Trent S. Uehling

Effect of contact metallization on electromigration reliability of Pb-free solder joints

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...

Electromigration statistics and damage evolution for Pb-free solder joints with Cu and Ni UBM in plastic flip-chip packages

A series of electromigration (EM) tests were performed as a function of temperature and current density to investigate lifetime statistics and damage evolution for Pb-free solder joints with Cu and Ni under-bump-metallizations (UBMs). The EM lifetime was found to depend on the failure criterion used, so the results were compared based on the first resistance jump and conventional open-failure criterion. Solder joints with Cu UBM had a longer lifetime than Ni UBM based on the open-failure criterion, but the lifetime with Ni UBM became comparable when the first resistance jump criterion was applied. To determine the temperature in solder joints, the Joule heating effect was investigated with experiments and finite element analysis. The temperature of solder joints was determined to be approximately 15°C higher than that at the Si die surface when 1 A of current was applied. With the appropriate temperature correction, the activation energies and the current density exponents were found to be Q = 1.11 eV, n = 3.75 and Q = 0.86 eV, n = 2.1 based on the open-failure criterion for solder joints with Cu and Ni UBM, respectively. Based on the first resistance jump criterion, Q = 1.05 eV, n = 1.45 for Cu UBM and Q = 0.94 eV, n = 2.2 for Ni UBM, respectively. For solder joints with Cu UBM, voids were formed initially at the Cu6Sn5/solder interface while the final open failure occurred at the Cu3Sn/Cu6Sn5 interface. For Ni UBM, voids were formed initially at the Ni3Sn4/solder interface leading to failure at the same interface. The formation of intermetallic compounds (IMCs) was enhanced under current stressing, which followed linear growth kinetics with time. The IMC growth was accompanied by volume shrinkage, which accelerated damage evolution under EM.

Electromigration lifetime statistics for Pb-free solder joints with Cu and Ni UBM in plastic flip-chip packages

A series of electromigration tests were performed as a function of temperature and current density to investigate lifetime statistics for Pb-free solder with Cu or Ni under-bump-metallization (UBM). Based on the overall shape of resistance traces, a conservative failure criterion for time-to-failure was defined and the results were compared with those based on the conventional open-failure criterion. Solder joints with Cu UBM had a longer lifetime than with Ni UBM, based on the open-failure criterion; however, the lifetime with Ni UBM became comparable when the conservative criterion was applied. The Joule heating effect was accounted for based on experiments and finite element analysis. The temperature of solder joints was determined to be approximately 15degC higher than that at the Si die surface when 1 A of current was passed. For solder with Cu UBM, voids formed initially at the Cu6Sn 5/solder interface while the final open failure occurred at the Cu3Sn/Cu6Sn5 interface. For Ni UBM, voids formed initially at the Ni3Sn4/solder interface leading to failure at the Ni3Sn4/solder interface

A study of electromigration failure in Pb-free solder joints

Electromigration (EM) lifetime and failure mechanisms have been investigated for SnAg Pb-free solder bumps with two types of under bump metallurgy (UBM). The activation energy was determined to be 0.64/spl sim/0.72 eV for Pb-free solders with Cu UBM and 1.03/spl sim/1.11 eV with Ni UBM. EM failure was observed only in solder bumps with electron current flow from UBM to the substrate. SEM and EDX analysis revealed a failure mechanism for solder bumps with Ni UBM caused by the dissolution of UBM as a result of Ni migration and subsequent solder cracking or de-wetting. The failure mechanism of Cu UBM samples showed temperature dependence. At higher temperatures, Cu UBM dissolved continuously while at lower temperatures, open failure was caused by crack formation at the Cu/sub 3/Sn/Cu/sub 6/Sn/sub 5/ interface with little damage to the UBM. This is attributed to a difference of Cu diffusivity in Cu/sub 3/Sn and Cu/sub 6/Sn/sub 5/. The EM lifetime of Pb-free solder was found to be much better than eutectic solders but worse than high-Pb solders at the same temperature.

Simulation and Reliability Study of Cu/Low‐k Devices in Flip‐chip Packages

The package impact to the mechanical integrity of the low dielectric constant (low‐k) dielectrics back end of the line (BEOL) structure has been proven to be significant in recent publications. This work reports a simulation study of the package‐induced delamination in low‐k structures by interfacial fracture mechanics combined with multi‐scale finite element method. The numerical simulation is validated by reliability test results of low‐k devices in different flip‐chip package configurations. The modeling result is compared to reliability test data of low‐k devices in organic, ceramic flip‐chip packages, and good correlation is found. Feasibility of flip‐chip packaging for low‐k devices is demonstrated. The risk of low‐k delamination on different package configurations is rated based on both reliability data and numerical simulations.

Effects of UBM thickness, contact trace structure and solder joint scaling on electromigration reliability of Pb-free solder joints

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.

Experimental Measurement of the Thermal Performance of a Two-Die 3D Integrated Circuit (3D IC)

Accurate measurement of the thermal performance of vertically-stacked three-dimensional integrated circuits (3D ICs) is critical for optimal design and performance. Experimental measurements also help validate thermal models for predicting the temperature field in a 3D IC. This paper presents results from thermal measurements on a two-die 3D IC. The experimental setup and procedure is described. Transient and steady-state measurements are made while heating the top die or the bottom die. Results indicate that passage of electrical current through the heaters in top/bottom die induces a measureable temperature rise. There appears to be a unique asymmetry in thermal performance between the top die and the bottom die. The top die is found to heat up faster and more than the bottom die. Results presented in this paper are expected to play a key role in validation of simulation-based and analytical thermal models for 3D ICs, and lead to a better fundamental understanding of heat transport in stacked systems. This is expected to lead to effective thermal design and characterization tools for 3D ICs.Copyright