Effect of Lanthanum Doping on the Microstructure Evolution and Intermetallic Compound (IMC) Growth during Thermal Aging of SAC305 Solder Alloy

The increasingly environmental concern over the toxicity of lead (Pb) combined with strict environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies and to adopt lead-free solder alloys [1-5]. The European Union Waste Electrical and Electronic Equipment (WEEE) Directive, published in 2002 and Restriction of Hazardous Substances (RoHS) Directive of European Community, published in 2003 restricted the usage of certain toxic materials including Lead (Pb) in production of electronic devices used in European Union effective on 1 July 2006 [6].


Introduction
The increasingly environmental concern over the toxicity of lead (Pb) combined with strict environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies and to adopt lead-free solder alloys [1][2][3][4][5]. The European Union Waste Electrical and Electronic Equipment (WEEE) Directive, published in 2002 and Restriction of Hazardous Substances (RoHS) Directive of European Community, published in 2003 restricted the usage of certain toxic materials including Lead (Pb) in production of electronic devices used in European Union effective on 1 July 2006 [6].
Owing to the enforcement of these directives, all electrical or electronic equipment and devices produced in or imported to EU member countries must meet the lead-free standards except those items that are exempted from the bans. Furthermore, several Japanese electronics manufacturers have successfully created a market differentiation and increased market share based on "green" products that use Pb-free solders and many Japanese companies have brought their lead-free products into the market much earlier than the EU directives' effective dates, including Panasonic in 2001, Sony in 2001: Toshiba in 2000, NEC in 2002, and Hitachi in 2001. In the United States, there are no specific government regulations regarding usage of Pb solder in electronic consumer products. However, the U.S. Environmental Protection Agency (EPA) has listed lead among the top 17 chemicals that conduce to threat for human health. The IPC (formerly known as the Institute of Interconnecting and Packaging Electrical Circuits) has also developed a roadmap for the lead-free movement in the U.S. [7]. Moreover many U.S. companies, including Motorola, Cisco, and Intel, have also been actively pursuing lead-free products in order to protect their world-wide market shares. Many universities have also been actively funding lead-free related research. Other countries, such as China and South Korea, which are emerging electronic manufacturing bases, have also adopted or are in the process of adopting directives similar to those of the European Union.
This multinational decision has led to vigorous development of alternative solder alloys and the most promising of these falls into the general alloy families of tin-silver (Sn-Ag) and tin-silver-copper (Sn-Ag-Cu). They were first discovered in 1996 by a research group at Ames Laboratory in USA [8]. Various type of Sn-Ag-Cu or SAC alloys have been the proposed by industrial consortiums e.g. In Japan, the Japan Electronic Industry Development Association (JEIDA) has recommended 96.5wt%Sn-3wt%Ag-0.5wt%Cu (SAC-305), in the EU, the European Consortium -the Industrial and Materials Technologies Program Brite-Euram, recommended 95.5wt%Sn-3.8wt%Ag-0.7wt%Cu (SAC-387), and in US, NEMI has recommended 95.5wt%Sn-3.9wt%Ag-0.6wt%Cu (SAC-396) for reflow soldering and 99.3Sn-0.7Cu for wave soldering [9]. soldering process, the conductive metal Cu is rapidly dissolved into a liquid solder until the solder becomes supersaturated with Cu at the Cu/liquid solder interface. At the same time the driving force for chemical reaction between Cu and Sn promotes the formation of a Cu-Sn intermetallic compound, often Cu 6 Sn 5 phase, by heterogeneous nucleation and growth at the Cu/liquid interface [19,36,37]. These IMCs usually have a scallop-like appearance [38] and change to a plate structure during thermal aging [39]. Thermodynamically, there should also be an intermetallic phase Cu 3 Sn formed between Cu and Cu 6 Sn 5 but it is usually very thin making it difficult to detect [19,[39][40][41]. The dominant diffusing element through both the Cu 3 Sn and the Cu 6 Sn 5 phases is copper [42]. This leads to a depletion of copper at the Cu-Cu 3 Sn interface and in the Cu 3 Sn layer resulting in the formation of Kirkendall voids. Many researchers have observed Kirkendall voids both at the Cu-Cu 3 Sn interface and within the Cu 3 Sn layer [19,[43][44][45][46][47]. Kirkendall voids usually appear in the Cu 3 Sn layer or at the Cu 3 Sn/Cu interface when exposed to temperatures above 100°C. The formation of Kirkendall voids greatly increases the chance of failure due to brittle fracture [48]. The size of the voids is about 0.1 to 0.2 μm. Owing to this small size, they are smeared during mechanical polishing and therefore it is not possible to observe them after polishing [43]. If it shall be possible to observe the Kirkendall voids, the surface needs to be prepared using methods such as Focused Ion Beam (FIB) or sputtering with Ar+ ions. Therefore, Kirkendall voids may be more common than reported [37]. Ag 3 Sn is the other IMC that can form in the matrix of the SAC solders as needle like structure. Large Ag 3 Sn plates is the least desirable phase since it extensively affect the mechanical properties of solder joints [49,50] by producing local plastic deformation [51] and conduce to stress concentration at the interface between the hard Ag 3 Sn particles and soft β-Sn [52] that results in catastrophic failures of electronic components. They are generally believed to be detrimental in both crack initiation and propagation, and numerous studies have attributed the failure of SAC305 solders to large plate-like Ag 3 Sn IMCs under impact and thermal cycling stimuli [10,17,53]. In addition, the formation of large plate like Ag 3 Sn IMCs causes solid dissolution and precipitation hardening, which in turn decreases the matrix strength [54,55]. The overall reliability of the solder joint can be greatly affected by the amount and size of Ag 3 Sn IMCs in the microstructure [56]. Since the Cu content in the SAC-305 alloy is very small, the majority of the IMCs formed in SAC305 are Ag3Sn particles.
To cater these problems, the rare earth (RE) elements have been added into the Sn-Ag-Cu based solder. The RE elements has some unique properties that make it have extensive applications in material and metallurgy field. Rare earth elements (RE) have been successfully used in the steel industry [57]. RE are the surface-active agents which greatly affect the metallurgy of materials e.g. refinement of microstructure, purification of materials and metamorphosis of inclusions [58]. They have been regarded as the vitamin of metals, whereby addition of a small quantity of rare earth elements may dramatically change the performance of metals e.g refining the microstructure . They can more easily agglomerate at the grain/ dendrite boundary and lower the grain/dendrite boundary energy to stabilize the boundaries and restrain the moving or sliding of the boundaries [25]. Some studies consider that RE elements are adsorbed at the grain/dendrite boundaries of the IMC and alter the relative correlation of the growth velocities between the crystalline directions of the polycrystalline IMC, which decrease the size of the IMC particles and distributes the IMCs more uniformly [81][82][83][84][85][86][87]. Several studies  have been conducted to find the effect of RE doping in solder alloys, the RE used generally are La, Ce, Y and Er. These studies demonstrate SAC-305 is the formation and growth of large intermetallic compounds (IMCs) in this alloy [17]. IMC are produced as result of the reaction of a molten solder with a conductive metal i.e. Cu. [18,19]. Although the presence of thin layer of IMCs between solders and conductor metals is desirable because it results in good metallurgical bonding [19]. However, a thick IMCs layer at the solder/conductor metal interface significantly reduces the reliability of the solder joints because of their inherent brittle nature and their tendency to generate structural defects because physical properties (such as elastic modulus and coefficient of thermal expansion) of IMCs are not compatible with parent metal i.e. SAC alloy [1,10,[19][20][21][22][23][24][25][26]. IMC have much higher strength than the bulk solder material [27,28]. "They are stoichiometric combinations of two or more metal atoms where the atomic fractions of the metals are generally fixed (for example Cu3Sn). This can be contrasted with solid solutions where the atomic fractions can sometimes very as widely as 0 -100%. Metals and alloys exhibit metallic bonding between the atoms, whereas IMCs exhibit a more covalent character. This is why IMCs tend to be much harder and have much higher elastic moduli than either of their respective metallic elements i.e. Sn, Ag & Cu" [29].
Prolonged exposure to high thermal environment causes these IMCs to grow [30,31]. Figure 1 shows how thermal aging leads to growth of the IMC layer between SAC305 solder and copper. The operating temperature of many new electronic systems could be as high as 200ºC, for example electronics in oil and gas exploration, avionics, automotive industry, and defense applications typically have more demanding thermal life cycle environments than consumer electronics [32,33]. In oil and gas well drilling nearly 15% of the wells have bottom hole temperature in the range of 150°C-175°C and 2 to 3% have temperature upto 200°C or higher [34]. A typical application of high thermal environmental conditions experienced by electronic system during oil and gas well drilling is running of wireline logging / slickline tools in high temperature wells. Memory gauges which are used for recording of bottom-hole pressure and temperature of oil & gas well encountered to a temperature range of 150°C-175°C with exposure time vary from 30 to 60 hours. Similarly high temperatures are encountered in the electronics used in supersonic aircrafts, military vehicles e.g. battle tanks. These demanding conditions of use, together with the need for greater reliability of all electronic systems motivate further research on the effects of high temperature aging of solder materials [35].
For SAC alloys most often IMC are Cu 6 Sn 5 & Ag 3 Sn. The IMC Cu 6 Sn 5 is important due to the large number of lead-free solder joints formed directly to copper. In addition, the Cu 6 Sn 5 IMC is a primary feature in the microstructure of SAC305 alloy. During that RE doping can significantly increase the wetting property of solder [14,[57][58]64,[75][76]81], it can reduce IMCs particle size and their growth on solder/pad interfaces, and thus greatly increase the solder joint reliability [73]. Some researchers have found that the difference in atomic radius between Sn and RE atoms makes it difficult for them to form replacement atom-type solid solutions, so the RE atoms gather at defects, such as dendrite boundaries. Since the mechanical behavior of solder alloys depends on the microstructure, it is critically needed to conduct a systematic quantitative microstructure study at all relevant length scales [57]. RE element enhances the mechanical strength and creep rupture life of solder alloys like Sn-Ag, Sn-Cu and SAC lead fee solder alloy because RE elements can promote chemical reactions at the interface and provide very strong bonding during soldering [14,58,[75][76]80,[82][83][84][85].
In the present study RE Lanthanum-La is selected for the investigation. The main objective of this paper is to study the effect of adding small amount of La on the microstructure and IMS formation of SAC-305 solder alloy with varying environmental conditions implemented during service. Lanthanum (La) is considered as the best doping element for SAC solder alloys due to their lower cost, wide availability and low melting point as compare to the other RE elements [88,89]. In steel industry Lanthanum is added to steel to improve its malleability, resistance to impact and ductility. Being surface active agents, even minor quantity of La significantly enhance the reliability of solder [90]. In addition the diameter of a La atom is 0.181 nm, while that of Sn is 0.141 nm which is 28% smaller than that of La. Owning to this it is difficult for La atoms to substitute Sn atoms in the matrix. Consequently it is easy for La atoms to agglomerate at dendrite boundaries, and refine dendrite of the grains [67]. This may enhance the resistance to grain growth which may results in the refinement of the microstructure.

Experimental Procedure
The base material used in this study is SAC305 lead free solder alloy and the RE used is Lanthanum -La. Three levels of RE doping were used and the final alloy compositions in weight percent are shown in Table 1 below: The 96.5Sn3.5Ag0.5Cu solder was obtained from Nathan Trotter & Co. in the form of solder bar. Other 96.5Sn3.5Ag0.5Cu-La alloys were obtained from Atlantic Metals & Alloys Inc. also in the form of solder bar. Some of the samples were then aged isothermally in high temperature oven. Two levels of temperatures ranges were used for the thermal aging: 100°C and 180°C. Similarly two levels 60 and 120 hours: were used as aging time factor. Including the case of no thermal aging condition (as-cast), there were 5 combinations of thermal treatment on the specimens and considering the four alloy compositions used in this study, there were 4x5=20 test conditions. Samples were cross-sectioned and mounted in epoxy, then ground by different grit sized silicon carbide (SiC) emery papers in a sequence of 320, 600, 800, 1000 and 1200 and subsequently mechanical polished with diamond paste (6 μm, 3 μm and 1 μm) and 0.05-μm alumina suspension. Pressurized water coolant is applied during grinding process to prevent sample heating. Ultrasonic cleaning for 2 minutes was carried out after each step of diamond paste polishing to remove any diamond abrasive on the sample surface. Polishing with alumina suspension does not reveal the precipitates. To reveal the precipitates, the sample was again polished with alkaline 0.05μm silica suspension. The alkaline solution preferentially etches the Tin-Sn grains at a higher rate and therefore helps to clearly identify the Ag3Sn precipitates [91]. The samples were then etched for several seconds using etching solution of 93% CH 3 OH + 5% HNO 3 + 2% HCl as used by most of the researcher for SAC alloys [12,14,67,88]. Etching time is tabulated below Table 2.
After thermal aging, microstructure of the polished samples was performed using optical microscope (Olympus GX51, Olympus Co. Japan) and Scanning Electron Microscope -SEM (JSM-5910, JEOL Japan) coupled to an Energy Dispersive X-ray Spectroscope -EDS (Oxfordins Inca-200). In order to hinder the charging of the epoxy mount, gold sputter coating was performed on the SEM sample with SPI Sputter coater JEE-420. Dispersion of different solder elements and the elemental analysis of IMC phases were performed using a JEOL JXA-733 superprobe electron probe microanalyzer equipped with Bruker AXS 4010 XFlash silicon drift energy dispersive X-ray (EDX) detector and QUANTX software. X-ray diffraction (XRD) measurements were carried out in order to determine the constitution of the phases forming the samples microstructure. XRD patterns were obtained utilizing a JDX-3532 (JEOL Japan) with a 2-theta range from 5° to 80°, Cu-Kα radiation with a wavelength, λ, of 0.15406 nm.

XRD analysis
XRD analysis was conducted to identify the type of IMC phases. Figure 2 exhibits the resulting XRD patterns for as-cost SAC305 with no La doping and that SAC305 with 0.5wt% La after thermal aging of 120 hours at 180°C of each sample. It was observed that the microstructure of un-doped SAC305 alloy contained two IMCs phases, i.e. Ag 3 Sn and Cu 6 Sn 5 while that of lanthanum doped SAC305 alloy contained three IMC phases i.e. Ag 3 Sn, Cu 6 Sn 5 and La 5 Sn 3 . Previous studies [25,57,61,86] found that Lanthanum doped SAC alloy will cause LaSn 3 IMCs while our results shows the presence of La 5 Sn 3 instead of LaSn 3.

Grain size
To study the grain size of the solder material, optical microscopy with cross polarized light was used. With the help of cross polarized light, grains with different shade can be viewed under the microscope. Figure  3 shows the typical images of 96.5Sn3Ag0.5Cu and 96.5Sn3Ag0.5Cu-0.5La after thermal aging of 120 hours at 180°C of each sample [92,93]. These Figures shows a significant decrease in grain size, due to the addition of lanthanum.
The grain size as a function of different La composition and aging temperature is plotted in Figures 4 and 5 respectively. The Figures illustrate that the grain size of lanthanum alloy is much smaller than un-doped SAC305 alloy. Also grain size decreases significantly up to 0.05% of La doping and then increase slightly with increasing amount of La doping. This is consistent with the previous studies conducted by Min et al. [25] and Sadiq et al. [94] for Sn-3.5Ag and SAC305 alloy respectively with similar La doping. Also a slight change in the grain size for La-doped alloy has been observed with thermal aging. This demonstrates that La-doping refines the grains which comply to  [25] and Sadiq et al. [94]. The philosophy of this refinement is because of the particular effect of La adsorption at different planes in the Sn-Ag-Cu alloys.

Microstructure analysis
The microstructure of the doped and un-doped SAC305 solder alloy was investigated in SEM and ordinary secondary electron microscopy images have been produced for each alloy composition. In the as-cast condition, the microstructure of SAC305 alloy comprises of dendritic β-Sn phase and Sn-Ag-Cu ternary eutectic network where intermetallic particles were finely distributed in the Sn matrix as shown in Figure 6. From the EDS and XRD analysis, the IMC found were mainly Ag 3 Sn and Cu 6 Sn 5 . There is very a clear boundary between dendritic β-Sn phase and Sn-Ag-Cu ternary eutectic regions. After thermal aging, the size of Ag 3 Sn and Cu 6 Sn 5 particles in Sn-Ag-Cu eutectic network becomes bigger and more uniformly dispersed. The reason for increase in the particle size of Cu 6 Sn 5 with thermal aging is the increase in the diffusion rate of Cu into Sn [92].
The "as cast" and "thermally aged" SEM micrographs for un-doped SAC-305 and SAC305 with 0.5wt% La doped alloy are shown in Figure  7. After thermal aging coarsening of IMCs particles take place. It could be observed from the images that the microstructure of La-doped sample is more refine the un-doped sample. After thermal aging at 180°C the size of Ag 3 Sn and Cu 6 Sn 5 particles in SnAgCu eutectic region increased and more uniformly dispersed. Owing to this dispersion it is difficult to identify the boundaries of β-Sn dendrite and eutectic region clearly.     1. In the as-cast condition, the microstructure of SAC305 alloy comprises of dendritic β-Sn phase and Sn-Ag-Cu ternary eutectic network where intermetallic particles were finely distributed in the Sn matrix.

2.
Addition of lanthanum to Sac305 solder alloy significantly reduced the IMC particle size.
3. La doping effectively suppresses the growth of Ag 3 Sn and Cu 6 Sn 5 intermetallic particle during thermal aging.

4.
La doping significantly reduce the grain size and keep the grain size stable during thermal aging.