Kirkendall’s Voids

Kirkendall’s Voids

Material Name: Solder
Record No.: 173
Primary Chemical Element in Material: Cu, Sn
Sample Type: Wafer
Uses: No data
Etchant Name: None
Etching Method: No data
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: Cross section TEM (XTEM) on the UBM pad without solder showed the original Cu coating thickness is about 500 nm. In the as-received sample (which has already been reflowed once), Cu layer has reacted to form Cu6Sn5 and Cu3Sn compounds. The Cu3Sn phase is small grains aggregated at the Ni(V) interface and mostly within large Cu6Sn5 grains. Kirkendall voids are accompanied and wrapped by Cu3Sn phase, as shown in Fig 1. As reported previously, the concave nature of the Cu6Sn5/Cu3Sn interface strongly suggested the reaction is going toward the Cu6Sn5 phase. Samples after reflow up to 10X did not show any major change in microstructure. Both Cu6Sn5 and Cu3Sn remained after 10 reflow cycles.
Reference: Chih-Hang Tung, et al., Microstructure Studies of Under Bump Metallization Systems Using Transmission Electron Microscopy, ISTFA 2002, Proceedings of the 28th International Symposium for Testing and Failure Analysis, 3-7 November 2002, Phoenix Civic Center, Phoenix, Arizona, pp. 505-511.

Figure 1: Cross section TEM on the as reflow Ni(V) UBM system showing Cu6Sn5, Cu3Sn intermetallics, and Kirkendall’s voids.