Transformation of solder joints into IMC joints

The entire microbump may transform into an IMC joint after reflow or reliability tests due to the small volume of solders.^{Reference Chang, Chen, Chang, Zhan, Juang and Huang5,Reference Lin, Zhan, Juang, Lau, Chen, Lo, Kao, Tian and Tu19–Reference You, Lee, Lee and Kang23} Nickel and copper are two popular UBM materials for microbumps. Three possible microbump structures are Ni/solder/Ni,^{Reference Lin, Chiou, Yang, Chang, Lin, Liao, Hung, Lin, Tsai, Shih, Wu, Wu, Tsai, Huang, Wang, Yu, Chang, Chen, Hou, Tung, Jeng and Yu6,Reference Hsu, Chou, Liu, Chen, Yao, Chou and Tu18} Ni/solder/Cu,^{Reference Huang, Zhan, Lin, Juang, Huang, Chen, Fan, Cheng, Chao, Lin, Lin and Chen31} and Cu/solder/Cu.^{Reference Labie, Limaye, Lee, Berry, Beyne and De Wolf22,Reference Park, Kim, Park, Kim, Son, Han, Oh, Kim and Yoo27} Cu₆Sn₅ IMCs have a much higher reaction rate, about five times that of Ni₃Sn₄ IMCs.^{Reference Kim and Tu32,Reference Kim, Jang, Lee and Tu33} Thus, Cu/solder/Cu microbumps can be easily transformed into IMC microbumps consisting of Cu/Cu₃Sn/Cu₆Sn₅/Cu₃Sn/Cu,^{Reference You, Lee, Lee and Kang23} as shown in Figure 2a. Some Kirkendall voids due to unbalanced atomic flux are formed in Cu₃Sn layers and may weaken the mechanical properties of the microbump. Further heat treatment will transform the microbump into Cu/Cu₃Sn/Cu.^{Reference Chiu, Liu, Haung and Chen34} Even though Ni has a lower reaction rate than Sn, the solder in Ni/solder/Ni can also be completely consumed and form a Ni/Ni₃Sn₄/Ni IMC joint after electromigration,^{Reference Chang, Chen, Chang, Zhan, Juang and Huang5,Reference Wei, Yu, Tung, Huang, Hsieh, Chiu, Hsiao, Chang, Lin, Liang, Chen, Yeh, Lin and Yu7} as shown in Figure 2b. Current stressing (i.e., applying high current densities) enhances the dissolution of Ni and accelerates the formation of the Ni₃Sn₄ IMC. For Ni/solder/Cu microbumps, ternary IMCs of (Cu,Ni)₆Sn₅ are formed at a rate between that of Ni₃Sn₄ and Cu₆Sn₅ IMCs.^{Reference Huang, Zhan, Lin, Juang, Huang, Chen, Fan, Cheng, Chao, Lin, Lin and Chen31}

Figure 2.

Cross-sectional scanning electron microscope images depicting intermetallic (IMC) microbumps. (a) IMC microbumps of Cu/Cu₃Sn/Cu₆Sn₅/Cu₃Sn/Cu formed after reflow at 260°C for 40 min, and (b) a Ni₃Sn₄ IMC microbump formed after current stressing by 9.6 × 10⁴ A/cm² at 164°C for 130 h.^{Reference Wei, Yu, Tung, Huang, Hsieh, Chiu, Hsiao, Chang, Lin, Liang, Chen, Yeh, Lin and Yu7}

Table I lists the resistivity, elastic modulus, and melting point for Ni₃Sn₄, Cu₆Sn₅, Cu₃Sn, and SnAg solders. Among the three IMCs, Ni₃Sn₄ is the most resistive compound. Nevertheless, when the solders react with Ni to form Ni₃Sn₄, the resistance of a microbump increases approximately 68%.^{Reference Chang, Chen, Chang, Zhan, Juang and Huang5,Reference Chang, Peng, Yang, Chen, Chang, Zhan, Juang and Huang35} However, formation of IMCs would neither cause excessive Joule heating nor accelerated electromigration damage. Instead, the three IMCs possess higher electromigration resistance than SnAg solders^{Reference Wei, Chen, Liu and Chen36,Reference Chen, Shih, Wei, Tung, Hsiao, Yu, Liang and Chen37} because they have higher melting points and elastic moduli than SnAg solders, as shown in Table I. With proper design of the thickness ratio of UBM materials to solders, the solders in microbumps can completely transform into IMCs, thus enhancing the electromigration resistance.^{Reference Chen, Shih, Wei, Tung, Hsiao, Yu, Liang and Chen37} In fact, it has been reported that the electromigration lifetime of microbumps surpasses that of flip-chip solder joints.^{Reference Lin, Chiou, Yang, Chang, Lin, Liao, Hung, Lin, Tsai, Shih, Wu, Wu, Tsai, Huang, Wang, Yu, Chang, Chen, Hou, Tung, Jeng and Yu6,Reference Syed, Dhandapani, Moody, Nicholls and Kelly38,Reference Wang, Lin, Chen, Chiu, Chen, Yeh, Lin, Hou, Lin, Chen, Jeng and Yu39} This is because IMC joints form during electromigration, and there is almost no current crowding effect in solders.^{Reference Wei, Yu, Tung, Huang, Hsieh, Chiu, Hsiao, Chang, Lin, Liang, Chen, Yeh, Lin and Yu7,Reference Chang, Peng, Yang, Chen, Chang, Zhan, Juang and Huang35,Reference Wang, Lin, Chen, Chiu, Chen, Yeh, Lin, Hou, Lin, Chen, Jeng and Yu39} The current distributes uniformly in the solder.

Table I.

Resistivity, elastic modulus, and melting points for Ni₃Sn₄, Cu₆Sn₅, Cu₃Sn, and SnAg solders.

On the other hand, IMCs are brittle in nature. Hence, the mechanical properties of IMC microbumps may be worse than those of microbumps of the remaining solders. Whether the three IMCs are brittle remains controversial,^{Reference Lee, Wang and Kim40} because both Ni-Sn and Cu-Sn microbumps can pass the temperature cycling test (TCT), in which the microbumps experience temperature cycling between –55°C and ∼125°C for 1000 cycles. Usually, when the resistance of the microbumps does not increase over 20% of their initial value after the test, the microbumps pass the test. The drop test can determine whether IMC microbumps are brittle. During drop tests, microbumps are dropped from a certain height and experience a much higher strain rate than during TCTs. However, no such results have been reported. In addition, the creep behavior of IMCs is not clear and merits more investigation.

Necking or voiding induced by side wetting/diffusion

Necking or voiding induced by side wetting/diffusion becomes a serious reliability issue in microbumps, and no solution has been reported so far. Liang et al. claimed that the solder may wet the circumference of the Ni/Cu UBM during reflow.^{Reference Liang, Chen and Tu24} Sn atoms tend to diffuse on the circumference of Ni and Cu UBM to form Ni₃Sn₄ and Cu₃Sn, respectively, to reduce the free energy of the system. Owing to the small solder volume, necking or voiding may occur when Sn is drawn away. Figure 3a presents an as-fabricated Cu/Ni/solder/Ni/Cu microbump with a solder height of 8 μm. After reflow at 260°C for 20 min, Sn diffused to the circumference of Ni and Cu UBM to form Ni₃Sn₄ and Cu₃Sn, respectively.^{Reference Liang, Chen and Tu24} As a result, the solders between the top and bottom UBM were depleted, and necking or voiding occurred in the joint. Necking or voiding weakens both mechanical and electrical properties of the microbump. In some cases, the solders were completely depleted, as shown in Figure 3b. This phenomenon not only happens during molten-state reflow, but also takes place in solid-state aging,^{Reference Cheng, Chung, Pai and Chen25} thermal cycling,^{Reference Chan, Liao, Lin, Chuang, Huang, Yeh, Tseng and Lai26} and electromigration tests.^{Reference Lin, Zhan, Juang, Lau, Chen, Lo, Kao, Tian and Tu19,Reference Park, Kim, Park, Kim, Son, Han, Oh, Kim and Yoo27} Therefore, it is an urgent problem, which needs to be solved.

Figure 3.

(a) As-fabricated Cu/Ni/solder/Ni/Cu microbump. (b) After reflow at 260°C for 20 min, side wetting occurred, and Sn diffused to the circumference of Ni and Cu under-bump metallization to form Ni₃Sn₄ and Cu₃Sn, respectively, resulting in the complete depletion of the solder.^{Reference Huang, Zhan, Juang, Lin, Huang, Chenl, Fan, Cheng, Chao, Hsieh, Chen and Lau55}

Formation of porous Cu₃Sn IMCs

For the Cu-Sn metallurgical reaction, layer-type Cu₃Sn is regarded as the terminal phase when there is a limited amount of Sn.^{Reference Tu4} However, Panchenko et al. reported that porous Cu₃Sn started to form in Cu/solder/Cu after reflow at 260°C for two min. Side wetting was also observed in the circumference of Cu UBM.^{Reference Panchenko, Croes, De Wolf, De Messemaeker, Beyne and Wolter28}Figure 4a shows an as-fabricated Cu/SnAg/Cu microbump with 8-μm-thick SnAg solder. After reflow for 12 h, porous Cu₃Sn formed in the microbump (Figure 4b). A mechanism has been proposed to explain the formation of the porous Cu₃Sn layer, as described later in the text. IMCs of Cu₆Sn₅ on Cu tend to transform into Cu₃Sn upon continuous annealing. The process involved is either

Figure 4.

(a) As-fabricated Cu/solder/Cu microbump with 8-μm-thick SnAg solder. (b) Porous Cu₃Sn is formed in the microbump attributed to side wetting on the circumference of Cu under-bump metallization after reflow at 260°C for 12 h.^{Reference Panchenko, Croes, De Wolf, De Messemaeker, Beyne and Wolter28}

(1)$${\rm{Cu}}_{\rm{6}} {\rm{Sn}}_{\rm{5}} \to {\rm{2Cu}}_{\rm{3}} {\rm{Sn + 3Sn}}$$

or

(2)$${\rm{Cu}}_{\rm{6}} {\rm{Sn}}_{\rm{5}} {\rm{ + 9Cu}} \to {\rm{5Cu}}_{\rm{3}} {\rm{Sn}}{\rm{.}}$$

During continuous high-temperature processing, the initially formed Cu₆Sn₅ joints transformed into Cu₃Sn. According to Equation 1, Sn is released. The released Sn in liquid state diffuses rapidly, leading to wetting of the side of Ni and Cu UBM to form IMCs. The microbump region is thus occupied by porous Cu₃Sn, indicating that porous Cu₃Sn is the terminal phase for microbumps when Sn atoms diffuse to the circumference of the Cu UBM during phase transformation of Cu₆Sn₅ to Cu₃Sn. Nevertheless, if there is enough Cu supply, the released Sn will react with Cu to form Cu₃Sn, as illustrated by Equation 2. Layer-type Cu₃Sn is formed in microbumps when there is no side wetting/diffusion.^{Reference Chiu, Liu, Haung and Chen34} The formation of porous Cu₃Sn also undermines the mechanical and electrical properties. Such porous structures have not been found in Ni₃Sn₄ and (Cu,Ni)₆Sn₅.

Early electromigration failures caused by specific orientations of Sn grains

Extremely fast Cu and Ni dissolution may occur along specific Sn grain orientations.^{Reference Yeh and Huntington41,Reference Dyson, Anthony and Turnbull42} Lu et al. found that rapid depletion of IMCs and Ni UBM occurs along the c-axis of Sn crystals, resulting in significant damage to the joints.^{Reference Lu, Shih, Lauro, Goldsmith and Henderson43} The dissolution of Cu and Ni is rapid because Cu interstitial diffusivity along the c-axis is 500 times faster than that along the a- or b-axis of Sn crystals at 25°C, and the diffusivity of Ni along the c-axis is ∼7 × 10⁴ times faster than that at right angles (a- or b-axis) at 120°C.^{Reference Yeh and Huntington41} Therefore, Sn grain orientation plays an important role in the electromigration failure modes and lifetime of SnAg solders. The effect of Sn grain orientation on reliability issues will become more significant because some of the microbumps consist of a single Sn grain.^{Reference Tu, Hsiao and Chen44,Reference Tian, Chen, Kunz, Tamura, Zhan, Chang and Tu45}

It has been reported that flip-chip joints with c-axis Sn grains parallel to the electron flow had shorter electromigration lifetimes than joints with Sn grains perpendicular to the electron flow.^{Reference Wang, Lu, Gupta, Stiborek, Shirley, Chae, Im and Ho46} This is because c-axis Sn grains parallel to the electron flow allow rapid dissolution of the Ni UBM, leading to deterioration of contact opening and shorter electromigration lifetime. The Ni UBM connects to the circuits in the chip through the contact opening, and voids may form after the Ni UBM is dissolved into the Sn grains. However, for microbumps, the effect of Sn grain orientation on electromigration lifetime is the reverse. Rapid dissolution of Ni UBM occurring in microbumps with c-axis Sn grains parallel to the electron flow transform them into IMC microbumps, resulting in a longer electromigration lifetime.^{Reference Lin, Chiou, Yang, Chang, Lin, Liao, Hung, Lin, Tsai, Shih, Wu, Wu, Tsai, Huang, Wang, Yu, Chang, Chen, Hou, Tung, Jeng and Yu6,Reference Chen, Shih, Wei, Tung, Hsiao, Yu, Liang and Chen37}

Figure 5a–f shows the effect of Sn grain orientation on the formation of Ni₃Sn₄ IMCs.^{Reference Wei, Yu, Tung, Huang, Hsieh, Chiu, Hsiao, Chang, Lin, Liang, Chen, Yeh, Lin and Yu7}c-axis Sn grains nearly perpendicular to the electron flow (high-angle grains) had a much lower IMC formation rate. The three Sn grain orientations in Figure 5b are 84.9°, 81.1°, and 85.2°. Thus, most of the solders were un-reacted, yet in the microbump in Figure 5c–d, the IMC formation rate in the center grain appeared much faster than the two grains on the periphery of the microbumps. The Ni₃Sn₄ IMC bridged the joint. However, some Sn grains at low angles to the electron flow were transformed into IMC grains, which have higher electromigration resistance than solder. The Sn grain in Figure 5c is at 89.9° to the electron flow, and most of the solder remained intact. Voids may form in this microbump after longer current stressing up to thousands of hours. The microbump in Figure 5c has a bridged Ni₃Sn₄ IMC, which will have a longer electromigration lifetime than the microbump in Figure 5e.

Figure 5.

Cross-sectional images of microbumps after current stressing by 9.6 × 10⁴ A/cm² at 164°C for 1–30 h. (a), (c), and (e) are scanning electron microscope images; (b), (d), and (f) are grain orientations mapped by electron backscatter diffraction (EBSD).^{Reference Lin, Chiou, Yang, Chang, Lin, Liao, Hung, Lin, Tsai, Shih, Wu, Wu, Tsai, Huang, Wang, Yu, Chang, Chen, Hou, Tung, Jeng and Yu6} The EBSD maps show the orientation of the remaining Sn grains after current stressing. The angle labeled in the maps represents the angle between the current direction and the c-axis of the Sn grain. Note: IMC, intermetallic.

Precipitation of plate-like Ag₃Sn IMCs

SnAg alloys have been adopted as solder materials in flip-chip solder joints due to their excellent mechanical properties and precise control of the composition during electroplating. Dispersed Ag₃Sn IMCs formed in the as-fabricated microbumps, as shown in Figure 6a.^{Reference Yang, Chang, Sung and Chen29} However, large plate-like Ag₃Sn IMCs form inside the solder joints when the concentration of Ag is higher than 3.5 wt%,^{Reference Jang, Frear, Lee and Tu47} and cracks may initiate at the interface of these plate-like Ag₃Sn IMCs and the solder when the solder joints are subjected to stress.^{Reference Zeng and Tu48} Therefore, the packaging industry has adopted SnAg alloys with a low Ag concentration (∼2.5 wt%) to avoid the formation of these plate-like Ag₃Sn IMCs. However, plate-like Ag₃Sn IMCs may still form even when low-Ag concentration solder is used. This is because during Cu-Sn or Ni-Sn interfacial reactions, the Ag atoms do not participate in the reactions, and they do not form Cu-Sn-Ag or Ni-Sn-Ag ternary IMCs. They are expelled to the molten solder during the interfacial reaction. In microbumps with small amounts of solder, when Sn is consumed by interfacial reactions, the concentration of Ag in the remaining solder continues to increase. At some point, the Ag concentration will exceed 3.5 wt%.^{Reference Chuang, Chen, Shih, Lai and Kao30,Reference Huang, Zhan, Lin, Juang, Huang, Chen, Fan, Cheng, Chao, Lin, Lin and Chen31} When the microbumps are cooled, plate-like Ag₃Sn IMCs will then form in the microbumps. Figure 6b shows 4.0-μm-thick Sn2.5Ag microbumps after reflowing for 10 min at 260°C, respectively. As can be seen, plate-like Ag₃Sn IMCs formed in the solder and Ni₃Sn₄ IMCs. These precipitates will adversely affect the mechanical properties of the microbumps. Therefore, the concentration of Ag in SnAg solders should be decreased with downward scaling of microbumps.

Figure 6.

Cross-sectional backscatter scanning electron microscope images of 4.0-μm-thick Sn2.5Ag microbumps after reflowing for (a) 0 min and (b) 10 min at 260°C.^{Reference Yang, Chang, Sung and Chen29}