| Nowadays, electronic products continue to develop towards miniaturization and higher performance. In compliant with this trend, the size of solder joint is decreasing and the density of current passing through the solder joint is increasing. As a result, electromigration (EM) becomes one of the most important reliability issues in flip chip solder joints. On the other hand, while the solder balls that connect the chips and substrates reduce in size, the interfacial reactions between the substrates and solders also show the sensitivity to solder volumes.In the present work, a multi-physics finite element (FE) simulation that combines electric, thermal and stress analysis was introduced to describe the EM process of flip chips. Furthermore, the interfacial reactions between Sn2.OAgO.8Cu solder balls with different size and Cu pads were also studied by experiment and simulation. The main results are as follows:1Simulation of electromigration:(a) The simulation results showed that there existed a rather high current density at where electrons entered (or exit) the solder balls from (to) UBM and the maximum value was triple the nominal current density. The maximum temperature located within the Cu interconnects was also showed and the Cu trace was the dominating source of joule heat. However, the temperature distribution of the solder balls was nearly uniform with a temperature difference of only0.14℃. This illustrated that no large temperature gradient presented in the solder balls and thus the thermal migration could be neglected.(b) The simulation results of thermal stress indicated that the substrate and the chip exhibited significant warpage behavior and the edge of the substrate deformed most. The warpage was primarily driven by the thermal expansion mismatch between silicon die and organic substrate. It’s obvious that the maximum hydrostatic stress always appeared in the outmost solder joint of the flip chip, and for the solder bumps, the localized stress concentration occurred at the marginal position.(c) The inlet for electrons from the Cu trace to the solder bump was the region where current crowding, as well as the concurrent areas of joule heating and stress concentration. Voids, cracks and other defects tended to initiate at this regions during EM process. The simulation results were consistent with the experiment phenomenon.2. Effect of solder size (volume) on interfacial reaction:(a) The experimental results showed that Cu6Sn5intermetallic compounds (IMC) precipitated at the interface after reflow. With increasing reflow times, the average thickness of interfacial IMC layer, the average diameter of IMC grains and the consumption of Cu pad all increased. The reactions between solders and Cu pads were related to the size (volume) of the solder balls. Specifically, compared with the larger solder balls, the smaller solder balls corresponded to thicker IMC layers and larger average grain sizes.(b) The concentration distribution of Cu in liquid solder during interfacial reaction was calculated by finite element software. The simulation results showed that the concentration of Cu in the200μm solder ball could quickly reached saturation, while there was still Cu dissolution for400μm solder ball that experienced an equal reaction time. The diffusion of Cu atoms into the solder ball limited the amount of Cu reacting with Sn at the interface. As a result, the growth of IMC grain would be restrained in larger solder balls. The simulation results gave an reasonable explanation of the ’size (volume) effect’on interfacial reaction and were in good accordance with the experimental results. |
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