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Dissolution of Sn in a SnPb solder bump under current stressing
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Image of FIG. 1.
FIG. 1.

(Color online) Schematic illustration of 95Pb5Sn/63Sn37Pb composite solder joint.

Image of FIG. 2.
FIG. 2.

The SEM images of the upper left corner of a solder bump show the dissolution of Sn-rich phases into the Pb matrix. Thinning, dissociation, partition, and dissolution of fibrous Sn-rich phase occur sequentially during the process of current stressing at 4.2 × 104 A/cm2.

Image of FIG. 3.
FIG. 3.

A local area nearby UBM of a 95Pb5Sn/63Sn37Pb solder joint showing micrographs before and after aging of 68 h at 88 °C. (a) The average concentration, 3.24 ± 0.54 wt. %, was analyzed by EDX and an elliptic Sn grain had an area of 3.83 μm2. (b) Aging after 68 h shows isotropic dissolution of Sn-rich phase. (c) Zoom-in micrograph showing the rectangular in Fig. 3(b).

Image of FIG. 4.
FIG. 4.

(Color online) A plot of area reduction of Sn-rich phase vs time at current densities. The wt. % values in bracket are the thermal equilibrium solubility of Sn in Pb at corresponding temperatures.

Image of FIG. 5.
FIG. 5.

In situ SEM observation of the solder joint under current stressing at 3.3 × 104 A/cm2. (a) The microstructure before current stressing. (b) Magnified images of rectangular region in (a). (c) Sn and Pb phase separation in the arrowed Sn grain at 60 h under current stressing. (d) Pb filled the arrowed grain at 70 h.

Image of FIG. 6.
FIG. 6.

The sketch shows the dissolution mechanism of Sn-rich phase in the Pb matrix during current stressing. The counter flow of Sn and Pb during current stressing results in complete replacement by Pb of the original Sn-rich grain.

Image of FIG. 7.
FIG. 7.

In situ SEM observation of the solder joint under current stressing at 5.0 × 104 A/cm2. (a) Morphology of solder joint before current stressing. (b) Rapid dissolution of Sn-rich phases leading clear surface of Pb matrix and voids formation at 5 min under current stressing. (c) A wrinkled Pb surface without Sn fine precipitates and dissolution of Cu pad resulted in Cu6Sn5 formation at 1 h under current stressing. (d) The high magnification of high Pb region near UBM showing in situ EDX analysis within 1 h under current stressing. The Sn content ranged between 5.1 ∼ 13.3 wt. % with an average concentration 9.22 ± 3.37 wt. %. This indicates thermomigration dominates supersaturation of Sn content with higher current density.

Image of FIG. 8.
FIG. 8.

(Color online) The average Sn concentration vs current stressing time with five current densities for the 95Pb5Sn solder. Aging at 88 °C shows the slope (dissolution rate) of 0.017 wt. %/h.

Image of FIG. 9.
FIG. 9.

(Color online) The temperature vs the average Sn concentration with five current densities in the Sn-Pb binary phase diagram. The symbol • represents the current stressing-induced Sn concentration and temperature, while the mark ♦ depicts the equilibrium theoretical temperature and concentration. ΔC describes the difference of Sn concentration between current stressing–induced and equilibrium concentration.

Image of FIG. 10.
FIG. 10.

(Color online) Exponent of dissolution rate (ln r) as a function of inverse of current density (1/I). [1/(4.2 × 104) = 2.38 × 10−5 cm2/A].


Generic image for table
Table I.

The measured and evaluated temperatures with five current densities under SEM after 2 hours of current stressing. The diffusivity of Sn in Pb at different temperatures in Ref. 23.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Dissolution of Sn in a SnPb solder bump under current stressing