Electromigration performance of Pb-free solder joints in terms of solder composition and joining path

The electromigration (EM) performance of Pb-free solder joints is investigated in terms of solder composition (Sn-0.5Cu vs. Sn-1.8Ag) and joining path to Cu vs. Ni(P) under bump metallization (UBM). In the double-sided joints of Ni(P)/solder/Cu, the micro-structure of solder joints and the interfacial intermetallic compound (IMC) layers are significantly affected by solder composition and joining path. The as-reflowed microstructure of Sn-0.5Cu joints consists of small columnar grains with thin IMC layers at both UBM interfaces, independent of the joining path, while Sn-1.8Ag joints have a few large grains of low-angle grain boundaries with thick IMC layers when joined first to Ni(P) UBM, but a few 60° twins with thin IMC layers when joined first to Cu UBM. The EM stressing under high current densities at 150°C reveals that Sn-1.8Ag joints have a superior lifetime compared to Sn-0.5Cu joints. In addition, the EM lifetime of Sn-1.8Ag joints reflowed first on Ni(P) UBM is the longest among four groups of the solder joints examined.     

Effect of pre-aging on precipitation behavior of Al-1.29Mg- 1.22Si-0.68Cu-0.69Mn-0.3Fe-0.2Zn-0.1Ti alloy

1 Introduction Aluminum alloys of the 6000 series, containing Mg and Si as the major solutes, are strengthened by the precipitation of metastable precursors of the equilibrium β(Mg2Si) phase. The precipitation of these metastable precursors occurs in on…     

Electromigration in solder joints and solder lines

Electromigration may affect the reliability of flip-chip solder joints. Eutectic solder is a two-phase alloy, so its electromigration behavior is different from that in aluminum or copper interconnects. In addition, a flipchip solder joint has a built-in currentcrowding configuration to enhance electromigration failure. To better understand electromigration in SnPb and lead-free solder alloys, the authors prepared solder lines in v-grooves etched on Si (001). This article discusses the results of those tests and compares the electromigration failure modes of eutectic SnPb and SnAgCu flip-chip solder joints along with the mean-timeto-failure.     

Effects of different tempers on precipitation hardening of 6000 series aluminium alloys

By means of Vickers hardness,mechanical property and formability tests,the effects of different tempers on precipitation hardening of 6000 series aluminium alloys for automotive body sheets were investigated.The results indicate that the short-time pre-aging at 170℃makes for subsequent artificial aging precipitation hardening.With the increase of pre-aging time,the artificial aging hardenability increases.The aging hardening rate reaches the maximum when pre-aging time is up to 10 min,and then it decreases.The short-time pre-aging at 170℃benefits sheets to obtain lower strength under delivery condition and consequently to improve stamping formability of automotive body sheets.The effects of different tempers on precipitation hardening are much more obvious than those of the alloying elements.It is a good treatment schedule to perform pre-aging for 5 min at 170℃right after solution treatment.     

具有纳米结构的增强颗粒对SnBi焊点电迁移的影响

探究了颗粒增强无铅复合钎料的电迁移特性。试验采用具有纳米结构的笼型硅氧烷齐聚物(POSS)颗粒作为增强颗粒,制备SnBi基复合钎料。在25oC,104A/cm2条件下,对钎料一维焊点实施不同时间的通电试验,并对焊点表面形貌和内部显微组织进行观察。结果表明,相对于共晶SnBi焊点,POSS颗粒增强复合钎料焊点的电迁移现象明显受到抑制。通电336h后,复合钎料焊点表面变化很小,界面处聚集的富Sn和富Bi层厚度很小,表明POSS颗粒能够有效抑制通电焊点中的物质扩散。     

Effects of the Delay Between Quenching and Aging on Hardness and Tensile Properties of A356 Aluminum Alloy

The aim of the study was to evaluate the accuracy of heat treatment guidelines, generally followed in industrial practices, about the T6 heat treatment of A356 aluminum alloy. In particular, the effect of the delay between quenching and artificial aging (pre-aging time) on microstructure, hardness, and tensile behavior was studied using specimens extracted from different locations of a cylinder head, characterized by different cooling rates and, consequently, by different secondary dendrite arm spacing values. Hardness and tensile tests confirmed the detrimental effect of pre-aging with a 20% reduction in hardness and strength after approximately 1 h of pre-aging, both for samples with fine and large SDAS. Differential scanning calorimetry analyses on samples that were solutionized, quenched, and pre-aged between 0 and 96 h, suggested that the nature and composition of the clusters formed during pre-aging, rather than their size, influenced the subsequent precipitation process and the final mechanical properties of the alloy.     

预时效处理对 Al-Zn-Mg-Cu 合金硬化响应及微观组织的影响

研究了预时效处理对Al-Zn-Mg-Cu铝合金薄板力学性能和微观组织的影响,测试了显微硬度和力学性能,表征了TEM微观精细组织。结果表明:固溶处理后在180 ℃立即进行人工时效处理,可获得Al-Zn-Mg-Cu合金的显著时效硬化效果,且在8 h后达到峰值硬度195 HV0.5,与基体呈共格关系的η′相为主要强化相;120 ℃×10 min为最佳的预时效处理制度,经14天的室温停滞后,相比于固溶+自然时效态,硬度降低了13 HV0.5,降幅达到10.7%,具有明显的抑制自然时效作用;预时效+自然时效态试样,经烘烤硬化处理后,屈服强度和抗拉强度分别达到了465和545 MPa,强度增量分别达到170和95 MPa,同时伸长率达到12.5%。     

预时效对Al-Zn-Mg-Cu系铝合金组织及硬度的影响

研究了Al-Zn-Mg-Cu系铝合金形变热处理过程中时效制度变化对其组织和性能的影响。结果表明,预时效过程中适当地提高时效温度和延长时效保温时间,可以得到预时效的优化工艺参数;金相观察发现,预时效后合金再结晶明显细化,可观察到明显的析出相;预时效后随着形变量的增加,合金硬度随之增加,当形变量达到40%左右时合金硬度最高,随形变量增加,合金硬度略有下降;变形后的试样再进行终时效处理后,硬度提升不明显。     

CuZnAl记忆颗粒对黄铜/锡/黄铜焊点组织与性能影响

研究了不同压力条件下(0.2 N,50 N)瞬时液相键合0.5%亚微米CuZnAl记忆颗粒对黄铜/锡/黄铜焊点组织和性能影响. 发现0.5%亚微米CuZnAl记忆颗粒对界面组织有一定的抑制作用,主要归因于亚微米颗粒的存在减小了元素之间的扩散速率. 另外相对0.2 N压力条件下,50 N压力下由于元素剧烈扩散易于使界面出现大量的空洞,而0.2 N压力界面组织则较为均匀,并且未出现明显的空洞. 力学性能结果表明,添加0.5%亚微米CuZnAl记忆颗粒,焊点的剪切力提高20%~25%. 在0.2 N压力、250 ℃保温10 min,随后200 ℃保温1 100 h可以将黄铜/锡-CuZnAl/黄铜焊点完全转化为金属间化合物,并未发现明显的空洞.     

电迁移对低银无铅微尺度焊点力学行为的影响

研究了电迁移条件下不同电流密度(0.63~1.0×104 A/cm2)和通电时间(0~48 h)对低银无铅Sn-Ag-Cu微尺度焊点的拉伸力学行为的影响. 结果表明,电迁移导致了低银无铅微尺度焊点的抗拉强度显著降低,并且随着电流密度的增加或通电时间的延长,焊点的抗拉强度均呈下降趋势;同时电迁移还导致焊点的拉伸断裂模式发生明显变化,在经历高电流密度或长时间通电的电迁移后,焊点在服役条件下会发生由韧性断裂向脆性断裂的转变. 此外含银量高的微尺度焊点的抗电迁移性能更强.     

结构变化对Cu/Sn-58Bi/Cu微焊点电迁移行为和组织演变的影响

利用SEM观察、聚焦离子束(FIB)微区分析和有限元模拟对比研究了直角型和线型Cu/Sn-58Bi/Cu微焊点在高电流密度下(1.5×10~4A/cm~2)的电迁移行为,从原子扩散距离和微区域电阻变化及阴阳极物相变化的角度研究了焊点结构变化对电迁移影响的机理.结果表明,2种焊点通电112和224 h后均发生了Bi向阳极迁移并聚集及Sn在阴极富集的现象;直角型焊点阳极由于Bi聚集后膨胀而产生压应力进而导致小丘状凸起和微裂纹出现,而阴极存在拉应力引发凹陷和微裂纹,且沿界面呈非均匀变化.微区组织分析表明,电迁移作用下焊点内部Bi原子的扩散速度大于Sn原子的扩散速度.观察分析和模拟结果还表明,具有结构不均匀性的直角型焊点中电子流易向电阻较小区域聚集而产生电流拥挤效应,这是引起直角型焊点电迁移现象严重的根本原因.     

电迁移对Ni/Sn3.0Ag0.5Cu/Cu焊点界面反应的影响

研究了温度为150℃,电流密度为5.0×10³A/cm²的条件下电迁移对Ni/Sn3.0Ag0.5Cu/Cu焊点界面反应的影响.回流焊后在Sn3.0Ag0.5Cu/Ni和Sn3.0Ag0.5Cu/Cu的界面上均形成了（Cu,Ni）₆Sn₅型化合物.时效过程中界面化合物随时效时间增加而增厚,时效800 h后两端的化合物并没有发生转变,仍为（Cu,Ni）₆Sn₅型.电流方向对Cu基板的消耗起着决定作用.当电子从基板端流向芯片端时,电流导致基板端Cu焊盘发生局部快速溶解,并导致裂纹在Sn3.0Ag0.5Cu/（Cu,Ni）₆Sn₅界面产生,溶解到钎料中的Cu原子在钎料中沿着电子运动的方向向阳极扩散,并与钎料中的Sn原子发生反应生成大量的Cu₆Sn₅化合物颗粒.当电子从芯片端流向基板端时,芯片端Ni UBM层没有发生明显的溶解,在靠近阳极界面处的钎料中有少量的Cu₆Sn₅化合物颗粒生成,电迁移800 h后焊点仍保持完好.电迁移过程中无论电子的运动方向如何,均促进了阳极界面处（Cu,Ni）₆Sn₅的生长,阳极界面IMC厚度明显大于阴极界面IMC的厚度.与Ni相比,当Cu作为阴极时焊点更容易在电迁移作用下失效.     

Observations of nucleation catalysis effects during solidification of SnAgCuX solder joints

While modification of a strong (high Cu) Sn-Ag-Cu (SAC) solder alloy with a substitutional alloy addition (X=Co, Fe, Zn, and Ni) for Cu has been demonstrated to enhance solder joint strength and ductility after aging at 150°C for 1,000 h, control of the as-solidified SAC+X solder joint microstructure is also needed to inhibit under cooling and nucleation of brittle pro-eutectic phases (e.g., Ag₃Sn). Bulk undercooling measurements of SAC+X alloys and microstructural analysis of SAC+X solder joints were used to rank the effectiveness and consistency of low-level (X < 0.15 wt.%) substitutional additions to a base SAC composition, Sn-3.5Ag-0.95Cu (wt.%). This SAC composition was selected to favor thermodynamically the nucleation of pro-eutectic Cu₆Sn₅ over that of Ag₃Sn and the formation of an enhanced ternary eutectic fraction in the joint microstructure, while retaining a pasty range that is only 3°C. Using differential scanning calorimetry with sample pans that serve as either inert (aluminum) or actively wetting (copper) substrates, reflow cycles were studied that simulated surface mount (1.5°C/s) and ball-grid array (0.17°C/s) cooling rates. Of the SAC+X solders tested with copper pans, X = Zn appeared to be most effective and consistent, providing catalytic enhancement of the nucleation temperature for even the minimum concentration (0.05 wt.%) and lowest cooling rate.     

预时效制度对Al-Mg-Si-Cu合金组织与性能的影响

通过TEM及拉伸性能试验研究了较宽的预时效工艺参数(80~200℃,2~30 m in)下,A l-Mg-S i-Cu合金的组织及性能演变规律。研究表明:实验合金在80~200℃进行预时效处理,能够抑制自然时效的不利影响。在80℃×(2~30 m in)进行预时效后,材料烘烤硬化效应提高有限;在120~160℃预时效5~10 m in后,材料的烘烤硬化效应得到较大的提高,且材料烤漆前的屈服强度较低,有利于冲压成形;在更高的温度如200℃进行预时效时,材料烤漆前屈服强度过高,但此时材料的烘烤硬化效应提高尤为显著。T4P下GP区的尺寸对材料烘烤硬化效应的提高至关重要。     

温度对振动载荷下互连微焊点寿命的影响

设计了温度-定频振动两场耦合可靠性试验，应用两参数weibull统计分析和物理失效分析方法，分析了温度（25，100℃）对振动载荷下微焊点寿命的影响.结果表明，温度由25℃升高到100℃，振动载荷下的微焊点寿命显著提高，U₁位置处焊点寿命提高了106%，U₂位置处焊点寿命提高了180%；失效模式统计分析表明，随着温度的升高，焊点由界面裂纹（裂纹产生在界面处的金属间化合物与铜焊盘之间）转变为体钎料裂纹（裂纹产生在体钎料处），失效机制由脆性断裂转变为韧性断裂，失效机制的转变与焊点寿命密切相关.     

Polarity effect of electromigration on intermetallic compound formation in SnPb solder joints

The polarity effect of electromigration on the interfacial reactions of micro ball grid array (μBGA) solder joints was studied in terms of microstructural evolution. A dummy μBGA package with the same pair of solder joints was used to obtain reproducible results for a practical application. The μBGA package with Ni(P)/SnPb/Ni(P) structure showed a polarity effect on intermetallic compound (IMC) formation after stress by a current density of 3.0 × 10³ A cm^?2 at 120 °C, compared to the no-current case. Electric current enhanced the growth of Ni₃Sn₄ at the anode and retarded it at the cathode because of the change in Sn diffusion induced by electromigration. The IMC growth at the anode was about one order of magnitude faster than that at cathode. The growth of IMC at the anode had a parabolic dependence on time since the square of thickness of IMC increases linearly with time. The real μBGA package has unique and more complicated geometry compared to ordinary diffusion couples. So not only the polarity growth of IMC but also the fast dissolution of Cu and Ni at specific positions was found with the μBGA package. The unique geometry of μBGA solder joints caused heat generation at the upside interfaces due to Joule heating. Because of the heat generation and the high interfacial energy at the triple point of upside interfaces, the electroless Ni(P) finishes and Cu trace are consumed rapidly in the region where the current is crowded. Thus electromigration induced the rapid dissolution of Ni and Cu into solder and then formed (Cu,Ni)₆Sn₅ at the triple point of the current crowding region at high temperature. This is a rapid failure process because the localized fast dissolution of Ni and Cu accelerated the solid-state reaction between solder and electroless Ni(P) or Cu trace.     

The creep properties of lead-free solder joints

This paper describes the creep behavior of three tin-rich solders that have become candidates for use in lead-free solder joints: Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-0.7Cu. The three solders show the same general behavior when tested in thin joints between copper and Ni/Au metallized pads at temperatures between 60‡C and 130°C. Their steady-state creep rates are separated into two regimes with different stress exponents. The low-stress exponents range from ∼3–6, while the high-stress exponents are anomalously high (7–12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95°C to 60°C. The anomalous creep behavior of the solders appears to be due to the dominant tin constituent. Research on creep in bulk samples of pure tin suggests that the anomalous temperature dependence of the stress exponent may show a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating tin-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.     

Interfacial reactions of Sn–Zn–Ag–Cu alloy on soldered Al/Cu and Al/Al joints

This work shows the effect on the soldering process of the addition of Ag and Cu to Sn–Zn alloys. Soldering of Al/Cu and Al/Al joints was performed for a time of 3?min, at a temperature of 250°C, with the use of flux. Aging was carried out at 170°C for Al/Cu and Al/Al joints for 1 and 10 days. During the aging process, intermetallic layers grew at the interface of the Al/Cu joint at the Cu substrate. Intermetallic layers were not observed during wetting of Al/Al joints. On the contrary, dissolution of the Al substrate and migration of Al-rich particles into the bulk of the solder were observed. The experiment was designed to demonstrate the effect of Ag and Cu addition on the dissolution of Al substrate during the soldering and aging processes. In the solder alloys, small precipitates of AgZn₃ and Cu₅Zn₈ were observed.     

Interactive effects of aging parameters of AA6056

The effect of thermomechanical treatment on the aging behavior of AA6056 aluminum alloy was modeled using response surface methodology (RSM). Two models were developed to predict the final yield stress (FYS) and elongation amounts as well as the optimum conditions of aging process. These were done based on the interactive effects of applied thermomechanical parameters. The optimum condition predicted by the model to attain the maximum strength was pre-aging at 80 °C for 15 h, followed by 70% cold work and subsequent final aging at 165 °C for 4 h, which resulted in the FYS of about 480 MPa. As for the elongation, the optimum condition was pre-aging at 80 °C for 15 h, followed by 30% cold work and final-aging at 165 °C for 4 h, which led to 21% elongation. To verify the suggested optimum conditions, the tests were carried out confirming the high accuracy (above 94%) of the RSM technique as well as the developed models. It is shown that the RSM can be used successfully to optimize the aging process, to determine the significance of aging parameters and to model the combination effect of process variables on the aging behavior of AA6056.     

Effects of in situ nickel particle addition on the microstructure and microhardness of Sn–Ag solder

Abstract

In this study, various amounts of Ni particles were added in situ to Sn–3·5 wt-%Ag lead free solder to form new composite solders. Copper substrates were then dipped into these solders and aged at 150°C for 0, 25, 225, or 1000 h. The microstructure and microhardness of the as solidified solder and the aged solder/copper couples were investigated. Experimental results revealed that the addition of Ni particles increased the microhardness of the composite solder. Ni additions of less than 3 wt-% yielded a microstructure of β-Sn grains surrounded by a eutectic mixture of Ag₃Sn and a Sn rich matrix. An intermetallic compound of Ni₃Sn₄ particles was dispersed throughout the eutectic. For 5 wt-%Ni addition, the Ni₃Sn₄ phase and the remaining Ni particles were agglomerated. In the case of copper substrate dipped with a thick layer of composite solder, water quenched and then aged at 150°C, the induced (Ni, Cu)₃Sn₄ particles coarsened and agglomerated. Additionally, the intermetallic (Cu, Ni)₆Sn₅ compound layer formed at the solder/Cu interface thickened with increasing Ni content.