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The purpose of this paper is to investigate the influence of pulse plating current density on the morphology and solderability of Pb-free Sn-Cu solder coatings prepared from alkaline stannate baths.

Sn-Cu solder coatings were produced from a plating solution containing sodium stannate, copper stannate, sodium hydroxide and sorbitol additive on copper substrates. The pulse plating experiments were conducted in galvanostatic mode. The plating current density was varied from 5 to 25 mA/cm², and the morphology of the coatings was studied. The solderability of the coatings was assessed by spread ratio measurement after reflowing the solder coatings at 250°C.

The composition control of eutectic solders is always a challenge in plating. The findings show that Sn-Cu coatings prepared by pulse plating are composed of tetragonal ß-Sn structure and Cu₆Sn₅ compounds irrespective of bath composition and conditions. The final coatings were very dense and smooth with nodular morphology. It was shown that a eutectic composition can be achieved if we apply a current density of ∼15-20 mA/cm². The solderability studies suggest that solder coatings plated at and beyond 15 mA/cm² are more suitable for solder finish applications.

The work presents key issues in pulse electroplating of Sn-Cu solder coatings from an alkaline bath. Possible strategies to control the eutectic Sn-Cu composition by plating process are recommended.

The purpose of this paper is to identify the solder joint with optimal mechanical properties among Cu/Sn/Cu, Ni/Sn/Ni and Cu/Sn/Ni solder joints.

Solder joints with the same specimen shape were prepared by reflow. The microstructures were observed and analyzed by scanning electron microscopy and tensile testing was carried out to investigate the mechanical properties.

The mechanical properties of solder joint correlate closely with the intermetallic compounds (IMC) layer structure and the dissociative IMC particles in the solder bulk. Under the influence of the opposite Cu bar, the Cu/Sn/Ni has a duplex IMC layer structure at the Ni side, involving a thin Ni‐Cu‐Sn IMC layer and a faceted (Cu,Ni)₆Sn₅ layer. The mechanical connection of the duplex IMC layers is weak due to the pores in the layers. The Cu/Sn/Ni fractures in the IMC layers in a brittle mode under tensile testing. Comparatively, the Ni/Sn/Ni also has duplex Ni₃Sn₄ layers, and they connect firmly with each other. The tensile fracture of the Ni/Sn/Ni occurs in the solder bulk in a ductile mode, as well as for the Cu/Sn/Cu. Compared with the Cu/Sn/Cu solder bulk, the solder bulk of the Ni/Sn/Ni and the Cu/Sn/Ni have higher ultimate tensile strengths, because the strengthening effect of the dissociative Ni₃Sn₄ and (Cu,Ni)₆Sn₅ particles on the solder bulk is stronger than that of the Cu₆Sn₅ particles. Among Cu/Sn/Cu, Ni/Sn/Ni and Cu/Sn/Ni, Ni/Sn/Ni has the optimal mechanical properties.

The paper offers insights into the significant influence of base material matching on the microstructure and mechanical properties of solder joints.

Solder bumps for chip interconnections are downsizing from current approximately 100 µm to the expected 1 µm in future. As a result, the Cu-Ni cross-interaction in Cu/Solder/Ni solder joints will be more complicated and then strongly influence the growth of the intermetallic compounds (IMCs). Thus, it is critical to understand the fundamental aspects of interfacial reaction in micro solder joints. This paper aims to reveal the effect mechanism of reflow temperature and solder size on the interfacial reaction in Cu/Solder/Ni solder joints.

The Cu-Ni cross-interaction in the Cu/Sn/Ni micro solder joints with 50 and 100 µm solder sizes at 250°C and 300°C were observed, respectively. The line-type interconnects were soaked in silicone oil, and the temperature of the line-type interconnects was 250 ± 3°C and 300 ± 3°C, which were monitored by a fine K-type thermocouple, and followed by an isothermal aging process at various times. After aging, the specimens were removed from the silicone oil and cooled in the air to room temperature.

The major interfacial reaction product on both interfaces was (Cu,Ni)₆Sn₅, and the asymmetric growth of (Cu,Ni)₆Sn₅, evidenced by the thickness of (Cu,Ni)₆Sn₅ IMCs at the Sn/Ni interface was always larger than that at the Sn/Cu interface, resulted from the directional migration of Cu atoms toward the Sn/Ni interface under Cu concentration gradient. The morphology of (Cu,Ni)₆Sn₅ IMC at Sn/Cu interface was columnlike at 250°C, and which changed from columnlike to scallop with large aspect ratio at 300°C, while that at Sn/Ni interface gradually evolved from needlelike to the mixture of needlelike and layered at 250°C, and which evolved from needlelike to scallop with large aspect ratio at 300°C. The evolution of morphology of (Cu,Ni)₆Sn₅ is attributed to the content of Ni. Furthermore, the results indicate that the Cu-Ni cross-interaction was stronger with small solder size and relatively low temperature in the Cu/Sn/Ni micro solder joints.

The asymmetric growth of (Cu,Ni)₆Sn₅ in the Cu/Sn/Ni micro solder joints, evidenced by the thickness of (Cu,Ni)₆Sn₅ IMCs at the Sn/Ni interface, was always larger than that at the Sn/Cu interface. The morphology evolution of (Cu,Ni)₆Sn₅ IMC at both interfaces was attributed to the content of Ni. The Cu-Ni cross-interaction was stronger with small solder size and relatively low temperature in the Cu/Sn/Ni micro solder joints.

Overall, selection maps about the extent of the eutectic growth projects the solidification velocities leading to given microstructures. This is because of limitations of most of the set of results when obtained for single thermal gradients within the experimental spectrum. In these cases, associations only with the solidification velocity could give the false impression that reaching a given velocity would be enough to reproduce a result. However, that velocity must necessarily be accompanied by a specific thermal gradient during transient solidification. Therefore, the purpose of this paper is to not only project velocity but also include the gradients acting for each velocity.

Compilation of solidification velocity, v, thermal gradient, G, and cooling rate, Ṫ, data for Sn-Cu and Sn-Bi solder alloys of interest is presented. These data are placed in the form of coupled growth zones according to the correlated microstructures in the literature. In addition, results generated in this work for Sn-(0.5, 0.7, 2.0, 2.8)% Cu and Sn-(34, 52, 58)% Bi alloys solidified under non-stationary conditions are added.

When analyzing the cooling rate (Ṫ = G.v) and velocity separately, in or around the eutectic composition, a consensus cannot be reached on the resulting microstructure. The (v vs. G) + cooling rate diagrams allow comprehensive analyzes of the combined v and G effects on the subsequent microstructure of the Sn-Cu and Sn-Bi alloys.

The present paper is devoted to the establishment of (v vs. G) + cooling rate diagrams. These plots may allow comprehensive analyses of the combined v and G effects on the subsequent microstructure of the Sn-Cu and Sn-Bi alloys. This microstructure-processing mapping approach is promising to predict phase competition and resulting microstructures in soldering of Sn-Cu and Sn-Bi alloys. These two classes of alloys are of interest to the soldering industry, whereas manipulation of their microstructures is considered of utmost importance for the metallurgical quality of the product.

This study aims to analyze the shear strength and fracture mechanism of full Cu-Sn IMCs joints with different Cu₃Sn proportion and joints with the conventional interfacial structure in electronic packaging.

The Cu-Sn IMCs joints with different Cu₃Sn proportion were fabricated through soldering Cu-6 μm Sn-Cu sandwich structure under the extended soldering time and suitable pressure. The joints of conventional interfacial structure were fabricated through soldering Cu-100 μm Sn-Cu sandwich structure. After the shear test was conducted, the fracture mechanism of different joints was studied through observing the cross-sectional fracture morphology and top-view fracture morphology of sheared joints.

The strength of joints with the conventional interfacial structure was 26.6 MPa, while the strength of full Cu-Sn IMCs joints with 46.7, 60.6, 76.7 and 100 per cent Cu₃Sn was, respectively, 33.5, 39.7, 45.7 and 57.9 MPa. The detailed reason for the strength of joints showing such regularity was proposed. For the joint of conventional interfacial structure, the microvoids accumulation fracture happened within the Sn solder. However, for the full Cu-Sn IMCs joint with 46.7 per cent Cu₃Sn, the cleavage fracture happened within the Cu6Sn5. As the Cu₃Sn proportion increased to 60.6 per cent, the inter-granular fracture, which resulted in the interfacial delamination of Cu₃Sn and Cu6Sn5, occurred along the Cu₃Sn/Cu6Sn5 interface, while the cleavage fracture happened within the Cu6Sn5. Then, with the Cu₃Sn proportion increasing to 76.7 per cent, the cleavage fracture happened within the Cu6Sn5, while the transgranular fracture happened within the Cu₃Sn. The inter-granular fracture, which led to the interfacial delamination of Cu₃Sn and Cu, happened along the Cu/Cu₃Sn interface. For the full Cu₃Sn joint, the cleavage fracture happened within the Cu₃Sn.

The shear strength and fracture mechanism of full Cu-Sn IMCs joints was systematically studied. A direct comparison regarding the shear strength and fracture mechanism between the full Cu-Sn IMCs joints and joints with the conventional interfacial structure was conducted.

With the established surface mount infrastructure and the temperature constraints of components and printed circuit boards, the melting temperatures of lead‐free solder alloys need to be designed as close to 63Sn/37Pb as practical, and not to exceed 215°C. However, metallurgically, the Sn‐based lead‐free solders cannot approach a melting point as low as 183°C without incorporating a high content of low‐melting elements such as In, Bi or Ga. Incorporating such high contents of these elements involves prohibitive mechanical properties and/or cost. Nonetheless, it has been found that a low dosage of one or more low‐melting point elements within a well designed alloy composition can achieve superior performance to 63Sn/37Pb without incurring an unacceptable cost. To avoid prohibitive cost and/or mechanical properties, whilst achieving the melting temperature requirement, the thresholds for each of these elements, as examples, are found to be In ≤q 6 wt.%, Bi < 4 wt.% or Ga ≤q 0.5 wt.%. This paper summarises the results for one of the lead‐free systems studied within a ten‐year research program.

Temperature cycling tests, and statistical analysis of the results, for various high‐density packages on printed‐circuit boards with Sn‐Cu hot‐air solder levelling, electroless nickel‐immersion gold, and organic solder preservative finishes are investigated in this study. Emphasis is placed on the determination of the life distribution and reliability of the lead‐free solder joints of these high‐density package assemblies while they are subjected to temperature cycling conditions. A data acquisition system, the relevant failure criterion, and the data extraction method will be presented and examined. The life test data are best fitted to the Weibull distribution. Also, the sample mean, population mean, sample characteristic life, true characteristic life, sample Weibull slope, and true Weibull slope for some of the high‐density packages are provided and discussed. Furthermore, the relationship between the reliability and the confidence limits for a life distribution is established. Finally, the confidence levels for comparing the quality (mean life) of lead‐free solder joints of high‐density packages are determined.

This study aims to review the effect of copper percentage in Sn-based solder alloys (Sn-xCu, x = 0–5 Wt.%) on intermetallic compound (IMC) formation and growth after laser soldering.

This study reviews the interfacial reactions at the solder joint interface, solder joint morphology and the theory on characterizing the formation and growth of IMCs. In addition, the effects of alloying and strengthening mechanism, including wettability, melting and mechanical properties are discussed.

This paper presents a comprehensive overview of the composition of tin-copper (Sn-Cu) solders with a potential to enhance their microstructure, mechanical characteristics and wettability by varying the Cu percentage. The study found that the best Cu content in the Sn-xCu solder alloy was 0.6–0.7 Wt.%; this composition provided high shear strength, vibration fracture life value and ideal IMC thickness. A method of solder alloy preparation was also found through powder metallurgy and laser soldering to improve the solder joint reliability.

This study focuses on interfacial reactions at the solder joint interface, solder joint morphology, modelling simulation of joint strength and the theory on characterising the formation and growth of IMC.

The paper comprehensively summarises the useful findings of the Sn-Cu series. This information will be important for future trends in laser soldering on solder joint formation.

The purpose of this paper is to investigate the effect of geometric size on intermetallic compound (IMC) growth and elements diffusion of Cu/Sn/Cu solder joint and establish the correlation model between the thickness of the IMC layer and size of the solder joint on the dozens of microns scale.

The sandwich-structured Cu/Sn/Cu solder joints with different gaps between two copper-clad plates (δ) are fabricated using a reflow process. The microstructure and composition of solder joints are observed and analyzed by scanning electron microscopy.

After reflow, the thickness of the IMC and Cu concentration in solder layers increase with the reduction of δ from 50, 40, 30, 20 to 10 μm. During isothermal aging, the thickness of the IMC fails to increase according to the traditional parabolic rule due to changes in Cu concentration. The reduction of δ is the root cause of changes in Cu concentration and the growth rule of the IMC layer. A correlation model between the thickness of the IMC layer and δ is established. It is found that the thickness of the IMC layer is the function of aging time and δ. With δ reducing, the main control element of IMC growth transfers from Cu to Sn.

This paper shows the changes of IMC thickness and elements concentration with the reduction of the size of solder joints on the dozens of microns scale. A correlation model is established to calculate the thickness of the IMC layer during aging.

This study aims to analyse the changes in the microstructure and grain orientation of the full Cu₃Sn solder joint (Cu/Cu₃Sn/Cu) during isothermal aging at 420°C.

The Cu₃Sn solder joint was fabricated through soldering Cu/Sn/Cu structure and then aged at 420°C. The microstructure evolution and grain orientation were studied by observing the cross-section and top-view surfaces of solder joints.

Original Cu₃Sn solder joint initially transformed into the full Cu₄₁Sn₁₁solder joint (Cu/Cu₄₁Sn₁₁/Cu) at 10 h and finally into the full α(Cu) solder joint (Cu/α(Cu)/Cu) at 150 h during aging. Micro-voids formed in the center of the solder joint interface during the conversion of Cu₄₁Sn₁₁to α(Cu), resulting in lower reliability of the solder joint. Cu₃Sn and Cu₄₁Sn₁₁ grains presented a column-like shape, while α(Cu) presented an irregular shape. The average grain sizes of interfacial phases first increased and then decreased during aging. Original Cu₃Sn solder joint exhibited two main textures: [100]//TD and [203]//TD. For Cu₄₁Sn₁₁, the preferred orientation of [111]//TD was found in the early nucleation stage, while the orientation of the formed full Cu₄₁Sn₁₁ solder joint was dispersed. Furthermore, α(Cu) grains exhibited {100}<100> preferred orientation.

Few researchers focused on the process of microstructure and grain orientation changes during high-temperature (> 300°C) aging of Cu₃Sn solder joint. To bridge the research gap, a high-temperature aging experiment was conducted on Cu₃Sn solder joints.