Electromigration in Line-Type Cu/Sn-Bi/Cu Solder Joints

In this study, the different electromigration (EM) behaviors of eutectic Sn-Bi solder in the solid and molten states were clarified using line-type Cu/Sn-Bi/Cu solder joints. When the eutectic Sn-Bi solder was in the solid state during the EM test, a Bi-rich layer formed at the anode side while a Sn-rich band formed at the cathode side, and the intermetallic compound (IMC) at the cathode side was thicker than that at the anode side. The growth of the Bi-rich layer exhibited a linear dependence on the time of stressing. While the actual temperature of the solder joint increased to 140°C and the solder was in a molten state or partially molten state, two separate Bi-rich layers formed at the anode side and a great many Cu₆Sn₅ IMC precipitates formed between the two Bi-rich layers. Also, the IMC layer at the cathode side was thinner than that at the anode side. With a current-crowding-reduced structure, the products of diffusivity and effective charge number of Bi in the eutectic Cu/Sn-Bi/Cu solder joints stressed with current density of 5 × 10³ A/cm² at 35°C, 55°C, and 75°C were calculated.     

Secondary Current Crowding Effect during Electromigration of Flip-Chip Solder Joints

In flip-chip interconnects under current stressing, the primary current crowding effect occurs at the entrance edge of the contact interface with the highest current density. In this study, an increased current density also occurred at the other edge of the contact interface, followed by a selective dissolution of under bump metallization. After primary current crowding, the rest of electrons flow to the metallization edge, followed by an abrupt change in direction toward the anode. Primary current crowding is attributed to the electrical field change whereas the secondary crowding effect is due to the physical blocking of the electron flow. Because this effect is not as great as that of primary current crowding, it must be assisted by thermal diffusion.     

Effect of Migration and Condensation of Pre-existing Voids on Increase in Bump Resistance of Flip Chips on Flexible Substrates during Electromigration

In Pb-free solder joints formed by reflowing a bump of solder paste, voids are formed within the solder due to the residue of flux in the reflow process. These voids migrate toward the cathode contact during electromigration under current stressing. Accompanying the electromigration, resistance jumps of a few 100 mΩ were observed. It was postulated that a jump occurs when a void touches the cathode contact. This study investigated the effect of the void migration and condensation on the change in bump resistance using three-dimensional (3D) simulations and finite element analysis. It was found that there was negligible change in bump resistance during void migration towards the high-current-density region before touching the cathode contact opening. When a small void condensed on the contact opening and depleted 18.4% of the area, the bump resistance increased only 0.4 mΩ. Even when a large void depleted 81.6% of the opening, the increase in bump resistance was 3.3 mΩ. These values are approximately two orders of magnitude smaller than those reported in the literature for the change in resistance due to void migration in flip chips on flexible substrates. We conclude that the reported change in resistance was most likely that of the Al or Cu interconnection in the flip-chip samples.     

Sn-Ag-Cu无铅钎料互连焊点的电迁移研究进展

电迁移问题作为影响焊点可靠性的关键问题之一,容易导致焊点出现裂纹、丘凸和空洞等焊接缺陷.其失效机制有电流拥挤效应、焦耳热效应、极化效应和金属间化合物失效等.聚焦Sn-Ag-Cu系无铅钎料焊点的电迁移问题,介绍了这一领域电迁移的失效机制、影响因素和防止措施的研究现状,并展望了今后的研究发展趋势.     

焦耳热作用下共晶锡铋焊点电迁移特性

电迁移可以引发芯片内部互连金属引线(单一元素)中的原子或离子沿电子运动方向移动.但是,在共晶锡铋焊点中,组成的元素为锡和铋而非单一元素.由于铋原子和锡原子在高电流密度下具有不同的迁移速率,因此共晶锡铋钎料具有独特的电迁移特性.实验中采用的电流密度为104A/cm2,同时焦耳热会引发焊点温度从25升高至49℃,富铋相在此温度下会发生明显粗化,除此之外,铋原子会首先到达正极界面处并形成坚硬的阻挡层,使得锡原子的定向运动受到阻碍,最终,富锡相会,凸起,其与负极界面问会有凹谷形成.     

焦耳热作用下共晶锡铋焊点电迁移特性

电迁移可以引发芯片内部互连金属引线（单一元素）中的原子或离子沿电子运动方向移动. 但是,在共晶锡铋焊点中,组成的元素为锡和铋而非单一元素. 由于铋原子和锡原子在高电流密度下具有不同的迁移速率,因此共晶锡铋钎料具有独特的电迁移特性. 实验中采用的电流密度为1E4A/cm2,同时焦耳热会引发焊点温度从25升高至49℃,富铋相在此温度下会发生明显粗化,除此之外,铋原子会首先到达正极界面处并形成坚硬的阻挡层,使得锡原子的定向运动受到阻碍,最终,富锡相会凸起,其与负极界面间会有凹谷形成.     

Local Melting during Electromigration in Cu Conductor Lines

We have observed abrupt, reversible resistance changes during electromigration (EM) testing of damascene Cu conductor lines. The tests were conducted at temperatures of 300°C or 350°C and with current densities from 0.6 × 10⁶ A/cm² to 1.6 × 10⁶ A/cm². In most cases, an incubation period with negligible resistance increase was followed by a period of continuous, gradual resistance increase, attributed to formation and growth of voids in the conductor line. If the direction of current was reversed, the resistances of the Cu lines decreased, due to the refilling of voids by the back flowing atoms. With further EM, the resistance curves showed spikelike features, with a sudden resistance increase followed by a resistance decrease, often to values close to those before the start of EM. In other cases, no resistance decrease occurred, and the line failed. We present resistance data, microstructural observations, and thermal calculations that suggest that the resistance decrease results from sudden, local Joule heating and melting of conductor line segments, and from voids being partially filled by the back-flowing liquid Cu, which then solidifies. In some cases, line failure results from liquid Cu erupting through the top surface passivation layer, rather than flowing back to the voids.     

焊料凸点回流时的桥接现象研究

在用回流焊料凸点时,常会发生凸点的桥接现象,致使芯片报废。此时,相邻的多个凸点彼此融合,聚集成一个更大的焊料球,并吸干先前各凸点中的焊料。本文研究了电镀PbSn凸点和蒸发铟凸点的回流过程中出现的桥接现象。介绍了桥接现象产生的过程及其背景,分析了桥接现象的机理,提出了改进措施。     

Temperature-Dependent Phase Segregation in Cu/42Sn-58Bi/Cu Reaction Couples under High Current Density

The effects of current stressing at 10⁴ A/cm² on Cu/42Sn-58Bi/Cu reaction couples with a one-dimensional structure at 23°C, 50°C, and 114°C were investigated. The microstructural evolution during electromigration was examined using scanning electron microscopy. The temperature dependence of the coarsening of the Bi-rich phase, the dominant migrating entity, and hillock/whisker formation in eutectic Sn-Bi were investigated under high current density. During current stressing at 10⁴ A/cm², the average size of the Bi-rich phase remained the same at 23°C, increased at 50°C, and shrank at 140°C. Bi accumulated near the anode side at both high (50°C, 140°C) and low temperature (23°C). At high temperatures, both Sn and Bi diffused towards the anode side, but Bi moved ahead of Sn during current stressing. However, at low temperatures, Sn reversed its direction of migration to the cathode side. Pure Bi hillocks/whiskers and a mixed structure of Sn and Bi hillocks were extruded as a consequence of compressive stress from electromigration- induced mass flow towards the anode side.     

Joule Heating Enhanced Phase Coarsening in Sn37Pb and Sn3.5Ag0.5Cu Solder Joints during Current Stressing

This paper investigated the effect of Joule heating on the phase coarsening in Sn37Pb and Sn3.5Ag0.5Cu ball grid array (BGA) solder joints stressed at −5°C and 125°C with a 6.0 × 10² A/cm² electric current. The phase growth under current stressing was also compared with those under aging at 125°C. It was found that the current stressing produced a substantial Joule heating in the solder joints and conductive traces. Hence, the solder joints underwent a considerable temperature rise by 30–35°C when stressed at −5°C and 125°C in this study. Coarsening of Pb-rich and Ag-rich phases was confirmed to be accelerated by the current stressing as a result of enhanced diffusion at elevated temperature and atomic stimulation due to numerous collisions between electrons and atoms. Different controlling kinetics were suggested for the cases stressed or aged at different temperatures.     

Nonuniform and Negative Marker Displacements Induced by Current Crowding During Electromigration in Flip-Chip Sn-0.7Cu Solder Joints

A quantitative analysis of the nonuniform distribution of current density and nonuniform rate of electromigration has been carried out by measuring the movement of an array of diffusion markers. Tiny marker arrays were fabricated by focused ion beam on the polished surface of flip-chip solder joints near the anode to measure the electromigration rate. The marker velocity at the current-crowding region was found to be at least five times larger than at locations far from the region. Some of the markers in the low-current-density region possess negative velocities, indicating that backflow occurs during the electromigration. The backflow, in which the atomic flow is against the electron flow, is explained by a constant-volume model as well as the back-stress induced by electromigration.     

The Influence of an Imposed Current on the Creep of Sn-Ag-Cu Solder

The work reported here included preliminary tests on the influence of an imposed current on the creep rate of the Pb-free solder Sn-Ag-Cu 305 (Sn-3Ag-0.5Cu in wt.%). The samples employed were double-shear specimens that contained paired solder joints, 400 μm × 400 μm in cross-section, 200 μm in thickness on Cu. Three tests were done. In the first, samples were tested under stress at room temperature with imposed current densities that ranged from 1 × 10³ A/cm² up to 6.5 × 10³ A/cm². As expected, because of Joule heating, the results show a sharp increase in creep rate with the imposed current density. A second set of tests was done to determine whether Joule heating fixed the creep rate. The steady-state temperature of the solder joints was measured under current, and samples were creep-tested at that temperature. Surprisingly, the creep rate under current was significantly below that measured in isothermal tests at the same temperature. The third set of tests studied the influence of microstructure. Samples were prepared with three starting microstructures: as cast, thermally aged by long-term isothermal exposure, and current aged by long-term exposure to a fixed current density. The three microstructures were then tested with and without current at two ambient temperatures. The different microstructures had very different creep rates in the absence of current but, surprisingly, imposing a current (5.5 × 10³ A/cm²) increased the creep rate by very nearly the same factor (~7×) in every case. Neither of these results is well understood at this time.     

Effects of Electromigration on Interfacial Reactions in the Ni/Sn-Zn/Cu Solder Interconnect

Electromigration in the Ni/Sn-Zn/Cu solder interconnect was studied with an average current density of 3.51 × 10⁴ A/cm² for 168.5 h at 150°C. When the electrons flowed from the Ni side to the Cu side, uniform layers of Ni₅Zn₂₁ and Cu₅Zn₈ were formed at the Ni/Sn-Zn and Cu/Sn-Zn interfaces. However, upon reversing the current direction, where electron flow was from the Cu side to the Ni side, a thicker Cu₆Sn₅ phase replaced the Ni₅Zn₂₁ phase at the Ni/Sn-Zn interface, whereas at the Cu/Sn-Zn interface, a thicker β-CuZn phase replaced the Cu₅Zn₈ phase.     

陶瓷封装倒装焊器件Sn-Pb微焊点的电迁移行为

高密度陶瓷封装倒装焊器件的焊点尺寸已降低至100μm以下,焊点电流密度达到10~4 A/cm~2以上,由此引发的电迁移失效成为不可忽视的问题。以陶瓷封装菊花链倒装焊器件为研究对象,开展了Sn10Pb90、Sn63Pb37焊点热电环境可靠性评估试验,通过电连接检测及扫描电子显微镜(SEM)等方法对焊点互连情况进行分析。结果表明,Sn63Pb37焊点阴极侧金属间化合物(IMC)增长明显,表现出明显的极化现象,IMC厚度的平方与通电时间呈线性关系。通电时间达到576 h后Sn63Pb37焊点阴极侧产生微裂纹,而Sn10Pb90焊点在通电576 h后仍未出现异常,表现出优异的电迁移可靠性。研究结果对于直径100μm微焊点的陶瓷封装倒装焊器件的应用具有重要的意义。     

Electromigration Reliability and Morphologies of Cu Pillar Flip-Chip Solder Joints with Cu Substrate Pad Metallization

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated ∼62 μm Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm² at 150°C, 10 kA/cm² at 160°C, and 15 kA/cm² at 125°C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)₆Sn₅ or Cu₆Sn₅ intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the ~52-μm-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of ~80% Cu-Ni-Sn IMC and ~20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.     

分步电镀法制备Sn-Ag-In三元凸点及其可靠性

本文介绍了一种制备细节距、元素分布均匀的Sn-Ag-In三元凸点的方法。其特征在于在Cu凸点下金属层上分步电镀Sn-Ag和In,通过精确控制回流过程,获得了Sn1.8Ag9.4In凸点。研究发现位于Sn-Ag-In焊料和Cu之间的金属间化合物厚度随回流时间的延长而生长,这使得焊料基体中Ag相对浓度增加,因此在凝固过程中,更多的Ag2In相析出,起到了颗粒增强的作用。     

Effect of Electromigration on the Mechanical Performance of Sn-3.5Ag Solder Joints with Ni and Ni-P Metallizations

The effect of moderate electric current density (1 × 10³ to 3 × 10³ A/cm²) on the mechanical properties of Ni-P/Sn-3.5Ag/Ni-P and Ni/Sn-3.5Ag/Ni solder joints was investigated using a microtensile test. Thermal aging was carried out at 160°C for 100 h while the current was passed. The interfacial microstructure and intermetallic compound (IMC) growth were analyzed. It was found that, at these levels of current density, there were no observable voids or hillocks. Samples aged at 160°C without current stressing failed mostly inside the bulk solder with significant prior plastic deformation. The passage of current was found to cause brittle failure of the solder joints and this tendency for brittle failure increased with increasing current density. Fractographic analysis showed that, in most of the electrically stressed samples, fracture occurred at the interface region between the solder and the joining metals. The critical current density that caused brittle fracture was about 2 × 10³ A/cm². Once brittle fracture occurred, the tensile toughness, defined as the energy per unit fractured area, was usually lower than ~5 kJ/m², compared with the case of ductile fracture where this value was typically greater than ~9 kJ/m². When comparing the two types of joint, the brittle failure was found to be more severe with the Ni than with the Ni-P joint. This work also found that the passage of electric current affects the IMC growth rate more significantly in the Ni than in the Ni-P joint. In the case of the Ni joint, the Ni₃Sn₄ IMC at the anode side was appreciably thicker than that formed at the cathode side. However, in the case of electroless Ni-P metallization, this difference was much smaller.     

Electromigration in the Flip Chip Solder Joint of Sn-8Zn-3Bi on Copper Pads

Electromigration in Sn-8Zn-3Bi flip chip solder bumps on Cu pads has been studied at 120°C with an average current density of 4 × 10³ A/cm² and 4.5 × 10⁴ A/cm². Due to the polarity effect, the thickness of the intermetallic compound Cu-Zn (γ-phase) formed at the anode is much greater than that at the cathode. The solder joint fails after 117 h of stressing at 4.5 × 10⁴ A/cm², and void formation at the cathode can clearly be seen after polishing. However, it is the melting at the edge of the bump that causes the solder joint to fail. A simulation of the current density distribution indicates that the current density is not distributed uniformly, and current crowding occurs inside the bump. The results indicate that the increase of current density associated with Joule heating has affected melting and enhanced damage in the solder joint during electromigration.     

Experimental Wettability Study of Lead-Free Solder on Cu Substrates Using Varying Flux and Temperature

The selection of soldering flux plays a critical role in promoting wetting and product reliability of printed circuit board assemblies. In this study, the effects of fluxes on the wetting characteristics of the Sn-3.0Ag-0.5Cu solder alloy on Cu substrates was researched by using various flux systems at different soldering temperatures. Because of the distinct characteristic of the lead-free solder—poor wettability—three kinds of fluxes [no-clean flux with high solid content (NCF), rosin mildly activated flux (RMA) and water-soluble flux (WSF)] were chosen for the wetting experiments. The wetting time and force were the evaluating indicators. The experimental observations indicated that the wettability clearly depended on the soldering temperature and flux system when using the same solder. Furthermore, the corrosion potential of flux residues was measured by surface insulation resistance (SIR) testing. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to determine the contents of the flux residues and corrosion products.     

Interfacial Reaction Between Eutectic Sn-Pb Solder and Electroplated-Ni as well as Electroless-Ni Metallization During Reflow

Electroplated-Ni (EP-Ni) has been adopted gradually as an underbump metallization layer due to its comparatively lower resistivity and higher deposition rate. In this study, the metallurgical reaction between eutectic Sn-Pb solder and EP-Ni as well as electroless-Ni (EL-Ni) was investigated at 200°C, 210°C, 220°C, and 240°C. It is found that the growth rate of Ni₃Sn₄ intermetallic compound (IMC) on EP-Ni was slower than that on EL-Ni. The consumption rate is measured to be 0.97 × 10⁻³ μm/s and 1.48 × 10⁻³ μm/s for EP-Ni and EL-Ni, respectively. The activation energy is determined to be 51 kJ/mol and 48 kJ/mol for EP-Ni and EL-Ni, respectively. The dense structure of EP-Ni may be responsible for the lower IMC formation rate.