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Topic Title: Electomigration and Void Formation Session
Topic Summary: A brief high level overview of this session's topics and presentations
Created On: 3/9/2007 4:17 PM

 3/9/2007 4:17 PM

Jason Walleser

Posts: 3
Joined: 2/19/2007

Disclaimer: electromigration is not my specialty; this is just what I learned from the Electromigration and Void formation session.

As silicon technology becomes smaller and smaller, so does the interconnect density that allow these chips to interact with the outside world. Unfortunately, as this happens, the current density in solder joints, particularly flip-chip (chip to package), increases (10^4A/cm^2) to the point where the electric field drives alloy segregation and migration within the joint. In Sn-Pb joints, Pb accumulates at the anode (-) and Sn, the cathode (+). During this voids often form and in general the joint is weakened.

One particular phenomena, current crowding, was discussed or referenced at length and is related to flip chip joint geometry. At the cusps between the pad and solder ball the current is locally higher. More than one impressive, or rather, alarming, micrograph of a crack/void forming near these edges was shown. One way to alleviate this problem is to use a thick UBM which somehow decreases the crowding ratio.

Multiple presentations discussed the idea of a critical length for electromigration and methods for testing this. Often this was studied through the use of Blech structure samples [1]. Under current stressing near 2x10^4 A/cm^2, for Sn-Pb, the critical length is around 20-30 microns.

One particularly striking presentation showed how electromigration can cause tin grains to rotate when stressed with a current in pure tin solder. This rotation is caused by tin’s anisotropy. Resistivity isn’t the same in all directions and grains will tend to align their low resistivity directions with the direction of the current flux. This grain rotation can be seen on the surface of these joints by raised and sunken grains. If x = grain rotation, then tan x = C*t, where C is a constant and t is time. The tangent of grain rotation is directly proportional to time.

[1] Blech IA. Electromigration in thin aluminum films on titanium nitride. Journal of Applied Physics 2006;47:1203.
[2] Hsu YC, Chou CK, Liu PC, Chen C, Yao DJ, Chou T, Tu KN. Electromigration in Pb-free solder stripes. Journal of Applied Physics 2005;98:33523.
[3] Chen WT, Ho CE, Kao CR. Effect of Cu concentration on the interfacial reactions between Ni and Sn-Cu solders. J. Mater. Res 2002;17:264.


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