Reflow soldering and isothermal solid-state aging of Sn-Ag eutectic solder on Au/Ni surface finish

The reaction between the eutectic Sn-3.5Ag solder and the Au/Ni surface finish during reflow as well as during isothermal aging was studied. The Au layer was electroplated and had a thickness of the one μm. The peak reflow temperature was fixed at 250 C while the reflow time was varied between 10 sec and one h. Samples that went through 90 sec reflow time were then subjected to 160 C isothermal aging for up to 875 h. It was found that during reflow the Au layer reacted very quickly with the solder to form AuSn₄. One μm of Au layer was consumed in less than 10 sec. As the aging time increased, AuSn₄ grains began to separate themselves from the Ni layer at the roots of the grains and started to fall into the solder. When, the reflow time reached 30 sec, all the Au intermetallic head left the interface, and Ni₃Sn₄ started, to form at the interface. The Ni₃Sn₄ growth rate followed linear kinetics initially (<240 sec), but the growth rate slowed down afterward. During the isothermal aging, only a small amount of (Au_xNi_1-x)Sn₄ resettled back to the interface, and a continuous (Au_0.45Ni_0.55)Sn₄ layer did not form at the interface, unlike the case for the Sn-37Pb solder. This is an important advantage for Sn-3.5 Ag over Sn-37Pb because a continuous (Au_0.45Ni_0.55)Sn₄ layer inevitably will weaken a solder joint. Our observation indicated that many (Au_xNi_1-x)Sn₄ particles were trapped by the Ag₃Sn particles, and were hindered from resettling back to the interface.     

Phase equilibria and the related properties of Sn-Ag-Cu based Pb-free solder alloys

We have recently developed a thermodynamic database for micro-soldering alloys which consists of the elements Pb, Bi, Sn, Sb, Cu, Ag, Zn, and In. In this paper, the phase equilibria and the related thermodynamic properties of the Sn-Ag-Cu base alloys are presented using this database, alloy systems being one of the promising candidates for Pb-free solders. The isothermal section diagrams of the Sn-Ag-Cu ternary system were experimentally determined by SEM-EDS, x-ray diffraction and metallographic techniques. Based on the present results as well as the previous data on phase boundaries and thermochemical properties, thermodynamic assessment of this system was carried out. The isothermal and vertical section diagrams, liquidus surface, mass fractions of the phase constitution, etc., were calculated. The predictions of surface energy and viscosity were also investigated. Moreover, a non-equilibrium solidification process using the Scheil model was simulated and compared with the equilibrium solidification behavior in some Sn-Ag-Cu base alloys. Calculated results based on the Scheil model were incorporated into a three-dimensional solidification simulation and the prediction of practical solidification procedures was performed.     

Interfacial reactions of Sn-Ag-Cu solders modified by minor Zn alloying addition

The near-ternary eutectic Sn-Ag-Cu alloys have been identified as leading Pb-free solder candidates to replace Pb-bearing solders in microelectronic applications. However, recent investigations on the processing behavior and solder joints reliability assessment have revealed several potential reliability risk factors associated with the alloy system. The formation of large Ag₃Sn plates in Sn-Ag-Cu joints, especially when solidified in a relatively slow cooling rate, is one issue of concern. The implications of large Ag₃Sn plates on solder joint performance and several methods to control them have been discussed in previous studies. The minor Zn addition was found to be effective in reducing the amount of undercooling required for tin solidification and thereby to suppress the formation of large Ag₃Sn plates. The Zn addition also caused the changes in the bulk microstructure as well as the interfacial reaction. In this paper, an in-depth characterization of the interfacial reaction of Zn-added Sn-Ag-Cu solders on Cu and Au/Ni(P) surface finishes is reported. The effects of a Zn addition on modification of the interfacial IMCs and their growth kinetics are also discussed.     

Reliability issues in Pb-free solder joint miniaturization

As solder joints become increasingly miniaturized to meet the severe demands of future electronic packaging, it is vitally important to consider whether the solder joint size and geometry could become reliability issues and thereby affect implementation of the Pb-free solders. In this study, three bumping techniques, i.e., solder dipping, stencil printing followed by solder reflow, and electroplating of solders with subsequent reflow, were used to investigate the interfacial interactions of molten Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn solders on a copper pad at 240°C. The resultant interfacial microstructures, coming from a variety of Cu pads, with sizes ranging from 1 mm to 25 μm, and representing different solder bump geometries, have been investigated. In addition, a two-dimensional thermodynamic/kinetic model has been developed to assist the understanding of the kinetics of interdiffusion and the formation of interfacial intermetallic compounds. Experimental results and theoretical predictions both suggest that the solder bump size and geometry can influence the as-soldered microstructure; therefore, this factor should be taken into consideration for the design of future reliable ultrafine Pb-free solder joints.     

Improved mechanical properties in new,Pb-free solder alloys

The mechanical properties of solders benefit from uniform dispersion of fine precipitates and small effective grain sizes. Metallurgical methods of attaining such a beneficial microstructure have been investigated in two new, near-eutectic, Pb-free solder alloys systems—Sn-Zn-In (m.p. ∼188°C) and Sn-Ag-Zn (m.p.∼217°C). It has been found that small alloying additions of Ag dramatically improve the mechanical properties of the ternary Sn-8Zn-5In alloy. The improvement is attributed to the elimination of the coarse and nonuniform distribution of plate-like dendrites and refining the effective grain size in the solidified microstructure. Also, small amounts of Cu dramatically improve the ductility in the ternary Sn-3.5Ag-lZn alloy. The quaternary Sn-3.5Ag-lZn-0.5Cu has better mechanical properties than the binary Sn-3.5Ag alloy because it has a uniform fine dispersion of precipitates and a small effective grain size. The combination of high mechanical strength and high ductility is likely to yield improved fatigue resistance properties in the interconnection of electronic components.     

Creep rupture of lead-free Sn-3.5Ag-Cu solders

Creep-rupture properties of lead-free Sn-3.5Ag-based alloys with varying amount of Cu were investigated using rolled and heat-treated dog-bone-shaped specimens. Nominal compositions of added copper were 0 wt.%, 0.5 wt.%, 0.75 wt.%, 1.0 wt.%, and 1.5 wt.%. During creep tests, the matrix hardness dropped significantly, and the minimum strain rates ( ) were lowest for the 0.75Cu specimens. The stress exponents (n) of were usually around 4, with the exception of the 0.5Cu and 0.75Cu alloys, which showed somewhat higher values of n. Fractographic analyses revealed typical creep rupture by the nucleation and growth of cavities in the matrix except the 1.5Cu specimens, which showed cavity nucleation at brittle Cu₆Sn₅ particles.     

Interfacial microstructure of Pb-free and Pb-Sn solder balls in the ball-grid array package

Ball-grid array (BGA) samples were aged at 155°C up to 45 days. The formation and the growth of the intermetallic phases at the solder joints were investigated. The alloy compositions of solder balls included Sn-3.5Ag-0.7Cu, Sn-1.0Ag-0.7Cu, and 63Sn-37Pb. The solder-ball pads were a copper substrate with an Au/Ni surface finish. Microstructural analysis was carried out by electron microprobe. The results show that a ternary phase, (Au,Ni)Sn₄, formed with Ni₃Sn₄ in the 63Sn-37Pb solder alloy and that a quaternary intermetallic phase, (Au,Ni)₂Cu₃Sn₅, formed in the Sn-Ag-Cu solder alloys. The formation mechanism of intermetallic phases was associated with the driving force for Au and Cu atoms to migrate toward the interface during aging.     

Creep deformation of Sn-3.5Ag-xCu and Sn-3.5Ag-xBi solder joints

Creep properties of lead-free Sn-3.5Ag-based alloys with varying amounts of Cu or Bi were studied by single lap-shear test. Solder balls with five different compositions of Cu (0 wt.%, 0.75 wt.%, 1.5 wt.%) and Bi (2.5 wt.%, 7.5 wt.%) were reflowed on Cu. The Cu-containing alloy had a lower creep rate than the Bi-containing alloy. The Sn-3.5Ag alloy showed the lowest creep rate on Cu, implying that the Cu element already dissolved in the Sn-3.5Ag alloy during reflow. The Cu-containing alloy was strengthened by dispersed small precipitates of Cu₆Sn₅. As the Cu content increased up to 1.5 wt.%, the Cu₆Sn₅ coarsened and platelike Ag₃Sn intermetallics were found, which deteriorated the creep resistance.     

Effects of Bi and Pb on oxidation in humidity for low-temperature lead-free solder systems

The oxidation behavior of various Sn-Zn(-Bi) alloys during 60°C/90% and 85°C/85% relative humidity (RH) exposure were investigated by microstructure observation and x-ray diffraction analysis. The mechanical property of the joints of resistor chips (1608R) with two kinds of terminations, Sn and Sn-10Pb, soldered on a printed circuit board with Sn-Zn(-Bi) were evaluated by a shear test. The heat/humidity exposure of Sn-Zn alloys promotes segregation into the grain boundary accompanying oxidation of Zn resulting in the ZnO formation. This segregation induces serious degradation of alloys and Sn whisker growth. Heat/humidity exposure of 85°C/85%RH seriously decreases the shear strength of the surface mounted chip joints, especially Sn-Zn-Bi solder, due to the formation of ZnO at the interface between the solder and the reaction layer. The presence of Bi or Pb in Sn-Zn alloys enhances the diffusion, resulting in severe degradation at 85°C/85%RH exposure. In contrast, the exposure at 60°C/90%RH does not influence the joint strength for up to 1000 h. Under this condition, the oxidation of Zn only reaches a few microns in depth from the free surface.     

Sn-Cu、Sn-Ag-Cu系无铅钎料的钎焊特性研究

制备了Sn-0.7Cu、Sn-3.5Ag-0.6Cu钎料,用润湿平衡法测量了钎料对铜的润湿曲线,研究了温度、钎剂活性、钎焊时间对润湿行为的影响,并与Sn-37Pb钎料进行了比较。结果表明:升高温度能显著改善无铅钎料对铜的钎焊性。当温度<270℃时,Sn-0.7Cu的钎焊性明显低于Sn-3.5Ag-0.6Cu钎料;而当温度≥270℃时,两种钎料对铜都会显示较好的润湿性,而Sn-0.7Cu略优于Sn-3.5Ag-0.6Cu钎料。提高钎剂活性能显著增强钎料对铜的润湿性,其卤素离子的最佳质量分数均为0.4%左右。随着浸渍时间的延长,熔融钎料与铜的界面间产生失润现象。无铅钎料的熔点和表面张力较高,是钎焊性较差的根本原因。     

电子组装用高温无铅钎料的研制

Bi-Ag合金是一种替代高铅钎料的芯片封装无铅焊料。研制了Bi-2.5Ag、Bi-2.5Ag-0.1RE、Bi-5Ag-0.1RE、Bi-7.5Ag-0.1RE、Bi-10Ag、Bi-10Ag-0.1RE钎料。结果表明,该合金系钎料的熔化温度范围随Ag含量的增加而增大,而且其润湿性能良好,润湿角都处于30o～40o。不同Ag含量的Bi-Ag/Cu接头在界面处发生断裂,剪切强度差别不大,都略大于30MPa。Bi-Ag/Cu界面没有金属间化合物形成,结合强度较弱。     

无铅的锡-银-铜焊料的发展——第二代低银含量SnAgCu体系

概要地评述了无铅焊料中低银含量的锡-银-铜（SnAgCu）体系的发展方向。由于高银含量的锡-银-铜（SnAgCu）体系存在着成本高和耐跌落（摔）性差的问题,它将被低银含量的锡-银-铜（SnAgCu）体系所取代。在低银含量的锡-银-铜（SnAgCu）体系中加入某些微量添加剂可以达到锡-铅焊料的性能水平。     

Dispersoid additions to a Pb-free solder for suppression of microstructural coarsening

The Bi-43%Sn eutectic solder alloy is a candidate for lead-free replacement of the widely used Pb-Sn solders. The alloy exhibits a microstructural instability at elevated service temperatures causing extensive coarsening and nonuniformity in microstructure and severe creep deformation. The addition of insoluble dispersoid particles using a novel magnetic distribution technique has been found to significantly reduce the coarsening and the onset of tertiary creep. With improved microstuctural stability, the useful service range of the Bi-Sn eutectic solder can be raised to a higher homologous temperature.     

Interfacial reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package applications, Pb-free Sn-Ag-X (X=In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and under bump metallurgy (UBM) systems, various UBMs were prepared by electroplating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8.5In solder was partially corroded and its shape was distorted. This phenomenon was observed in a Sn3Ag10In 1Cu solder system, too. Their fractured surface, microstructure of solder joint interface, and of bulk solder ball were examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepared with various Pb-free solders, then a temperature cycle test (−65∼ 150°C) was performed. All tested Pb-free solders showed better board level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn-9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability than Sn-36Pb-2Ag solder ball, respectively.     

Studies of the mechanical and electrical properties of lead-free solder joints

The mechanical and electrical properties of several Pb-free solder joints have been investigated including the interfacial reactions, namely, the thickness and morphology of the intermetallic layers, which are correlated with the shear strength of the solder joint as well as its electrical resistance. A model joint was made by joining two “L-shaped” copper coupons with three Pb-free solders, Sn-3.5Ag (SA), Sn-3.8Ag-0.7Cu (SAC), and Sn-3.5Ag-3Bi (SAB) (all in wt.%), and combined with two surface finishes, Cu and Ni(P)/Au. The thickness and morphology of the intermetallic compounds (IMCs) formed at the interface were affected by solder composition, solder volume, and surface finish. The mechanical and electrical properties of Pb-free solder joints were evaluated and correlated with their interfacial reactions. The microstructure of the solder joints was also investigated to understand the electrical and mechanical characteristics of the Pb-free solder joints.     

Interfacial reactions between Ni substrate and the component Bi in solders

The reactions between Ni and liquid Bi at 300, 360, 420, and 480°C were studied. Bismuth is an important element in many electronic solders, while Ni is used in many printed circuit board surface finishes. It was found that the only intermetallic compound formed was NiBi₃. The other intermetallic compound NiBi, which is therm odynamically stable at these temperatures, did not form. Reaction at 300°C produced a thick reaction zone, which is a two-phase mixture of NiBi₃ needles dispersed in Bi matrix. The thickness of the reaction zone increased rapidly with reaction time, reaching 400 μm after 360 min. Reactions at 360 and 420°C produced very thin reaction zones, and the major interaction was the dissolution of Ni into liquid Bi. Reaction at 480°C produced extremely thin reaction zone, and the dissolution of Ni into liquid Bi was very fast and was the major interaction. It is proposed that the formation of the reaction zone is controlled by two factors: the solubility limit and the diffusivity of Ni in liquid Bi. Small diffusivity and small solubility limit, i.e., lower temperature, tend to favor the formation of a thick reaction zone. In addition to the NiBi₃ formed within the reaction zone, NiBi₃ also formed outside the reaction zone in the form of long needles with hexagonal cross section. The dissolution rate of Ni into Bi is comparable to that of Ni into Sn at the same temperature, and is much slower than the dissolution rates for Au, Ag, Cu, and Pd into Sn.     

Effect of Cu concentration on the reactions between Sn-Ag-Cu solders and Ni

The reaction between the Sn-Ag-Cu solders and Ni at 250°C for 10 min and 25 h was studied. Nine different Sn-Ag-Cu solders, with the Ag concentration fixed at 3.9 wt.% and Cu concentrations varied between 0.0–3.0 wt.%, were used. When the reaction time was 10 min, the reactions strongly depended on the Cu concentration. At low-Cu concentrations (≦0.2 wt.%), only a continuous (Ni_1−xCu_x)₃Sn₄ layer formed at the interface. When the Cu concentration increased to 0.4 wt.%, a continuous (Ni_1−xCu_x)₃Sn₄ layer and a small amount of discontinuous (Cu_1−yNi_y)₆Sn₅ particles formed at the interface. When the Cu concentration increased to 0.5 wt.%, the amount of (Cu_1−yNi_y)₆Sn₅ increased and (Cu_1−yNi₆)₆Sn₅ became a continuous layer. Beneath this (Cu_1−yNi_y)₆Sn₅ layer was a very thin but continuous layer of (Ni_1−xCu_x)₃Sn₄. At higher Cu concentrations (0.6–3.0 wt.%), (Ni_1−xCu_x)₃Sn₄ disappeared, and only (Cu_1−yNi_y)₆Sn₅ was present. The reactions at 25 h also depended strongly on the Cu concentration, proving that the strong concentration dependence was not a transient phenomenon limited to a short reaction time. The findings of this study were rationalized using the Cu-Ni-Sn isotherm. This study shows that precise control over the Cu concentration in solders is needed to produce consistent results.     

Sn42Bi57Ag1无铅焊膏用活性剂的优化设计

针对Sn42Bi57Ag1无铅焊料开发出一种低温无铅焊锡膏,着重研究助焊剂中活性剂对焊锡膏润湿性能的影响。以铺展性能作为主要评价指标,通过回流焊接实验,对用于助焊剂的7种有机酸进行了筛选。最终选取了性能较好的甲基丁二酸、己二酸、水杨酸与丁二酸四种有机酸进行正交试验,通过量化分析选取正交试验水平,根据方差分析确定助焊剂中活性剂的最佳组合。结果表明:有机酸的复配能显著改善焊点铺展情况,当甲基丁二酸、己二酸、水杨酸与丁二酸质量比为2∶3∶5∶2时,Sn42Bi57Ag1无铅焊膏的润湿性能优异,焊点铺展率达86.52%。     

SnAgCu无铅焊膏用活性物质研究

以SnAgCu无铅焊膏铺展性能为主要指标,通过回流焊接实验,对用于助焊剂的17种有机酸活性物质进行了筛选。选取两种性能较好的有机酸配成复合活性物质,并对该活性物质的组成进行优化。用湿热试验测试焊后残留物的腐蚀性。结果表明:以羧基官能团比例为3:7的丁二酸和一元酸A混合物作为活性物质,并添加质量约0.66%的乙醇胺调整酸度,得到了pH值约为3的助焊剂;此助焊剂性能优良,可使焊点铺展率达到84%。使用含有此助焊剂的焊膏,采用回流焊接工艺在PCB板上贴装片式元件,所得焊点光亮饱满,且焊后残留物无腐蚀性,可以实现免清洗。     

Microstructure and mechanical properties of Pb-free solder alloys for low-cost electronic assembly: A review

Lead-free solders, including Sn-58Bi, Sn-52In, and Sn-3.5Ag, are potential replacements for Sn-37Pb solder in low-cost electronic assembly. This paper reviews the literature on the microstructure and mechanical properties of these alloys. Because of the processing and testing conditions, many of the data are not predictive for electronic assembly applications. However, eutectic Sn-Bi seems to have properties approaching those of eutectic Sn-Pb under most conditions, while eutectic Sn-In seems far inferior in most respects. Eutectic Sn-Ag has many promising characteristics, but its relatively high melting temperature may preclude its use for this type of application.