Effects of metallic nanoparticle doped flux on interfacial intermetallic compounds between Sn-3.0Ag-0.5Cu and copper substrate

S. K. Ghosh, A. S. M. A. Haseeb, and Amalina Afifi

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya 50603 kuala Lumpur, Malaysia

E-mail: haseeb@um.edu.my

Abstract Intermetallic compounds (IMCs) formed between solder

and substrate play a vital role in determining the long term reliability of microelectronic packages. Various attempts have been made by the researchers to control the morphology and thickness of IMC layers. The aim of this study is to investigate the effects of nanoparticle dopants into flux on the morphology and thickness of interfacial intermetallic compounds layers. Different types of nano-sized metallic particles were studied to understand their effects on the wetting characteristics and interfacial microstructural evaluations after first reflow by adding nanoparticles to flux at various percentages. Nanoparticles were dispersed manually with a water soluble flux to prepare a nanoparticles doped flux which was placed on the copper substrate. Lead-free Sn-3.0Ag-0.5Cu (SAC 305) solder balls of diameter 0.45mm were then placed on top of the flux and were reflowed in a reflow oven at a peak temperature of 240°C for 45s. Wetting area, contact angle and interfacial microstructure were investigated by optical microscopy, scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and energy-dispersive x-ray spectroscopy (EDX). It was found that doping of cobalt (Co) and nickel (Ni) nanoparticles with flux was successful in incorporating Co and Ni into the solder joint. Microstructural observations showed that both Co and Ni nanoparticles changed the interfacial morphology from a scallop to a planer type. This was suggested to be caused by alloying effect of these elements. In case of Co, this morphological change was evident down to 0.25 wt% Co addition to flux. For Ni, this effect was notable even at 0.1 wt% Ni addition to flux. Therefore, Nano doping of flux can be successfully used to cause in situ targeted alloying at the solder/substrate interface.

Introduction Restrictions on the use of lead (Pb) based solder alloys in

microelectronic assemblies due to the inherent toxicity and harmful aspects of Pb on human health and environment [1] pave the path to develop various Pb-free tin based solder alloys. Among the lead-free solders, the hypoeutectic Sn-3.0Ag-0.5Cu solder (SAC 305) appears to be the most promising candidate at present to replace the eutectic Sn-Pb solder due to its lower eutectic temperature, enhanced strength, improved creep and thermal fatigue characteristics, lower cost having low silver content and good compatibility with device components as compared with other Pb-free solder systems [2].

Intermetallic compounds (IMCs) formed between solder and substrate play a vital role in determining the long term reliability of microelectronic packages. A thinner, planer and continuous intermetallic compound (IMC) layer is essential as

it ameliorates a strong bonding at the interface [3]. However, excessive IMC layer can degrade the mechanical properties of the solder joint leading to a catastrophic failure [4-5]. Lead free solders developed so far are known to lead to the formation of thick interfacial intermetallic compound layers with rough morphologies. At lead free solder/substrate interface, the IMCs also coarsen rapidly during use. Thus, the control of the morphology and growth of interfacial IMC is necessary to ensure solder joint reliability.

Alloy addition to bulk of the solder by conventional metallurgical alloying is one of the way of controlling the morphology and growth of IMC layer [6]. This may require special and complex apparatus, conditions and exceptional processing expertise that tend to result in costly final application in some cases [7]. Moreover, this method affects not only the interface but also the bulk of the solder forming IMC within the bulk that may induce brittleness in the bulk. As a result, ductility of the solder bulk can be lowered. Ductility has a significant importance in solder joint reliability. Ductile solder material can minimize the thermal stresses that generate due to the mismatch of the thermal coefficient of expansion in the solder system.

Reinforcement of lead-free solder alloys through the addition of particles leading to a composite solder is another method to control the morphology and growth of IMC at the solder/substrate interface. Both micro scale [8-9] and nano scale [1, 3] reinforcing particles have been utilized to form composite solder. In recent years, with the pitch size in electronic packages decreasing, nanoparticle reinforcements are being increasingly investigated. Both ceramic [2, 10] and metallic nanoparticles [3, 11-12] are being studied. However, nano-composite solder methods may suffer the reduction in ductility of the solder bulk due to the formation of IMC within the bulk as described above.

Solder paste mixing method is the most simple and direct approach to prepare nano-composite solder. But, this method has some unique drawbacks. A major drawback of this method is the relatively high consumption of nanoparticles on the formation of IMC at the solder/substrate interface. Also, the waste of nanoparticles is high as only a fraction of nanoparticles enter the solder alloy while the majority is expelled to the flux residue. Another major drawback of this method is the increment of the melt viscosity upon nanoparticles addition on the solder paste during heating. As the solder paste already have dispersed micron-sized lead-free solder particles within flux, addition of nanoparticles will eventually enhance the stickiness of the resultant paste which will hamper the wettability of the molten solder during reflow [3,12-13].

Thus, a suitable approach to control the morphology and growth of IMC thickness will be a process to deposit a small

21978-1-4799-2834-7/13/$31.00 c©2013 IEEE

amount of suitable metallic nanoparticles on the substrate prior to the application of solder material wherein metallic nanoparticles undergo reactive dissolution only at the interface without affecting the solder bulk. The present study aims to provide a novel method of in-situ targeted alloying of the solder/substrate interface through the reactive dissolution of metallic nanoparticles during reflow to control the morphology and growth of IMC thickness by dispersing metallic nanoparticles into a water soluble flux [14].

Experimental Procedures Commercially available cobalt (Co-99.8% trace metal

basis) and nickel (Ni-99.9% trace metal basis) nanoparticles (Accumet Materials, Co., USA) were used in this study to control the morphology and growth of IMC layer. The crystallite size and phases of the Co and Ni nanoparticles were investigated by X-Ray diffractometer (XRD-PANalytical). Field emission scanning electron microscopy (Zeiss Ultra60 FESEM) was conducted to determine the nanoparticles size and morphology using an in-lens detector of 1kV EHT voltage. The nanoparticles were dispersed ultrasonically (Ultrasonic cleaner JP-040S, 10L) in ethanol for 15 minutes and then 6-8 droplets of 5μL dispersed nanoparticles-ethanol suspension was placed onto a clean copper (Cu) sheet (15x15x2 mm3) by using a digital micropipette (Socorex Acura 825, 10-100 μL).

Lead-free Sn-3.0Ag-0.5Cu (SAC 305) solder balls (Duk San Hi-metal., Ltd. Korea) of diameter 0.45mm were used as the solder material in this experiment. Water soluble flux (Sparkle Flux WF-6317, Japan) was used to prepare a nanoparticle doped flux by mixing the nanoparticles manually for 30 minutes with the flux at various percentages i.e., from 0 wt% to 2 wt%. Commercial polycrystalline copper sheet (15mm x15mm x 2mm) was used as the substrate. Before reflow, the Cu sheet was polished using a 2000 grit abrasive paper (Riken Corundum Co., Ltd. Japan) to minimize the surface topography effect of Cu on the wettability. Then it was dipped in 10 vol.% H2SO4 solution for 30s to remove the oxide layers. After that, the sheet was dipped in 2-Propanol solution (R & M Marketing, Essex, U.K.) for 30s to clean the substrate. Finally, the substrate was rinsed thoroughly with deionized water and dried in acetone.

To prepare the samples for reflow, 0.025 gm of nanoparticle doped flux was placed and spread on the clean Cu substrate. Solder balls were then placed on top of the flux and were reflowed in a reflow oven (Forced Convection, FT02) at a peak temperature of 240°C for 45s. The flux residue was removed from the reflowed samples by applying tap water. The samples were then dried in acetone.

Wettability of the solder was evaluated by measuring the wetting angles and wetting areas of the reflowed solder joints for every percentage of nanoparticles addition. For each condition, wetting area was determined from the average diameter of the solder bumps. At least fifteen top surface SEM micrographs of the solder bumps were used to calculate the average diameter with the aid of Olympus SZX10 image analyzer. To minimize the test error in wetting area calculation, the diameter of each solder bump was calculated four times in four different directions. Then, the average value was taken as the final diameter of that particular solder bump. After measuring the wetting area, the reflowed sample was

mounted in epoxy resin and polished up to 0.02 μm silica suspension for wetting angle measurement and interfacial morphology characterization. The average value of wetting angles and images of whole cross-section solder bumps were done by using Olympus SZX10 image analyzer. The FESEM micrographs were used to study the morphology of IMC layer and also to measure the IMC thickness. The IMC thickness for each FESEM micrograph was determined by dividing the total IMC area with the length of IMC layer. Al least, five FESEM micrographs were used to determine the average IMC thickness for every condition. The elemental compositions of the IMC layer were evaluated by using the energy dispersive X-ray spectroscopy (EDX).

Results Figure 1 shows the X-ray diffraction patterns for the as-

received Co and Ni nanoparticles. In Figure 1(a), three dominant peaks were observed at 44.13°, 51.40° and 75.79° indicating the presence of (111), (002) and (022) characteristic crystal planes for metallic Co. In addition, a single peak of Co3O4 appears at 36.63° representing (113) crystal plane. The presence of Co3O4 oxide peak in the XRD pattern for Co nanoparticles indicates oxidation of Co occurred to some extent. On the other hand, three sharp peaks were observed at 44.69°, 52.05° and 76.51° representing (111), (002) and (022) crystal planes of Ni respectively in Figure 1(b). It is to be noted that no oxide peaks were found in case of Ni nanoparticles by XRD analysis. Scherrer’s formula was utilized to calculate the crystallite size of Co and Ni nanoparticles by using full-width at half maximum (FWHM) data [15]. The average crystallite sizes of Co and Ni nanoparticles calculated were 42.65 nm and 39.13 nm respectively.

Figure 1: XRD patterns for (a) Co and (b) Ni nanoparticles.

22 2013 IEEE 15th Electronics Packaging Technology Conference (EPTC 2013)

Figure 2 shows the FESEM micrographs of Co and Ni nanoparticles used in this study. It seems that both Co and Ni agglomerated. The morphology seems to be more or less spherical in both cases. The average particle size was found to be 58 nm and 44 nm for Co and Ni nanoparticles. It is to be noted that the calculated average particle sizes for Co and Ni nanoparticles were higher under FESEM than that of XRD analysis. The average, maximum and minimum particle sizes are tabulated in Table 1.

Figure 2: FESEM micrographs for (a) Co and (b) Ni nanoparticles.

Metallic nanoparticle

Average particle size (nm)

Maximum particle size (nm)

Minimum particle size (nm)

Cobalt (Co) 58 119 23

Nickel (Ni) 44 93 24

Table 1: Average particle sizes of various metallic nanoparticles along with maximum and minimum particle

sizes in FESEM. Figure 3 shows the top view images of reflowed solder

joints on Cu substrate prepared with undoped flux and fluxes doped with Co and Ni nanoparticles. Cross-sectional micrographs are inserted to show the wetting angle. It is seen that solder ball spreads very poorly on Cu substrate for 2wt% Co addition into flux giving high contact angle. On the contrary, wettability of solder ball improves significantly for 2 wt% Ni addition into flux as they spread very well on Cu substrate giving low wetting angle.

In case of doping cobalt nanoparticles with water soluble flux, it is seen from Fig. 4(a) that a gradual decrease in wetting area observes from 0.4365 ± 0.04 mm2 for 0wt% Co to 0.412 ± 0.0489 mm2 for 0.5wt% Co. Then the wetting area drastically decreases to 0.228 ± 0.05 mm2 for 1wt% Co and remains relatively constant up to 2wt% Co. On the other hand, wetting angle increases successively from 39.3 ± 4.933° for 0% Co to 46.03 ± 5.07° for 0.5wt% Co. Then it suddenly rises to 66.52 ± 6° for 1wt% Co and 70.2 ± 6.35° for 2% Co addition respectively.

Figure 4(b) shows the effects of Ni nanoaprticles doping into flux on the wettability of the solder as a function of wt% of Ni nanoparticles. It can be seen that wetting area increases gradually from 0.4365 ± 0.04 mm2 for 0wt% Ni to 0.55 ± 0.06 mm2 for 0.5wt% Ni and remains relatively constant at 1wt% Ni addition and then rapidly increases to 1.132 ± 0.039 mm2 for 2wt% Ni. The wetting angle decreases gradually from 39.3 ± 4.933° for 0% Ni to 25.5 ± 4.9° for 2% Ni.

Figure 5 shows the FESEM cross sectional images for the solder joints with cobalt and nickel nanoparticle doping on the morphology of intermetallic compounds (IMCs) between Sn-3.0Ag-0.5Cu (SAC305) and Copper (Cu) substrate. It is seen that the morphology of IMCs has been changed from a scallop

type for 0% nanoparticle to a planer type for Co and Ni nanoparticle doping. In case of Co doped flux, this morphological change is evident down to 0.25wt% Co addition. But, this effect is notable even at 0.1wt% Ni addition into flux. EDX analysis confirmed the presence of Co and Ni in the formation of complex compound (Cu,Co)6Sn5 and (Cu,Ni)6Sn5 at the interface as shown in Fig. 6. Table 2 shows the elemental composition of the interfacial IMCs. Furthermore, a small amount of entrapped Sn-rich phase was observed in the IMC layer with addition of 2wt% Co nanoparticles into [16] which was not seen in case of Ni doped flux in this study. In all cases, a thin Cu3Sn layer with dark contrast was observed after reflow. Also, the detachment of (Cu,Co)6Sn5 and (Cu,Ni)6Sn5 IMCs is observed in Fig. 6 when Co and Ni nanoparticles are added to flux and the extent of detachment increases with increase in the amount of doping.

Figure 3: SEM top view images of reflowed solder balls for (a) 0wt%, (b) 0.25wt% Co , (c) 2wt% Co, (d) 0.1wt% Ni and

(e) 2wt% Ni nanoparticles addition into flux (Inscribed micrographs show cross-section of respective solder balls for

wetting angle measurement). Figure 7 shows the variation in IMC thickness with

different Co nanoparticles addition to flux. It is seen that the total IMC thickness decreases from 3.395 ± 0.26 μm for undoped condition to the lowest value of 1.667 ± 0.255 μm for 0.25wt% Co. Then it increases to 3.25 ± 0.56 μm for 0.5wt% Co and remains relatively constant upto 2wt% Co addition. From Figure 8, it is seen that total IMC thickness decreases with the doping of Ni nanoparticles in flux compared to undoped condition. The total IMC thickness decreases from 3.395 ± 0.26 μm for 0wt % Ni to the lowest value of 1.378 ± 0.17 μm for 0.1wt% Ni. Then it gradually increases with increasing the amount of doping of Ni nanoparticles with flux.

Discussion

(b)(a)

(a)

(d) (e)

(b) (c)

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Wettability of solder can be defined as the ability of the molten solder material to spread over on a substrate during reflow. Generally, a solder joint of low wetting angle and high wetting area is desirable. The degree of wettability is indicated by the value of the wetting angle ( ): very good wetting if 0° < < 30°, good wetting if 30° < < 40°, acceptable wetting if

40° < < 55°, poor wetting if 55° < < 70°, and very poor wetting if > 70° [17]. According to Young’s Equation, the contact angle or, wetting angle, is dependent on three interfacial energies f / s, f / l and s / l , where f, s and l represent the flux, the substrate and the liquid respectively [18]. So, we can write from Figure 9,

f/s= s/l+ f/lcos (1) From Eq. 1, the value of cos will increase or the value of

, wetting angle, will decrease if the interfacial energy between substrate and liquid, s/ l or, the interfacial energy between flux and liquid, f / l or, both decrease.

Figure 4: Effects of (a) Co and (b) Ni nanoparticles

addition into flux on the wettability of the solder alloy with respect to wetting angle and wetting area.

In Figure 4(a), addition of Co nanoparticles decreases the wettability of the solder joints. The dissolution of Co nanoparticles in the formation of (Cu,Co)6Sn5 (Fig. 5) might have a higher interfacial energy between the molten solder and substrate compared to Ni as Co is less effective than Ni at stabilizing Cu6Sn5 phase [19], thus reducing the wettability. The cobalt nanoparticles were also partially oxidized as shown in Fig. 1. This might cause the difficulty in the flow of molten solder.

On the other hand, nickel addition improves the wettability of the solder joints as shown in Figure 4(b). Improvement in wettability due to addition of Ni was also found in a previous study [20]. In general, the addition of reinforcements in solder alloy shows negative effect on wettability [12, 21].

One of the reasons might be a decrease in interfacial energy between molten solder and substrate through the formation of more thermodynamically stable (Cu,Ni)6Sn5 phase as shown in Figure 5. The dissolution of Ni nanoparticles might lower the liquid/substrate interfacial energy [22] and thus improved the wettability of the solder.

Figure 5: FESEM images of cross-sections of IMCs formed by doping flux with (a) 0wt%, (b) 0.25wt% Co, (c) 2wt% Co,

(d) 0.1wt% Ni and (e) 2wt% Ni nanoparticles.

Figure 6: FESEM cross-sectional images for (a) 0.25wt% Co and (b) 0.1wt% Ni nanoparticles addition into flux, (c) and (d)

show EDX and elemental analysis on (Cu,Co)6Sn5 and (Cu,Ni)6Sn5 IMC layers respectively.

Another possible reason would be that nickel nanoparticles were not oxidized as shown in Fig. 1. Thus, doping of nickel nanoparticles with flux might improve the effectiveness of the flux to cover the molten solder during reflow by lowering the interfacial energy between flux and molten solder. It was

(a)

Cu3Sn

Cu6Sn5

(e)

(Cu,Ni)6Sn5

Cu3Sn

(b)

(Cu,Co)6Sn5

Cu3Sn

(c)

Sn(Cu,Co)6Sn5

Cu3Sn

(d)

(Cu,Ni)6Sn5

Cu3Sn

Cu3Sn

(Cu,Ni)6Sn5

(b) (d)

Cu3Sn

(Cu,Co)6Sn5

(a) (c)

24 2013 IEEE 15th Electronics Packaging Technology Conference (EPTC 2013)

reported by Shen, J. et al [23] that doping of nanoparticles with the flux would improve the effectiveness of the flux if the doped nanoparticles themselves wetted the molten solder alloy during reflow by adsorbing on the molten solder. As nanoparticles are considered to be very effective surface active agents, the adsorption of nanoparticles would eventually lower the interfacial energy between molten solder and flux, consequently, improved the wettability of the solder joint. Sample name

Cu (at%)

Sn (at%)

Co (at%)

Ni (at%)

Phases identified

SAC 57.4 ± 0.6

41.7 ± 0.2

-- -- Cu6Sn5

SAC-0.25wt% Co

50.43 ± 0.3

47.96± 0.4

1.62 ± 0.12

-- (Cu,Co)6Sn5

SAC-0.1wt%Ni

54.5 ± 1.6

45.2 ± 1.9

-- 1.84 ± 0.63

(Cu,Ni)6Sn5

Table 2: EDX and elemental compositions of IMC layers for different samples after reflow.

Figure 7: The Relationship in between the IMC thickness

and amount of Co nanoparticles added into flux after reflow.

Figure 8: The Relationship in between the IMC thickness

and amount of Ni nanoparticles added into flux after reflow. The experimental results discussed above for Figure 5

show good agreement with the previous studies that were done with cobalt and nickel as alloy addition [16] and nanoparticles reinforcement [3, 12].

With increasing the amount of nanoparticles in the flux, the IMC layer grew thicker as more nanoparticles dissolved into the molten solder along with consumption of Cu atoms of the solder [24]. The drop in Cu concentration of the solder might destabilize the interfacial product causing detachment or spalling of the interfacial product. The phenomenon of

detachment or spalling of IMC is prominent in case of low volume solder joint [25]. In this study, the solder ball diameter was 450 μm which was relatively low in volume. As a result, supply of Cu atoms would be limited and Cu concentration drop of the solder near the interface occurred readily causing detachment of the IMCs. It is to be noted that the intensity of detachment or spalling of IMC layer for Co doped flux was much lower than that of Ni doped flux.

Figure 9: Schematic diagram illustrating the relationship

among wetting angle and three interfacial energies at solid-liquid interface.

In this study, doping Co and Ni nanoparticles into flux has significantly lowered the total IMC thickness. The characteristic morphological change and reduction in thickness have been achieved for Co doped flux at 0.25wt%. But, these are achievable even at 0.1wt% addition for Ni doped flux. Thus, it can be said that an in-situ targeted alloying method is an effective way to control the morphology and growth of IMC layer by reactively dissolving metallic nanoparticles at solder/substrate interface.

Conclusions From this work, the following conclusions can be drawn: 1. Doping of flux with metallic nanoparticles has been

successful in incorporating them into the solder joint interface.

2. Microstructural morphology has been changed from a scallop to a planer type with Co and Ni addition through alloying effect.

3. This morphological change is evident down to 0.25 wt% Co addition. But for Ni, this effect is notable even at 0.1 wt% Ni addition.

4. Nano doping of flux can be successfully used to cause in situ targeted alloying at the solder/substrate interface.

Acknowledgments The authors acknowledge the financial support of High

Impact Research grant (UM.C/HIR/MOHE/ENG/26, Grant No. D000026-16001).

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f / l

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