The effect of underbump metallization (UBM) on electromigration (EM) lifetime and failure mechanism has been investigated for Pb-free solder bumps of 97Sn3Ag composition in the temperature range of 110–155°C. The EM lifetime of the SnAg Pb-free solders with either Cu or Ni UBM was found to be better than the eutectic SnPb (63Sn37Pb) solders but worse than high-Pb (95Pb5Sn) solders. In the test temperature range, the EM lifetimes were found to be comparable for Cu and Ni UBMs but with different activation energies: 0.64–0.72eV for Cu UBM and 1.03–1.11eV for Ni UBM. EM failure was observed only in solder bumps with electron current flow from UBM to the substrate. Failure analysis revealed that EM damage was initiated by the formation of intermetallic compounds (IMC) at the UBM∕solder interface which was found to be significantly enhanced by mass transport driven by the electron current. Under EM, the continued growth of IMC with the dissolution of the UBM and the accumulation of Kirkendall voids resulted in the formation of interfacial cracks and eventual EM failure of the solder bump. For Ni UBM, the IMC formation was dominated by the Ni3Sn4 phase while for Cu UBM, a bilayer of Cu3Sn∕Cu6Sn5 was found. Void formation at the Cu6Sn5∕solder interface was found to be important in controlling the EM lifetime of the Cu UBM solder.
MechanicaI reliability is widely recognized as the primary obstacle to productization of porous low-k materials. The combination of weak bulk and interfacial properties with increasingly complex geometries poses a considerable challenge at the 65-nm node. The final solution must be sufficiently robust so as to ensure compatibility with multiple substrate types, interconnect configurations and packages. In this work, material engineering, modeling, design rule tailoring, and assembly optimization are employed to achieve required assembly reliability for both wirebond and flip-chip packages, for both bulk and SO1 substrates.
By analyzing the composition dependence of the total amount of Ag photodissolved into amorphous GexS1-x and the fragility of GexS1-x, it is found that at the composition where the amount of Ag photodissolved exhibits a maximum, the fragility shows a minimum, that is, there is an inverse correlation between these two quantities.The possible origin of the correlation found has been discussed by using the constraint theory and the model of fragility proposed by one of the authors.
Electromigration (EM) tests were performed on Pb-free solder joints having different thicknesses of Ni UBM, to examine the effect of UBM thickness on EM reliability. The UBM thickness dependency of EM lifetime was explained in terms of the current crowding effect with the help of finite element analysis (FEA). Based on the experimental results as well as FEA, the maximum current density at the UBM/solder interface was found to be a critical factor controlling EM reliability. Further analyses were conducted by FEA to evaluate the dependency of current density distribution on a contact trace structure.The results showed that an appropriate selection of a contact trace structure was as important as the UBM thickness. The effects of solder joint scaling on current crowding were also investigated. It was found that the maximum current density did not increase as much as the average current density when solder joints were scaled down.
IntroductionAs device feature size scaling continues with increasing demands for high I/O density, the solder size and pad pitch in flip-chip packages are also subject to scaling. As a result, current density in solder joints will increase to about 10 4 A/cm 2 when the solder diameter decreases below 100 µm in the near future [1]. Added to the problem of increased current density, the implementation of Pb-free solders raises serious electromigration (EM) reliability concerns within solders, especially because their inferior current carrying capability compared with Pb-based solders.Significant current crowding can occur in solder joints due to the geometry of under-bump metallurgy (UBM) and that of solder contact trace, which plays an important role in influencing EM failure [1][2][3][4][5]. Previous simulation studies have reported that a thicker UBM could relieve current crowding effectively [3,6]. However, a direct correlation between current crowding and experimentally determined EM lifetime, particularly for Pb-free solders with different UBM thickness, has not been reported. Moreover, there exists little information on the effects of the contact trace structure and scaling on current crowding.In this study, we investigated these issues on Pb-free solder EM reliability. First, EM experiments were performed on Sn2.5Ag solder joints with Ni UBM of 1, 2, and 3 µm in thickness. 3-D finite element analysis (FEA) was employed to determine the current density distribution in solder joints, and the maximum current density was correlated to the EM lifetime. Second, the effect of a solder contact trace structure
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