Electromigration-induced failure in flip-chip solder joints

The electromigration failure mechanism in flip-chip solder joints through the rapid dissolution of the Cu metallization was studied in detail. The ambient temperature was found to be a very important factor in this failure mechanism. When the ambient temperature was changed from 100°C to 70°C, the time to failure changed from 95 min to 31 days. The results of this study indicate that temperature, as an experimental variable, is not less important than the current density in electromigration study. The surface temperatures of the chip and substrate during electromigration were also measured. The temperature of the Si chip was reasonably homogeneous because of the fact that Si is a very good thermal conductor. It was also reasoned that the high thermal conductivity of the PbSn solder could not support a temperature gradient large enough to induce thermomigration across the solder joint in the present study. Experimentally, no evidence of mass transport caused by thermomigration was observed.     

Simulation study on thermo-fatigue failure behavior of solder joints in package-on-package structure

In this work, the simulating analysis of PoP structure under the temperature range from 0 °C to 125 °C is carried out using direct thermal-cycle analysis and Coffin-Manson method. The results show that the maximum accumulating inelastic hysteresis energy appears on the solder ball in the bottom fine-pitch ball grid array (FBGA) structure. The thermal-fatigue crack initiates in the two symmetrical corners of solder ball in FBGA structure. The thermal-fatigue damage evolves fast in the outer row corner's balls then slowly propagates into the inner row balls in FBGA structure. By analyzing the failure data of solder balls, a thermal-fatigue failure criterion is defined where the critical failure probability value is about 80%.     

热-电耦合下倒装芯片封装焊点的电迁移失效研究

热电交互作用下产生的电迁移现象成为倒装芯片封装关键的可靠性问题。建立了FCBGA(倒装芯片球栅陈列封装)三维封装模型,研究了热-电交互作用下倒装芯片互连结构中的温度分布、电流密度分布以及焦耳热分布;发现焊料凸点中存在严重的焦耳热和电流聚集现象;分析了焊料凸点中热点出现的原因,并发现热点在焊料凸点空洞形成过程中起到了关键作用。     

电迁移作用下的微焊点振动疲劳行为研究

研究了不同电迁移时间(0～96 h)和电流密度(0～1.52×104A/cm2)对Sn-3.0Ag-0.5Cu微焊点振动疲劳行为的影响.结果显示,在电迁移时间和电流密度均为0时,微焊点的振动疲劳循环次数大于1 170次,疲劳寿命大干234 min;而在125℃服役温度下,当振动频率为0.8 Hz,交变应力为0～20 M...     

On the failure path in shear-tested solder joints

Failure of solder joints under shear frequently occurs along a path near and parallel to, but not necessarily exactly at, the interface between the alloy and the bonding material. A numerical finite element analysis was employed to simulate the deformation in solder during cyclic lap-shear testing. High magnitudes of equivalent plastic strain were seen to initiate from the corner regions and spread into the solder along a concentrated band close to the interface. Damage can be expected to occur along the localized band, thus rationalizing the experimentally observed failure path. The effect of solder geometry was also explored in this study.     

Mapping the failure envelope of board-level solder joints

Single solder interconnects were subjected to a series of combined tension-shear and compression-shear tests to determine their failure load. The failure envelope of these interconnects was obtained by plotting the normal component against the shear component of the failure load. The interconnect failure force map was found to be elliptical like the failure envelopes of many materials. The failure map can be described by a simple mathematical expression to give a simple force-based criterion for combine loading of solder joints. Post mortem analyses were conducted on the solder joint specimens to identify the failure mechanisms associated with various segments of the failure map. Computational simulations of actual board tests show that the failure map obtained for joint tests provides good predictions of board-level interconnect failures and hence suggest that such failure maps are useful in the design and analysis of board assemblies subjected to mechanical loads. The industry could adopt the methodology to obtain failure envelopes for solder joints of different alloys, bump size and reflow profiles which they could later use to aid in board-level and system-level designs of their products for mechanical reliability.     

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.     

Thermal gradient in solder joints under electrical-current stressing

The thermal gradient and temperature increase in SnAg3.5 solder joints under electrical-current stressing have been investigated by thermal infrared microscopy. Both positive and negative thermal gradients were observed under different stressing conditions. The magnitude of the thermal gradient increases with the applied current. The measured thermal gradients reached 365°C/cm as powered by 0.59 A, yet no obvious thermal gradient was observed when the joints were powered less than 0.25 A. The temperature increase caused by joule heating was as high as 54.5°C when powered by 0.59 A, yet only 3.7°C when stressed by 0.19 A. The location of heat generation and path of heat dissipation are believed to play crucial roles in the thermal gradient. When the major heat source is the Al trace, the thermal gradient in the solder bumps is positive; but it may become negative because the heat generated in the solder itself is more prominent.     

Effects of process conditions on reliability, microstructure evolution and failure modes of SnAgCu solder joints

In this study, microstructure evolution at intermetallic interfaces in SnAgCu solder joints of an area array component was investigated at various stages of a thermal cycling test. Failure modes of solder joints were analyzed to determine the effects of process conditions on crack propagation. Lead-free printed-circuit-board (PCB) assemblies were carried out using different foot print designs on PCBs, solder paste deposition volume and reflow profiles. Lead-free SnAgCu plastic-ball-grid-array (PBGA) components were assembled onto PCBs using SnAgCu solder paste. The assembled boards were subjected to the thermal cycling test (−40 °C/+125 °C), and crack initiation and crack propagation during the test were studied. Microstructure analysis and measurements of interface intermetallic growth were conducted using samples after 0, 1000, 2000 and 3000 thermal cycles. Failures were not found before 5700 thermal cycles and the characteristic lives of all solder joints produced using different process and design parameters were more than 7200 thermal cycles, indicating robust solder joints produced with a wide process window. In addition, the intermetallic interfaces were found to have Sn–Ni–Cu. The solder joints consisted of two Ag–Sn compounds exhibiting unique structures of Sn-rich and Ag-rich compounds. A crystalline star-shaped structure of Sn–Ni–Cu–P was also observed in a solder joint. The intermetallic thicknesses were less than 3 μm. The intermetallics growth was about 10% after 3000 thermal cycles. However, these compounds did not affect the reliability of the solder joints. Furthermore, findings in this study were compared with those in previous studies, and the comparison proved the validity of this study.     

银含量对随机振动条件下无铅焊点可靠性的影响

为了探究银含量对无铅焊点在随机振动条件下的可靠性的影响,对Sn-3.0Ag-0.5Cu、Sn-1.0Ag-0.5Cu和Sn-0.3Ag-0.7Cu三种不同Ag含量材料的焊点做窄带范围内的随机振动疲劳实验,并对失效焊点进行分析。结果表明:三种材料焊点的失效位置基本都在靠近PCB侧,最外围焊点最容易失效,失效模式均为脆性断裂,并且随着Ag含量的降低,金属间化合物的厚度逐渐减小,焊点的疲劳寿命逐渐延长。     

Reliability model for bridging failure of Pb-free ball grid array solder joints under compressive load

The ever increasing power density in high performance microelectronic devices for applications such as large business computing and telecommunication infrastructure has led to several new reliability challenges for solder interconnects. One of them is the creep collapse and bridging of ball grid array (BGA) solder joints under heatsink compressive loads. For characterizing the solder joint response to compressive load and model the corresponding reliability failure, the compressive creep behavior of Sn3.8Ag0.7Cu solder was first investigated. A viscoplastic constitutive model developed from the creep characterization was then incorporated into numerical finite element (FE) analysis to predict solder joint creep collapse and bridging under heatsink compressive load. The numerical analysis results were validated by experimental studies of solder joint collapse under compressive load and isothermal aging condition. A simplified power-law formula is also provided for modeling the creep collapse of Sn3.8Ag0.7Cu solder joint. The model may be applied for predicting solder joint compressive reliability under a prescribed heatsink compression, or to determine the maximum allowable heatsink load for a given life expectancy.     

片式电阻混合焊点热循环负载可靠性研究

对在不同工艺参数下形成的、并且经过不同周数热循环负载的片式电阻混合焊点、有铅焊点和无铅焊点进行了外观检测和剪切测试。结果显示,在不同工艺参数下形成的混合焊点的剪切力,随热循环周数的变化趋势有所不同,但是在保证片式电阻焊端和焊料充分熔融的情况下,部分混合焊点的平均剪切力比有铅焊点高,热循环1 000周后,为9.1～11.1 N。     

Fatigue-creep crack propagation path in solder joints under thermal cycling

The present study investigates the thermal fatigue crack propagation path in a eutectic solder joint between a 2512 leadless chip resistor and a printed wiring board which had experienced thermal cycling between -55 and 125°C. This was achieved through the microstructural examination of fractured surfaces of the joints. Patches of finely spaced striations were observed in a predominant shear strain field in the joints. These striations were attributed to the tensile strain components in the field and used to add the identification of the fatigue crack propagation direction. It was observed that cracks did not simply propagate across the depth of the joint from the inner end (the heel) to the outer end (the toe) in the longitudinal direction, but from a corner point on the free edge of the heel to the center across the joint depth, making an angle of about 70° with respect to the longitudinal direction.     

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.     

Effects of intermetallic morphology at the metallic particle/solder interface on mechanical properties of Sn-Ag-based solder joints

Mechanical incorporation of metallic particles in the Sn-Ag-based solder resulted in various intermetallic compound (IMC) morphologies around these particles during reflow. Unlike with the Ni particles, the IMCs formed around Cu and Ag particles are relatively insensitive to reflow profiles employed. The IMC formed around the Ni particles ranges from “sunflower” morphology to “blocky” morphology with increasing time and temperature above liquidus during the heating part of the reflow profile. Mechanical properties, such as simple shear strength and creep behavior, of these composite solders were affected by the IMC morphologies in the composite solders investigated. Sunflower-shaped IMC formed around an Ni particles resulted in higher simple shear strength and better creep properties.     

内生无铅复合钎料在不同工艺条件下的制备及性能研究

通过采用不同工艺的内生法在Sn-3.5Ag共晶钎料基体中引入弥散分布的Cu6Sn5颗粒,制得了内生无铅复合钎料。研究了不同工艺条件对该钎料力学性能及电迁移行为的影响。结果表明,在冷却速度(0.1℃/s)较慢时制备的钎料,其内生Cu6Sn5颗粒细小,分布最均匀,且团聚程度较轻,另外,其钎焊接头力学性能最好;通5 A电流384 h后,其正负极金属间化合物层厚度的差异保持在约1.2μm,说明Cu6Sn5颗粒的引入提高了钎料的抗电迁移性能。     

Mechanical behavior of solder joints under dynamic four-point impact bending

Reliability of solder joints under drop impact loading is important to mobile electronic products. In this paper, dynamic four-point impact bending tests of board level electronic packages are carried out to investigate mechanical behavior of solder joints. In the test, strain gauges, a high speed camera and the digital image correlation method are used to acquire strain and deflection of the printed circuit board (PCB). After validated by the test data, a finite element model of the dynamic four-point impact bending test is used to obtain strain and stress in the solder joints. Then, failure predictions of the solder joints are made by strain index, and the predictions are compared with the experimental observations. Furthermore, a strain rate dependent Johnson-Cook material model and rate independent elastic-plastic model of lead-free solder are used to investigate the effect of strain rate on behavior of solder joints under drop impact loading. We find that the material model has insignificant influence on the deflection of the PCB during the drop impact but severely affect the stress and strain in solder joints. The rate independent elastic-plastic solder material model always underestimates the stress and overestimates the strain of the solder joints. The index of equivalent plastic strain computed by the strain rate dependent Johnson-Cook model can predict more realistic failure behavior of the solder joints.     

剪切蠕变下无铅焊点厚度的尺寸效应

利用自制的电子测试系统,测量分析了试样焊点厚度(0.05～0.50mm)对电阻应变的影响。结果表明:在剪切蠕变条件下,焊点厚度为0.25mm时,电阻应变最小,蠕变寿命最长。利用有限元软件ANSYS对焊点的蠕变应变进行仿真分析。结果显示:随着焊点厚度变化,焊点蠕变应变的变化趋势与实验结果一致。将相同厚度下的电阻应变与蠕变应变进行拟合,得到了电阻应变与蠕变应变之间的定量关系式。     

Damage accumulation under repeated reverse stressing of Sn-Ag solder joints

To better understand the effect of repeated reverse stress in solder joints, a new testing method was developed. Tin-silver solder joints were fabricated, constrained between Cu blocks, and then subjected to repeated shear loading in a tensile tester. Constant strain amplitudes were applied to simulate service conditions. However, large loads were used to accelerate the damage accumulation. Microstructural features of the damage were very similar to those found with studies on thermomechanical fatigue (TMF) of small, single shear lap samples. Concentrated-shear banding or striations were observed to form along Sn dendrites. The load behavior of the solder with each cycle and during hold times at the extreme strain amplitude was consistent with damage accumulating with each successive cycle. Effects of strain amplitude, hold times at the stress extremes, number of cycles, and solder-joint thickness were found to play significant roles on the stress-strain behavior and surface damage.     

Microstructure, creep properties, and failure mechanism of SnAgCu solder joints

The effect of microstructure on the creep properties and the failure mechanism of SnAgCu solder joints was studied. Single overlap shear specimens made of FR-4 printed circuit boards (PCBs) with organic solderability preservative (OSP), NiAu, and immersion Sn surface finish were reflow-soldered with hypoeutectic, eutectic, and hypereutectic SnAgCu solder paste. Creep tests of the solder joints were performed at 85°C and 105°C under constant load. The effect of microstructure on the creep behavior of the joints was studied by examining the fracture surfaces and cross-sectional samples of the tested joints. Results show that the intermetallic compound at the interface between the PCB and solder affects the fracture behavior of SnAgCu solder joints, thus creating a significant difference in the creep properties of solder joints on different surface finishes. Composition of SnAgCu solder was also found to affect the creep properties of the joints.