Ultrasonic Assisted Soldering of Low Ag SAC Lead Free Solder Paste at Low Temperature

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Ultrasonic-Assisted Soldering of Low-Ag SAC Lead-Free Solder Paste

at Low-Temperature

Gui-Sheng Gan

1,2,*

, Lin-Qiao Gan

3

, Ji-Zhao Guo

2

, Da-Quan Xia

2

, Chunhong Zhang

2

, Donghua Yang

2

,

Yiping Wu

1

and Cong Liu

2

1College of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

2Chongqing Municipal Engineering Research Center of Institutions of Higher Education for Special Welding Materials and Technology (Chongqing University of technology), Chongqing 400054, China

3School of Mechanical Electronic & Information Engineering, China University of Mining & Technology, beijing, Beijing 100083, China SAC0307 (Sn-0.3mass%Ag-0.7mass%Cu) lead-free solder pastes were used to complete Cu/Cu joint by ultrasonic-assisted. Effect of

nano-Ni particles and temperature on the performance of joint by ultrasonic-assisted with lead-free solder paste were investigated. The experi-mental results have shown that interfacial IMC of joint with SACP were typical scallop-type, but the type of interfacial IMC were serrated and became smoother after added nano-Ni particles. The IMC thickness was not more than 6.00 μm and the IMC thickness of SACP joint was slightly thick comparing with SACPC joint at the same temperature. Interfacial IMC of SACP joint at 210 C was mainly composed of a layer of thick Cu6Sn5 and thin Cu3Sn, and the concentration gradients of Cu and Sn in IMC were also obviously. The composition of IMC was Cu3Sn7 in SACPC joint at 210 C, there were no different and the composition was Cu42.5Sn57.5 whether nano-Ni particles were added at 240 C. The strength of two kinds of joints were increased first and then decreased with the increase of ultrasonic time at 210 C 220 C, but decreased in both joints at 240 C. The shear strength of SACP joint was only 31.59 MPa, but the shear strength of SACPC joint reached the peak value of 41.20 MPa at 5 s by ultrasonic-assisted at 210 C, were about 0.32 and 19.80 percent more than that of no ultrasonic-assisted. The shear strength of SACP joint and SACPC joint reached the peak value of 38.80 MPa and 41.96 MPa after ultrasonic vibration for 0 s at 240 C, but decreased to the minimum of 22.47 MPa and 21.11 MPa after ultrasonic-assisted at 5 s and 10 s, were about 42.09 and 49.69 per-cent less than that of no ultrasonic-assisted respectively. Ultrasound can help fill the solder seam to increase the shear strength, but slag inclu-sion and foreign gases would gather and grow up to lead decreasing of the shear strength at 210 C 220 C. Slag incluinclu-sion and foreign gases were wrapped into the seam and not easy to overflow under the liquid state in the liquidus nearby, would gather to the interface of IMC/solder and grow up to lead decreasing of the shear strength under the the ultrasonic vibration at 240 C. Nano-Ni particles also played a part in in-creasing the strength of joints. [doi:10.2320/matertrans.M2017255]

(Received August 14, 2017; Accepted December 13, 2017; Published February 2, 2018)

Keywords: solder paste, nano-particles, ultrasound-assisted, low-temperature soldering

1. Introduction

Low-silver Sn-Ag-Cu (SAC) lead-free solder has been widely used in soldering due to its relatively high performance- price ratio, which has been considered as the most promis-ing alternative to the Sn-Pb solder alloy.1,2) However, with the Ag content decreased, low-silver SAC solder will lead to a series of problems like increased the melting range, de-clined the wettability and reduced the mechanical proper-ties.3–8) A lot of researches have been done to solve these problems.9–14) However, these researches had little effect on the improvement of solders performance, the wettability and the melting temperature of solder were difficult to achieve satisfactory results. Therefore, it is urgent to develop new soldering technology to overcome the bottleneck of lead-free solder. A low-temperature stirring soldering technique was developed in the prophase work, which was found that the addition of nano-Ni particles could improve the wettabil-ity of solder, the growth of interface intermetallic com-pounds (IMC) was obviously inhibited and the mechanical properties of joints were improved.15–17) During stirring, slag and porosity were easy to be found, which affected the im-provement of joints. In this paper, SAC0307 lead-free solder paste were used to complete Cu/Cu low-temperature solder-ing, nano-Ni particles and ultrasonic were used to improve the performance of joints.

2. Experimental Methods

99.99% Copper with the size of 60 mm ×  20 mm ×  10 mm polished by 400# sandpaper smooth, then cleaned with alcohol, finally placed in the ultrasonic-assisted low-temperature soldering device (Fig. 1) and the soldering gap is controlled to 0.3 mm. The ultrasonic assisted solder-ing system consisted of an ultrasonic generator, an ampli-tude transformer and an induction heating units. The fre-quency of the ultrasonic vibration was 20 kHz with a power of 1 kW. Two kinds of solders were SAC0307 lead-free sol-der paste (denoted by SACP) and its composites with 0.5% nano-Ni particles (almost 80 nm) (denoted by SACPC). Rosin-based flux whose composed of opropanol, AXE rosin

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foral and diethylamine hydrochloride were used to help sol-dering. The specimens were heated to the soldering tempera-ture at 210 C ±  2 C(solid state), 220 C ±  2 C(semi-solid state) and 240 ±  2 C(liquid state) by the heating table after filled the solder paste and rosin-based flux at first, and then vibrated 0 s, 5 s, 10 s and 15 s by ultrasonic, finally the sol-dering process was completed after holding 10 mins. Optical metallographic microscope (OM) and scanning electron mi-croscope (SEM) were used to analyze the microstructures of interfacial. The thickness of IMC layer was calculated by the Image-pro Plus software. The average shear strength of sev-eral joints (the shear velocity was 10 mm/min) were iden-tied by the PTR-1101 bonding strength tester.

3.  Results and Discussion

3.1 Interfacial IMC of soldered joints by ultrasonic- assisted

The melting temperature of SAC0307 solder was 217 C 227 C, the soldering temperature of 210 C, 220 C and 240 C were in the solid state, semi-solid state and liquid state of the liquidus nearby respectively. All processes were referred to as soldering, although under strict etiquette rules,

it cannot be called soldering at 210 C. Material softening were important reasons causing soldering at the low tem-perature with the help of ultrasonic wave. Interfacial IMC of joints with SACP and SACPC at 210 C, 220 C and 240 C can be seen from Figs. 2 4. Interfacial IMC of joint with SACP were typical scallop-type, but the type of interfacial IMC were serrated and became smoother after added nano- Ni particles at 210 C and 220 C. Scallop-type IMC of joint with SACP became more obvious when the soldering tem-perature increased to 240 C, and interfacial IMC of joint with SACPC became denser and more flat. Ultrasonic did not have obvious influence on interfacial IMC at 210 C and 220 C, interfacial IMC of joint with SACP at 240 C were shattered by ultrasonic, but not in the SACPC joint.

When the ultrasonic time was changed from 0 s to 15 s at 210 C in Table 1, the IMC thickness of SACP joint were 3.89 μm and 3.38 μm, 4.81 μm and 3.67 μm respectively, and the IMC thickness of SACPC joint were 3.39 μm and 3.18 μm, 3.71 μm and 4.34 μm respectively. The IMC thick-ness at 220 C were between 3.26 μm and 4.00 μm in SACP joint, were between 3.05 μm and 3.70 μm in SACPC joint. With the increase of ultrasonic time at 240 C, the IMC thickness of SACP joint were 3.47 μm and 5.17 μm,

Fig. 2 Interfacial IMC of joint with SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at 210 C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s)

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5.81 μm and 3.64 μm respectively, the IMC thickness of SACPC joint were 3.01 μm and 3.70 μm, 3.93 μm and 3.83 μm respectively, and the IMC thickness increased first and then decreased. It can be seen that the IMC thickness of SACP joint was slightly thick comparing with SACPC joints at the same soldering temperature, but the IMC thickness were not more than 6.00 μm in all joints.

Under ultrasonic condition, interfacial IMC of SACP joint at 210 C was mainly composed of a layer of thick Cu6Sn5 and thin Cu3Sn, and the concentration gradients of Cu and Sn in IMC were also obviously by the aid of EDX in

Fig. 5(a). There was an obvious step by the line-scanning in Fig. 5(b), more Sn atoms entered into the interfacial IMC of SACPC joint and whose composition was stable and Cu3Sn7 (30.2 at% Cu). It can be seen that there were no different in the composition of IMC at 240 C whether nano-Ni particles were added, Less Cu (or more Sn) atoms entered into the IMC whose composition were 42.41% Cu atoms and 42.58% Cu atoms (75.00 at% Cu in Cu3Sn and 54.55 at% Cu in Cu6Sn5) in Fig. 5(c) and Fig. 5(d) respectively because Sn atoms diffused more rapidly than relatively Sn atoms at the same temperatures.

On the one hand, ultrasonic intensified the diffusion of Sn atoms in solder and Cu atoms in the base metal, and pro-moted the growth of IMC, so interfacial IMC thickness in-creased as ultrasonic time. On the other hand, the vibration wound intensify the breaking of interfacial IMC, interfacial IMC shattered by ultrasonic and wound enter into the sol-dering seam when the ultrasonic time was prolonged, so in-terfacial IMC thickness decreased as ultrasonic time again. The IMC thickness difference of SACP joint with ultrasonic time were 1.14 μm, 0.74 μm and 2.17 μm at 210 C, 220 C and 240 C respectively, and the IMC thickness difference of SACPC joint were 1.16 μm and 0.65 μm and 0.82 μm re-spectively. At the low temperature (210 C 220 C), the growth rates of interfacial IMC with ultrasonic time was small due to the slow diffusion of Cu atoms and a longer heat preservation time than ultrasonic time, the variation of interfacial IMC thickness was no obvious with ultrasonic time. At the high temperature (240 C), the growth rates of interfacial IMC increased due to the fast diffusion of Cu at-oms, the variation of interfacial IMC thickness became big with ultrasonic time due to severely shaken by liquid solder to the thicker IMC under ultrasonic vibration.

3.2 The shear strength of soldered joints by ultrasonic- assisted

The shear strength of SACP joint and SACPC joint at 210 C 240 C were showed in Table 2. After adding nano- particles, the shear strength of SACP joint at 210 C in-creased from 31.49 MPa to 34.39 MPa with no ultrason-ic-assisted. The shear strength of SACP joint was only 31.59 MPa after ultrasonic-assisted for 5 s, but the shear strength of SACPC joint reached the peak value of 41.20 MPa, were about 0.32 and 19.80 percent more than that of no ultrasonic-assisted respectively. The shear strength of two kinds of joints decreased to 28.92 MPa and 30.89 MPa respectively after 15 s by ultrasonic-assisted, were about 8.16 and 10.17 percent less than that of no ultrasonic-assisted.

The shear strength of SACP joint was only 28.47 MPa, and increased to 31.37 MPa after adding nano-particles without ultrasonic vibration at 220 C. The shear strength of SACP joint and SACPC joint reached the peak value of 33.84 MPa and 37.49 MPa after 10 s by ultrasonic-assisted, were about 18.86 and 19.51 percent more than that of no ultrasonic-assisted respectively. After 15 s by ultrasonic- assisted, the strength of SACP joint and SACPC joint de-creased to 28.59 MPa and 30.02 MPa respectively. The strength of two kinds of joints were increased first and then decreased with the increase of ultrasonic time at 210 C and Fig. 4 Interfacial IMC of joint with SACP (a) (b) (c) (d) and SACPC (e)

(f) (g) (h) at 240 C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s)

Table 1 The IMC thickness (μm) of SACP joint and SACPC joint respectively.

Solders joints Temperature Ultrasonic time

0 s 5 s 10 s 15 s

SACP

210 C 3.89 3.38 4.81 3.67

220 C 3.39 3.18 3.71 4.34

240 C 3.26 3.65 3.54 4.00

SACPC

210 C 3.70 3.31 3.41 3.05

220 C 3.47 5.17 5.81 3.64

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220 C, the maximum increasing amplitude of joints was SACPC joint at 220 C and the maximum decreasing ampli-tude of joints was SACPC joint at 210 C, but the range were less than 20%.

The shear strength of SACP joint and SACPC joint de-creased with the increase of ultrasonic time at 240 C. The shear strength of SACP joint and SACPC joint reached the peak value of 38.80 MPa and 41.96 MPa without ultrasonic vibration. The shear strength of SACP joint and SACPC joint decreased to the minimum of 22.47 MPa and 21.11 MPa after ultrasonic-assisted at 5 s and 10 s, which were 42.09 and 49.69 percent less than that of no ultrasonic- assisted respectively. It found that nano-particles had no ob-vious change in the shear strength, but ultrasonic can greatly decrease the shear strength of joints.

3.3 The fracture morphology of soldered joints by ultrasonic-assisted

The microstructure of soldering seam became compact first and then became loose and porous with the increase of ultrasonic time from Fig. 6. Softening materials was not easy to fill the seam without ultrasonic vibration, slag inclu-sion and foreign gases were easy to wrap into the seam. Ultrasound can effectively improve foreign gases escape

from the seam and help solder to fill the solder seam, the mechanical property of joints will increase. On the other hand, slag inclusion and foreign gases would gather and grow up to lead decreasing of the shear strength under the the ultrasonic vibration. Whence the strength of two kinds of joints increased first and then decreased with the increase of ultrasonic time in the solid soldering and semi-solid soldering.

The appearance of SACP joint and SACPC joint at 240 C can be seen from Fig. 7, there were more compact without ultrasonic-assisted, more slag inclusion and pore under ultrasonic-assisted. Slag inclusion and foreign gases were wrapped into the seam and not easy to overflow under the liquid state in the liquidus nearby because of bad liquidity at low superheat. Slag inclusion and foreign gases would gather to the interface of IMC/solder, and grow up to lead decreasing of the shear strength under ultrasonic vibration. Fig. 6 The appearance of the SACPC joint at 210 C. (Ultrasound (a) 0 s,

(b) (e) 5 s, (c) 10 s, (d) 15 s) Table 2 The shear strengths (MPa) of SACP joint and SACPC joint

respectively.

Solders joints Temperature Ultrasonic time

0 s 5 s 10 s 15 s

SACP

210 C 31.49 31.59 32.17 28.92

220 C 28.47 31.22 33.84 28.59

240 C 38.80 22.47 27.54 23.27

SACPC

210 C 34.39 41.20 30.54 30.89

220 C 31.37 33.38 37.49 30.02

240 C 41.96 31.65 21.11 25.01

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The size of slag inclusion and pore decreased after adding nano-particles, so the mechanical property of joints would increase.

The shear fracture morphology of SACP joint and SACPC joint at 210 C under different ultrasonic time were showed in Fig. 8 and Fig. 9 respectively, and all joints showed toughness fracture. The fracture presented more pores in joint without ultrasonic vibration, because solder cannot completely melt at low temperature. The fracture presented the parabolic dimple, and its orientation was the same with the shear orientation when ultrasonic time was 5 s. The pore of the samples with ultrasonic for 5 s decreased obviously as compared to the shear fractures without ultrasonic. After

that, foreign gases or shrinkage defects began to increase with the increase of ultrasonic time. Finally, the loss of sol-der at the solsol-dering edge can be seen by the naked eye when the ultrasonic time was 15 s. The loss of solder at the solder-ing edge filled the soldersolder-ing seams again, and gas cannot es-cape from the joint which resulting in the emergence of many pores defects. This is consistent with the variation of shear strength of the corresponding joint. The shear fracture morphology of SACPC joint at 240 C showed toughness fracture in Fig. 10. With the increase of ultrasonic time, for-eign gases or shrinkage defects began to increase, the me-chanical properties of joints fell down evidently.

4.  Conclusions

(1) Interfacial IMC of joint with SACP were typical scallop- type, but the type of interfacial IMC were serrated and be-came smoother after added nano-Ni particles, but scallop- type IMC of joint with SACP became more obvious at 240 C. The IMC thickness were not more than 6.00 μm, and the IMC thickness of SACP joint was slightly thick compar-ing with SACPC joint at the same temperature. Interfacial IMC of SACP joint at 210 C was mainly composed of a layer of thick Cu6Sn5 and thin Cu3Sn, and the concentration gradients of Cu and Sn in IMC were also obviously. The composition of IMC was Cu3Sn7 in SACPC joint at 210 C, there were no different for the composition was Cu42.5Sn57.5 whether nano-Ni particles were added at 240 C.

(2) The strength of two kinds of joints were increased first and then decreased with the increase of ultrasonic time at 210 C and 220 C, but decreased in both joints at 240 C. The shear strength of SACP joint was only 31.59 MPa, but the shear strength of SACPC joint reached the peak value of Fig. 7 The appearance of SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at

240 C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s)

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41.20 MPa at 5 s by ultrasonic-assisted at 210 C, which were 0.32 and 19.80 percent more than that of no ultrasonic- assisted. The shear strength of SACP joint and SACPC joint reached the peak value of 33.84 MPa and 37.49 MPa after 10 s by ultrasonic-assisted at 220 C, which were 18.86 and 19.51 percent more than that of no ultrasonic-assisted re-spectively. The shear strength of SACP joint and SACPC joint reached the peak value of 38.80 MPa and 41.96 MPa after ultrasonic-assisted for 0 s at 240 C, but decreased to the minimum of 22.47 MPa and 21.11 MPa after ultrasonic- assisted at 5 s and 10 s, which were 42.09 and 49.69 percent

less than that of no ultrasonic-assisted respectively.

(3) Ultrasound can help fill the solder seam to increase the shear strength, but slag inclusion and pore would gather and grow up to lead decreasing of the shear strength at 210 C and 220 C. Slag inclusion and foreign gases were wrapped into the seam and not easy to overflow under the liquid state in the liquidus nearby, would gather to the interface of IMC/

solder and grow up to lead decreasing of the shear strength under the the ultrasonic vibration at 240 C. All joints showed toughness fracture.

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Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 61774066) and the China Postdoctoral Science Foundation (NO. 2015M582221), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (NO. KJ1600943 and KJ1600912) and the Program for Science and Technology of Banan District in Chongqing (NO. 2017TJ08 and 2017TJ09) respectively, the Innovate Program of Common Special Key Technology in Focus Industries of

Chongqing (NO. cstc2016zdcy-ztzx0047 and cstc2016zd-cy-ztzx0036) and the Research Projects of Experimental Techniques of Chongqing University of technology(NO. SK201708)respectively.

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Figure

Fig. 1 Ultrasonicassistedsoldering system consists.
Fig. 1 Ultrasonicassistedsoldering system consists. p.1
Fig. 2 Interfacial IMC of joint with SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at 210°C
Fig. 2 Interfacial IMC of joint with SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at 210°C p.2
Fig. 3 Interfacial IMC of joint with SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at 220°C
Fig. 3 Interfacial IMC of joint with SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at 220°C p.2
Fig. 5(a). There was an obvious step by the line-scanning in Fig. 5(b), more Sn atoms entered into the interfacial IMC of Sn atoms diffused more rapidly than relatively Sn atoms at (30.2 at% Cu)
Fig. 5(a). There was an obvious step by the line-scanning in Fig. 5(b), more Sn atoms entered into the interfacial IMC of Sn atoms diffused more rapidly than relatively Sn atoms at (30.2 at% Cu) p.3
Table 1 The IMC thickness (μm) of SACP joint and SACPC joint respectively.

Table 1

The IMC thickness (μm) of SACP joint and SACPC joint respectively. p.3
Fig. 6 The appearance of the SACPC joint at 210°C. (Ultrasound (a) 0 s, (b) (e) 5 s, (c) 10 s, (d) 15 s)
Fig. 6 The appearance of the SACPC joint at 210°C. (Ultrasound (a) 0 s, (b) (e) 5 s, (c) 10 s, (d) 15 s) p.4
Fig. 5 The EDX of interfacial IMC with SACP (a) (c) and SACPC (b) (d) at 210°C (a) (b) and 240°C (c) (d) for 5 s by ultrasonic-assisted.
Fig. 5 The EDX of interfacial IMC with SACP (a) (c) and SACPC (b) (d) at 210°C (a) (b) and 240°C (c) (d) for 5 s by ultrasonic-assisted. p.4
Table 2 The shear strengths (MPa) of SACP joint and SACPC joint respectively.

Table 2

The shear strengths (MPa) of SACP joint and SACPC joint respectively. p.4
Fig. 7 The appearance of SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at 240°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s)
Fig. 7 The appearance of SACP (a) (b) (c) (d) and SACPC (e) (f) (g) (h) at 240°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s) p.5
Fig. 8 The shear fracture fracture morphology of SACP joint at 210°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s)
Fig. 8 The shear fracture fracture morphology of SACP joint at 210°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s) p.5
Fig. 9 The shear fracture fracture morphology of SACPC joint at 210°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s)
Fig. 9 The shear fracture fracture morphology of SACPC joint at 210°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s) p.6
Fig. 10 The shear fracture fracture morphology of SACPC joint at 240°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s)
Fig. 10 The shear fracture fracture morphology of SACPC joint at 240°C. (Ultrasound (a) (e) 0 s, (b) (f) 5 s, (c) (g) 10 s, (d) (h) 15 s) p.7

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