Bulk Solder Research Articles

The Effect of Ni and Bi Additions on the Solderability of Sn-0.7Cu Solder Coatings

The present investigation explores the influence of Ni and Bi on the solderability of Sn-0.7Cu solder coatings. The minor addition of 0.05 wt.% Ni into the Sn-0.7Cu solder alloy results in an improvement in the wettability based on dipping tests. The solderability investigation using a globule mode shows the influence of Ni and Bi on the interfacial intermetallic compound (IMC). The addition of Ni to a Sn-0.7Cu solder coating resulted in a (Cu,Ni)6Sn5 interfacial IMC, which enhanced the solderability performance during the globule test. With an increasing amount of Bi in the Sn-0.7Cu-0.05Ni-xBi solder ball, the surface energy of the solder alloy can be reduced, and this improves the solderability. The synchrotron micro-XRF results indicate that Ni is found in a relatively high concentration in the interfacial layer. Additionally, Bi was found to be homogenously distributed in the bulk solder, which improved solderability.

Wettability, Interfacial Behavior and Joint Properties of Sn-15Bi Solder

Sn-15Bi solid solution solder provides a potential replacement for Sn-40Pb solder due to lower cost and similar melting point. Sn-15Bi solder was compared with Sn-40Pb solder to assess the microstructure, wettability, bulk tensile properties, interfacial microstructures in solder joints with a Cu substrate, the interfacial evolution in joints during isothermal aging, and the shear strength on ball solder joints with the effect of aging conditions. The microstructure of Sn-15Bi bulk solder was composed of the primary β-Sn phase with Bi in solid solution and with precipitated Bi phases along the grain boundaries. The wettability of solder alloys was evaluated with wetting balance testing, and the results showed that Sn-15Bi solder had a lower wetting force and longer wetting time than Sn-40Pb solder. The poorer wettability of Sn-15Bi solder was not influenced by different types of soldering flux, but improved with increasing temperature. Tensile tests on bulk solder alloys indicated that Sn-15Bi solder had a higher tensile strength but lower elongation than Sn-40Pb solder. Also, the tensile strength of Sn-Bi solder decreased with decreasing strain rate and with increasing temperature, while the elongation of Sn-Bi solder was independent of the temperature and strain rate. In as-soldered joints, Sn-15Bi solder produced a thicker Cu6Sn5 layer than Sn-40Pb solder. Following isothermal aging, the thickness of the interfacial Cu-Sn intermetallic compound (IMC) increased with the aging time, and the growth rate of the IMC layer in Sn-15Bi solder joints was slightly higher than that in Sn-40Pb solder joints during aging. Ball shear test on solder joints illustrated that Sn-15Bi solder joint had a higher shear strength than Sn-40Pb solder joint, and that with increased aging time, the solder became stronger due to precipitation of Bi, leading to a change in the fracture mode from ductile failure in the solder to brittle fracture at the IMC interface.

Comprehensive Properties of a Novel Quaternary Sn-Bi-Sb-Ag Solder: Wettability, Interfacial Structure and Mechanical Properties

Sn-58Bi eutectic solder is the most recommended low temperature Pb-free solder but is also limited from the interfacial embrittlement of Bi segregation. Since the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides a similar melting point as Sn-58Bi eutectic, this paper systematically investigated the properties of this solder from wettability, bulk tensile properties, interfacial microstructure in solder joints with a Cu substrate, interfacial evolution in joints during isothermal aging and the shear strength on ball solder joints with effect of aging conditions. The results were also compared with Sn-58Bi solder. The wettability of solder alloys was evaluated with wetting balance testing, and the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a better wettability than Sn-58Bi solder on the wetting time. Tensile tests on bulk solder alloys indicated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a higher tensile strength and similar elongation compared with Sn-58Bi solder due to the finely distributed SnSb and Ag3Sn intermetallics in the solder matrix. The tensile strength of solder decreased with a decrease in the strain rate and with an increase in temperature, while the elongation of solder was independent of the temperature and strain rate. When soldering with a Cu substrate, a thin Cu6Sn5 intermetallic compound (IMC) is produced at the interface in the solder joint. Measurement on IMC thickness showed that the quaternary Sn-38Bi-1.5Sb-0.7Ag had a lower IMC growth rate during the following isothermal aging. Ball shear test on solder joints illustrated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints had higher shear strength than Sn-58Bi solder joints. Compared with the serious deterioration on shear strength of Sn-58Bi joints from isothermal aging, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints presented a superior high temperature stability. Therefore, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides better performances and the possibility to replace Sn-58Bi solder to realize low temperature soldering.

Microstructure and shear behavior of solder joint with Sn58Bi/Sn3.0Ag0.5Cu/Cu superposition structure

Solder joint with Sn58Bi/Sn3.0Ag0.5Cu/Cu superposition structure was designed and fabricated by two-step soldering process. Sn, Ag, and Cu atoms diffused from the Sn3.0Ag0.5Cu bulk into the molten Sn58Bi solder paste during the second soldering process. The Sn3.0Ag0.5Cu bulk in the composite solder joint increased the concentration and grain size of β-Sn in the Sn58Bi bulk. Moreover, a large amount of tiny Bi-rich particles were found in the Sn58Bi solder bulk. The formation and growth of the β-Sn phases in the Sn58Bi solder bulk was affected by the microstructure of the Sn3.0Ag0.5Cu bulk. Additionally, the β-Sn dendritic in Sn58Bi bulk grew along the Sn3.0Ag0.5Cu bulk like the sunlight. The composite solder joint showed more ductile features than the traditional Sn58Bi eutectic solder joint due to its microstructural transformation occurred during the second soldering process. The Sn3.0Ag0.5Cu bulk with superposition structure works as a barrier to propagate cracks and effectively suppresses the brittle failure of the solder joint.

Investigation of ultrasound-assisted soldering of SiC ceramics by Zn-Al-In high-temperature solder

This work deals with the study of ultrasonic soldering of SiC ceramics by use of Zn-Al-In solder destined for higher application temperatures. The structure and properties of this soldering alloy were assessed. The Zn5Al1In solder consists of βZn matrix, where the βZn + αAl a (In) eutectics in the form of oblong and spherical clusters are non-uniformly distributed. The tensile strength of soldering alloy attained the value of 52 MPa. The soldered joint of SiC/Zn5Al1In/SiC was fabricated at the temperature of 420 °C at simultaneous acting of ultrasonic vibrations. The bond was formed owing to the formation of two new thin layers of Al + Si and Zn + C on the surface of SiC substrate. The average shear strength of SiC/Zn5Al1In/SiC joint attained 49 MPa. The fracture has occurred not only in the solder bulk but also in a close vicinity of SiC substrate. In this zone, similarly as on the fractured surface in the solder zone, the presence of In was identified, in which a considerable measure affected the resultant strength values.

Effect of Cu, Ag on the microstructure and IMC evolution of Sn5Sb–CuAgNi/Cu solder joints

In this paper, three new type Sn5Sb-based solders were used to the study. The three sets of solders are Sn–5Sb–0.5Cu–0.1Ni–0.5Ag, Sn–5Sb–0.5Cu–0.1Ni–0.1Ag and Sn–5Sb–1Cu–0.1Ni–0.1Ag. Experimental results indicated that with the increase of Cu and Ag, the solidus temperature reduced due to the increase of the eutectic phase of low melting point Ag3Sn+β-Sn and Cu6Sn5+β-Sn in the solder. The adding elements of Cu, Ni, Ag can improve the wettability of Sn5Sb while excessive Cu, Ag content inhibit the wettability. The Similar microstructures exist in the three groups of solders, which consist of β-Sn–Sb solid solutions, (Cu1−xNix)6Sn5, Ag3Sn and a small amout of β-Sn. The interfacial intermetallic compound (IMC) is composed of (Cu1−xNix)6Sn5. The lower Cu content suppresses the IMC growth by increasing the dissolution of interfacial IMCs into bulk solder but it also increase the IMC growth by elevating the diffusion speed. The thickness of the IMC layer of Sn–5Sb–0.5Cu–0.1Ni–0.1Ag/Cu grow faster than that of Sn–5Sb–1Cu–0.1Ni–0.1Ag/Cu, Which illustrates the Cu atoms affect the IMC mainly through inhibiting the reaction diffusion. The addition of Ag in the solder significantly suppressed the IMC layer in Sn–5Sb–0.5Cu–0.1Ni–0.5Ag/Cu, Which is attributes to the concentration of Ag3Sn near the IMC layer. The increase of Cu, Ag content in the solder will increase the ductility of the solder. The toughness decrease when the liquid keeping time over 30 s and the shear strength decrease correspondingly. The fracture mechanism changed from ductile fracture to tough and brittle hybrid fracture when the liquid keeping time from 10 s to 90 s.

The effect of the size of NbTi filaments on interfacial reactions and the properties of InSn-based superconducting solder joints

Interactions of superconducting Nb(47 wt% Ti) with liquid binary In-35 wt% Sn and Bi-containing ternary (In, 15 wt% Bi)-35 wt% Sn solders have been examined for the first time up to 450 °C. Reaction of NbTi with InSn results in an intermetallic phase, (Nb,Ti)Sn2, the morphology and chemical composition of which depends on the size of NbTi filaments. Larger filaments react to form a thin uniform (Nb,Ti)Sn2 phase with a Nb:Ti ratio of 6:1, while for finer filaments large (Nb,Ti)Sn2 grains grow with a Nb:Ti ratio of 0.5:1. Differences for interfacial phase layers have been explained by a diffusion-controlled growth model based on metallurgical state of NbTi filaments. Neither filamentary types exhibit any change in critical temperature (~9.2 K) after soldering treatment at 350 °C, but the irreversibility field and flux line pinning force of NbTi changes by a higher factor for the larger filaments. Interaction of NbTi with ternary (In,Bi)Sn results in a lack of any detectable interaction zone, and characteristic nucleation behaviour of phases in the bulk solder at NbTi–(In,Bi)Sn interface is rationalized on the basis of thermodynamic arguments. Transport measurements suggest that properties of solder are the limiting factor for joint behaviour and not any interfacial reactions.

Investigation of the interfacial reactions and growth behavior of interfacial intermetallic compound between Sn37Pb solder and Au/Ni/Kovar substrate

The interfacial reactions and microstructural evolution of Sn37Pb/Kovar with Au/Ni platings solder joints were investigated during isothermal aging at 120 °C, 150 °C and 170 °C for different duration, respectively. The experimental results indicated that a thin interfacial intermetallic compounds (IMCs) layer formed at the Sn37Pb/Kovar interface after reflowing process. The accurate compositions of thin IMCs were double-layers of (Ni, Au)3Sn4 and AuSn4, which were confirmed by utilizing TEM-EDS analysis. However, during the aging process, the compositions of interfacial IMCs of solder joint obviously changed, which transformed from AuSn4/(Ni, Au)3Sn4 to (Au, Ni)Sn4/Ni3Sn4 and the AuSn4 IMC formed in the bulk solder gradually changed to the (Au, Ni)Sn4 IMC. Furthermore, the total IMC thickness increased dramatically with increasing isothermal aging time, as well as aging temperature. The growth process of IMC layer is mainly dominated by the volume diffusion mechanism during the solid-state aging. The square of growth rate constant (K) equals to 2.559 × 10−17, 1.307 × 10−16 and 7.781 × 10−16 m2s−1 for the aging temperature of 120 °C, 150 °C and 170 °C, respectively. The activation energy (Q) value of the Sn37Pb/Kovar couple was calculated to be approximate 95.96 kJ·mol−1.

Interfacial reaction, microstructure and mechanical properties of Sn58Bi solder joints on graphene-coated Cu substrate

Abstract The wettability, interfacial reaction, microstructure and hardness of Sn58Bi (SnBi) solder paste on Cu, high-temperature Cu (H-Cu) and graphene-coated Cu (G-Cu) were investigated in this study. Experimental results indicate that the wettability of Sn58Bi solder on G-Cu is higher than that on Cu and H-Cu substrates. The thickness of the interfacial intermetallic compound (IMC) on Cu, H-Cu and G-Cu substrates increased during aging. The IMC on G-Cu showed the lowest growth rate due to the interfacial diffusion suppressed by the graphene-coated layer. Large dendrite β-Sn phase formed and grew up in the solder bulks on Cu and H-Cu substrates. In contrast, the dissolved graphene inhibits the growth and segregation of β-Sn in the solder bulk on G-Cu during aging. The solder bulk on this substrate shows more uniform microstructure and lower hardness than that on the others.

Isothermal aging and shear creep behavior of a novel lead-free solder joint with small additions of Bi, Sb and Ni

In this paper, we investigate the effects of isothermal aging on the microstructure and creep properties of a novel lead-free SAC387-3Bi-1.5Sb-0.15Ni (InnoLot)/Cu solder joints. Single-lap shear solder joints of the InnoLot solder were aged up to 1 year at 100 °C, and then loaded to failure in shear creep tests under different applied shear stresses and testing temperatures. The findings show that both aging time and temperature lead to the global increase of the interfacial IMC thickness significantly, but the growth of the Sn-rich IMC layer during aging remains limited in comparison with other solder materials due to the Ni content. In addition, shear creep tests reveal that the shear creep strength of the aged solder joint decreases initially before remaining dramatically stable in the long term. In terms of failure modes, shear creep tests at room and elevated temperatures show distinct rupture features. While rupture within the bulk solder happens at higher temperatures, rupture through the interface occurs at lower temperatures. When the specimens are exposed to isothermal aging, creep deformation becomes more predominant. The aged microstructure exhibits a brittle-to-ductile transition with occurrence of intergranular fracture due to the dynamic recrystallization inside the solder matrix. It is observed that the mechanical factor is of first-order importance in the formation of the new grain structure of the solder.

Interfacial reaction, microstructure and hardness between graphene-coated Cu substrate and SAC305 solder doped with minor Ni during isothermal aging

This study investigated the influences of Cu, high temperature-treated Cu (H-Cu) and graphene-coated Cu (G-Cu) substrates on interfacial reaction, microstructure and hardness of Sn-3.0g-0.5Cu (SAC305) solder alloy. Intermetallic compound (IMC) layer evolution and mechanical property of Sn-3.0g-0.5Cu-0.3Ni (SAC305-0.3Ni) solder joints were also studied under different aging duration. A continuous scallop-like IMC layer was observed at SAC305/Cu, SAC305/H-Cu, SAC305/G-Cu interfaces during reflow and isothermal aging. After adding Ni in the SAC305-0.3Ni solder alloy, the roughness of IMC layer on Cu, H-Cu substrates increased. In contrast, the addition of Ni had a limited impact on the roughness of IMC layer on G-Cu substrates. The total thickness of IMC layer grew as aging time increases, proportionated to the square root of aging duration. The addition of Ni in the solder alloy promoted the growth of IMC layer on Cu and H-Cu substrates, but it was restrained on G-Cu substrate. The amount of the IMC phases in SAC305 and SAC305-0.3Ni solder bulks on the three substrates increased significantly as aging time prolonged. Thus, the hardness of SAC305 and SAC305-0.3Ni solder bulks on the three substrates rose. The addition of Ni in the solder bulks on the three substrates sharply enhanced the formation of [Formula: see text]-Sn phases and increased the quantity of the IMCs. Consequently, the hardness of SAC305-0.3Ni solder bulks was higher than that of SAC305 solder bulk on the three substrates under same aging condition. In addition, the graphene-coated layer on G-Cu substrate could improve the hardness of SAC305 and SAC305-0.3Ni solder bulks.

Board-level vapor phase soldering (VPS) with different temperature and vacuum conditions

Purpose Vapor phase soldering (VPS), also known as condense soldering, is capable of improving the mechanical reliability of solder joints in electronic packaging structures. The paper aims to discuss this issue. Design/methodology/approach In the present study, VPS is utilized to assemble two typical packaging types (i.e. ceramic column grid array (CCGA) and BGA) for electronic devices with lead-containing and lead-free solders. By applying the peak soldering temperatures of 215°C and 235°C with and without vacuum condition, the void formation and intermetallic compound (IMC) thickness are compared for different packaging structures with lead-containing and lead-free solder alloys. Findings It is found that at the soldering temperature of 215°C, CCGA under a vacuum condition has fewer voids but BGA without vacuum environment has fewer voids despite of the existence of lead in solder alloy. In light of contradictory phenomenon about void formation at 215°C, a similar CCGA device is soldered via VPS at the temperature of 235°C. Compared with the size of voids formed at 215°C, no obvious void is found for CCGA with vacuum at the soldering temperature of 235°C. No matter what soldering temperature and vacuum condition are applied, the IMC thickness of CCGA and BGA can satisfy the requirement of 1.0–3.0 µm. Therefore, it can be concluded that the soldering temperature of 235°C in vacuum is the optimal VPS condition for void elimination. In addition, shear tests at the rate of 10 mm/min are performed to examine the load resistance and potential failure mode. In terms of failure mode observed in shear tests, interfacial shear failure occurs between PCB and bulk solder and also within bulk solder for CCGA soldered at temperatures of 215°C and 235°C. This means that an acceptable thicker IMC thickness between CCGA solder and device provides greater interfacial strength between CCGA and device. Originality/value Due to its high I/O capacity and satisfactory reliability in electrical and thermal performance, CCGA electronic devices have been widely adopted in the military and aerospace fields. In the present study, the authors utilized VPS to assemble a typical type of CCGA with the control package of conventional BGA to investigate the relation between essential condition (i.e. soldering temperature and vacuum) to void formation.

Enhancement of hardness of bulk solder by doping Cu nanoparticles at the interface of Sn/Cu solder joint

In this study, Cu NPs prepared by chemical reduction method,were doped into flux at weight proportions (0, 0.2, 0.5, 1 and 2 wt.%) and then reflow soldering was performed for the pure Sn solder (initial diameter = 1400 μm) with Cu substrate at 250 °C for 120 s. The presence of Cu nanoparticles (NPs) in soldering flux was observed to enhance the spreading of solder on Cu substrate. Solder with larger base spread area, characterized with faster diffusion of Cu from substrate attained quicker supersaturation during reflow and larger precipitation of primary Cu6Sn5 intermetallics (IMCs) upon air cooling. The solders containing greater area proportion of primary IMCs are characterized with enhanced effective elastic modulus. Experimentally, the solders prepared with Cu nanoparticles composed flux,were characterized with the increase in Vicker's microhardness. Solder processed with flux containing 2 wt.% Cu nanoparticles possessed Vicker's hardness of 18.6 HV, whereas solder prepared with undoped flux only had hardness of 13.8 HV. Numerical computation of the Cu diffusion and the elasticity have been done using finite element method.

Size effect on interface reaction of Sn–xCu/Cu solder joints during multiple reflows

At present, electronic products are developing in the direction of miniaturization and integration, which leads to the downsizing of solder bump in the packaging process. Moreover, micro solder bumps often require undergoing multiple reflow processes due to the improvement of packaging technology, which has a great influence on interface reaction. Hence, it is necessary to study the effects of solder composition, bump size and reflow cycle on interfacial reaction between solder alloys and Cu substrates. In this experiment, Sn–xCu (x = 0, 0.7, 2.0 wt%) alloys with diameter of 200 µm, 500 µm, and 800 µm were soldered to Cu substrates at 250 °C for 1 min, and then reflowed 20 cycles totally. The size effect of micro solder joints on the growth of IMC after multiple reflows was analyzed. At the same time, the impact of Cu concentration inside the bulk solder on the interfacial reaction during multiple reflows was explored. This experiment finds that the diameter of IMC grains increases with the decrease of solder ball diameter after one reflow cycle, and a significant size effect occurs in Sn/Cu solder bump. As the number of reflow cycle increases, the size effect on interface reaction is more pronounced. The most direct kinetic factor of this phenomenon is that the average Cu concentration in the small-sized solder ball rises faster than the others. When the number of reflow cycle reach to nine times, the lateral growth rate of IMC grains begins to surpass the longitudinal growth rate, and the morphology of IMC grains becomes flat. This phenomenon is especially evident in the small-sized solder ball. Moreover, the addition of Cu element in solder promotes ripening reaction resulting in the lateral growth of IMC. Cu6Sn5 micro particles appearing at the Sn/Cu, Sn–0.7Cu/Cu interface hinder the grain boundary motion and inhibit the lateral annexation of IMC grains, thereby suppressing the lateral growth of IMC.

Critical Review of Size Effects on Microstructure and Mechanical Properties of Solder Joints for Electronic Packaging

With the miniaturization of electronic packaging and devices, the size of solder joints in electronics is also decreasing from bulk solder joints to micro-bumps. Both the microstructure and mechanical properties of the solder joints are also evolving with the decreasing size, which brings great concern for the reliability of different sizes of solder interconnections. In this paper, the effect of solder size on the microstructure (i.e., interfacial intermetallic compound (IMC) growth, precipitation in the solder matrix, dendrite arms, and undercooling) and mechanical properties (i.e., tensile property, shear and compression strength, fracture toughness, and creep deformation) are reviewed from the mechanical point of view. In addition, some areas for further researches about size effects on solder joints are discussed.

Effect of Ni concentration on solderability, microstructure and hardness of SAC0705-xNi solder joints on Cu and graphene-coated Cu substrates

In this paper, solderability, microstructure and hardness of SAC0705-xNi solder joints on Cu and graphene-coated Cu (G-Cu) substrates were studied. As Ni content increases in the solder, the solderability improves gradually on both the Cu and G-Cu substrates. The solderability of SAC0705-xNi is better on G-Cu substrate than that on Cu substrate. The increasing Ni content in the solder has a positive effect on the microstructure refinement of both the kinds of substrates. Such effect is more significant on G-Cu substrate than that on Cu substrate. With the increase of Ni content, the thickness of the interfacial intermetallic compound (IMC) shows an increasing trend first and then decreasing trend on the two kinds of substrates. Since the graphene layer works as a diffusion barrier, the IMC on G-Cu is thinner than that on Cu substrate. The addition of Ni leads to the strengthening of the microstructure and thus increases the hardness of the solder bulks.

Effect of In Addition on Microstructure, Wettability and Strength of SnCu Solder

Abstract Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength.

Abstracts

Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength.

Evolution of the Microstructure of Sn58Bi Solder Paste with Sn-3.0Ag-0.5Cu Addition During Isothermal Aging

Eutectic Sn58Bi (SnBi) solder paste mixed with 0 wt.%, 3 wt.%, 5 wt.%, 8 wt.% and 15 wt.% of Sn-3.0Ag-0.5Cu (SAC) paste was prepared by mechanical mixing. The effects of SAC paste additions on the microstructure evolution of SnBi-SAC/Cu composite solder joints during isothermal aging were investigated. The results indicated that the number of large Bi-rich phases decreased and the relative areas of β-Sn increased with increasing SAC content. Moreover, 1-μm Bi-rich particles were found near the Bi-rich phases. During the isothermal aging process, the diameter of the 1-μm Bi-rich particles in the solder bulk increased by about 50% with aging time by Ostwald ripening. The thickness of the interfacial intermetallic compound in all the solder joints increased slightly during the aging process. The formation of Cu6Sn5 was suppressed by the Bi-rich phases above the Cu6Sn5 layer with the aging time increasing. In addition, the solder bulk showed many cracks along the β-Sn grain boundaries after isothermal aging when the content of SAC paste was 5 wt.%. With 8 wt.% or 15 wt.% SAC, fractures were more obvious near the interface than away from the interface.

Failure study of Sn37Pb PBGA solder joints using temperature cycling, random vibration and combined temperature cycling and random vibration tests

In this paper, the tin-lead (Sn-37wt%Pb) eutectic solder joints of plastic ball grid array (PBGA) assemblies are tested using temperature cycling, random vibrations, and combined temperature cycling and vibration loading conditions. The fatigue lives, failure modes for the solder joints and the typical locations of the failed solder joints for single-variable loading and combined loading conditions are compared and analyzed. The results show much earlier solder joint failure for combined loading than that for either temperature cycling or pure vibration loading at room temperature. The primary failure mode is cracking within the bulk solder under temperature cycling, whereas the crack propagation path is along the intermetallic compound (IMC) layer for vibration loading. The solder joints subjected to combined loading exhibit both types of failure modes observed for temperature cycling and vibration loading; in addition, cracking through the IMC and the bulk solder is observed in the combined test. For temperature cycling and vibration loading, the components in the central region of the printed circuit board (PCB) have more failed solder joints than other components, whereas for combined loading, the number of failed solder joints in the components in different locations of the PCB is approximately the same.