Hsiao-Yun Chen

Failure induced by thermomigration of interstitial Cu in Pb-free flip chip solder joints

Thermomigration in Pb-free SnAg solder joints is investigated during accelerated electromigration tests under 9.7 × 103 A/cm2 at 150 °C. Different from Pb-containing solders, it is found that Cu-Sn intermetallic compounds (IMCs) migrate toward the cold end on the substrate end and thus voids accumulate in the passivation opening for the bump with current flowing from the substrate end to the chip end. Theoretical calculation indicates that the thermomigration force is greater than the electromigration force at a thermal gradient above 400 °C/cm. Copper atoms may migrate against current flow and become the dominant diffusion species. Also, the Q* can be estimated around 11 kJ/mole. On the other hand, a control experiment shows that Ni-Sn IMCs did not migrate even under a huge thermal gradient over 1400 °C/cm.

Generic rules to achieve bump electromigration immortality for 3D IC integration

An immortal solder micro-bump (μbump) electromigration (EM) lifetime has been demonstrated for 3D IC integration. This ultimate goal was achieved under strictly controlled conditions, including the optimal design of bump metallurgy, geometry, optimized processes, along with well-defined stressing conditions and manufacturing window. The current carrying capability and EM lifetime of μbump have been investigated as functions of stressing conditions which are correlated with the degradation mechanisms. When stressed under the appropriate g conditions, all μbump test samples survived prolonged stressing, some over 13,000 hours, without a failure. Cross-sectional analyses revealed that the entire solder joint almost all transformed into intermetallic compounds (IMCs) with very minor or no voids. The resistance plots showed an initial fast rise in resistance due to IMC formation, then gradually leveled off and eventually reached a steady state. The observed degradation mechanism is dominated by IMC formation, which is the same as that of the user conditions. On the other hand, void formation that eventually led to open failure was the dominant degradation mechanism when samples were aggressively stressed. In other words, when all other conditions were the same, the stressing conditions make a huge difference in determining between an almost immortal EM lifetime vs. a short lifetime using the same high quality μbumps. The boundary that separates the stressing conditions is roughly defined and will be discussed. In addition, since full IMC μbump will become inevitable in the future miniaturized solder interconnect structure, the EM behavior of IMC dominated μbump has also been evaluated in this study. Under highly accelerated stressing conditions of 174°C, at 1.6×105 A/cm2 current density, the IMC dominated μbumps were able to survive more than 600 hours and are still going strong. In comparison, solder μbump failed quickly after just 107 hours when stressed under the same condition. This comparison study clearly demonstrated that IMC dominated joint has significantly higher current carrying capability than that of the solder joint. After reviewing all the data, we have concluded that the failure criteria for solder μbump should be raised significantly higher than the 20% criteria traditionally used for the much larger C4 bumps.

Electromigration immortality of purely intermetallic micro -bump for 3D integration

The progress of three-dimensional integrated circuit (3D IC) micro-bump joining technology has led to an increased volume fraction of intermetallics (IMC) in the post reflow joints, to an extent that a solder micro-bump may consist almost entirely of IMCs. Therefore, the current carrying capability and electromigration (EM) life time of the purely IMC micro-joint needs to be understood as functions of stressing conditions and degradation mechanisms. Superior EM performance and robustness of IMC joints is demonstrated with no resistance fluctuation under ultra-high stressing condition for over 9000 hrs while solder micro-bumps led to an open failure within 500 hrs. At least an order of magnitude greater current carrying capability of IMC micro-joint compared with solder micro-joint is observed experimentally. The observed degradation mechanism is void formation within Al trace rather than damage inside IMC joint. IMC joint is not the EM reliability bottle neck of the test circuit.

Measurement of electromigration activation energy in eutectic SnPb and SnAg flip-chip solder joints with Cu and Ni under-bump metallization

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.