Seung-Hyun Chae

Electromigration enhanced intermetallic growth and void formation in Pb-free solder joints

A kinetic analysis was formulated for electromigration enhanced intermetallic evolution of a Cu–Sn diffusion couple in the Sn-based Pb-free solder joints with Cu under bump metallurgy. The simulated diffusion couple comprised the two terminal phases, Cu and Sn, as well as the two intermetallic phases, Cu3Sn and Cu6Sn5, formed between them. The diffusion and electromigration parameters were obtained by solving the inverse problem of the electromigration enhanced intermetallic growth, and they were compatible with the literature values. Finite difference method was applied to the whole simulated domain to solve for the mass transport kinetics within the intermetallic phases and across each interface of interest. Simulation showed that, when electromigration effect was absent (zero current), intermetallic growth followed a parabolic law, suggesting a diffusion controlled mechanism for thermal aging. However, under significant current stressing (4×104A∕cm2), the growth of the dominant intermetallic Cu6Sn5 cle...

Recent advances on kinetic analysis of electromigration enhanced intermetallic growth and damage formation in Pb-free solder joints

A comprehensive kinetic analysis was established to investigate the electromigration (EM) enhanced intermetallic compound (IMC) growth and void formation for Sn-based Pb-free solder joints with Cu under bump metallization (UBM). The kinetic model takes into account Cu–Sn interdiffusion and current stressing. Derivation of the diffusion coefficients and the effective charge numbers for the intermetallic compounds is an essential but challenging task for the study of this multi-phase multi-component intermetallic system. A new approach was developed to simultaneously derive atomic diffusivities and effective charge numbers based on simulated annealing (SA) in conjunction with the kinetic model. A consistent set of parameters were obtained, which provided important insight into the diffusion behaviors driving the IMC growth. The parameters were used in a finite difference model to numerically solve the IMC growth problem and the result accurately correlated with the experiment. EM reliability test revealed that the ultimate failure of the solder joints was caused by extensive void formation and subsequent crack propagation at the intermetallic interface. This damage formation mechanism was analyzed by first considering vacancy transport under current stressing. This was followed by a finite element analysis on the crack driving force induced by void formation. This paper is concluded with a future perspective on applying the kinetic analysis and damage mechanism developed to investigate the structural reliability of the through-Si-via in 3D interconnects.

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

Effect of Dicing Technique on the Fracture Strength of Si Dies With Emphasis on Multimodal Failure Distribution

The ball-on-ring (BOR) and three-point bending (3PB) tests were used in this paper to characterize the effect of the dicing process on the fracture strength of Si dies. Dies prepared by blade- and laser-dicing processes were studied. The edge-initiated fracture was distinguished from the surface-initiated fracture by fractographic analysis. The fracture-strength distributions related to surface flaws in the 3PB test, as well as the BOR test, were consistent regardless of the dicing process. For the edge-defect-induced failure mode, on the other hand, blade-sawn dies showed wider spread distribution than laser-sawn dies. It was due to the scattered nature in size and location of the edge flaws induced by blade dicing. Laser-sawn dies showed a tighter distribution of die strength, although the average die strength was slightly lower than that of blade-sawn dies. This paper has successfully demonstrated that the die failure caused by edge defects can be deconvoluted from the 3PB test data by using fractographic observation.

Investigation of Intermetallic Compound Growth Enhanced by Electromigration in Pb-Free Solder Joints

An efficient numerical method based on simulated annealing was developed to extract the diffusion and electromigration (EM) parameters for multi-phase intermetallic compounds (IMCs) formed between under-bump-metallurgy (UBM) and solder bumps. This method was applied to the growth of Cu-Sn IMCs during thermal aging and electromigration in Pb-free solder joints with Cu UBM. Diffusion coefficients and effective charge numbers of Cu and Sn in IMCs were derived in this study. The simulated annealing approach provided superior efficiency and accuracy over the conventional grid heuristics, and was proven to be particularly useful for analyzing multiple parameters in multiphase systems, such as solder joints. The effect of EM-enhanced IMC growth on stress fields in solder joints was also studied using finite element analysis. The negative volumetric strain appeared to be an important factor contributing to the degradation of EM reliability of solder joints.

Verification of ball-on-ring test using finite element analysis

The ball-on-ring (BOR) test is an effective technique used to characterize the biaxial fracture strength of brittle materials. In particular, damages induced by wafer backgrinding process can be evaluated using the BOR test. It is difficult to measure directly the radius of contact area between the loading ball and a specimen, which is needed for stress determination by an analytic solution. In this study, parametric finite element analyses were performed to compare with known closed-form solutions. It was found that the effect of small loading area must not be ignored and that the radius of contact could be precisely determined using Hertzs contact theory. This can serve as a guideline to accurately obtain the fracture strength of a BOR specimen.

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.

Current Carrying Capability of Sn0.7Cu Solder Bumps in Flip Chip Modules for High Power Applications

The primary objective of this work was to characterize the current carrying capability of Sn0.7Cu solder bumps for use in high power flip chip module applications. The factors to be considered in using modules as test vehicles for current carrying capability studies are explored. Experimental data on the current carrying capability of Sn0.7Cu solder bumps in a module test vehicle is complemented with thermal simulations to understand the phenomena occurring the first level solder interconnects. The associated electromigration characteristics of the Sn0.7Cu on plated Cu was also evaluated. Current densities of 6.0x103, 9.42x103, 1.06x104, and 1.18x104 A/cm2 were investigated at 115C, 125C and 135C. No failures were observed for up to 2550 hours with the lowest current level of 6.0x103 A/cm2 at 135C. However, a variety of failure modes were seen at the other current levels. In this work a thermocouple was used to approximate the temperature of the solder bump. While this method has produced excellent results in obtaining the electromigration of solder bumps in single die applications, failure analysis and thermal simulation indicated that this is not the case with flip chip modules. A Computational Fluid Dynamics (CFD) tool was employed and thermal modeling confirmed that the bump temperature was significantly higher than the temperature measured on the surface of the mold compound. Temperature gradients played a significant role in determining the electromigration characteristics and extra considerations are needed to examine the multiple effects resulting from it.

Kinetic Analysis of Current Enhanced Intermetallic Growth and Its Effect on Electromigration Reliability of Solder Joints

A kinetic analysis was formulated for electromigration induced intermetallics evolution of a Cu-Sn diffusion couple. Simulation of the time dependency of intermetallic growth is consistent with experimental data showing growth and accumulation of a high concentration of vacancies and formation of Kirkendall voids at the Cu₆Sn₅ side of the Cu₃Sn/Cu₆Sn₅ interface

Effect of backside scratch direction on the Si die strength

The biaxial fracture strength of back-ground Si dies was measured using the ball-on-ring (BOR) test. A quarter of a wafer was divided into groups according to the angle of back-grinding scratches from the <110> direction in order to investigate the effect of this scratch angle on the die strength. As the scratch angle became larger, higher die strength was obtained. For example, when the scratch angle was larger than 34° the die strength was ∼60–70% higher than that when the angle was smaller than 11°. This is because the preferred fracture surface of Si is parallel to the <110> direction. The possible die strength improvement by an appropriate modification of the conventional back-grinding process was also compared against the additional backside finishing process such as polishing and etching.