Microstructure Evolution during Electromigration

Figure19 through 26 show the typical microstructure evolution during current stressing for various current density at 80℃, 100℃ and 120℃. After the reflow of the solder, Pb-rich region was surrounded by Sn-rich region. During current stressing Pb-rich region grew up due to solid state aging. After current stressing, depletion occurred at the cathode end of the stripe, and extrusions formed at the anode end of the stripe.

Figure 19 (b) shows the SEM images of the cathode end of the solder stripes after the under current stressing of 9.7 x 10³A/cm² at 80℃ for 24 hrs Pb-rich region grew up due to solid state aging. With longer current stressing for 72hrs, the sizes of Pb-rich region do not change very much. There was some residual flux on the Ti at right side. Figure 20 (a) show the SEM images of the cathode end of the solder stripes before current stressing of 4.9 x 10⁴A/cm² at 80℃. At the corners of the edge, the solder did not wetting well during reflow process. After the current stressing of 4.9 x 10⁴A/cm² at 80℃, it could be observed that IMCs appear near the edge. The deplete region which is defined by the IMCs region. It was found that, at upper and lower corners of the edge, the depletion length was longer than that at center. It was due to polishing process, which made the solder at ther corners much thinner than that at center. Figure 20 (c) shows SEM image of the cathode end of the solder stripe after the current stressing of 4.9 x 10⁴A/cm² at 80℃ for 72hrs. The depletion was much more than that under the stressing for 24hrs. A hillock formed 60 μm away form the edge. Figures 21 (a) and (b) show the SEM images of the cathode end of the solder stripe before and after the current stressing of 8 x 10⁴A/cm² at 80℃ for 72hrs, respectively.The delpletion region observed by SEM was enlarged than that in Fig. 20 (c).

There was also a hillock formed at approximately 50 μm away from the edge. Figures 22 (b) and (c) show the SEM images of the cathode end of the solder stripe after the current stressing of 9.1 x 10³A/cm² at 100℃ for 72hrs and 144hrs, respectively. The depleted

region was slightly increasing for 144hrs current stressing. Figures 23 (b) shows the SEM image of the cathode end of the solder stripe after the current stressing of 3 x 10⁴A/cm² at 100℃ for 72 hrs. The depleted region was measured to be 17 μm. At upper-right hand side of the depleted region ,some residual Pb-rich region was found among IMCs. Figure 24 (b) shows the SEM image of the cathode end of the solder stripe after the current stressing of 5.4 x 10⁴A/cm² at 100℃ for 72 hrs. The depletion region was measured to be 27 μm. Figure 25 (a) and 25 (b) show the SEM images of the cathode end of the solder stripe before the current stressing of 9.9 x 10³A/cm² at 120℃ for 72hrs. Some IMCs were observed before the current stressing, which was due to polish process. After the current stressing of 9.9 x 10³A/cm² at 120℃ for 72hrs, the IMCs region slightly increased. Figure 26 (b) shows the SEM image of the cathode end of the solder stripe after the current stressing of 3 x 10⁴A/cm² at 120℃ for 72 hrs. The IMCs appeared after the current

stressing. Again, there were some residual Pb-rich regions among the IMCs. It is hard to calculate the depleted volume by SEM images only because it hard to define the depletion region. Therefore, we employed AFM to measure the depletion volume.

Figure 27 shows the cross-sectional SEM image for the solder Blech structure. The morphology of the IMC is scallop-type and the average thickness is 0.87μm. This scallop-type IMC was analyzed to be Cu⁶Sn⁵. According to the literature [2], the eutectic SnPb solder joints consist of Cu⁶Sn⁵ (η-phase) and Cu³Sn (ε-phase) intermetallic layers at the interface of eutectic SnPb solder and a copper foil. However, only η-phase presents and no ε-phase was formed in this study. The absence of the Cu³Sn may be due to that the copper layer in the UBM was only 0.4 μm. There is no enough Cu for the formation of the Cu³Sn IMC.

In conclusion, as shown in Figs. 20(b) (c), Fig. 21 (b) (c), Fig. 23 (b), Fig. 24 (b), Fig.

25 (b) and Fig. 26 (b). The IMC appeared on the cathode end of the solder after current stressing. However, IMCs could not be observed at cathode end after the current stressing

of 9.7 x10³ at 80℃ up to 162hrs, as shown in Figs. 19(a) to 19(d). As shown in Figs. 20(a) to (c), solder depleted form the edge and broadened the depletion region as stressing time increased. Due to lower drift velocity at lower stressing temperature and current density, volume depletion in the cathode end was not serious.

Figures 28 (a) and 28 (b) show the AFM 3D- images of the cathode end before and after the current stressing of 3x10⁴A/cm² at 100℃ for 72hrs, it was clearly seen that the morphology of IMC is scallop type after solder was depleted. By estimating average height of this region, we can obtain the average thickness of IMC to be 0.894μm.

Figures 29 to 36 show the microstructure evolution at the anode end during current stressing. For most cases, hillocks were observed at the anode end of the solder. However, no obvious hillock formation was found in sample with lower stressing current and temperature, as shown in Fig. 29.

(a) 0 hr (b) 24 hrs

(c) 72 hrs (d) 162 hrs

Fig. 19 SEM images of the cathode end of the solder stripes after the under current stressing of 9.7Χ10³A/cm² at 80℃ for (a) 0 hr (b) 24 hrs(c) 72 hrs (d) 162 hrs

，

e^- e^-

e^- e^-

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(a) 0 hr (b) 24 hrs

(c) 72 hrs

Fig. 20 SEM images of the cathode end of the solder stripes after current stressing of 4.9Χ10⁴A/cm² at 80℃ (a) 0 hr (b) 24hr (c) 72 hrs.

(a) 0 hrs (b) 72 hrs

Fig. 21 SEM images of the cathode end of the solder stripes after current stressing of 8Χ10⁴A/cm² at 80℃. (a) 0 hr (b) 72 hrs.

e^-

e^-

e^-

e^-

e^- Hillock

Hillock

(a) 0 hr (b) 72 hrs

(c) 144 hrs

Fig. 22 SEM images of the cathode end of the solder stripes after the current stressing of 9.1Χ10³A/cm² at 100℃ for (a) 0 hr (b) 72 hrs (c) 144 hrs

(a) 0 hr (b) 72 hrs

Fig. 23 SEM images of the cathode end of the solder stripes after current stressing of 3Χ10⁴A/cm² at 100℃. (a) 0 hrs (b) 72 hrs

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e^-

e^- e^-

(a) 0 hr (b) 72 hrs

Fig. 24 SEM images of the cathode end of the solder stripes after the current stressing of 5.4Χ10⁴A/cm² at 100℃ for (a) 0 hr (b) 72 hrs

(a) 0 hr (b) 72 hrs

Fig. 25 SEM images of the cathode end of the solder stripes after current stressing of 9.9Χ10³A/cm² at 120℃ for (a) 0 hr (b) 72 hrs

(a) 0 hr (b)72 hrs

Fig. 26 SEM images of the cathode end of the solder stripes after current stressing of 3Χ10⁴A/cm² at 120℃ for (a) 0 hr (b) 72 hrs

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Fig. 27 Cross-sectional SEM image of the solder stripe after the reflow at 210℃ for 4s

(a) 0 hr (b) 72 hrs

Fig. 28 3-D AFM images of the cathode end of the solder stripes after the sample after under current stressing of 3Χ10⁴A/cm² at 100℃. (a) before current stressing (b) After 72 hr s current stressing.

E-PbSn

IMC

(a) 0 hr (b) 24 hrs

(c) 72hrs (d) 162 hrs

Fig. 29 SEM images of the anode end of the solder stripes after current stressing of 9.7Χ 10³A/cm² at 80℃ for (a) 0 hr (b) 24 hrs(c) 72 hrs (d) 162 hrs

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e^- e^-

(a) 0 hr (b) 24 hrs

(c) 72 hrs

Fig. 30 SEM images of the anode end of the solder stripes after current stressing of 4.9 Χ 10⁴A/cm² at 80℃ for (a) 0 hr (b) 24 hrs (c) 72hrs

(a) 0 hr (b)72 hrs

Fig. 31 SEM images of the anode end of the solder stripes after current stressing of 8 Χ 10⁴A/cm² at 80℃ for (a) 0 hr (b) 72 hrs

e^- e^-

e^-

e^- e^-

(a) 0 hr (b) 72 hrs

(c) 144 hrs

Fig. 32 SEM images of the anode end of the solder stripes after current stressing of 9.1 Χ 10³A/cm² at 100℃ for (a) 0 hr (b) 72 hrs (c) 144 hrs

(a) 0 hr (b)72 hrs

Fig. 33 SEM images of s the anode end of the solder stripes after current stressing of 3Χ10⁴A/cm² at 100℃ for (a) 0 hr (b) 72 hrs

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e^-

e^- e^-

(a) 0 hr (b) 72 hrs

Fig. 34 SEM images of the anode end of the solder stripes after current stressing of 5.4Χ10⁴A/cm² at 100℃ for (a) 0 hr (b) 72 hrs

(a) 0 hr (b) 72 hrs

Fig. 35 SEM images of the anode end of the solder stripes after current stressing of 9.9Χ10³A/cm² at 120℃ for (a) 0 hr (b) 72 hrs

(a) 0 hr (b) 72 hrs

Fig. 36 SEM images of the anode end of the solder stripes after current stressing of 3Χ10⁴A/cm² at 120℃ for (a) 0 hr (b) 72 hrs

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