Direct measurement of hot-spot temperature in flip-chip solder joints with Cu columns under current stressing using infr
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1249-F08-02
Direct measurement of hot-spot temperature in flip-chip solder joints with Cu columns under current stressing using infrared microscopy Yu-Chun Liang1, Chih Chen1* and D. J. Yao2 1
Department of Material Science & Engineering, National Chiao Tung University, Hsin-chu 30010, Taiwan, ROC 2 Institute of Microelectromechanical System, National Tsing Hua University, Hsinchu 30010, Taiwan, ROC
Abstract In this study, the temperature map distribution in the Sn3.0Ag0.5Cu solder bump with Cu column under current stressing is directly examined using infrared microscopy. It is the radiance changes between the different materials of the surface that cause the unreasonable temperature map distribution. By coating a thin layer of black optical paint which is in order to eliminate the radiance changes, we got the corrected temperature map distribution. Under a current stress of 1.15 × 10 4 A/cm2 at 100 ℃, the hot-spot temperature is 132.2℃ which surpasses the average Cu column temperature of 129.7 ℃ and the average solder bump temperature of 127.4 ℃. Thermomigration in solder may still occur under a large current stressing. Introduction Flip chip technology has been adopted for high-density packaging due to its excellent electrical performance and better heat dissipation ability.1 Furthermore, to meet the miniaturization trend of portable devices, the dimensions of the solder bumps continue to shrink, causing the current density in each solder joint to increase abruptly. Therefore, electromigration in the solder bumps has become an important issue.2 Several investigations on electromigration in flip-chip solder joints have been carried out.3-10 Current crowding effect occurs seriously near the entrance point of the Al trace into the solder joint, which is responsible for the failure in the chip/anode side of the solder joint. 4-5 In addition, Joule heating effect also takes place during accelerated electromigration tests.6-10 The temperature increase due to current stressing may be over 30C when a 1.0 A current is applied to the solder bump. 5-7 Besides, several simulation studies point out that there exists a hot spot in the flip-chip solder bump near the entrance of the Al trace accelerating electromigration.7-9 The Cu column is a thick under-bump-metallization (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions and the hot-spot temperature in the solder bump decreases for the solder joints with a thicker UBM.11 The temperature may affect the mean-time-to-failure (MTTF) significantly, as depicted by Black’s equation12:
MTTF A
1 Q exp ( ) n kT j
(1)
where A is a constant, j is the current density in amperes per square centimeter, n is a model parameter for current density, Q is the activation energy, k is Boltzmann’s constant, and T is the average bump temperature in degrees Kelvin. Yet, there are no experimental data to verify that whether the hot-spot
temperature is also decreased in the solder bumps with Cu columns. This is because the solder joi
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