AN2035 - Application note

5/11

rate is virtually uniform across the Tin/Copper interface the resulting Cu6Sn5 structure has

practically no “wedges”.

The difference in the Cu6Sn5 structure is clearly demonstrated in the micrographs in

Figure 2. In these micrographs the Tin was selectively removed by chemical etching, thus

In the micrographs on the left hand side of Figure 2, the intermetallic was allowed to form at

room temperature over a period of one month. The result is a large, blocky and irregular

Note:

In this image the large flat spaces between the blocky intermetallic are exposed copper.

formed by baking the part within 24 hours of plating at a temperature of 150°C. In this case

Note:

after etching.

Figure 2.

Microscope View of Protection by Post-bake Treatment (1 hour at 150°C)

The ability of a 150°C bake to virtually eliminate the intermetallic “wedges” that lead to large

compressive stress at the grain boundaries serves as the basis for the second component of

the whisker mitigation strategy adopted by STMicroelectronics.

The second component of the strategy to mitigate whisker growth is to apply a 1-hour 150°C

bake within 24hrs to freshly plated Matte-Tin finishes. This has several beneficial effects: the

film is annealed and stress due to atomic-level plating defects is reduced. In addition, and

localized penetration of the intermetallic at grain boundaries, thus avoiding the “wedge”

effect that creates compressive stress. Furthermore, the bake creates an additional

Large and Irregular IMC

(1 month at Room Temperature)

Thin and Uniform IMC

(1 hour at 150°C)