| Summary: | 博士 === 國立交通大學 === 材料科學與工程系所 === 98 === 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. With the increase of applied current, the temperature increased rapidly due to Joule heating. Therefore, temperature measurement in the solder joint becomes a important issue. Because the solder joints are completely surrounded by an IC chip, underfill and a substrate. Thus it is difficult to measure the temperature distribution around the solder joints. This dissertation focuses on the measurement of the temperature distribution in the solder bump at various stressing conditions by thermal infrared microscopy. It can also help us to explore the thermal characteristics in the solder joint, such as temperature increment, temperature gradient and hot spot temperature. We used eutectic SnAg3.5 solder joints with typical dimensions to measure the thermal characteristics in solder bump. Two clear hot spots are observed in the bump. One is located at the region with peak current density, and the other one is at the bump edge under the current-feeding metallization on the chip side. Under a current stress of
1.06 �e 104 A/cm2, the temperature in the two hot spots are 161.7 �aC and 167.8 �aC, respectively, which surpass the average bump temperature of 150.5 �aC.
In addition, effect of under-bump-metallization (UBM) thickness on the hot spots is also examined. It indicates that the hot-spot temperature in the solder bump increases for the solder joints with a thinner UBM.
During electromigration test, Joule heating effect in the solder bump plays an important role. For this reason, the Joule heating effect at various stages of electromigration of flip-chip Sn3.5Ag solder joints was investigated under a current of 0.5 Amp at 100 °C. During various stages of electromigration, voids may form and propagate. Thus Joule heating effect may vary at different void sizes. To verify the void nucleation and propagation on Joule heating effect during electromigration process, the solder bump was stressed for different lengths of time and then examined by Kelvin bump probes and infrared microscopy. We found that voids started to form at approximately 1.2 times of the initial bump resistance. Then the voids propagated when the bump resistance increased. In addition, the temperature of the solder joints is also increased with the increase of bump resistance. In the last stage, the temperature of the solder bump increased rapidly due to the dramatic increase in the bump resistance and local Joule heating effect.
Joule heating in the silicon chip generates a thermal gradient in a flip chip solder joint. Since Al traces serve as the major source of heat during accelerated electromigration tests, high current stressing also produces a non-uniform temperature distribution, creating a large thermal gradient in a flip chip solder joint.
Therefore, we used alternate current (AC) to the joint to decouple the thermomigration from electromigration effect, since there is no electromigration effect under the AC stressing. Yet the AC produces the same amount of Joule heating as the direct current dose. Eutectic SnPb and lead-free solders have been adopted by the microelectronics industry. To measure the thermomigration rate directly, markers fabricated by focus ion beam are employed. The thermomigration flux of Pb is measured to be 3.3×1013 atoms/cm2, when the solder bump was stressed by 0.55 Amp at 100 �aC. With the known thermal gradient, the molar heat of transport of Pb can be obtained as 26.8 kJ/mole.
About the thermomigration in lead free solder joint, it is found that Sn atoms migrated toward the hot end. The thermomigration flux and molar heat of transport are measured to be 5.0×1012 atoms/cm2 and 1.36 kJ/mole, respectively, when the solder bump are stressed by 0.57Amp at 100 �aC.
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