Journal of Alloys and Compounds 599 (2014) 114–120

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Journal of Alloys and Compounds

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Effect of Ag content and the minor alloying element Fe on the electricalresistivity of Sn–Ag–Cu solder alloy

http://dx.doi.org/10.1016/j.jallcom.2014.02.1000925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +60 3 7967 5399.E-mail address: smsaid@um.edu.my (S.M. Said).

Nur Aishah Aminah Mohd Amin a, Dhafer Abdulameer Shnawah b, Suhana Mohd Said a,⇑,Mohd Faizul Mohd Sabri b, Hamzah Arof a

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

a r t i c l e i n f o

Article history:Received 26 August 2013Received in revised form 14 February 2014Accepted 17 February 2014Available online 25 February 2014

Keywords:Sn–1Ag–0.5Cu solderFe additionElectrical resistivity

a b s t r a c t

In this paper, we investigated the electrical resistivity of the high-Ag-content Sn–3Ag–0.5Cu, the low-Ag-content Sn–1Ag–0.5Cu solder alloy, and the three quaternary solder alloys Sn–1Ag–0.5Cu–0.1Fe,Sn–1Ag–0.5Cu–0.3Fe, and Sn–1Ag–0.5Cu–0.5Fe. The electrical resistivity was characterized by thefour-point probe technique. The results show that both the Ag content and the addition of Fe significantlyaffected the electrical resistivity. The electrical resistivity was found to increase with a reduction in theAg content in the solder alloy. The electrical resistivity of Fe-modified Sn–1Ag–0.5Cu solder alloysdecreased initially with the addition of 0.1 wt.% Fe, and then gradually increased with 0.3 wt.% and0.5 wt.% Fe addition. Microstructural changes resulting from the content of Ag and the addition of Fe havebeen correlated to the electrical resistivity change.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

In the electronics industry, solders are used as interconnectmaterials between electronic components and printed circuitboards. As the joining material, solders provide the necessary elec-trical, mechanical, and thermal continuity in electronics assem-blies [1–4]. Therefore, the performance and quality of the solderare crucial to ensure the proper functioning of the electronicsassembly. Consumer electronics are becoming increasingly porta-ble with rapidly growing functionality. Portable electronic prod-ucts are also becoming smaller, cheaper, lighter, and faster, asshown in the trend towards personalization in terms of their de-sign and visual appeal [5–8]. These shifts in electronic portabilityhave supported the development of area array packages such asball grid arrays (BGAs), chip-scale packages (CSPs), and flip chips,due to their smaller package size, higher number of input/outputpins, and superior electrical performance [1,9–11]. In these area ar-ray packages, the solder joints provide the electrical and mechan-ical connections between the integrated circuit (IC) chip andprinted circuit board, and a pathway to dissipate the heat gener-ated by the IC chip.

The increased functional complexity of portable electronic de-vices leads to new reliability concerns, such as fitting additional

functionalities into a smaller package will generate higher heatdensities inside the product [1,5,12–14]. This increase in heat willnot only induce thermal–mechanical fatigue but also drive changesin the microstructure of the solder joints, therefore affecting theirmechanical properties over time. Moreover, portable electronicproducts are prone to accidental drops resulting from mishandlingduring customer usage, and these impacts can cause mechanicaland electrical failure of solder joints [1,9,15–19]. Hence, themechanical and electrical properties of the solder joints have be-come a crucial issue in the electronics industry. Sn–Pb solder alloyshave been used extensively as joining materials for assembly andpackaging in modern electronic components and devices over re-cent decades because of its superior performance and low cost.The presence of Pb in Sn-based solder alloys, mostly in the compo-sition of eutectic 63Sn–37Pb, makes the solder superior in terms ofits thermal and mechanical characteristics for electronics assemblyand reliability [20–22]. However, Electrical and Electronic Equip-ment (EEE) manufacturers are showing increasing interest in thedevelopment of Pb-free solders and their associated soldering pro-cesses due to increasing environmental awareness and impendinglegislation against Pb usage. This environmental concern is alsoraised by the directives on Waste of Electrical and ElectronicEquipment (WEEE) and the Restriction of Hazardous Substances(RoHS), in line with the tremendous market potential for greenproducts.

N.A.A.M. Amin et al. / Journal of Alloys and Compounds 599 (2014) 114–120 115

Recently, Sn–xAg–Cu with high silver content has become oneof the industry standards for Pb-free solder alloys because of theirlow available melting temperature, near-eutectic composition, andfavorable thermal–mechanical fatigue properties [23–26]. How-ever, this choice has been encumbered by the fragility of the solderjoints, which has been observed in drop testing, as well as the highcost of Ag [27,28]. Therefore, low-Ag-content Sn–Ag–Cu solder al-loys were considered as a solution for both issues [29,30]. A re-duced Ag content resulted in a soft and highly compliant solderalloy and a reduction in the overall cost of raw materials. However,these low Ag-content Sn–Ag–Cu alloys exhibit poor creep and ther-mal fatigue performance and thus are limited in their potentialapplications in the electronics industry [23,31]. Investigations haverevealed that the addition of a fourth alloying element, such as Mn,Ce, Ni, Ti, Co, Al, Fe or Bi, to low-Ag-content Sn–Ag–Cu alloys pro-vides a marked improvement in microstructural modifications andmechanical properties [4,32–35,15], and for these reasons, thesealloys have attracted considerable attention. In our previous study[9], the addition of Fe has been shown to effectively enhance andstabilize the mechanical properties of Sn–1Ag–0.5Cu solder alloy.It is also important to ensure that the electrical properties of thesemodified solders are not compromised at the expense of enhancedmechanical properties. In order to function adequately as an elec-trical interconnects, the electrical resistivity is the property ofinterest.

Kim et al. [36] studied the effect of small additions of up to2 wt.% of Sb on the electrical resistivity of the Bi–2.6Ag–0.1Cu sol-der. The results showed that the resistivity of the Bi–2.6Ag–0.1Cu–xSb solder increased with the Sb content, with the solders indicat-ing resistivity of the order of 360–480 lX cm, whereas pure Bi hasa resistivity of 129 lX cm. Even though the solders were alloyedwith elements having resistivity smaller than that of pure Bi, theaddition of Sb adversely affected the electrical resistivity of the sol-ders. The observed increase in the electrical resistivity when com-pared with the Bi–2.6Ag–0.1Cu solder can be attributed to thesubstitutional solute (Sb) that induced lattice defects as an impu-rity factor and the increased amount of a large resistivity phase(the Bi-rich solid solution).

Lashin and co-workers [37] similarly reported that addition ofZn in the range of 0.5–2.5 wt.% to the Sn–10Sb–Cu solder alloy in-creased the electrical resistivity. This observation was attributed tothe formation of intermetallic compounds (SbSn/SnSb and Cu3Sn)and the hexagonal closed packed structure of Zn in the matrixwhich acted as scattering centers for the electrons, hence the in-crease in electrical resistivity. Shalaby [38] investigated the effectof addition of silicon of 0.1–0.5 wt.% silicon on the electrical prop-erties of Sn–Zn based alloys which have been rapidly quenchedfrom melt. The results revealed that the electrical resistivity ofSn–Zn–Bi–Si solder alloys, 9–14 lX cm, is much lower than thatof Sn–37Pb solder. Moreover, it was revealed that the resistivitydecreased from 14 to 9 as the Si content increased from 0.1 to0.5 wt.%. The decrease in resistivity was attributed to the scaveng-ing of dissolved oxygen in the solder alloy matrix, due to a muchhigher oxygen affinity of Si.

The effect of Ge addition on the electrical resistivity of the Sn–13Zn–3Bi solder alloy was investigated by Wang et al. [39]. Theauthors reported that the addition of Ge in the range of 0.1–0.3 wt.% to the Sn–13Zn–3Bi solder alloy showed a much lowerelectrical resistivity than the Sn–13Zn–3Bi solder alloy. However,the addition of Ge, over 0.3 wt.%, resulted in a higher electricalresistivity. The decreasing effect at low Ge concentrations wasagain attributed to the scavenging of dissolved oxygen in the sol-der-alloy matrix, due to the much higher oxygen affinity of Ge.However, the opposite effect of increased resistivity due to overd-oping was attributed to the precipitation of Ge-rich phase, whichcontributed to extra electronic scattering during electrical trans-

port in the alloy matrix. In another investigation, Babaghorbaniet al. [40] reported that there is no effect on the electrical resistiv-ity of Sn–3.5Ag and Sn–3.5Ag–0.7Cu solders reinforced with a widerange of reinforcements (SnO2, Cu, Y2O3, ZrO2 + 8 mol.% Y2O3, andTiB2) at the nanometer length scale. This finding was analyzed withregards to the presence of low volume fraction of porosity in thecomposite solders. Nai et al. [14] investigated the effect of carbonnanotubes in the range of 0.01–0.07 wt.% on the electrical resistiv-ity of Sn–3.5Ag–0.7Cu solder alloy. The results revealed that theaddition of carbon nanotubes in various amounts did not degradethe resistivity of the Sn–3.5Ag–0.7Cu solder matrix.

Cook et al. [41] observed that the addition of 1 wt.% Bi toSn–3.7Ag–0.9Cu increased the bulk resistivity by approximately8%. This increase was attributed to the solubility effect of Bi inthe b-Sn matrix according to the Linde–Norbury rule [42]. Theeffect of small additions (0.5–2.5 wt.%) of Zn on the electrical resis-tivity of the Sn–3.5Ag solder alloys was investigated by Kamal et al.[43]. The authors observed that the resistivity increased continu-ously with Zn content to its maximum value (19.1 lX cm) at 2and 2.5 wt.% Zn. This observation was attributed to the formationof the intermetallic compounds of higher electrical resistivity thanthat of its components, which hinder the motion of conductionelectrons from one site to another. Negm et al. [44] reported thatthe electrical resistivity was increased by 29% with the additionof 3.5 wt.% of Ag, and was further increased by 5.7% with the addi-tion of 4 wt.% Ag into Sn–0.7Cu solder. Increasing the electricalresistivity was considered to have been attained by the combina-tion of the uniform distribution of fine precipitation and due tothe introduction of internal defects such as dislocations and grainboundaries. This present study investigates the electrical resistivityof the high-Ag-content Sn–3Ag–0.5Cu with the low-Ag-contentSn–1Ag–0.5Cu solder alloy and the three quaternary solder alloysSn–1Ag–0.5Cu–0.1Fe, Sn–1Ag–0.5Cu–0.3Fe, and Sn–1Ag–0.5Cu–0.5Fe in order to understand the effect of Ag content and the addi-tion of Fe on the electrical resistivity of the Sn–Ag–Cu solder alloys.

2. Experimental procedures

The bulk solder specimens of Sn–3Ag–0.5Cu (SAC305), Sn–1Ag–0.5Cu (SAC105),Sn–1Ag–0.5Cu–0.1Fe (SAC105–0.1Fe), Sn–1Ag–0.5Cu–0.3Fe (SAC105–0.3Fe), andSn–1Ag–0.5Cu–0.5Fe (SAC105–0.5Fe) were fabricated. The raw materials for thesolders were supplied by Accurus Scientific. The bulk solder specimens were pre-pared by melting pure ingots of Ag, Cu, and Fe in an induction furnace at more than1000 �C for 40 min. The molten alloys were then mixed with pure liquid Sn in amelting furnace at 290–300 �C for 60 min. The bulk solder specimens of Sn–3Ag–0.5Cu (SAC305), Sn–1Ag–0.5Cu (SAC105), Sn–1Ag–0.5Cu–0.1Fe (SAC105–0.1Fe),Sn–1Ag–0.5Cu–0.3Fe (SAC105–0.3Fe), and Sn–1Ag–0.5Cu–0.5Fe (SAC105–0.5Fe)were prepared according to the desired compositions. Then, the molten alloys werepoured into stainless-steel molds that were preheated at 120–130 �C, and subse-quently, the molds were naturally air-cooled to room temperature (25 �C). Themolds were disassembled, and the bulk solder specimens were removed and visu-ally inspected to ensure that the surface of the parallel area was free of damage andvoids.

The dimensions of the bulk solder rod specimens were 5.0 mm in diameter and21 mm long. The disk-shaped ingots were sent to a third party laboratory (SGS) toverify the Fe element concentration. Chemical composition analyses were carriedout to determine the exact composition of the cast ingots (Table 1). The bulk solderrod specimens were cut into 8 mm thick solder slices. Finally, the solder slices wereground on circular faces with four grades of SiC paper (#800, #1200, #2400, and#4000) to obtain flat surfaces and then washed in acetone solution by ultrasonicmeans. The electrical resistivity (q) of the sliced solder samples were measuredat ambient temperature using a four-point probe setup. A schematic of the fourpoint probe configuration used for the electrical resistivity measurements is shownin Fig. 1. A probe spacing (s) of 0.05 cm was used. The current used was 1A which iswithin the range of values (10 mA–50 A) used and reported by other investigatorsfor four-point probe testing [45,46]. The advantage of using the four-point probemethod is the possibility to measure the sample resistance without any interfer-ence from the contact resistance at the probe contacts. A total of three sampleswere tested for each solder alloy with 10 readings for each sample. In this study,the correction factor to be used in calculating the resistivity was determined byreferring to the geometric factors in four point resistivity measurement provided

Table 1Chemical composition of the solder alloys.

Solder alloy (wt.%) SAC105 SAC305 SAC105–0.1Fe SAC105–0.3Fe SAC105–0.5Fe

Sn 98.3815 96.5079 98.2544 98.0889 97.9212Ag 1.0446 2.9726 1.0957 1.0676 1.0576Cu 0.5070 0.5019 0.5230 0.5200 0.5096Fe 0.0021 0.0015 0.1099 0.3064 0.4943Pb 0.0067 0.0058 0.0059 0.0061 0.0063Sb 0.0042 0.0046 0.0045 0.0046 0.0043Al 0.0005 0.0004 0.0005 0.0005 0.0005As <0.0001 <0.0001 0.0015 0.0013 0.0012Bi 0.0019 0.0018 0.0021 0.0022 0.0022Cd 0.0001 0.0001 0.0001 0.0001 0.0006Zn 0.0002 0.0002 0.0001 0.0001 0.0002In 0.0019 0.0017 0.0022 0.0022 0.0021Ni 0.0493 0.0015 – – –

Fig. 1. Schematic diagram showing the four-point probe configuration used for theelectrical resistivity measurements.

Fig. 2. Electrical resistivity of the SAC305, SAC105, SAC105–0.1Fe, SAC105–0.3Fe,and SAC105–0.5Fe solder alloys.

Table 2Electrical resistance and electrical resistivity of the SAC305, SAC105, SAC105–0.1Fe,SAC105–0.3Fe, and SAC105–0.5Fe solder alloys.

Solder alloy (wt.%) Resistance, R (lO) Resistivity, q (lO cm)

SAC305 39.66 12.46SAC105 43.70 13.73SAC105–0.1Fe 41.63 13.08SAC105–0.3Fe 43.16 13.56SAC105–0.5Fe 44.43 13.96

116 N.A.A.M. Amin et al. / Journal of Alloys and Compounds 599 (2014) 114–120

by Haldor Topsoe [47]. The ‘‘Probe Array Parallel to Edge, Thick Sample’’ geometricfactor was chosen as it provides the closest approximation to our sample geometry.Fundamentally, the electrical resistivity is expressed as

q ¼ GðRÞ; G ¼ 2ps � D2Ls

� �� F3

ts;

Ls

� �ð1Þ

where s is the probe spacing, 0.5 mm, t is the thickness of sample, 8 mm, L is thelength from the parallel probe array to the edge of sample, 2.5 mm, 2ps is the geo-metric factor for a semi-infinite volume. Provided that

2ps � D2 ¼2ps

1þ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 2L

sð Þe2ð Þp � 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ Lsð Þe2ð Þ

p ð2Þ

yields

D2Ls

� �! 1 as

Ls!1 ð3Þ

Assuming that the thickness, t of the sample is finite, the final equation of theelectrical resistivity can be expressed as

q ¼ 2ps � R ð4Þ

3. Results and discussion

The electrical resistivities of SAC305, SAC105, SAC105–0.1Fe,SAC105–0.3Fe, and SAC105–0.5Fe bulk solder alloys are shown inFig. 2. As illustrated, both the Ag content and the addition of Fe sig-nificantly affect the electrical resistivity. The lowest bulk resistivitywas observed in the SAC305 solder alloy with a value of 12.46 lXcm, as shown in Table 2. Conversely, the 0.5 wt.% Fe-modifiedSAC105 solder alloy was observed to exhibit the highest resistivitywith a value of 13.96 lX cm. There are a number of complex fac-tors that determine the bulk resistivity of solder alloys. In the sol-der alloys, the alloying elements can lead to lattice-stain scattering.

The degree to which the lattice-strain scattering becomes a contri-bution to resistivity depends on whether the alloying elements isdissolved in the solvent or forms secondary phases. The increasein resistivity caused by the lattice-strain scattering (if the soluteis assumed to form a substitutional solid solution with the solvent)is proportional to the square of the valence difference between sol-ute and solvent, (DV)2, according to the Linde–Norbury rule. In thecase of the Sn–Ag–Cu-Fe solder alloys, the magnitude of (DV)2 is 4for Fe and 9 for Ag and Cu. Thus, to a first approximation, a minoramount of Ag and Cu substitution in a b-Sn solvent would be re-quired to significantly increase the bulk resistivity, whilst a moder-ate amount of Fe would have the same effect. Indeed,microstructural results indicate that a significant fraction of theAg, Cu, and Fe solute forms secondary intermetallic phases, consis-tent with the limited solid solubility of Ag, Cu, and Fe solutes in b-Sn. The bulk resistivity of the solder alloys is affected by a combi-nation of the following factors: (i) the presence of intermetallicphase, (ii) the volume fraction of intermetallic phase, (iii) the shapeof intermetallic phase, (iv) the size of intermetallic phase, and (v)the type of intermetallic phase and matrix.

Fig. 3. SEM micrographs of (a) SAC105 and (b) SAC305 solder alloys.

Table 3Electrical resistivity of specified elements and compounds at 300 K.

Phase or composition Electrical resistivity (300 K) (lX cm) Refs.

Sn 12 [49]Ag 1.59 [49]Cu 1.66 [49]Fe 10 [49]Cu6Sn5 17.5 [50]Ag3Sn 6.08 [51]FeSn2 100 [52]63Sn–37Pb 14.5 [53]96.5Sn–3.5Ag 12.5 [45]96.5Sn–3Ag–0.5Cu 12.34 [45]

N.A.A.M. Amin et al. / Journal of Alloys and Compounds 599 (2014) 114–120 117

In our previous studies [9,19,35], we investigated the micro-structure properties of the SAC305 and SAC105 solder alloys. Theresults showed that the microstructures of the two solder alloysare similar, both consisting of primary b-Sn dendrites and inter-dendritic regions consisting of Ag3Sn and Cu6Sn5 intermetalliccompounds (IMCs) particles dispersed within an Sn-rich matrix(Fig. 3). It is noteworthy that the number of Ag3Sn IMCs signifi-cantly increases with a corresponding increase in Ag content. The

Fig. 4. SEM micrographs of Fe-be

microstructure of the SAC105 solder alloy (Fig. 3a) consists of rel-atively large primary b-Sn dendrites and fine Ag3Sn IMCs sparselydistributed within the interdendritic regions. The microstructure ofthe SAC305 solder alloy (Fig. 3b) consists of small primary b-Sndendrites and, remarkably, a large number of Ag3Sn IMCs are finelydispersed within the interdendritic regions. Increasing Ag contentfrom 1 wt.% to 3 wt.% was shown to decrease the bulk resistivity(Table 2). This decrease can be attributed to the increase in thenumber of Ag3Sn IMCs, which has lower electrical resistivity thanthat of Sn-matrix (Table 3). The Ag3Sn IMCs is an electron com-pound of electron atomic ration 7/4, which has one incompletebond. This bond is responsible for the conduction of the solder al-loy [48]. Moreover, increasing Ag content from 1 wt.% to 3 wt.%was observed to decrease the number of Cu6Sn5 IMCs, which hashigher electrical resistivity than that of Sn-matrix (Table 3). How-ever, there are far fewer Cu6Sn5 IMCs than Ag3Sn for both theSAC305 and SAC105 solder alloys. Therefore, the bulk resistivityof both SAC305 and SAC105 solder alloys is dominated by Ag3SnIMCs and primary Sn-matrix.

In our previous study [9], we also investigated the effect of Feaddition in the range of 0.1–0.5 wt.% on the microstructural prop-

aring SAC105 solder alloys.

Fig. 5. SEM element mapping of Fe-bearing SAC105 solder alloy.

Table 4Area Fraction (%) of SAC105–0.1Fe, SAC105–0.3Fe, and SAC105–0.5Fe solder alloys.

Solder alloy(wt.%)

Total area(lm2)

Area ofFeSn2 (lm2)

Areafraction (%)

SAC105–0.1Fe 305623.348 447.3205 0.146SAC105–0.3Fe 306646.744 7297.513 2.380SAC105–0.5Fe 306646.744 11323.814 3.693

118 N.A.A.M. Amin et al. / Journal of Alloys and Compounds 599 (2014) 114–120

erties of the SAC105 solder alloy. The results showed that the addi-tion of Fe leads to the formation of large circular FeSn2 IMCs lo-cated in the interdendritic regions. Fig. 4 shows microstructuresof the SAC105–0.1Fe, SAC105–0.3Fe, and SAC105–0.5Fe solder al-loys [9]. Fe has a very low solubility in the b-Sn matrix (and viceversa) below 200 �C. Consequently, most of the Fe precipitates asthe FeSn2 phase or in other forms, such as pure Fe, in the eutectic

Fig. 6. Area fraction of FeSn2 and resistivity of Fe-modified SAC105 solder alloys.

N.A.A.M. Amin et al. / Journal of Alloys and Compounds 599 (2014) 114–120 119

regions. In our previous study, Fe was only identified in the FeSn2

phase. The FeSn2 IMCs are sparsely distributed within the micro-structure located in the interdendritic regions; hence, these FeSn2

IMCs are not always observed in the interdendritic regions. More-over, the number of FeSn2 IMCs increases with increasing theamount of Fe addition, as shown in Fig. 4. The formation of theFeSn2 IMCs leads to a higher resistivity value than that of Fe orSn itself (Table 3). Consequently, bulk solder microstructures con-taining a large number of FeSn2 IMCs would be expected to exhibita higher resistivity, as is borne out in this study. However, in air-cooling after casting, all excess Fe atoms in our previous study donot immediately precipitate due to the rapid cooling rate. There-fore, more Fe atoms remain in the interdendritic regions. In otherwords, not only does Fe exist in FeSn2 IMCs (higher Fe concentra-tion) but it is also observed in the primary b-Sn and interdendriticregions (lower Fe concentration).

The bulk resistivity of the Fe-modified SAC105 solder alloyswere observed to decrease initially with the addition of 0.1 wt.%Fe and then gradually increase with increasing the amount of Feaddition. The electrical resistivity of solder alloys must be suffi-ciently low to permit current flow without heating a solder joint.The addition of 0.1 wt.% Fe in the current study causes approxi-mately 4.7% reduction in the resistivity of the SAC105 solder. Thisreduction is not so significant but it is very important as the pushfor miniaturization continues. The exact reason for the initial de-crease of resistivity is not clear at this stage of the investigation.However, this decrease may be explained as follows: the resistivityof Fe is lower than Sn as shown in Table 3. Therefore, the presenceof Fe in the interdendritic regions (Fig. 5) is possibly responsible forthe initial decrease of resistivity. The presence of Fe may also leadto scavenging of dissolved oxygen in the solder-alloy matrix due tomuch high oxygen affinity of Fe. Moreover, few number of highresistivity FeSn2 IMCs are seen in the lower resistivity 0.1 wt.%Fe-modified SAC105 solder alloy (Fig. 4). In contrast, the higherresistivity 0.3 and 0.5 wt.% Fe-modified SAC105 solder alloys areseen to contain many FeSn2 IMCs of quite large size located inthe interdendritic regions (Fig. 4). Imaging analysis on the SEMmicrographs in Fig. 4 was carried out to calculate the area fractionof the solder alloy surfaces for every amount of Fe addition. Table 4depicts the calculated area fraction of SAC105 solder alloy with theaddition of 0.1 wt.%, 0.3 wt.%, and 0.5 wt.% of Fe. It can be seen thatthe percentage of area fraction of FeSn2 increases as the amount ofFe addition increases from 0.1 wt.% to 0.5 wt.%. Thus, the amount ofthe FeSn2 IMCs affects the electrical resistivity of the solder alloy(Fig. 6). The large number of the FeSn2 IMCs leads to a large volume

fraction of scattering centers for conduction electrons, which inturn hinders the motion of conduction electrons from one site toanother. Therefore, the high electrical resistivity of the 0.3 and0.5 wt.% Fe-modified SAC105 solder alloys is the result of the dis-turbance of normal motions of electrons, which is primarily attrib-uted to the lattice scattering of FeSn2 IMCs. The lattice scatteringreduces the mean free path of electron motion. This consequentlyleads to a reduction in electron mobility and hence an increase inthe resistivity value. A quantitative, semi-predictive descriptionof the electrical resistivity can be obtained by employing an appro-priate model, which accounts for the volume fraction of eachphase. However, a more detailed analysis of the phase constituencyand each corresponding volume fraction in the solder matrixneeded to provide sufficient information for a generalized rule-of-mixtures resistivity model is beyond the scope of this paper. Be-sides, Fe atoms may exist in the b-Sn matrix with the increase inthe amount of added Fe into the solder composition. The presenceof Fe atoms in the b-Sn matrix decreases the mean free path ofelectrons and consequently increases the electrical resistivityaccording to the Linde–Norbury rule.

4. Conclusions

(1) The Ag content and the addition of Fe significantly affect theelectrical resistivity.

(2) The electrical resistivity decreases with increasing the Agcontent from 3.0 wt.% to 1.0 wt.%. This is because the num-ber of low resistivity Ag3Sn IMCs increases with an increasein the content of Ag.

(3) The electrical resistivity of Fe-modified Sn–1Ag–0.5Cu solderalloys decrease initially with the addition of 0.1 wt.% Fe, andthen gradually increase with increasing the amount of Feaddition. The initial decrease in electrical resistivity wasattributed to the low resistivity of Fe compared to Sn. Thegradual increase in electrical resistivity was attributed tothe presence of a large number of high resistivity FeSn2

IMCs.(4) The electrical resistivity of the Fe-bearing Sn–1Ag–0.5Cu sol-

der alloys is comparable with the resistivity of the Sn–1Ag–0.5Cu solder alloy, and is sufficient for a normal operation ofthe electrical circuit. In other words, the differences are nei-ther too high nor too low to significantly affect the overallfunctionality of any electronic circuit in current use withinthe context of electrical performance. However, the consid-erable improvement in the mechanical properties of Fe-added Sn–Ag–0.5Cu has been elaborated in previous investi-gations. Thus, this work confirms the reasonable electricalperformance of the Fe-added SAC1-5 solder whilst achievingsuperior mechanical performance through this formulation.

Acknowledgement

The authors would like to acknowledge the financial supportprovided by the University of Malaya under the HIR Fund ProjectNo.: UM.C/625/1/HIR/M/OHE/ENG/29, and UMRG Grant No:RP003C-13AET.

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