properties. Also selected 1 and 2 wt.% Bi toadd to SAC105 because Liu et al. [16] showed that more than 3 wt.%Bi make solder too brittle.

bismuth addition on the microstructural, mechanical, and thermal5), http://dx.doi.org/10.1016/j.microrel.2015.06.134

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2. Experimental procedure

This study used Sn–1Ag–0.5Cu (SAC105), Sn–1Ag–0.5Cu–0.05Fe–1Bi (SAC105–Fe–1Bi), and Sn–1Ag–0.5Cu–0.05Fe–2Bi (SAC105–Fe–2Bi) bulk solder specimens with flat dog-bone shape. The dimensionof dog bone samples were 5.0 mm thick × 5.0 mmwide × 21 mm long.

Pure ingots of Sn, Cu, Ag, Fe and Bi melted in an induction furnace atmore than 1000 °C for 40 min to prepare alloys. Then, themolten alloyswere mixed with liquid pure Sn for 60 min in a melting furnace at290–300 °C. Subsequently, the molten alloys were cast to disk shapedingots to verify the exact composition of the casting ingots by atomicemission spectrometry (AES) with nominally ppm accuracy using aSpectro LAB device. This was done to ensure that the percentages ofSn, Ag, Cu, Fe, Bi and impurities of the alloy composition compliedwith the specifications (per JIS-Z-3282:1999, see table 1). Then, themolten alloys were poured into stainless steel molds that were pre-heated at 120–130 °C, and the molds were air cooled naturally toroom temperature (25 °C). Finally, the dog-bone sampleswere removedand visually evaluated to be without damage or voids.

Differential scanning calorimetry (DSC) was used to measure melt-ing and solidification temperature of solder alloys. The sample sizewas of approximately 10 mg, and the scanning rate was 5 °C/min. Foreach alloy, the samplewasfirst scanned from30 °C up to 300 °C, followedby cooling down to 30 °C at the same rate. In this study, the amountof undercooling was measured by the difference of each onset temper-ature in the heating and cooling curve. For each sample the test wasrepeated 5 times.

The solder bar was set onto a testing grip at two ends of the speci-men using an Instron 5569A universal testingmachine which equippedwith extensometer for 10mm gauge length. The tensile force applied tothe specimen was measured by a load cell for stress calculation. Fivesamples were tested under the same testing conditions for each solderspecimen and the tensile properties were obtained by averaging thetest data. The tensile tests were conducted at room temperature(25 °C) for all solders composition under a strain rate of 10−3 s−1 toinvestigate the effects of the added Fe and Bi on the mechanical prop-erties of the solder, yield stress, ultimate tensile strength (UTS), andelongation.

In this paper, the yield stress of the solder was considered to be thestress value at which 0.2% plastic strain occurred. The UTS of the solderwas considered to be the maximum stress in the stress–strain curve.

A field-emission scanning electron microscope (FESEM; FEI HeliosNanoLab 650) with a concentric backscatter detector (CBS) was usedto take backscatter electron imaging and examine the microstructures.Additionally, energy dispersive X-ray spectroscopy (EDX) and X-rayDiffraction (XRD) were adopted to determine the phase compositions.To obtain the microstructure, the solder samples were prepared by dic-ing, resin molding, grinding and polishing. The samples were ground

Table 1Chemical composition of the alloys (wt.%).

Quality SAC105 SAC105–Fe–1Bi SAC105–Fe–2Bi

Ag 0.9401 1.0105 0.9159Al 0.0005 0.0005 0.0005As 0.0008 0.0008 0.0008Bi 0.0016 1.0124 1.9510Cd 0.0001 0.0001 0.0002Co 0.0004 0.0006 0.0006Cu 0.4947 0.4960 0.4747Fe 0.0005 0.0460 0.0503In 0.0013 0.0013 0.0013Ni 0.0002 0.0000 0.0000Pb 0.0039 0.0047 0.0045Sb 0.0034 0.0038 0.0039Zn 0.0001 0.0001 0.0001Sn 98.5524 97.4232 96.5962

Please cite this article as: M.H. Mahdavifard, et al., The effect of iron andproperties of Sn–1Ag–0.5Cu solder alloy, Microelectronics Reliability (201

with SiC papers and then polished with a 3 μm diamond and a0.04 μm colloidal silica suspension.

3. Result and discussion

3.1. Tensile properties of the alloys

The stress–strain curves of the as-cast SAC105, SAC105–Fe–1Bi andSAC105–Fe–2Bi solder alloys under a strain rate of 10−3 s−1 areshown in Fig. 1. The mechanical properties of solder alloys yield stress,ultimate tensile strength (UTS), and total elongation are shown inFig. 2. The result showed 21.1 MPa yield strength, 27.5 MPa UTS and60% total elongation for SAC105, which is in agreement with previouswork by Che et al. [17]. As illustrated, doping 0.05 wt.% Fe and 1 wt.%Bi to SAC105 solder increased the yield strength to 31.7 MPa and in-creased UTS to 40.5 MPa, while the total elongation was slightlydecreased to 43%. By increasing Bi to 2 wt.% the same trends continue,increase yield strength to 41.8 MPa and UTS to 56 MPa, and also totalelongation decrease to 33.8%. Jie Zhao et al. [18] and also Xiaowu Huet al. [19] showed the similar effect in mechanical properties by addingBi to Sn–Ag–Cu and Sn–0.7Cu respectively. Also D.A. Shnawah et al.showed that just by adding 0.1 wt.% Fe to SAC yield stress and UTSdecreased, while total elongation maintained at the SAC105 level [15].

3.2. Microstructures of the alloys

The as-cast microstructures of the standard SAC105, SAC105–Fe–1Bi, and SAC105–Fe–2Bi alloys are shown in Fig. 3 (a, b, c). The micro-structures of SAC105 in Fig. 3(a) include β-Sn dendrites andinterdendritic regions consisting of Cu6Sn5 and Ag3Sn intermetalliccompounds (IMCs) particle dispersed within a Sn-rich matrix. Also,Fig. 4 shows X-ray diffraction (XRD) result of composition in the solderalloys. Fig. 3(b) shows SAC105–Fe–1Bi which contain β-Sn, Cu6Sn5 andAg3Sn. Moreover, addition of 0.05 wt.% Fe and 1 wt.% Bi to SAC105makes FeSn2 in bulk of solder and 1 wt.% Bi scatter on whole bulk ofsolder. On the basis of the phase diagrams of Bi–Ag, Bi–Sn, Bi–Cuand Bi–Fe, Bi cannot make any intermetallic compound with otherelement, so there is not any Bi compound in solder. By adding 2 wt.%Bi to SAC105, FeSn2, Ag3Sn, and Cu6Sn5 detected in the solder bulk,Fig. 3(c). Elemental mapping of SAC105–Fe–2Bi solder alloy in Fig. 5shows clear distribution of all elements in the area. Huang et al. showedthat the solid solubility limit of Bi in the Sn–Ag-based solder is about4wt.% at room temperature, andwith a higher Bi addition, the supersat-urated Bi would precipitate in the form of the pure Bi phase from the Snmatrix. So it is not possible to show onepoint as a Bi in the FESEM imagewith 1 and 2 wt.% Bi [16].

Adding Bi to SAC105–Fe increase β-Sn and decrease Cu6Sn5. As it isclear in Fig. 3 (b and c) eutectic regions degenerated into a chain-likearrangement. Lin et al. also showed the same result by adding Mn and

Fig. 1. Stress–strain curves of SAC105, SAC105–Fe–1Bi and SAC105–Fe–2Bi solders.

bismuth addition on the microstructural, mechanical, and thermal5), http://dx.doi.org/10.1016/j.microrel.2015.06.134

Fig. 2. Tensile properties of SAC105, SAC105–Fe–1Bi and SAC105–Fe–2Bi solders: (a) yield stress, (b) UTS, and (c) total elongation.

Fig. 3. FESEM micrographs of as-cast SAC105, SAC105–Fe–1Bi and SAC105–Fe–2Bi solder alloys.

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Ti to Sn–Ag–Cu solder [20]. Vianco and Rejent [21] showed that the Biparticles in Sn–3.5Ag solder pinned several grain boundaries. Grainboundary pinning would contribute to the high mechanical strengthof this particular composition, which attributed to solid-solution andprecipitation strengthening effects by Bi in the Sn-rich phase.

Fig. 4. XRD result: (a) SAC105, (b) SAC105–Fe–

Please cite this article as: M.H. Mahdavifard, et al., The effect of iron andproperties of Sn–1Ag–0.5Cu solder alloy, Microelectronics Reliability (201

3.3. Fracture surfaces of the alloys

To further study the effect of Bi and Fe addition to the SAC105 bulksolder, the fracture surfaces were examined after tensile tests. Fig. 6shows SEM micrographs of the fracture surfaces of alloys after tensile

1Bi, and (c) SAC105–Fe–2Bi solder alloys.

bismuth addition on the microstructural, mechanical, and thermal5), http://dx.doi.org/10.1016/j.microrel.2015.06.134

Fig. 5. Elemental mapping analysis of SAC105–Fe–2Bi solder alloy.

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tests. The fractograph of the SAC105, SAC105–Fe–1Bi, and SAC105–Fe–2Bi alloys (Fig. 6) shows different cross-sectionals areas. For the stan-dard SAC105 solder cross-sectional area shows necking. The fracture

Fig. 6. SEM fractographs of the alloys after tensile tests (a and b)

Please cite this article as: M.H. Mahdavifard, et al., The effect of iron andproperties of Sn–1Ag–0.5Cu solder alloy, Microelectronics Reliability (201

surface of the SAC105 alloy consists of large ductile dimples as shownin Fig. 6(b). These results demonstrate that the fracture mechanismfor SAC105 is ductile. As shown in Fig. 6(c) adding 0.05 wt.% Fe and

SAC105, (c and d) SAC105–Fe–1Bi, (e and f) SAC105–Fe–2Bi.

bismuth addition on the microstructural, mechanical, and thermal5), http://dx.doi.org/10.1016/j.microrel.2015.06.134

Table 2Differential scanning calorimetry (DSC) test results of the alloys.

Alloys Solidus temp. Ts (°C) Liquidus temp. Tl (°C) Pasty range Onset soldifi. temp. Tc (°C) Under cooling, Ts–Tc (°C)

SAC105 219.3 231.7 12.4 207 12.3SAC105–0.05 Fe–1 Bi 217.6 230 12.4 207.5 10.1SAC105–0.05 Fe–2 Bi 214 229 15 204.2 9.8

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1 wt.% Bi make the fracture surface clearly brittle with very little plasticdeformation and no necking. Also Fig. 6(d) shows that the number ofdimples decrease in compare to Fig. 6(b). By increasing Bi to 2 wt.%the surface become more brittle, Fig. 6(e), and there is not any dimpleon the surface, Fig. 6(f). Previous works [22] showed that adding0.3 wt.% Fe to SAC105 make large circular FeSn2 IMC particles insidethe bulk of solder which can be nucleation sites for failure, even thoughdoes not have effect onmode of ductile fracture for SAC105.Whereas inthe present work by adding just 0.05 wt.% Fe there is few FeSn2 IMCparticles inside the β-Sn. Therefore, this non-ductile fracture behaviorcan be attributed to Bi, which is scatter inβ-Sn and prevent solder to de-form in β-Sn. Also, the reduction of elongation of the SAC105–Fe–1Bi,and SAC105–Fe–2Bi alloys can be justified by Bi brittle properties.

3.4. Thermal behavior

Differential scanning calorimetry (DSC) analysis was carried out todetermine the effects of the Fe and Bi additions on the melting temper-ature and pasty range of the SAC105 alloy. The solidus and liquidustemperatures and pasty range of the solder alloys were measured andare listed in Table 2. DSC test shows a decrease in solidus temperaturefrom 219.33 °C for SAC105 to 214 °C for SAC105–0.05Fe–2Bi. Adding0.05 wt.% Fe and 2 wt.% Bi to SAC105 decreased undercoolingfrom 12.3 to 9.8. These results are in agreement with previous work byEl-Daly et al. [23] which showed a 6 °C decrease of melting temperatureby adding 3wt.% Bi to SAC157 anddecrease undercooling 4 °C.Moreover,previous works. [15] showed that Fe does not have considerable effecton pasty range, solidus and liquidus temperature, while can increaseundercooling slightly.

4. Conclusion

Addition of Bi to SAC105–Fe increased yield strength and ultimatetensile strength while decreased total elongation. Bi degenerated theeutectic region into a chain-like arrangement, which decreased Cu6Sn5and increase β-Sn in solder. 0.05 wt.% Fe made few FeSn2 IMC particlesin the solderwhichdoes not have considerable effect onmechanical andmicrostructural properties. 1 wt.% or 2 wt.% Bi scattered in the whole ofsolder without concentration at any position and strengthen solder by asolid solution effect. The surface fracture of solder does not shownecking by addition of Bi which is related to the Bi brittle propertiesand prevention of β-Sn deformation by Bi. The solidus temperature ofSAC105Fe–2Bi is 214 °C, which is 5 °C less than SAC105, while pastyrage increased 3 °C. Also undercooling decreased from 12.3 °C to9.8 °C by adding 2 wt.% Bi to SAC105–Fe.

Acknowledgments

The authors acknowledge thefinancial supports provided byUniver-sity of Malaya under PPP Fund project No: PG079/2014A and UMRGFund project No: RP003A-13AET.

Please cite this article as: M.H. Mahdavifard, et al., The effect of iron andproperties of Sn–1Ag–0.5Cu solder alloy, Microelectronics Reliability (201

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