Ni Under Bump Metallurgy Research Articles

Effect of Tin Grain Orientation on Electromigration-Induced Dissolution of Ni Metallization in SnAg Solder Joints.

In this study, symmetrical solder joints (Cu/Ni/SnAg2.3/Ni/Cu) were fabricated. They were electromigration (EM)-stressed at high (8 104 A/cm2) or low (1.6 104 A/cm2) current densities. Failures in the solder joints with different grain orientations under EM stressing were then characterized. Results show that Ni under-bump-metallurgy (UBM) was quickly dissolved into the solder joints possessing low angles between Sn c-axis and electron direction and massive NiCuSn intermetallic compounds formed in the Sn matrix. The diffusion rate of Ni increased with decreasing orientation grain angle. A theoretical model was also established to analyze the consumption rate of Ni UBM. Good agreement between the modeling and experimental results was obtained. Additionally, we found that voids were more likely to form in the solder joints under high EM stressing.

High-strength and thermal stable Cu-to-Cu joint fabricated with transient molten Ga and Ni under-bump-metallurgy

Copper (Cu)-to-Cu bonding is an essential process for most advanced electronic devices, such as Cu-pillar joining in flip-chip packaging, through-silicon-via interconnections in three-dimensional integrated circuits, and die attachments in wide band-gap electronics. Here we propose an approach for fabricating face-centered cubic solid-solution joints without formation of intermetallic compounds by using a trace amount of gallium (Ga) and nickel (Ni) under-bump-metallurgy (UBM) for the reactive diffusion bonding at 300 °C. The formation of solid-solution joints involves interactions with the NiCuGa mixture at the bonding interface. A shear-strength of 43.5 MPa was achieved in the as-bonded joint due to the formation of grains across the bonding interface, while it does not degrade after prolonged post-annealing at 300 °C for 200 h. Instead, an even stronger joint with a shear-strength of 48.2 MPa was obtained. We demonstrate the fabrication of strong, thermally stable Cu-to-Cu joints using transient molten Ga and Ni UBM.

Sputtered Ni–Zn under bump metallurgy (UBM) for Sn–Ag–Cu solders

We developed a new sputtered under bump metallurgy (UBM) based on Ni-20wt% Zn thin films for Pb-free solders. This study focuses on the interfacial reactions between two Pb-free solders (Sn3.0Ag0.5Cu and Sn1.0Ag0.7Cu) and a Ni–Zn alloy UBM. By adding Zn to Ni UBM, Zn is incorporated into intermetallic compounds (IMCs) to form a quaternary Cu–Ni–Zn–Sn phase at the solder/Ni–Zn interface after reflow and subsequent isothermal aging. The Ni–Zn UBM sufficiently reduces the interfacial reaction and IMCs formation rates as well as UBM consumption rates compared to a Ni UBM. In particular, the formation of (Ni, Cu)3Sn4 IMC was significantly retarded by adding Zn into UBM.

Intermetallic compound spalling characteristics of Sn-3.5Ag solder over ternary electroless Ni under-bump metallurgy

Abstract

Effect of UBM Thickness on the Mean Time to Failure of Flip-Chip Solder Joints under Electromigration

Flip-chip solder joints with Cu/Ni/Al underbump metallurgy (UBM) on the chip and an Au/Ni surface finish on the substrate were studied under current stressing at an ambient temperature of 150°C. Three different Ni thicknesses in the Cu/Ni/Al UBM (0.3, 0.5, and 0.8 μm) were used in order to investigate the effect of the Ni thickness on reliability. The solder used was eutectic Pb-Sn, and the applied current density was 5 × 103 A/cm2. The results show that the combined effect of current crowding and the local Joule heating near the entry points of electrons into the joints induced asymmetric Ni UBM consumption. Once the Ni was exhausted in a certain region, this region became nonconductive and the flow of electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the Ni UBM there was consumed at an accelerated rate. This process repeated itself, and the Ni-depleted region continued to extend, creating an ever larger nonconductive region. The solder joints eventually failed when the nonconductive region extended across the entire contact window of the joints. This failure model supports the observation that joints with a thicker Ni tend to have a longer average lifetime.

Effect of surface finish on the failure mechanisms of flip-chip solder joints under electromigration

Two substrate surface finishes, Au/Ni and organic solderable preservative (OSP), were used to study the effect of the surface finish on the reliability of flip-chip solder joints under electromigration at 150°C ambient temperature. The solder used was eutectic PbSn, and the applied current density was 5×103 A/cm2 at the contact window of the chip. The under bump metallurgy (UBM) on the chip was sputtered Cu/Ni. It was found that the mean-time-to-failure (MTTF) of the OSP joints was six times better than that of the Au/Ni joints (3080 h vs. 500 h). Microstructure examinations uncovered that the combined effect of current crowding and the accompanying local Joule heating accelerated the local Ni UBM consumption near the point of electron entrance. Once Ni was depleted at a certain region, this region became nonconductive, and the flow of the electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the local Ni UBM consumption was accelerated. This process repeated itself, and the Ni-depleted region extended further on, creating an ever-larger nonconductive region. The solder joint eventually, failed when the nonconductive region became too large, making the effective current density very high. Accordingly, the key factor determining the MTTF was the Ni consumption rate. The joints with the OSP surface finish had a longer MTTF because Cu released from the substrate was able to reduce the Ni consumption rate.

Electromigration-induced UBM consumption and the resulting failure mechanisms in flip-chip solder joints

Eutectic PbSn flip chip solder joint was subjected to 5×103 A/cm2 current stressing at 150°C and 3.5 × 104 A/cm2 current stressing at 30°C. The under bump metallurgy (UBM) on the chip was sputtered Ni/Cu, and the substrate side was a thick Cu trace. It was shown through in-situ observation that the local temperature near the entrance of electrons from the Al interconnect to the solder became higher than the rest of the joint. The accelerated local Ni UBM consumption near the entrance was also observed. Once the Ni was consumed at a location, a porous structure formed, and the flow of the electrons was blocked there. It was found that the formation of the void and the formation of the porous structure were competing with each other. If the porous structure formed first, then the void would not be able to nucleate there. On the other hand, if the void could nucleate before the UBM above lost its conductivity, then the joint would fail by the void formation-and-propagation mechanism.

Intermetallic compound formation and diffusion path evolution in eutectic tin‐lead flip chip solder bumps after aging

PurposeIn the flip‐chip technology (FCT) used in current microelectronic packages, a Ni‐based under‐bump metallurgy (UBM) is widely used due to its slow reaction rate with Sn. In this study, solders joints of eutectic Pb‐Sn with a Ni UBM were employed to investigate the intermetallic compound (IMC) formation after aging at 150°C for various periods of time.Design/methodology/approachThe compositions and elemental re‐distribution in the IMC formed due to the interfacial reaction between the Ni/Cu UBM and eutectic Sn‐Pb solders were evaluated with an electron probe microanalyzer. The interfacial morphologies were revealed with the aid of a field‐emission scanning electron microscope through a special etching technique.FindingsAt the centre of the chip side, two IMCs were found between the solder and Ni metallization. The scalloped‐like IMC was determined to be (Cu, Ni)6Sn5, while the nodule‐like IMC was (Ni,Cu)3Sn4. However, at the edge of the chip side, three IMCs were revealed. The scalloped‐like IMC was (Cu1−y,Niy)6Sn5, the nodule‐like IMC was (Ni1−x,Cux)3Sn4, and the layer‐type IMC was (Cu1−z,Niz)3Sn.Originality/valueOn the basis of the elemental distributions from the quantitative analysis of the IMC and the related phase transitions during the IMC formation, two distinct diffusion paths are proposed to illustrate the interfacial reaction and phase transformation between IMCs and solder in Sn‐Pb joints aged at 150°C. These diffusion paths demonstrated two kinds of phase equilibrium, including (Cu1−z,Niz)3Sn/(Cu1−y,Niy)6Sn5/solder and (Ni1−x,Cux)3Sn4/(Cu1−yNiy)6Sn5/solder.

A reliability comparison of electroplated and stencil printed flip-chip solder bumps based on UBM related intermetallic compound growth properties

The effects of under bump metallurgy (UBM) microstructures on the intermetallic compound (IMC) growth of electroplated and stencil printed eutectic Sn-Pb solder bumps were investigated. The process parameters and their effects on UBM surface morphology and UBM shear strength were studied. For the electroplating process, the plating current density was the dominant factor to control the Cu UBM microstructure. For the stencil printing process, the zincation process has the most significant effect on the Ni UBM surface roughness and Ni grain sizes. In both processes, the good adhesion of UBM to aluminum can be obtained under suitable UBM processing conditions. Samples with different UBM microstructures were prepared using the two processes. The resulting samples were thermal aged at 85/spl deg/C, 120/spl deg/C, and 150/spl deg/C. It was observed that the Cu UBM surface roughness had larger effect on the IMC growth and solder ball shear strength than the Ni UBM surface roughness. The thickness of Cu/sub 3/Sn and Cu/sub 6/Sn/sub 5/ IMC depended strongly on the UBM microstructure. However, for Ni/Au UBM, no significant dependence was observed. More likely, the thickness of Au-Ni-Sn IMC near the IMC/solder interface was controlled by the amount of gold and the gold diffusion rate in the solder. Shear tests were performed after thermal aging tests and thermal/humidity tests. Different failure modes of different sample groups were analyzed. Electroless Ni UBM has been developed because it is a mask-less, low-cost process compared to electroplated Cu UBM. This study demonstrated that the process control was much easier for Ni UBM due to its lower reactivity with Sn material. These properties made Ni UBM a promising candidate for the lead-free solder applications.

Comparison of electroplated eutectic Bi/Sn and Pb/Sn solder bumps on various UBM systems

The effect of a reflow process and under bump metallurgy (UBM) systems on the growth of intermetallic compounds (IMC) of the 57Bi/43Sn and 37Pb/63Sn solder bump/UBM interfaces was investigated. The selected UBM systems were sputtered Al/Ti/Cu, sputtered Al/NiV/Cu, Al/electroless Ni/immersion Au, and Al/Ti/electroless Cu. An alloy electroplating method was used for the solder bumping process. The microstructure and composition of intermetallic compound (IMC) phases and their morphologies were examined using scanning electron microscopy and X-ray diffraction. The Cu/sub 6/Sn/sub 5/ /spl eta/'-phase IMC appeared on all Cu containing UBM cases with Pb/Sn and Bi/Sn solders and the Cu/sub 3/Sn /spl epsiv/-phase was detected only with Pb/Sn solder bumps. The Ni/sub 3/Sn/sub 4/ IMC was found to be the main IMC phase between Ni and solder. The Ni/sub 3/Sn secondary IMC was also detected on the electroless Ni UBM with PbSn solder after ten times reflow. Through the bump shear test, Al/NiV/Cu, Al/elNi/Au, and Al/Ti/elCu UBMs showed good stability with Bi/Sn and Pb/Sn solder in terms of metallurgical aspects.

Methodology for studying the impact of intrinsic stress on the reliability of the electroless Ni UBM structure

A methodology for evaluating the reliability and the impact of intrinsic stresses on the electroless Ni under bump metallurgy (UBM) structure is presented. The first part of this work will address the testing methodology, which uses a pressure sensing device to determine the intrinsic stress in Ni due to the plating process. An optical method is used to capture deformation in the sensing device due to the Ni plating process. A finite element model is then used to calculate the intrinsic stress in the Ni film using the deformation output from the optical measurements. The second part of this work will address a predictive model used to determine the reliability of applying intrinsic stress values to a low cost electroless Ni UBM structure during the bump formation and solder reflow process. The combined work of the testing and predictive methodology provides a more effective and accurate method of predicting the Ni UBM reliability.