The effects of Mn powder additions on the microstructures and tensile property of SnAgCu/Cu solder joints

In this paper, the effects of Mn powder on fusion property of Sn3.0Ag0.5Cu solder alloy and microstructures as well as tensile property of the solder joints of Sn3.0Ag0.5Cu/Cu were investigated by differential scanning calorimetry analysis, scanning electron microscopy and tensile tests. The results showed that the addition of Mn dramatically suppressed under cooling of SnAgCu solder alloy. Mn addition contributed to the growth of Cu₆Sn₅ intermetallic compound layers since it provided nucleation sites for Cu₆Sn₅ at the solder joints. Moreover, Mn addition increased the hardness of the solder alloys and reduced the tensile strength of SnAgCu/Cu solder joints. During aging, the growth of IMC layers of SnAgCuMn/Cu solder joints was slower than that of SnAgCu/Cu solder joints, and the tensile strength of all the solder joints increased after aging.     

Properties enhancement of SnAgCu solders containing rare earth Yb

In order to enhance the properties of SnAgCu lead-free solders in microelectronic packaging, various contents of rare earth Yb were incorporated into the alloys. Results indicated that the addition of Yb can improve the wettability, tensile strength, thermal fatigue behavior of lead-free alloys. The lead-free solder with 0.05%Yb addition exhibited the best comprehensive properties as compared to the alloys with other Yb weight fractions. And found that after soldering, the initial interfacial IMC thickness of SnAgCuYb solder joint was smaller than that of SnAgCu solder joints, and this signified that the addition of Yb was effective in retarding the growth of the IMC layer. In addition, the Yb can refine the microstructures of SnAgCu solder, excessive Yb added can form bulk Sn–Yb phase and deteriorate the properties.     

Effect of thermal cycles on interface and mechanical property of low-Ag Sn1.0Ag0.5Cu(nano-Al)/Cu solder joints

Low-Ag content SnAgCu solder has drawn more and more researchers’ attention due to the low cost. In this paper, the effect of 0.1 wt% nano-Al particles on interface reaction between Sn1.0Ag0.5Cu and Cu substrate was investigated, and the growth of intermetallic compounds (IMC) and mechanical property of solder joints during ??55 to 125 °C thermal cycling were also analyzed. The results show that the Cu₆Sn₅ IMC formed at the as-soldered interface and grow obviously with the increase of thermal cycling. The growth rate of IMC in the SnAgCu–0.1Al/Cu is lower than that of SnAgCu/Cu, which indicates that the nano-Al particles can inhibit the diffusion coefficient of IMC layers. Moreover, the shear force of two kinds of solder joints decrease during thermal cycling, but the shear force of SnAgCu–0.1Al is higher than that of SnAgCu.     

Effects of Aging on Interfacial Microstructure and Reliability Between SnAgCu Solder and FeNi/Cu UBM

Effects of thermal aging on the interfacial microstructure and reliability of the SnAgCu/FeNi‐Cu joint are investigated. It is found that aging effects depends strongly on the temperature. Aging at low temperature, e.g., at 125 °C, a submicron meter thick FeSn₂ IMC layer formed at the SnAgCu/FeNi‐Cu interface during reflowing grows at a rate twenty times slower than the growth rate of the IMC at the SnAgCu/Cu interface. At high temperature, e.g., at 180 °C, the Cu element is found to diffuse through FeNi layer to produce the (Cu, Ni)₆Sn₅ IMC and this IMC layer grows even faster than the IMC at the SnAgCu/Cu interface. Solder ball shear test results show that the SnAgCu/FeNi‐Cu joint has a comparable strength to the SnAgCu/Cu joint after reflowing, and the strength drop after aging at 125 °C is less than that of the SnAgCu/Cu joint. However, after aging at 180 °C, the strength of the SnAgCu/FeNi‐Cu joint is degraded to a low value, along with a shift in failure mode from the solder fracture to the brittle intermetallics fracture.     

热循环条件下SnAgCu/Cu焊点金属间化合物生长及焊点失效行为

研究了热循环过程中SnAgCu/Cu焊点界面金属间化合物的生长规律及焊点疲劳失效行为。提出了热循环条件下金属间化合物生长的等效方程以及焊点界面区不均匀体模型,并用有限元模拟的方法分析了热循环条件下焊点界面区的应力应变场分布及焊点失效模式。研究结果表明:低温极限较低的热循环,对应焊点的寿命较低。焊点的失效表现为钎料与金属间化合物的界面失效,且金属间化合物厚度越大,焊点中的累加塑性功密度越大,焊点越容易失效。     

不同缓蚀剂对SnAgCu焊膏焊接性能的影响

通过对SnAgCu焊膏/Cu焊接界面IMC层和力学性能进行分析,研究了助焊剂中添加咪唑类缓蚀剂A和喹啉类缓蚀剂B及其复配对SnAgCu焊膏焊接性能的影响.利用扫描电镜(SEM)和能量色散谱仪(EDS)分别对IMC层的微观结构和焊点的组织成分进行观察和分析,采用力学试验机测试焊点的剪切强度和拉伸强度,并通过SEM观察其断口形貌.研究结果表明:缓蚀剂对界面IMC层的生长起到一定控制作用,不添加任何缓蚀剂时,IMC层厚度不均匀,部分呈粗大的柱状结构,平均厚度为7.6μm;而添加0.5%A和0.5%B复配缓蚀剂的焊膏,IMC层最薄而且致密均匀,厚度为3.4μm;添加0.5%A和0.5%B复配缓蚀剂的焊膏,获得了最大的剪切强度和抗拉强度,其中剪切强度为47.92 MPa,剪切断裂模式为韧性断裂,抗拉强度为99.28 MPa,拉伸断裂模式为脆性断裂.     

SnAgCu(SnPb)/Ni和SnAgCu(SnPb)/Cu界面金属间化合物在热冲击过程中的生长规律

通过对SnAgCu（SnPb）／Ni和SnAgCu（SnPb）／Cu界面热）中击过程中金属间化合物（Intermetallic compound，IMC）生长规律的研究，以期更好的了解SnAgCu无铅焊料与Ni或Cu金属化层的相互匹配。结果表明固溶在焊料中的Cu或者Ni影响热）中击过程中界面IMC的演化。采用NiAu镀层时，IMC的生长速率比HASL（Hot Air Solder Level）镀层慢，并且界面没有Kirkendall孔洞生成，所以有更好的长期可靠性。采用SnAgCu焊料时，界面Kirkendall孔洞较SnPb焊料少，界面IMC的生长速率较SnPb焊料慢。     

Intermetallic growth study on SnAgCu/Cu solder joint interface during thermal aging

The intermetallic compound (IMC) growth behavior at SnAgCu/Cu solder joint interface under different thermal aging conditions was investigated, in order to develop a framework for correlating IMC layer growth behavior between isothermal and thermomechanical cycling (TMC) effects. Based upon an analysis of displacements for actual flip-chip solder joint during temperature cycling, a special bimetallic loading frame with single joint-shear sample as well as TMC tests were designed and used to research the interfacial IMC growth behavior in SnAgCu/Cu solder joint, with a focus on the influence of stress–strain cycling on the growth kinetics. An equivalent model for IMC growth was derived to describe the interfacial Cu-Sn IMC growth behavior subjected to TMC aging as well as isothermal aging based on the proposed “equivalent aging time” and “effective aging time”. Isothermal aging, thermal cycling (TC) and TMC tests were conducted for parameter determination of the IMC growth model as well as the growth kinetic analysis. The SnAgCu/Cu solder joints were isothermally aged at 125, 150 and 175 °C, while the TC and TMC tests were performed within the temperature range from ?40 to 125 °C. The statistical results of IMC layer thickness showed that the IMC growth for TMC was accelerated compared to that of isothermal aging based on the same “effective aging time”. The IMC growth model proposed here is fit for predicting the IMC layer thickness for SnAgCu/Cu solder joint after any isothermal aging time or thermomechanical cycles. In addition, the results of microstructure evolution observation of SnAgCu/Cu solder joint subjected to TMC revealed that the interfacial zone was the weak link of the solder joint, and the interfacial IMC growth had important influence on the thermomechanical fatigue fracture of the solder joint.     

Reliability of lead-free solder joints in CSP device under thermal cycling

Finite element method and Garofalo–Arrheninus creep model were combined and used to evaluate the reliability of different lead-free solder joints (SnAgCu, SnAg, SnSb and SnZn) and SnPb solder joints in chip scale package (CSP) 14 × 14 device under thermal cyclic loading. The results show that von Mises stress and equivalent creep strain in each of the four lead-free solder joints and SnPb solder joints were strongly different, increasing in the order SnPb < SnAg < SnSb < SnZn < SnAgCu. It is found that maximum stress–strain concentrates on the top-surface of corner solder joints in the CSP device for all solder joints, and SnAgCu solder joints shows the highest fatigue life among those five kinds of solder joints.     

Effects of trace rare earth Nd addition on microstructure and properties of SnAgCu solder

Effects of rare earth Nd on solderability of the Sn3.8Ag0.7Cu alloy were studied by wetting balance method, and the mechanical properties (such as pull-force and shear-force) of the joints soldered with SnAgCu–XNd solders were determined using STR-1000 joint strength tester. Moreover, the microstructures of SnAgCu–XNd solders bearing different amount of Nd as well as the intermetallic compounds (IMCs) formed at solder/Cu interface during soldering have been investigated using optical microscopy, scanning electron microscopy and energy dispersive X-ray analysis, respectively. The results indicate that trace amount of Nd addition can remarkably improve the solderability and mechanical properties of SnAgCu solder. At the same time, it is found that rare earth Nd in SnAgCu solder could refine and improve microstructure of the solder, some bigger IMC plates in SnAgCu solder were replaced by fine granular IMCs. Moreover, the thickness of the intermetallic layer at the Cu/solder interface was reduced significantly. In summary, we suggest that the most suitable content of rare earth Nd is about 0.05 wt% and it will be inadvisable when the Nd exceeds 0.25 wt%.     

Mechanical and microstructural properties of SnAgCu solder joints

Mechanical and microstructural properties of SnAgCu solder joints with hypoeutectic, eutectic and hypereutectic compositions were studied. Eutectic SnPb joints were used as the reference. Reflowed lap shear specimens made of FR-4 glass epoxy printed circuit boards with OSP and NiAu surface finishes were used in the tests. Mechanical properties and microstructural features of the joints were examined in the as-reflowed condition and after isothermal aging at 85 °C for 1000 h. Both the composition and PCB surface finish had a notable effect on the mechanical behaviour of the SnAgCu solder joints. The shear strength value of SnAgCu solder joints was mainly dependent on the size and distribution of Ag₃Sn dispersions. The coarseness of the dispersions depends strongly on the amount of Ag in the solder alloy, the cooling rate after the reflow and the aging history of the solder joints.     

Investigation on high temperature mechanical fatigue failure behavior of SnAgCu/Cu solder joint

In this paper, high temperature mechanical fatigue tests on SnAgCu/Cu solder joints were carried out under three test temperatures (100, 125, 150 °C). Failure mechanism was analyzed through observation of micro-crack evolution and fracture morphology. The results show that the deformation curve of solder joint under high temperature mechanical fatigue tests can be divided into three stages: strain hardening stage, stable deformation stage and accelerated failure stage, which is similar to the curve under creep test condition. In addition, the cyclic life decreases rapidly with increasing temperature. Deformation field in the solder joint is non-uniform and shear strain concentration occurs in solder close to the intermetallic compound (IMC) layer. Micro-crack initiates at the corner of the solder joint and then tend to propagate along interface between Cu substrate and solder. The fracture morphology under three temperatures all exhibits ductile fracture mode and the failure path transforms from cutting through the top of Cu₆Sn₅ to propagation in solder matrix close to IMC layer with increasing temperature.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

回流次数对无铅焊点组织演变及可靠性的影响

目的研究不同回流次数对焊点形貌以及组织演变的影响,并通过力学性能来表征不同回流次数下焊点的可靠性。方法利用置球法将Sn3Ag0.5Cu小球置于Cu基板表面,随后在回流焊机中形成焊点,并进行不同次数回流焊接得到所需焊点,横截镶样打磨腐蚀后,利用光学显微镜、扫描电子显微镜进行显微组织观察,并用推拉试验机进行剪切测试。结果在焊点反应过程中,由于熔融焊料中析出的过饱和Cu和Sn会在焊料部分形成中空的Cu6Sn5管状物和片状的Sn基体,但随着反应的持续,这些物质逐渐消失。在IMC层的形成过程中,伴随着大量Cu6Sn5颗粒的产生,随着反应的持续,颗粒数量逐渐减少,IMC层厚度逐渐增加,但增加速度减缓。结论在IMC层的生长过程中,大块IMC会吞噬Cu6Sn5小颗粒来增加自身体积,从而抑制小颗粒的产生,最终减缓自身的生长。此外随着回流次数的增加,焊点由韧性断裂逐渐转变为韧脆性混合断裂,对焊点可靠性的降低具有一定影响。     

Influence of Thermal Cycling on the Microstructure and Shear Strength of Sn3.5Ag0.75Cu and Sn63Pb37 Solder Joints on Au/Ni Metallization

The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu （SAC） and Sn63Pb37 （SnPb） solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound （IMC） layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and （Au, Ni, Cu）Sn4 and a very thin AuSn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin （Ni, Cu, Au）3Sn4 IMC layer and a Pb-rich layer formed below and above the （Au, Ni）Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.     

Effects of rare earths on properties and microstructures of lead-free solder alloys

In order to further enhance the properties of lead-free solder alloys such as SnAgCu, SnAg, SnCu and SnZn, trace amount of rare earths were selected by lots of researchers as alloys addition into these alloys. The enhancement include better wettability, physical properties, creep strength and tensile strength. For Sn3.8Ag0.7Cu bearing rare earths, when the rare earths were La and Ce, the creep-rupture life of solder joints can be remarkably improved, nine times more than that of the original Sn3.8Ag0.7Cu solder joints at room temperature. In addition, creep-rupture lifetime of RE-doped solders increases by over four times for SnAg and seven times for SnCu. This paper summarizes the effects of rare earths on the wettability, mechanical properties, physical behavior and microstructure of a series of lead-free solders.     

Properties and microstructure of Sn–0.7Cu–0.05Ni solder bearing rare earth element Pr

Effects of trace amount of rare earth element Pr on properties and microstructure of Sn–0.7Cu–0.05Ni solder were investigated in this paper. The solderability of Sn–Cu–Ni–xPr alloy and shear strengh of Sn–Cu–Ni–xPr soldered micro-joints were determined by means of the wetting balance method and shear test, respectively. Moreover, microstructure of solder alloys bearing Pr, as well as intermetallic compound (IMC) layer formed at solder/Cu interface after soldering were observed. It was concluded that the major benefits of rare earth element Pr on Sn–Cu–Ni lead-free solder are: improving solderability, refining microstructure, and depressing IMC (IMC) growth, which exhibited improved mechanical properties. It also revealed that (Cu,Ni)₆Sn₅ is the majority IMC phase at the interface of Sn–Cu–Ni–xPr/Cu solder joints. Ni added into the solder effectively suppressed the growth of Cu₃Sn and consequently also the total IMC layer thickness. Above all, the thickness and morphology of the interfacial (Cu,Ni)₆Sn₅ IMC were optimized due to alloying Pr. It can be inferred that Pr and Ni would play an important role in improving the reliability of Sn–Cu–Ni lead-free solder joints.     

Effect of Bismuth on Intermetallic Compound Growth in Lead Free Solder/Cu Microelectronic Interconnect

The intermetallic compound (IMC) growth kinetics in pure Sn/Cu and Sn10 wt%Bi/Cu solder joints was studied, respectively, after they were aged at 160-210°C for different time. It was found that the total IMC in Sn10 wt%Bi/Cu joint developed faster than it did in pure Sn/Cu solder joint, when they were aged at the same temperature. And the activation energy Qa for total IMC in Sn10 wt%Bi/Cu joint was lower than that for pure Sn/Cu interconnect. The IMC growth process was discussed. The IMC Cu6Sn5 was enhanced in compensation of reduced IMC Cu3Sn growth. The work reveals that Bi element containing in lead free solder alloys with 10 wt% can enhance IMC growth in lead free solder/Cu joint during service.     

Simulation of dynamic fracture along solder–pad interfaces using a cohesive zone model

In this paper, dynamic fracture of a single solder joint specimen is numerically simulated using the finite element method. The solder–IMC and IMC–Cu pad interfaces are modeled as cohesive zones. The simulated results show that under pure tensile loading, damage typically starts at the edge of the solder–IMC interface, then moves to IMC–Cu pad interface. Eventual failure is typically a brittle interfacial failure of the IMC–Cu interface.     

Direct robust bonding between Sn-based solder and Si substrate

The robust bonding between silicon and low melting point solders composed of Sn–Ag solder with addition of Ce and Nd rare earth (RE) element is obtained. The content of the RE element is 2, 5 wt.%, respectively. The RE elements are stored as island compounds in the solder matrix and their addition has little effect on the melting point of the Sn–Ag solder. When soldering at 250 °C, the intermetallic compounds (IMC) layer is discontinuous and the bonding strength is lower. While soldering at 300 °C, the IMC layer become continuous and stronger bonding strength is obtained. The addition of 5 wt.% RE can result in higher bonding strength than that of 2 wt.% RE. In the present research work, we find that in order to get strong bonding between the Si substrate and the composite solders, suitable pressure should be employed to overcome the oxide skin of the liquid solders formed during soldering. The interfacial structure and fracture analysis of the solder joints are investigated to reveal the bonding nature. Complex IMC are found in the residual IMC layer after shear test. Therefore it is clear that RE elements diffuse to the interface and react with thin SiO₂ layer that exists on the Si surface to form IMC during soldering.