Science Forum Vols. 475-479 (2005) pp. 1869-1872 http://www.scientific.net

Trans Tech Publications, Switzerland

ApJtliCiiltioln of TLP(Transient Liquid Phase) Bonding Method to the High T m Lead-Free Solder

J.S. Lee1· a, W.H. Bang2

·b, J.P. Jung1·c and K.H. Oh2·d

1Department of Materials Science and Engineering, University of Seoul, Siripdae-gil(90

cheonnong-dong) , Dongdaemun-Ku, Seoul, 130-7 43, Korea.

'school of Materials Science and Engineering, Seoul National University, San 56-1, Shinrim-Dong, Kwanak-Ku, Seoul, 151-742, Korea.

'j751ee@engmail.uwaterloo.ca, busk2@snu.ac.kr, cjpjung@uos.ac.kr, dkyuhwan@snu.ac.kr,

Keywords: TLP(Transient Liquid Phase) method, soldering, electroplating, reflow

Abstract. TLP method was applied to the lead free soldering process; Sn-3 .5Ag core solder (Tm of 221 °C} I Sn-Bi coating layer (T m of 139°C at eutectic) was reflowed on UBM pad. Sn-Bi was formed by electroplating method at current range of 2-6A/dm2

, which had varied the coating layer composition from 96wt% to 28wt% Bi. Specimens were reflowed on copper pad at temperature range of200 and 220°C, and then interface reaction was examined. It was claimed that high Bi adding in the plated layer improved the wetting performance on Cu. Even at the reflow temperature of200°C, SEM observation revealed well-defined interface junctions between solder and copper pad, i.e. nucleation and growth of CuxSn intermetallic compound. After heating for 5min at 220 ·c, Bi was resolved into Sn-3.5Ag core solder and completely homogenized, which is favorable from the viewpoint of long tenn fatigue reliability.

Introduction.

For environmental issue, Sn alloyed with Ag or Cu, has been intensively researched as possible substitute for the Ph-containing solder. However, higher soldering temperature (by -40°C compared to conventional eutectic Sn-37Pb) of these alloys could thermally damage the soldered matters of chips or boards in electronics, which is one of the major limits at present time. Many researches have been conducted for the decrease of the T m of these lead-free systems. Possible suggestion is the addition of third or fourth elements such as 'Bi' and 'In'. 'In' can lower the melting temperature of lead-free solder but its high price blocks the practical application. Bi can decrease the melting temperature and also improves the both of strength and wettability. But the excessive addition of Bi, higher than 3wt% in total system, could cause brittleness and form low melting phase.

Recently, there has been interested in application of transient liquid phase (TLP) [1,2] bonding technique to the soldering process. The solder, which has low melting temperature, places between the base metal and solder insert. During the heating up, the liquid phase forms between the interfaces of solder and base material then is consumed by the diffusional reaction with substrate and solder. Finally, solder contact is developed at lower temperature than its melting point.

This study reports the application of diffusion soldering like transient liquid phase diffusion bonding (TLP) process for reflow process. The Sn-Bi layer, where Bi is acted as a transient region for melting depressant, was coated on Sn-3.5mass%Ag alloy as an effort to lower soldering temperature. The effect of electroplated Sn-Bi layer thickness and composition on the shear strength and microstructure was studied

Experimental procedure.

Electroplating was used to fabricate a melting depressant layer on the Sn-3.5wt%Ag solder (hereafter wt% was omitted)surface. In order to study characteristics of the electroplated Sn-Bi layer, Bi

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20min

Fig. 1 SEM images showing the cross-sectional view of deposited layer on the Sn-3.5Ag solder with plating times.

composition ranged from 25wt% to 97wt% was electroplated on the Sn-3 .5Ag rolled plate. For the purpose of assisting the plating, the Sn-3.5Ag was polished using 500, 1000, and 2400 grit sand papers followed by I,um and 0,04~m Si02 colloidal suspension. The rolled solder was cleaned with 10% HCl solution for lOseconds, and then with ethanol to remove oxide on the Sn-3 .5Ag plate. The plating bath was stirred at 250rpm to maintain uniform distribution of the solution. The electric currents used for the plating to form an identical thickness of Sn-Bi layers were 2, 4, 6A/dm2 for the plating time of 2.5, 5, and lOminitues, respectively. The Sn-Bi electroplated Sn-3.5Ag alloy was rinsed with DI water at 80 ·c. The compositions of the electroplated Sn-Bi layers were analyzed with an electron probe X-ray analyzer(EPMA).

The Sn-Bi electroplated Sn-3 .5Ag rolled plates (1 x lcm) were mounted on a Cu coupon and reflowed by an infrared reflow machine with RMA-typed BGA flux to look for the optimistic soldering temperature, and to evaluate the effect of Sn-Bi composition and thickness on the microstructure. The soldering peak temperatures used were 200, 220, and 250 ·c. To evaluate Bi homogenization, the Sn-Bi electroplated Sn-3 .5Ag solder was heated at 220 "C on the hot plate and area-analyzed by EPMA.

Results and Disccusion.

The final composition of the solder using TLP method depends not only on the cored solder, but also on the composition and thickness of electroplated layers. As seen at Fig. I showing the cross-section of Sn-Bi layers on the Sn-3.5Ag solder with electroplating time for 5, 10, 20minitues at 2A/dm2, the Sn-Bi layers were formed uniformly on the Sn-3 .5Ag solder without any defects on the interface between Sn-Bi layer and Sn-3 .5Ag solder. The experimental results showed the linear correlation between time and plating thickness. It is usually believed that the plating thickness is proportional to the plating time in electroplating. It appears that application of the linear correlation made it possible to predict roughly the plating time required to achieve aimed plating thickness.

(a) (b) (c) Fig. 2 SEM images showing the wettabilities of Sn-Bi coated Sn-3.5Ag solder with various Bi

compositions. ((a) 96Bi, (b) 80Bi, and (c) 28Bi)

Materials Science Forum Vols. 475-479 1871

Fig. 2 shows the comparison of the contact angles of the Sn-Bi coated Sn-3.5Ag solders at the various Bi compositions after soldering at 200 'C . As seen at Fig.2 (a)-( c), measured contact angles were 24°, 31 °, and 45° at the composition of96Bi, 80Bi, and 28Bi, respectively. It was also seen-from Fig. 2(c) that the Sn-Bi coated Sn-3 .5Ag solder was partially melted.

In general, Yong's equation can be applied to the interface between substrate and solder.

Ysv - rLS = YLv cosB (1)

Where, , r SV, r LS and r LV are Surface tension( Or interfacial tension) between the SUbStrate and vapor,

between liquid solder and the substrate, and between liquid solder and vapor, respectively. B is an contact angle at the triple point. Then, a contact angle is determined by the surface tension. It is usually known that the wettability increase with decreasing contact angle( B). The contact angle

decreases as r sv increases and r LS and r LV decreases. In this study, r sv and r LV were constant.

From this result, it was confirmed that the higher Bi contents in the Sn-Bi layer, the more improved wetability on Cu coupon. That is to say that Bi adding increase the wettability of solder.

Fig. 3 and Fig. 4 shows the microstructures of the Sn-Bi coated Sn-3 .5Ag solders on the Cu coupon after soldering 200 'C. As seen in these figures, the plated layers seems to be effective in formation of

fine solder contact even at a low reflow temperature of200 'C as well as 220 'C as no defects along the bonded interface between the solder and Cu coupon were not observed. However, the Cu6Sn5 IMC was barely developed in case of the 28Bi coated solder at even 220 'C, while Well-defined Cu6Sns IMC can be observed between the Cu coupon and the Sn-3 .5Ag solder coated with Sn-96Bi and Sn-80Bi layer. It was clearly observed from Fig. 3(b) and Fig. 4(b) that relatively big Cu6Sns IMCs were formed at 220 'C , showing the temperature effect on the IMC formation. It is believed that Cu6Sn5 IMC in the solder was formed by Cu diffusion into the solder[3].

(a) (b) (c) Fig. 3 SEM images showing the microstructures of (a) 96%Bi, (b) 80%Bi, and (c) 28%Bi

coated Sn-3.5Ag solders after soldering at 200 'C

w ~ ~ Fig. 4 SEM images showing the microstructures of (a) 96%Bi, (b)80%Bi, and (c)28%Bi

coated Sn-3.5Ag solders after soldering at 220 'C

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Interestingly, it is seen from Fig.3 that large Ag3Sn intermetallic compounds seem to form or at the interface between the solder and the Cu coupon, while large Ag3Sn IMC was disllpp:anl the reflow temperature of 220 ·c as seen at Fig.4, regardless of the Bi contents.

To clarity the existence of Sn, Cu, and Ag elements on the intermetallic compounds, investigate the distribution of Bi, which affects the mechanical strength of the solder, EPMA analysis was employed after reflowed at 220 ·c . Figure 5 showed the intermetallic compounds distribution on the cross-sectioned solder joint and inner solder. Sn, Cu, and Ag elements composed of the intermetalic compounds. Ag and Cu elements are observed in the inner solder. It not clear that Cu6Sns intermetallic compounds were built up by spalling of Cu6Sn5 or Cu uu.•u:.•uu•

the solder [4]. Ag3Sn intermetallic compounds formed due to existence of Ag in the solder. however, was not observed on the layer by EPMA mapping. Therefore, it could be understood that the melting depressant, is well dissolved and homogenized into the solder.

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Fig. 5 Bi homogenization across Sn-80Bi solder with soldering time at 220 "C

Conclusion

Solderability evaluation of Sn-Bi coated Sn-3.5Ag core solder (T m of221 oq I Cu coupon was carried out. A high Bi in the plated layer improved the wetting performance on Cu. SEM observation revealed that solder joints were well-formed on the interface at 2,4A/dm2 (96Bi, and 80Bi containing Sn-Bi layer), regardless of the various temperature, and Ag3Sn IMCs were formed in the solder and the solder joint at 200°C, but were not developed at 220°C. After heating for 5min at 220 ·c , EPMA result showed that Bi was resolved into Sn-3 .5Ag core solder and completely homogenized

References

[1] X.Qiao, S.F. Corbin: Mat. Sci. Eng., Vol. A283, (2000) p. 38

[2] T. Korhonen, V. Vuorinen, and J.K. Kivilaht: , IEEE Trans.Vol.24, (2001) p. 515

[3] J.H. Lee, D.H. Shin and Y.S. Kim: Met. Mater. Int. Vol.9 (2003), p.577

[4] Ann A. Liu, H.K. Kim, and K.N.Tu: J. Appl. Phys, Vol. 80 (1996) p. 2774