Cross-Interaction Study of Cu/Sn/Pd and Ni/Sn/Pd Sandwich Solder Joint Structures

Cross-interactions between Cu/Sn/Pd and Ni/Sn/Pd sandwich structures were investigated in this work. For the Cu/Sn/Pd case, the growth behavior and morphology of the interfacial (Pd,Cu)Sn₄ compound layer was very similar to that of the single Pd/Sn interfacial reaction. This indicates that the growth of the (Pd,Cu)Sn₄ layer at the Sn/Pd interface would not be affected by the opposite Cu/Sn interfacial reaction. We can conclude that there is no cross-interaction effect between the two interfacial reactions in the Cu/Sn/Pd sandwich structure. For the Ni/Sn/Pd case, we observed that: (1) after 300 s of reflow time, the (Pd,Ni)Sn₄ compound heterogeneously nucleated on the Ni₃Sn₄ compound layer at the Sn/Ni interface; (2) the growth of the interfacial PdSn₄ compound layer was greatly suppressed by the formation of the (Pd,Ni)Sn₄ compound at the Sn/Ni interface. We believe that this suppression of PdSn₄ growth is caused by heterogeneous nucleation of the (Pd,Ni)Sn₄ compound in the Ni₃Sn₄ compound layer, which decreases the free energy of the entire sandwich reaction system. The difference in the chemical potential of Pd in the PdSn₄ phase at the Pd/Sn interface and in the (Pd,Ni)Sn₄ phase at the Sn/Ni interface is the driving force for the Pd atomic flux across the molten Sn. The diffusion of Ni into the ternary (Pd,Ni)Sn₄ compound layer controls the Pd atomic flux across the molten Sn and the growth of the ternary (Pd,Ni)Sn₄ compound at the Sn/Ni interface.     

Electromigration studies on Sn(Cu) alloy lines

The Cu alloying effect in the Sn(Cu) solder line has been studied. The Sn0.7Cu solder line has the most serious electromigration (EM) damage compared to pure Sn and Sn3.0Cu solder lines. The dominant factor for the fast EM rate in Sn0.7Cu could be attributed to the relatively small grain size and the low critical stress, i.e., the yielding stress of the Sn0.7Cu solder line. Also, we found that the shortest Sn0.7Cu solder line, 250 μm, has the most serious EM damage among three solder lines of different lengths. The back stress induced by EM might not play a significant role on the EM test of long solder lines. A new failure mode of EM test was observed; EM under an external tensile stress. The external stress is superimposed on the stress profile induced by EM. As a result, the hillock formation was retarded at the anode side, and void formation was enhanced at the cathode.     

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.     

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.     

Electromigration and Thermomechanical Fatigue Behavior of Sn0.3Ag0.7Cu Solder Joints

The anisotropy of Sn crystal structures greatly affects the electromigration (EM) and thermomechanical fatigue (TMF) of solder joints. The size of solder joint shrinkage in electronic systems further makes EM and TMF an inseparably coupled issue. To obtain a better understanding of failure under combined moderately high (2000 A/cm²) current density and 10–150°C/1 h thermal cycling, analysis of separate, sequential, and concurrent EM and thermal cycling (TC) was imposed on single shear lap joints, and the microstructure and crystal orientations were incrementally characterized using electron backscatter diffraction (EBSD) mapping. First, it was determined that EM did not significantly change the crystal orientation, but the formation of Cu₆Sn₅ depended on the crystal orientation, and this degraded subsequent TMF behavior. Secondly, TC causes changes in crystal orientation. Concurrent EM and TC led to significant changes in crystal orientation by discontinuous recrystallization, which is facilitated by Cu₆Sn₅ particle formation. The newly formed Cu₆Sn₅ often showed its c-axis close to the direction of electron flow.     

Effect of Stand-Off Height on Microstructure and Tensile Strength of the Cu/Sn9Zn/Cu Solder Joint

To investigate the effect of stand-off height (SOH) on the microstructure and mechanical behavior of certain solder joints, Cu/Sn9Zn/Cu solder joints with a SOH of 100 μm, 50 μm, 20 μm, and 10 μm were prepared and studied. It was found that, as the SOH is reduced, the Zn content in the solder bulk experiences a rapid decrease due to consumption by metallurgical reaction in the reflow process; hence, the microstructure of the solder bulk is changed significantly from a Sn-Zn eutectic structure to a hypoeutectic structure. By contrast, Cu content in the solder bulk experiences a rapid increase with reducing SOH, and this leads to more dissociative intermetallic compounds (IMCs) in the solder bulk. These compositional and microstructural changes induced by reducing the SOH correlate closely with the mechanical properties of the solder joints. In our study it is found that, as SOH is reduced, the tensile strength of the solder joints decreases, the fracture path of the solder joint transfers from the solder bulk into the interface between the IMC layer and the solder bulk, and the fracture mode tends to change from ductile to brittle. These findings point to a probable way to improve the mechanical properties of miniaturized solder joints by controlling the composition and dissociative IMCs in the solder bulk.     

无铅焊料热风整平（锡／镍／铜）可焊性问题的探讨

无铅热风整平作为一种PCB新的无铅表面处理方式，绿色环保先进，很受业界青睐。但是PCB至客户端上锡不良却是一个问题。其不良率时高时低，而且反复发生，多数初采用此工艺的业者无法马上找到问题所在；同时目前国内关于此问题的研究较少，可借鉴文献不足。文章依据实际工作中发生的问题，及通过与星马公司技术人员的探讨成功解决的实例，对无铅热风整平可焊性问题从原理、过程、影响IMC厚度的因素几个方面进行了详细的分析。最后对如何在生产过程中进行管理给出了建议。     

A Study on the Breakdown Mechanism of an Electroless-Plated Ni(P) Diffusion Barrier for Cu/Sn/Cu 3D Interconnect Bonding Structures

This study examined the thermal stability of an electroless-plated Ni(P) barrier layer inserted between Sn and Cu in the bonding structure of Cu/Sn/Cu for three-dimensional (3D) interconnect applications. A combination of transmission electron microscopy (TEM) and scanning electron microscopy allowed us to fully characterize the bonding morphology of the Cu/Ni(P)/Sn/Ni(P)/Cu joints bonded at various temperatures. The barrier suppressed Cu and Sn interdiffusion very effectively up to 300°C; however, an interfacial reaction between Ni(P) and Sn led to gradual decomposition into Ni₃P and Ni₃Sn₄. Upon 350°C bonding, the interfacial reaction brought about complete disintegration of the barrier in local areas, which allowed unhindered interdiffusion between Cu and Sn.     

Frequency-Dependent Low Cycle Fatigue of Sn1Ag0.1Cu(In/Ni) Solder Joints Subjected to High-Frequency Loading

The low-cycle-fatigue characteristics of solder joints, formed by reflowing Sn98.8/Ag1.0/Cu0.1/In0.05/Ni0.02 solder over electroless nickel immersion gold-plated copper pads, were investigated by dynamic cyclic bending of printed circuit boards (PCBs). The PCB strain amplitudes were varied from 1.2 × 10^?3 to 2.4 × 10^?3 and the flexural frequencies ranged from 30 Hz to 150 Hz, to simulate drop impact-induced PCB resonant frequencies. A trend of drastically decreasing fatigue life with cyclic frequency was observed, in contrast with previous reports indicating the reverse; this is attributed to the different failure mechanisms activated. A systematic procedure involving optimization followed by transformation was used to condense the strain–frequency–life data into a master curve expressed in strain–life space.     

Improvement in Joint Reliability of SiC Power Devices by a Diffusion Barrier Between Au-Ge Solder and Cu/Ni(P)-Metalized Ceramic Substrates

The long-term joint reliability of SiC power devices bonded on a ceramic substrate metalized with copper (Cu) and electroless nickel-phosphorus [Ni(P)] using a gold-germanium (Au-Ge) eutectic solder was investigated at 330°C in air. Rapid growth of Ni-Ge intermetallic compounds (IMCs) at the solder/Ni(P) interface and subsequent oxidation of the conductive Cu layer at the IMCs/Cu interface led to a dramatic decrease in bond strength and an increase in the electrical resistance of the joint. To improve the joint reliability, a 250-nm-thick tungsten (W) diffusion barrier (DB) was prepared on the surface of the substrate using a sputtering process. SiC Schottky barrier diode (SBD) power devices were then die-bonded to the W-DB-modified substrate with the Au-Ge eutectic solder using a vacuum reflow system. The bonded samples were aged at 330°C in air. After 1600 h, the joint strength was two times higher than that on the W-DB-free substrate, and no change was observed in the electrical resistance.     

Electromigration-Induced Failure of Ni/Cu Bilayer Bond Pads Joined with Sn(Cu) Solders

The effect of electromigration (EM) on Sn(Cu)/Ni/Cu solder joint interfaces under current stressing of 10⁴ A/cm² at 160°C was studied. In the pure Sn/Ni/Cu case, the interfacial compound layer was mainly the Cu₆Sn₅ compound phase, which suffered serious EM-induced dissolution, eventually resulting in serious Cu-pad consumption. In the Sn-0.7Cu case, a (Cu,Ni)₆Sn₅ interfacial compound layer formed at the joint interface, which showed a strong resistance to EM-induced dissolution. Thus, there was no serious consumption of the Cu pad under current stressing. In the Sn-3.0Cu case, we believe that the␣massive Cu₆Sn₅ phase in the solder matrix eased possible EM-induced dissolution at the interfacial compound layer due to current stressing.     

Solid-State Reactions between Cu(Ni) Alloys and Sn

Solid-state interfacial reactions between Sn and Cu(Ni) alloys have been investigated at the temperature of 125°C. The following results were obtained. Firstly, the addition of 0.1 at.% Ni to Cu decreased the total thickness of the intermetallic compound (IMC) layer to about half of that observed in the␣binary Cu/Sn diffusion couple; the Ni addition decreased especially the thickness of Cu₃Sn. Secondly, the addition of 1 to 2.5 at.% Ni to Cu further decreased the thickness of Cu₃Sn, increased that of Cu₆Sn₅ (compared to that in the binary Cu/Sn couple) and produced significant amount of voids at the Cu/Cu₃Sn interface. Thirdly, the addition of 5 at.% Ni to Cu increased the total thickness of the IMC layer to about two times that observed in the binary Cu/Sn diffusion couple and made the Cu₃Sn disappear. Fourthly, in contrast to the previous case, the addition of 10 at.% Ni to Cu decreased the total IMC (Cu₆Sn₅) thickness again close to that of the Cu/Sn couple. With this Ni content no voids were detected. The results are rationalized with the help of␣the thermodynamics of the Sn-Cu-Ni system as well as with kinetic considerations.     

Effects of Solder Volume on Formation and Redeposition of Au-Containing Intermetallics in Ni/Au-SnAgCu-Ni(P) Solder Joints

SnAgCu is one of the most promising candidates for lead-free solders to replace conventional eutectic SnPb solders. The effects of solder volume on interfacial reactions and microstructure evolution in Ni/Au-SnAgCu-Ni(P) solder joints have been investigated under soldering and thermal aging conditions. The results show that solder volume has a strong effect on the formation of Au-containing intermetallic compounds (IMCs) and their redeposition at the interfaces. The size and volume fraction of Au-containing IMCs dispersed in the solder bulk increased with decreasing solder joint dimensions. For the small solder joint with 300-μm solder ball, the (Au,Ni)Sn₄ IMCs redeposited to the interfaces after thermal aging at 150°C for 9 days, but this was not observed for the other two solder joints with large solder volume. These results also indicate that the redeposition of (Au,Ni)Sn₄ is closely associated with the migration of Cu toward the interfaces.     

Cu-Ni/Solder/Ni-Cu互连结构的电迁移

采用Cu-Ni/Solder/Ni-Cu互连结构,在加载的电流密度为0.4×104 A/cm2的条件下,得到了界面阴极处金属原子的电迁移.数值模拟揭示了其原因是由于凸点互连结构的特殊性,电子流在流经凸点时会发生流向改变进而形成电流聚集,此处的电流密度超过电迁移的门槛值,从而诱发电迁移.运用高对流系数的热传导方法降低了互连焊点的实际温度,在电迁移的扩展阶段显著减小了高温引起的原子热迁移对电迁移的干扰;因此电迁移力是原子迁移的主要驱动力.在电迁移的快速失效阶段,原子的迁移是热迁移和电迁移共同作用的结果:电迁移力驱动阴极处原子的迁移,造成局部区域的快速温升,从而加剧此处原子的热迁移.     

Critical Current Density for Inhibiting (Cu,Ni)6Sn5 Formation on the Ni Side of Cu/Solder/Ni Joints

The Cu/solder/Ni structure is a common joint configuration used in microelectronic packages today. In high-temperature operation of this joint structure, an appreciable amount of Cu can readily diffuse across the entire solder to the Ni side, where it might grow into a brittle bilayer structure of (Cu,Ni)₆Sn₅/(Ni,Cu)₃Sn₄. The driving force for Cu diffusion is the chemical potential gradient. To counterbalance this chemical-force-induced Cu flux (J _chem^Cu), an attempt to apply a reverse electric current into a Cu/Sn(50 μm)/Ni structure was made in this study. Ten current densities (j = 0 A/cm² to 2 × 10⁴ A/cm²) were examined at 150°C upon current stressing. The results indicated that, under current stressing of <10⁴ A/cm², Cu atoms were still driven to the Ni side, resulting in noticeable increase in the amount of Cu (N _Cu) or (Cu,Ni)₆Sn₅ at the Sn/Ni interface. In contrast, N _Cu decreased significantly, and the (Cu,Ni)₆Sn₅ was further converted into another low- Cu-content phase, (Ni,Cu)₃Sn₄, when j exceeded 1.25 × 10⁴ A/cm². Under ~10⁴ A/cm², the (Cu,Ni)₆Sn₅ thickness and N _Cu remained relatively unchanged over time, suggesting that 10⁴ A/cm² is close to the critical current density (j _crit) that can counterbalance J _chem^Cu. Growth of (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ under the conditions (I) j < j _crit, (II) j > j _crit, and (III) j ≈ j _crit were also examined in this study.     

Cu-Ni/Solder/Ni-Cu互连结构的电迁移

采用Cu-Ni/Solder/Ni-Cu互连结构,在加载的电流密度为0.4×104 A/cm2的条件下,得到了界面阴极处金属原子的电迁移.数值模拟揭示了其原因是由于凸点互连结构的特殊性,电子流在流经凸点时会发生流向改变进而形成电流聚集,此处的电流密度超过电迁移的门槛值,从而诱发电迁移.运用高对流系数的热传导方法降低了互连焊点的实际温度,在电迁移的扩展阶段显著减小了高温引起的原子热迁移对电迁移的干扰;因此电迁移力是原子迁移的主要驱动力.在电迁移的快速失效阶段,原子的迁移是热迁移和电迁移共同作用的结果:电迁移力驱动阴极处原子的迁移,造成局部区域的快速温升,从而加剧此处原子的热迁移.     

Electromigration-Induced Interfacial Reactions in Cu/Sn/Electroless Ni-P Solder Interconnects

The effect of electromigration (EM) on the interfacial reaction in a line-type Cu/Sn/Ni-P/Al/Ni-P/Sn/Cu interconnect was investigated at 150°C under 5.0 × 10³ A/cm². When Cu atoms were under downwind diffusion, EM enhanced the cross-solder diffusion of Cu atoms to the opposite Ni-P/Sn (anode) interface compared with the aging case, resulting in the transformation of interfacial intermetallic compound (IMC) from Ni₃Sn₄ into (Cu,Ni)₆Sn₅. However, at the Sn/Cu (cathode) interface, the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.2 wt.% Ni) and Cu₃Sn. When Ni atoms were under downwind diffusion, only a very small quantity of Ni atoms diffused to the opposite Cu/Sn (anode) interface and the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.6 wt.% Ni) and Cu₃Sn. EM significantly accelerated the dissolution of Ni atoms from the Ni-P and the interfacial Ni₃Sn₄ compared with the aging case, resulting in fast growth of Ni₃P and Ni₂SnP, disappearance of interfacial Ni₃Sn₄, and congregation of large (Ni,Cu)₃Sn₄ particles in the Sn solder matrix. The growth kinetics of Ni₃P and Ni₂SnP were significantly accelerated after the interfacial Ni₃Sn₄ IMC completely dissolved into the solder, but still followed the t ^1/2 law.     

Effect of (Au, Ni)Sn 4 Evolution on Sn-37Pb/ENIG Solder Joint Reliability Under Isothermal and Temperature-Cycled Conditions

The ternary intermetallic compound Au_0.5Ni_0.5Sn₄ forms at the Sn-37Pb/ENIG solder interface during aging and temperature cycling, leading to increased interfacial cracking and a corresponding decrease in solder joint reliability for 15 mm ball grid array (BGA) structures. (Au,Ni)Sn ₄ forms at both the board finish (bottom) and component side (top) of the solder joint for isothermally aged, temperature-cycled, and (aged + cycled) joints. For control specimens (reflow only), no cracks or interfacial Au are observed. For isothermally aged joints (170 and 340 h at 125degC), a broken, discontinuous layer of (Au,Ni)Sn₄ is present, but no cracking. For temperature-cycled joints, lowered reliability and interfacial cracking occurs along a continuous (Au,Ni)Sn₄ intermetallic layer on the solder side of the interface after ~450 h of cycling. Aging + cycling did little to inhibit cracking or formation of (Au,Ni)Sn₄. Development of a continuous (Au,Ni)Sn₄ film at the interface is the key failure mechanism. At low cycle numbers where high joint reliability is observed, the (Au,Ni)Sn₄ layer is discontinuous and not fully developed. At higher cycle numbers and longer aging times, the (Au,Ni)Sn ₄ layer becomes continuous and encourages crack growth along the intermetallic interface and consequent lower reliability. The correlation of interfacial smoothness with lowered reliability is consistent with recent work showing that, when intermetallic compounds form smoothly at the solder interface, the mechanical properties are degraded (compared to a rough intermetallic) due to the decreased resistance to shear along the interface     

Effect of Cu content on interfacial reactions between Sn(Cu) alloys and Ni/Ti thin-film metallization

The effect of Cu content in Sn(Cu) alloys on the interfacial reaction between Ni thin film and Sn(Cu) alloys has been investigated. We have found that the variation of Cu content has a strong influence on the spalling of the Ni thin film. With small Cu additives in the Sn, spalling was deferred to longer reflowing time. When the Cu content increased to about 1.0 wt.%, a layer of Cu-Sn compound formed on the Ni thin film, and no spalling was observed after 20-min reflowing. The possible mechanism of spalling deferring is proposed. A Cu flux from the solder to the interface compensated the ripening flux of the semispherical compound grains; therefore, spalling was retarded. The driving force of the Cu flux was attributed to the reduction of Cu solubility caused by the presence of Ni at the interface of the Ni thin film. The Cu flux from solder to the interface is calculated to be in the same order with the ripening flux of the Cu₆Sn₅ compound grains, which confirms the proposed mechanism of spalling deferring. For the Sn(Cu) alloys having Cu content over 1.0 wt.%, the Cu-Sn compound layer grew so fast that the surface of the interfacial compound layer was free of Ni. There was no Cu flux to compensate the ripening flux; therefore, the ripening flux dominated, and spalling occurred after a short reflowing time.