倒装芯片凸焊点的UBM

介绍了倒装芯片凸焊点的焊点下金属（UBM）系统,讨论了电镀Au凸焊点用UBM的溅射工艺和相应靶材、溅射气氛的选择,给出了凸焊点UBM质量的考核试验方法和相关指标。     

Effect of multiple reflows on interfacial reaction and shear strength of Sn-Ag electroplated solder bumps for flip chip package

The effect of displacement rate and intermetallic compound (IMC) growth on the shear strength of electroplated Sn-2.5Ag (in wt.%) flip chip solder with Cu under-bump metallization (UBM) were investigated after multiple reflows. Cu₆Sn₅ IMC was formed at the interface after one reflow. After five reflows, two different IMC layers, consisting of a scallop-shaped Cu₆Sn₅ phase and a planar Cu₃Sn phase, and their thicknesses increased with increasing reflow number up to 10. The shear strengths peaked after four reflows, and then decreased with increasing reflow number. Increasing displacement rate increased the shear force. The tendency toward brittle fracture characteristics was intensified with increasing displacement rate and reflow number.     

Failure mechanism of lead-free solder joints in flip chip packages

The failure mechanisms of SnAgCu solder on Al/Ni(V)/Cu thin-film, underbump metallurgy (UBM) were investigated after multiple reflows and high-temperature storage using a ball shear test, fracture-surface analysis, and cross-sectional microstructure examination. The results were also compared with those of eutectic SnPb solder. The Al/Ni (V)/Cu thin-film UBM was found to be robust enough to resist multiple reflows and thermal aging at conditions used for normal production purposes in both SnAgCu and eutectic SnPb systems. It was found that, in the SnAgCu system, the failure mode changed with the number of reflows, relating to the consumption of the thin-film UBM because of the severe interfacial reaction between the solder and the UBM layer. After high-temperature storage, the solder joints failed inside the solder ball in a ductile manner in both SnAgCu and SnPb systems. Very fine Ag₃Sn particles were formed during multiple reflows in the SnAgCu system. They were found to be able to strengthen the bulk solder. The dispersion-strengthening effect of Ag₃Sn was lost after a short period of thermal aging, caused by the rapid coarsening of these fine particles.     

Fabrication on uniform plating-based flip chip solder bumps after reflow process using a polishing mechanism

This paper presents an innovative polishing process aimed at leveling rough surface of plating-based flip chip solder bumps so as to get uniform coplanarity across the whole substrate after both electroplating and reflow processes. This polishing mechanism is characteristic of combining mechanical-dominated polishing force with slight chemical reaction together. A large number of extremely but inevitably rugged mushroom-like structures after electroplating are drastically smoothed down with the help of this newly-developed polishing process. Nearly 70 μm solder bumps in height with two different profiles as square and circle on the substrates reach as flatly as ±3 μm between different substrates after reflow process; ±2.5 μm in single substrate; and even ±1 μm in die, respectively. Besides, surface roughness among the solder bumps is simultaneously narrowed down from Ra 0.6 to Ra 0.03 along with the coplanarity improvement. Excellent uniformity and smooth surface roughness in solder bumps are absolutely beneficial to pile up and deposit in the following steps in MEMS and semiconductor fields.     

Anisotropic behavior of the capillary action in flip chip underfill

An underfill encapsulant can be used to improve the long-term reliability of flip chip interconnecting system by filling the gap between the chip and substrate around the solder bumps. The underfill encapsulant was filled by a capillary flow. This study was devoted to investigate the anisotropic effects of the capillary action induced by the solder bumps. A modified Hele-Shaw flow model, considering the flow resistance in both the thickness direction and the restrictions between solder bumps, was used. A capillary force model, depending on the direction of filling flow, for full array solder bumps was proposed. The capillary force was formulated based on quadrilateral arrangement of solder bumps. It was found that the capillary action is not the same for different directions. In the 45° direction, enhancement of the capillary flow was noticed for a bump pitch within a critical value. The edge preferential flow during the underfill experiment could be attributed to the anisotropic behavior of the capillary action.     

High density of electrodeposited Sn/Ag bumps for flip chip connection

As peripheral pads in commercial chips have a pitch in the neighbourhood of 40-50 μm, a technique that could deposit solder paste directly in such pitch would be of great interest to reduce the overall cost of flip chip.This paper describes a new technique that can considerably reduce the final pitch. The main new feature of this process is that the bump pads can be built directly onto the peripheral ones. An electroplating process allows solder bump formation with a final pitch goal of 40-50 μm and after an accurate reflow process, eutectic Sn-3.5 wt%Ag solder bumps are obtained. In fact, the typical re-routing process can be eliminated and the process cost considerably reduced.     

Characterization of flip chip bonded structure with Cu ABL power bumps

Power distribution in both 2D and 3D integrated circuit (IC) devices becomes one of the key challenges in device scaling, because the on-chip power dissipation becomes significantly severe and causes thermal reliability issues. In this study, the process solution to resolve the on-chip power dissipation by improving power distribution was investigated through newly designed power bumps called ABL (advanced bump layer) bumps. Rectangular-shaped Cu ABL bumps were fabricated and bonded on Si substrate using flip chip bonding process. The bump height difference in signal and ABL power bumps, bonding interface, and electrical resistivity of flip chip bonded structure were evaluated. The lowest electrical resistivity of Cu ABL bump system was estimated to be 3.3E−8 Ω m. The process feasibility of flip chip bonded structure with Cu ABL bumps has been demonstrated.     

Thermal and mechanical effects of voids within flip chip soldering in LED packages

This paper investigates the effect of void percentage in the solder layer on the shear strength and thermal property of DA3547 packages by SAC soldering technology. X-ray observation and shear tests revealed that the increase of solder paste volume significantly decreases the void percentage in the solder layer and thus improved the shear strength of the packages. Furthermore, packages with lower void percentage showed a lower junction temperature based on the results of IR test and finite element simulation. The temperature difference due to the effect void percentage shows a correlation with the input power. For the DA3547 packages studied in this research, voids show limited influence on the junction temperature under 50 mA, the typical current recommended by Cree.     

Investigation of the delamination mechanism of the thin film dielectric structure in flip chip packages

As the electronics industry continues its efforts in miniaturizing the integrated circuit (IC), an IC chip with copper/low-k stacked Back End of Line (BEoL) structures has been developed for reducing R-C delay in order to obtain high-speed signal communication. However, its reliability might become a concern owing to the considerably lower adhesive strength, as well as the greater coefficient of thermal expansion (CTE) of the low-k materials. In this paper, the global-local finite element method, specified boundary condition (SBC) method, is employed as a bridge to estimate the impact from package level to the deep submicron BEoL structure of the flip chip package. The results show that the defect in the stacking structure at the center of the silicon has a lower tendency to crack than that at the corner region. In addition, the higher underfill CTE shows the disadvantage of the defect.     

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.     

Studies of electroless nickel under bump metallurgy—Solder interfacial reactions and their effects on flip chip solder joint reliability

The electroless-deposited Ni-P under bump metallurgy (UBM) layer was fabricated on Al pads for Sn containing solder bumps. The amount of P in the electroless Ni film was optimized by controlling complexing agents and the pH of plating solution. The interfacial reaction at the electroless Ni UBM/solder interface was investigated in this study. The intermetallic compound (IMC) formed at the interface during solder reflowing was mainly Ni₃Sn₄, and a P-rich Ni layer was also formed as a by-product of Ni-Sn reaction between the Ni-Sn IMC and the electroless Ni layer. One to four microns of Ni₃Sn₄ IMC and a 1800–5000 Å of P-rich Ni layer were formed in less than 10 min of solder reflowing depending on solder materials and reflow temperatures. It was found that the P-rich Ni layer contains Ni, P, and a small amount of Sn (～7 at.%). Further cross-sectional transmission electron microscopy (TEM) analysis confirmed that the composition of the P-rich Ni layer was 75 at.% Ni, 20at.%P, and 5at.%Sn by energy-dispersive x-ray spectroscopy (EDS) and the phase transformation occurred in the P-rich Ni layer by observing grain size. Kirkendall voids were also found in the Ni₃Sn₄ IMC, just above the P-rich Ni layer after extensive solder reflow. The Kirkendall voids are considered a primary cause of the brittle fracture; restriction of the growth of of the P-rich Ni layer by optimizing proper processing conditions is recommended. The growth kinetics of Ni-Sn IMC and P-rich Ni layer follows three steps: a rapid initial growth during the first 1 min of solder reflow, followed by a reduced growth step, and finally a diffusion-controlled growth. During the diffusion-controlled growth, there was a linear dependence between the layer thickness and time^1/2. Flip chip bump shear testing was performed to measure the effects of the IMC and the P-rich Ni layers on bump adhesion property. Most failures occurred in the solder and at the Ni₃Sn₄ IMC. The brittle characteristics of the Ni-Sn IMC and the Kirkendall voids at the electroless Ni UBM-Sn containing solder system cause brittle bump failure, which results in a decreased bump adhesion strength.     

Interfacial reaction and joint reliability of fine-pitch flip-chip solder bump using stencil printing method

This study was focused on the formation and reliability evaluation of solder joints with different diameters and pitches for flip chip applications. We investigated the interfacial reaction and shear strength between two different solders (Sn-37Pb and Sn-3.0Ag-0.5Cu, in wt.%) and ENIG (Electroless Nickel Immersion Gold) UBM (Under Bump Metallurgy) during multiple reflow. Firstly, we formed the flip chip solder bumps on the Ti/Cu/ENIG metallized Si wafer using a stencil printing method. After reflow, the average solder bump diameters were about 130, 160 and 190 μm, respectively. After multiple reflows, Ni₃Sn₄ intermetallic compound (IMC) layer formed at the Sn-37Pb solder/ENIG UBM interface. On the other hand, in the case of Sn-3.0Ag-0.5Cu solder, (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs were formed at the interface. The shear force of the Pb-free Sn-3.0Ag-0.5Cu flip chip solder bump was higher than that of the conventional Sn-37Pb flip chip solder bump.     

Reliability of nickel flip chip bumps with a tin-silver encapsulation on a copper/tin-silver substrate during the bonding process

This study focused on the feasibility of using Ni flip chip bumps with a Sn-2.5Ag (wt.%) solder encapsulation. The interfacial reaction and die shear property of the Ni flip chip bump with a Sn-Ag solder cap bonded on the electroplated Cu/Sn-Ag substrate were investigated with increasing bonding time. After bonding for 1 s (Cu_xNi_1−x)₆Sn₅ and Cu₆Sn₅ intermetallic compound (IMC) layers were formed at the upper and lower interfaces, respectively, with the former IMC being the predominant phase during bonding. The transformation of the solder cap into the (Cu_xNi_1−x)₆Sn₅ IMC depleted the solder after bonding for 30 s, and then the Ni concentration in the IMC gradually decreased with increasing bonding time. The shear property peaked after 30 s, and then decreased with increasing bonding time. The fractures occurred at the solder/Cu₆Sn₅ interface and inside the (Cu_x Ni_1−x)₆Sn₅ IMC after bonding for 1 s and 30 s, respectively, after which the fracture location shifted toward the (Ni_xCu_1−x)₃Sn₄/(Cu_x Ni_1−x)₆Sn₅ interface with increasing bonding time.     

Morphological stability of solder reaction products in flip chip technology

We have studied two kinds of solder reactions between eutectic SnPb and Cu. The first is wetting reaction above the melting point of the solder, and the second is solid state aging below the melting point of the solder. In wetting reaction, the intermetallic compound (IMC) formation has a scallop-type morphology. There are channels between the scallops. In solid state aging, the IMC formation has a layer-type morphology. There are no channels but grain boundaries between the IMC grains. Why scallops are stable in wetting reactions has been an unanswered question of fundamental interest. We have confirmed that the scallop-type morphology is stable in wetting reaction by re-wetting the layer-type IMC by molten eutectic SnPb solder. In less than 1 min, a layer-type Cu₆Sn₅ is transformed back to scallops by the molten solder at 200 C. In analyzing these reactions, we conclude that the scallop-type morphology is thermodynamically stable in wetting reaction, but the layer-type morphology is thermodynamically stable in solid state aging, due to minimization of interfacial and grain boundary energies.     

Evaluation of solder joint reliability in flip-chip packages during accelerated testing

The microstructural investigation and thermomechanical reliability evaluation of the Sn-3.0Ag-0.5Cu solder bumped flip-chip package were carried out during the thermal shock test of the package. In the initial reaction, the reaction product between the solder and Cu mini bump of chip side was Cu₆Sn₅ intermetallic compound (IMC) layer, while the two phases which were (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ were formed between the solder and electroless Ni-P layer of the package side. The cracks occurred at the corner solder joints after the thermal shocks of 400 cycles. The primary failure mechanism of the solder joints in this type of package was confirmed to be thermally-activated solder fatigue failure. The premature brittle interfacial failure sometimes occurred in the package side, but nearly all of the failed packages showed the occurrence of the typical fatigue cracks. The finite-element analyses were conducted to interpret the failure mechanisms of the packages, and revealed that the cracks were induced by the accumulation of the plastic work and viscoplastic shear strains.     

Electromigration studies of flip chip Sn95/Sb5 solder bumps on Cr/Cr-Cu/Cu under-bump metallization

The electromigration-induced failure of Sn95/Sb5 flip chip solder bumps was investigated. The failure of the joints was found at the cathode/chip side after current stressing with a density of 1×10⁴ A/cm² at 150°C for 13 sec. The growth of intermetallic compounds (IMCs) was observed at the anode side after current stressing. Voids were found near the current crowding area in the cathode/chip side, and the (Cu,Ni)₆Sn₅ IMC at the cathode/chip end was transformed into the Sn phase. The failure mechanism for Sn95/Sb5 flip chip solder joint is proposed in this paper.     

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.     

Interfacial reactions and compound formation in the edge of PbSn flip-chip solder bumps on Ni/Cu under-bump metallization

Flip-chip technology with the layout of ball grid array has been widely used in today’s microelectronics industry. The elemental distribution in the edge of the solder bump is crucial for its correlation with the bump strength. In this study, Ni/Cu under-bump metallization (UBM) was used to evaluate the intermetallic compound (IMC) formation in the edge of the solder bump between the UBM and eutectic Sn-Pb solder in the 63Sn-37Pb/Ni/Cu/Ti/Si₃N₄/Si multilayer structure. During reflows, layered-type (Ni_1−xCu_x)₃Sn₄ and island-like (Cu_1−yNi_y)₆Sn₅ IMCs formed in the interface between the solder and UMB, while only the (Cu_1−yNi_y)₆Sn₅ IMC was observed in the sideway of the Ni/Cu UBM. After high-temperature storage (HTS) at 150°C for 1,000 h, both (Cu_1−yNi_y)₆Sn₅ and (Cu_1−zNi_z)₃Sn were found in the sideway of the Ni/Cu UBM. Two other IMCs, (Ni_1−xCu_x)₃Sn₄ and (Cu_1−yNi_y)₆Sn₅, formed in the interface between the solder and UBM. The growth of the (Cu_1−yNi_y)₆Sn₅ IMC was relatively fast during HTS.     

elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). The (Cu_1−y,Ni_y)₆Sn₅ IMC gradually grew with the rate constant of 4.63 × 10⁻⁸ cm/sec^0.5 before 500 h aging had passed. After 500 h aging, the (Cu_1−y,Ni_y)₆Sn₅ IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu₆Sn₅ IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu_1−y,Ni_y)₆Sn₅ IMC was down to 1.74 × 10⁻⁸ cm/sec^0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.     

Influence of Cu content on compound formation near the chip side for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump during aging

Sn-Ag-Cu solder is a promising candidate to replace conventional Sn-Pb solder. Interfacial reactions for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder joints were investigated after aging at 150°C. The under bump metallization (UBM) for the Sn-3.0Ag-(0.5 or 1.5)Cu solders on the chip side was an Al/Ni(V)/Cu thin film, while the bond pad for the Sn-3.0Ag-0.5Cu solder on the plastic substrate side was Cu/electroless Ni/immersion Au. In the Sn-3.0Ag-0.5Cu joint, the Cu layer at the chip side dissolved completely into the solder, and the Ni(V) layer dissolved and reacted with the solder to form a (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). For the Sn-3.0Ag-1.5Cu joint, only a portion of the Cu layer dissolved, and the remaining Cu layer reacted with solder to form Cu₆Sn₅ IMC. The Ni in Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. At the plastic substrate side, three interfacial products, (Cu_1−y,Ni_y)₆Sn₅, (Ni_1−x,Cu_x)₃Sn₄, and a P-rich layer, were observed between the solder and the EN layer in both Sn-Ag-Cu joints. The interfacial reaction near the chip side could be related to the Cu concentration in the solder joint. In addition, evolution of the diffusion path near the chip side in Sn-Ag-Cu joints during aging is also discussed herein.