Microstructure Of Solder Research Articles

Influence of the fluxes properties on quality and the microstructure of lead-free solder joints executed by selective soldering

Purpose – The purpose of this paper is to investigate the influence of the properties of new compositions of fluxes for selective soldering on lead-free solder joints quality and microstructures as well as showing which flux properties are the most important. Design/methodology/approach – The three different types of fluxes were tested, which differed in composition, solids content, amount and type of activators added. The selective soldering process was done with the use of lead-free solder SAC 305. The test boards had two coatings SnCu (HASL) or Au/Ni. Basic chemical and physical properties of fluxes were examined according to the relevant standards. Different types of components from the bulky ones, demanding more heat, to the smaller ones were used during the assembly process. AOI and X-ray analyses as well as cross-sections and SEM analyses were utilized to deeply assess the quality and microstructure of the investigated solder joints. Findings – The results showed that information about density or static activity of flux is not enough for correct flux assessment. The dynamic activity of flux measured by wetting balance method is the best for this, especially in the case when there is short soldering time and heat transfer is hindered. The quality and the microstructure of lead-free solder joints are related not only with wetting properties of the flux used for soldering but also with other properties like solids content in a flux. Research limitations/implications – It is suggested that further studies are necessary for the confirmation of the practical application, especially of the reliability properties of the joints obtained with the use of the elaborated fluxes. Originality/value – The results showed that type of flux (ORL or ROL) as well as minor changes in their dynamic activity and solids content might have significant influence on quality of solder joints and their microstructure. It was noted that selective soldering is demanding technique and optimization of soldering process for different type of components and fluxes is important.

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.

Effects of Al and Fe additions on microstructure and mechanical properties of SnAgCu eutectic lead-free solders

In this study, Sn–3.5Ag–0.9Cu (wt%) lead-free solder was modified with minor additions of Al and Fe. The thermal, microstructural and mechanical behaviors after and before compositional modifications were investigated by a combined study of differential scanning calorimetry (DSC), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and shear strength measurements. EDS results and Darken–Gurry predictions indicated a high concentration of Fe substitution within the Cu6Sn5 at the solder/copper interface which induced desirable effects on undercooling and microstructure evolution. Eutectic SAC and SAC+0.05wt% Al solder joints exhibit considerable number of brittle proeutectic phases (i.e., Ag3Sn). Proeutectic Ag3Sn formation was found to be suppressed after Fe modification. A new type of Al–Sn–Cu intermetallic compound was detected for Al added specimens. The rod-like morphology of this IMC appears to cause a sharp decrease in the shear strength of Al modified solder joints. The shear strength values for Fe modified solder joints were found to be higher in a wider composition range (0.01–0.1wt% Fe) as compared to eutectic SAC and SAC+0.05wt% Al.

Development of solidification microstructure and tensile mechanical properties of Sn-0.7Cu and Sn-0.7Cu-2.0Ag solders

Non modified and Ag-modified eutectic Sn-0.7Cu solder alloys were directionally solidified under transient heat flow conditions. The microstructure of the Sn-0.7Cu alloy has been characterized and the present experimental results include the cell/primary dendrite arm spacing (λ1) and its correlation with: the tip cooling rate (\(\mathop T\limits^{ \bullet }\)) during solidification, ultimate tensile strength (σu) and elongation to fracture (δ). Distinct morphologies of intermetallic compounds have been associated with the solidification cooling rate for both alloys examined. For the Sn-0.7Cu alloy, cellular regions were observed to occur for cooling rates lower than 0.9 K/s, being characterized by aligned eutectic colonies. On the other hand, the alloy containing 2.0 wt %Ag enabled the launch of tertiary branches within the dendritic arrangement. The comparison of results allows stating that finer solder microstructures are shown to be associated with higher ultimate tensile strengths (σu) for both alloys although a more complex microstructure was found for the SAC alloy. In contrast the elongation (δ) exhibited opposite tendencies. The growth of coarse Ag3Sn fibers and platelets within interdendritic regions seems to contribute for the reduction on ductility observed for the SAC alloy.

Effect of Metal Bond-Pad Configurations on the Solder Microstructure Development of Flip-Chip Solder Joints

Various microstructural zones were observed in the solidified solder of flip-chip solder joints with three metal bond-pad configurations (Cu/Sn/Cu, Ni/Sn/Cu, and Cu/Sn/Ni). The developed microstructures of the solidified flip-chip solder joints were strongly related to the associated metal bond pad. A hypoeutectic microstructure always developed near the Ni bond pad, and a eutectic or hypereutectic microstructure formed near the Cu pad. The effect of the metal bond pads on the solder microstructure alters the Cu solubility in the molten solder. The Cu content (solubility) in the molten Sn(Cu) solder eventually leads to the development of particular microstructures. In addition to the effect of the associated metal bond pads, the developed microstructure of the flip-chip solder joint depends on the configuration of the metal bond pads. A hypereutectic microstructure formed near the bottom Cu pad, and a eutectic microstructure formed near the top Cu pad. Directional cooling in the flip-chip solder joint during the solidification process causes the effects of the metal bond-pad configuration. Directional cooling causes the Cu content to vary in the liquid Sn(Cu) phase, resulting in the formation of distinct microstructural zones in the developed microstructure of the flip-chip solder joint.

Development of a lead-free composite solder from Sn–Ag–Cu and Ag-coated carbon nanotubes

The microelectronic applications of lead-free solders pose ever-increasing demands. We seek to improve the solder by forming composites with Ag-coated single-walled carbon nanotubes (Ag-coated SWCNTs). These were incorporated into 96.5Sn–3.0Ag–0.5Cu solder alloy with an ultrasonic mixing technique. Composite solder pastes with 0.01–0.10 wt% nanotube reinforcement were prepared. The wettability, melting temperature, microstructure and mechanical properties of the composite solders were determined, and their dependency on nanotube loading assessed. Loading with 0.01 wt% Ag-coated SWCNTs improved the composite solder’s wetting properties, and the contact angle was reduced by 45.5 %, while over loading of the coated nanotubes up to 0.10 wt% degraded the wettability. DSC results showed only slight effects on the melting behavior of the composite solders. Cross-section microstructure analysis of the spreading specimens revealed uniform distribution of the intermetallic compounds throughout the solder matrix, and EDS analysis identified the phases as β-Sn, Ag3Sn and Cu6Sn5. The mechanical properties of composite specimens, compared with those of unloaded 96.5Sn–3.0Ag–0.5Cu solder, had a maximal improvement in the shear strength of 11 % when the nanotube loading was 0.01 wt% of Ag-coated SWCNTs.

Massive spalling and morphological change of intermetallic compound affected by adding Pd in Co-based surface finishes

Abstract This study investigates the interfacial reactions and microstructures of SAC305 solders on four different Co-based surface finishes, electroless Co (EC), electroless Co/immersion Au (ECIG), electroless Co/electroless Pd (ECEP), and electroless Co/electroless Pd/immersion Au (ECEPIG). The evolution of microstructure at different reflow conditions revealed that Pd not only changed the morphology of the intermetallic compound but also generated higher interfacial energy between solders and Co-based substrates. High interfacial energy and thick reaction phase could induce compressive stress within the (Co,Cu,Pd)Sn3 layer during the reflow process after exhausting Co(W,P). The reduction of the stress could be the possible driving force of massive spalling of the (Co,Cu,Pd)Sn3 intermetallic compound.

Interfacial microstructure and mechanical properties of In–Bi–Sn lead-free solder

The interfacial microstructure and mechanical properties of a low melting temperature lead-free solder of In-18.75Bi-22.15Sn (in at.%) (In–Bi–Sn) were investigated. The microstructure analysis of bulk In–Bi–Sn revealed that irregular lamellar γ-Sn phases distributed in the In2Bi matrix. There was only a single endothermic peak with an onset temperature of 62 °C on the DSC curve, indicating that In–Bi–Sn is close to a ternary eutectic solder. The ultimate tensile strength of the bulk In–Bi–Sn was 21.76 MP at a strain rate of 10−2s−1 at 25 °C. The elongation of the bulk In–Bi–Sn solder reached 87 %, indicating an excellent ductility of the In–Bi–Sn solder. Two intermetallic compounds (IMCs), needle-like Cu(In,Sn)2 and laminar Cu6(In,Sn)5, formed at the In–Bi–Sn/Cu interface. An IMC layer of polyhedral crystallites of InNi formed at the In–Bi–Sn/Ni interface. The shear strength of Cu/In–Bi–Sn/Cu solder joints was 21.15 MP, and the shear fractograph showed that the ductile fracture with dimples appearance occurred in the solder.

Effects of Electrical Current on Microstructure and Interface Properties of Sn–Ag–Cu/Ag Photovoltaic Ribbons

This study presents an electrical current testing that is based on the Sn­xAg­0.5Cu (x = 1, 3mass%) photovoltaic (PV) ribbon, and investigates the growth mechanism of the intermetallic compounds (IMCs). The microstructure of both alloy solders contains the eutectic region (¢-Sn+Cu6Sn5+Ag3Sn) and the base phases (¢-Sn). The eutectic phases in the Sn­3Ag­0.5Cu (SAC305) alloy presented a continuous distribution, and its amount was higher. After soldering, the Cu6Sn5 and Ag3Sn IMCs were found at the interfaces, and their morphologies were dominated by Ag contents in the Sn­Ag­Cu (SAC) solder. The whole interfacial characteristics of IMCs were affected after biasing for 40h. The growth behavior of these IMCs was controlled by the bias-induced thermal diffusion mechanism and the evolution of IMC morphology was dominated by the growth dynamics. The IMCs, formed at the interfaces (SAC/Cu, SAC/Ag), dominated the series resistance of the PV ribbon. [doi:10.2320/matertrans.M2013110]

Effects of minor alloying additive on the shear strength of Sn-58Bi solder joint

The use of Pb has been limited in the electronic industry because of the toxicity of lead. As the result, the research and development activities in developing lead-free solders have been accelerated and some lead-free solders were introduced as alternative. Sn-Bi-based alloys are an alternative to replace Pb-based solders in the low temperature ranges. However, Sn-Bi-based solders show brittle behavior in both the bulk and the interface. Generally, the addition of impurity elements to the solders can improve the ductility of the solder joint in both the bulk and the interface by refining the solder microstructure and suppressing the intermetallics at the interface, respectively. In this study, the effect of third elements, i.e., Ni and In (1 and 0.5wt.%) on the microstructure of the Sn-58Bi solder and thickness of the interface between Sn-58Bi solder and Cu substrate were investigated. Solid state IMC compound layer growth was also examined following the thermal aging of the solder joint. Results after aging show that minor added elements were effective in suppressing the intermetallics at the interface during solid state growth. The results also show that the added In was highly effective in limiting the Bi phase coarsening. Mechanical properties of solder joints, before and after heat treatment, were also investigated by shear and micro-vickers tests.

Real Time Synchrotron X-Ray Imaging for Nucleation and Growth of Cu<sub>6</sub>Sn<sub>5</sub> in Sn-7Cu-0.05Ni High Temperature Lead-Free Solder Alloys

This paper demonstrates how recent progress for real-time solidification observation at SPring-8 synchrotron has contributed to the development of Sn-7wt%Cu-0.05wt%Ni high temperature lead-free solder alloys. Lead-free solder alloys in the composition range Sn-0.7 to 7.6wt%Cu that consist of primary Cu6Sn5in a eutectic Sn-Cu6Sn5matrix have been proposed as solders for application at temperatures up to 400°C for the assembly high current semiconductors. It is shown that trace levels of Al have a marked effect on the solder microstructure and refine the size of the primary Cu6Sn5. The solidification pathway that leads to the refinement was observed in real-time using X-ray synchrotron observations.

Research Advances of Composite Solder Material Fabricated via Powder Metallurgy Route

Researches and studies on composite solder have been done by many researchers in an effort to develop viable lead-free solders which can replace the conventional lead-based solders as lead is considered as toxic. Solder materials developed by composite approach showed improvement in their properties and importantly it improved their service performance when compared with solder materials developed by other methods. This paper reviews the solder properties of various types of composite lead-free solder that were fabricated via powder metallurgy route. The fabrication processes of the composite solder material by using powder metallurgy route which involved mixing the powder homogeneously, compaction of the mixed powder and sintering the green body were discussed in detail. The types of reinforcements used in order to enhance its properties and the roles of the reinforcement used were also discussed in detail. Properties of a desirable composite solder and the effects of the reinforcement addition to the composite solder microstructure, changes in its wettability and improvement of its mechanical properties were later discussed in this paper. In conclusion, by reviewing various research advances in composite solder material, a solder material with high solder joint reliability at elevated temperature have yet to be found. Thus, a novel composite solder material with higher solder joint reliability at room and elevated temperature was proposed.

The effect of homogenization treatment on microstructure and properties of ZnAl15 solder

The effect of homogenization treatment before extrusion process on microstructure and properties of ZnAl15 solders was investigated. The results of observing microstructure shown that homogenization treatment increased the structural homogeneity and stability of ZnAl15 solder. The results of microhardness measurements and reverse bend test indicated that the homogenized ZnAl15 solder resulted in lower microhardness and much higher ductility. Corrosion behavior of non-homogenized and homogenized ZnAl15 solders was studied in 3.5wt.% NaCl solution using immersion test and Tafel plots and resulted in higher corrosion resistance of the homogenized ZnAl15 solder.

Influence of Composition on the Morphology of Primary Cu6Sn5 in Sn-4Cu Alloys

Alloys from the composition range Sn-(0.7 wt.% to 7.6 wt.%)Cu consist of primary Cu6Sn5 surrounded by a eutectic Sn-Cu6Sn5 mixture. The primary Cu6Sn5 intermetallics commonly adopt a coarse elongated morphology, which is not optimal for the mechanical properties of the soldered joint. This paper investigates the effect of trace elemental additions on the size and morphology of the primary Cu6Sn5 in Sn-4 wt.%Cu alloy with and without Ni additions. Elements investigated include ppm additions of Al, Ag, Ge, and Pb. It is shown that Al has a marked effect on the solder microstructure and refines the size of the primary Cu6Sn5, even at very low addition levels, in both binary Sn-Cu alloys and those containing additional Ni. The effect of Al is confirmed using real-time x-radiographic synchrotron observations of solidification.

Microstructures and properties of SnZn-xEr lead-free solders

The Sn9Zn eutectic alloy is the nontoxic lead-free solders alternative having a melting temperature which is closest to that of the eutectic SnPb alloy. In order to improve the properties of SnZn lead-free solders, 0–0.5 wt.% of rare earth Er was added to the base alloys, and the microstructures were studied. Results showed that the addition of rare earth Er could enhance the wettability of SnZn solders, with 0.08%Er addition, the spreading area gave an 19.1% increase. And based on the mechanical testing, it was found that the tensile force and shear force of SnZn-xEr solder joints could be improved significantly. Moreover, the oxidation resistance of SnZn0.08Er solder was better than that of SnZn solder. In addition, it was found that trace amounts of rare earth Er could refine the microstructures of SnZn solders, especially for Zn-rich phases, and excessive amount of rare earth Er led to a coarse microstructure.

Fracture Behavior of Sn-3.5Ag-0.7Cu and Pure Sn Solders as a Function of Applied Strain Rate

The demand for environmentally benign Pb-free solders is increasing, and the push toward smaller portable electronics will make it more likely for solder interconnects to encounter mechanical shock through dropping or mishandling. Thus, fundamental understanding of the relationship between solder microstructure and mechanical shock resistance is essential for developing reliable numerical models of mechanical shock behavior. In this paper we report on the strain rate-dependent mechanical behavior of pure Sn and Sn-3.5Ag-0.7Cu solders, measured from tensile tests conducted in the strain rate range from 10 3 s 1 to 30 s . Local strain and strain rate distributions were measured by digital image correlation. Finally, the strain rate dependence of fracture mechanisms is discussed. For a given strain rate, waterquenched tin–silver–copper (SAC) had the greatest ultimate tensile strength (UTS), followed by furnace-cooled SAC, then pure Sn. Furnace-cooled SAC had lower ductility than water-quenched SAC, due to large Ag3Sn needles that nucleated elongated voids which easily coalesced.

Interfacial reaction and shear strength of Ni-coated carbon nanotubes reinforced Sn–Ag–Cu solder joints during thermal cycling

In this study, varying weight percentages of Ni-coated carbon nanotubes (Ni-CNTs) were incorporated into Sn–Ag–Cu (SAC) solder matrix, to form composite solders. Up to 0.05 wt.% of Ni-CNTs were successfully incorporated. The interfacial microstructure and shear strength of solders on Ni/Au finished Cu substrates were investigated after thermal cycling (from −40 °C to +125 °C) for up to 2000 cycles. The thermomechanical property results showed an improvement in thermal stability for the composite solders. Results also revealed that after soldering and thermal cycling, the interfacial IMC thickness of the unreinforced solder joint was observed to grow more significantly than that of the composite solder joints. Shear tests results revealed that both composite solder joints which were thermally cycled and as-soldered had better ultimate shear strength than their monolithic counterparts. The shear strength of all thermally cycled solder joints decreased with increasing thermal cycles.

Effect of Alloying Elements on the Microstructure and IMC Formation of SnAgCu Solder on Ni (P) Substrate

When the use of lead was banned, the development of lead-free solders began to rise and was greatly explored. Presently, majority of the eutectic Pb-free solders are Sn-based and the most promising is ternary SnAgCu (SAC) solder. However, SnAgCu solder tends to become brittle in nature and excessive solder interfacial reactions. An attractive approach is by introducing alloying elements such as Ce, Fe and Zn into the SnAgCu solder, which allows for an increase of the mechanical properties and refine the solder microstructure as well as improve the wetting properties. The present work focuses on the effects of a series of elemental additions of 0.5% Ce, Fe and Zn into the basic Sn3.0Ag0.5Cu solder, in attempt to reduce the intermetallic compound (IMC) growth as well as joint reliability and reflow properties of the solder on Ni-P surface finish. The intermetallic compound Cu6Sn5, containing a small amount of dissolved Ni, was found to form preferentially on the Ni coating. This compound layer served as a barrier for direct reaction of Sn with the Ni-P coating. The P-rich Ni layer acts as a good diffusion barrier layer, which decreases the dissolution rate of the Ni-P layer hence decreases the growth of IMC thickness layer. It is also expected that with the addition of alloying elements, the wetting properties will be further enhanced.

Properties of SnZn lead free solders bearing rare earth Y

Rare earth (RE) Y has been incorporated into eutectic SnZn solders to study the wettability, mechanical properties, oxidation resistance, microstructures and thermal fatigue behaviours. The results indicate that adding trace amount of RE Y can remarkably improve the wettability of SnZn solders. When the content of RE Y is 0·06 wt-%, the tensile and shear forces of SnZn-xY solder joints give 24·1% and 27·2% increase after soldering. In addition, it is found that SnZn0·06Y has better oxidation resistance than that of SnZn solder, and the microstructures of SnZn solders can be refined by the addition of RE Y; the size of Zn rich phase can be reduced obviously. Moreover, SnZn0·06Y shows superior thermal fatigue property over SnZn solders. It can be concluded that the optimum content of RE Y is 0·06 wt-% for SnZn solders.

Effect of Ni-Coated Carbon Nanotubes on Interfacial Reaction and Shear Strength of Sn-Ag-Cu Solder Joints

In this study, varying weight percentages of Ni-coated carbon nanotubes (Ni-CNTs) were incorporated into Sn-Ag-Cu (SAC) solder matrix, to form composite solder. Up to 0.05% of Ni-CNTs were successfully incorporated. The interfacial microstructure and shear strength of solders on Ni/Au-finished Cu substrates were investigated after aging at 150°C for up to 42 days. Results revealed that, after soldering and aging for various lengths of time, the interfacial intermetallic compound (IMC) thickness of the unreinforced solder joint was observed to grow more significantly than that of the composite solder joints. Furthermore, the composite solder joints also showed lower diffusion coefficients (2.5 × 10−15 cm2/s to 3.2 × 10−15 cm2/s) compared with that of the SAC solder joints (4.9 × 10−15 cm2/s). Shear test results revealed that as-soldered and aged composite solder joints had better ultimate shear strength than their monolithic counterparts and the shear strength of all aged solder joints decreased with increasing aging time.