Measurement of electromigration activation energy in eutectic SnPb and SnAg flip-chip solder joints with Cu and Ni under
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Electromigration activation energy is measured by a built-in sensor that detects the real temperature during current stressing. Activation energy can be accurately determined by calibrating the temperature using the temperature coefficient of resistivity of an Al trace. The activation energies for eutectic SnAg and SnPb solder bumps are measured on Cu under-bump metallization (UBM) as 1.06 and 0.87 eV, respectively. The activation energy mainly depends on the formation of Cu–Sn intermetallic compounds. On the other hand, the activation energy for eutectic SnAg solder bumps with Cu–Ni UBM is measured as 0.84 eV, which is mainly related to void formation in the solder. I. INTRODUCTION
To meet the demand for high-performance microelectronic devices, miniaturization processes continue to scale down solder-bump pitch and diameter.1 This has resulted in a dramatic increase in the current density in flip-chip solder bumps. Recently, the electromigration (EM) of flipchip solder joints has been realized as a serious reliability issue1,2 because it can lead to failure due to interfacial void formation and large intermetallic compound (IMC) formation inside solder joints.3–7 Furthermore, because of the combination of the serious current crowding and the Joule heating effect, which cause a nonuniform temperature distribution inside solder joints,8 it is difficult to predict the failure time of flip-chip solder joints. Choi et al.9 found that the measured mean time to failure (MTTF) was much smaller at higher current density than the values calculated from Black’s equation. They proposed that Black’s equation should be modified due to the serious current crowding and the Joule heating effect during current stressing. According to Choi et al., the MTTF equation can be represented as 1 Q MTTF ¼ A ; ð1Þ exp ðcjÞn kðT þ DTÞ where MTTF is the current stressing time until failure, A represents a constant that contains a factor involving the cross-sectional area of the joints, j is the current density in amperes per centimeter squared, n is a model parameter for current density, Q is the activation energy, k is Boltzmann’s constant, T is the average temperature a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0230 J. Mater. Res., Vol. 25, No. 9, Sep 2010
of the bump, and D T is the temperature increase in the solder joint due to the Joule heating effect. The measurement of real temperature is critical in determining the activation energy, since the temperature and activation energy are both located in the exponential term. Several studies have measured activation energy without calibrating the real stressing temperature,10–12 while others have tried to measure bump temperature by placing a thermocouple or temperature crayon on the surface of a Si die,10 although these measurements may deviate from the bump temperature. On the other hand, Gee et al. calibrated bump temperature using a special Al trace design to measure the nearest stressing solder bump temperature. Using the te
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