A simple method for testing the electromigration resistance of solders

Abstract

Recently, the size of solder bump interconnects have been significantly reduced with the advent of highdensity packaging, and thus the evaluation of electromigration in solder bumps has become necessary. The present paper proposes a simple method to test the electromigration resistance of Pb-free solders. One of the key points of the present method is the fabrication of a simple solder sample that can produce sufficient current density to cause electromigration. Moreover, the actual local temperature of a small area subjected to electromigration in the sample was measured by a direct method. A right-angled diamondshaped hole was introduced in a thin film of solder using a focused-ion-beam system. A direct current was supplied to the film far from the hole, perpendicular to the diagonal of the hole. In this way, the current density was concentrated near to the corner of the hole, and the value of this was obtained through a theoretical analysis. It was noted that the steady temperature in the film along a line extending from the diagonal, remained constant, although the current density decreased gradually far from the corner. Therefore, the temperature at a position near the corner, where electromigration takes place, can easily be found by measuring the temperature far from the corner. The temperature was measured directly under current flow conditions by utilizing the chemical reagents with known melting points. Finally, by measuring the ratio of hillock volume to the time for the current supply as a measure of the atomic flux divergence due to electromigration, the corresponding resistances of some Pb-free solders to electromigration were evaluated.

A technique utilizing chemical reagents for direct measurement of temperature at a local area and its engineering applications

“…[22] that the steady temperature in the film along a line extending from the diagonal, [17,19] remains constant and is the same as that near the corner, although the current density decreases gradually far from the corner. Therefore, the temperature at a position near the corner, where electromigration takes place, can easily be found by measuring the temperature away from the corner.…”

“…Our research group has proposed a simple method for testing the electromigration resistance of solders which necessitates application of the present method for measuring temperature [22].…”

“…[22], in order to produce sufficient current density to cause electromigration, a right-angled diamond-shaped hole is introduced in a thin film of solder and the actual local temperature of a small area, subjected to electromigration in the sample, is measured by the present method. Additionally, the atomic flux divergence due to electromigration is predicted by measuring the ratio of hillock volume to the time for the current supply, which is then used to evaluate the corresponding resistances of some Pb-free solders to electromigration.…”

“…Fig. 8 A strip subjected to a current of density, J ∞ , supplied from the input and output current pads, where a right-angled diamond-shaped hole has been introduced at the center of the strip [22] Regarding the thermal characteristics of a sample which is symmetric about the diagonal BB, the steady temperature T at any point in the sample can be expressed as follows [22,23]:…”

“…8, which is subjected to a current of density J ∞ , supplied from the input and output current pads [22]. A right-angled diamondshaped hole is located at the center of the strip, where the length of the diagonal of the hole is denoted by 2a.…”