Soldering Alloys

Eu RoHS Directive for environmental and health concerns have resulted in significant activities to find substitutes for lead-contained solders for microelectronics. The potential candidates such as Sn-Ag 1 and Sn-Ag-Cu 1 eutectic solders with melting temperatures of 221°C and 217°C, respectively are the most prominent solders because of their excellent mechanical properties as compared with that of eutectic Sn-Pb solder2. Other candidates as drop-in replacements for eutectic Sn-Pb solder, such as Sn-In-Zn alloys, may have melting point close to 185°C, though not eutectic, and an acceptable solidification range but have received only limited attention 1. Among the many possible lead-free solder alloy candidates, three commonly used alloys to meet automotive thermal cycling requirement are Sn3.0Ag0.5Cu, Sn3.8AgO.7Cu and Sn4.0Ag0.5Cu. However, industry has found these commonly applied solder alloys to have certain level of ball drop problem which affects production yield, product quality as well as customers satisfaction. This paper reports a study which was conducted on BGA lead-free C5 solder joint system to compare SnAgNiCo solder alloy versus the conventional Sn3.8AgO.7Cu solder alloy, with the objective to strengthen the solder joint so as to eliminate ball drop encountered on the conventional Sn3.8AgO.7Cu solder alloy. This study showed that SnAgNiCo C5 solder system performed better than Sn3.8AgO.7Cu in terms of joint strength and intermetallic brittle mode failure rate. Experimental works were carried out to observe the melting properties, micro structure and elemental analysis that were obtained by Differential Scanning Calorimetry (DSC), SEM and EDX respectively. Shear and pull strength was measured by Dage which is representative of solder joint strength between the C5 solder sphere and Cu/Ni/Au pad finishing. Drop Tests were done per Tray & Packing methods to gauge solder joint performance against impact force. A comprehensive study was done to study the effect of microstructure and interface intermetallic of both solder system at ambient, high temperature storage (HTS) at 150°C for 168 hours and 504 hours, and 6× multiple reflow towards the joint integrity. Microstructure studies on SnAgNiCo solder reveals that formation of rod shape Ag 3Sn IMC distributed across the solder surface helps to act as dispersion hardening that increases the mechanical strength for the SnAgNiCo solder. Through the microstructure study with SEM, it was also found that the addition of Ni and Co into the SnAg solder alloy also helps to minimize growth of grain size and prevent increase in intermetallic thickness after thermal aging. EDX analysis confirmed that in SnAgCu solder/Ni interface, Cu-rich IMC formed on top of the Ni-rich IMC. For SnAgNiCo system, only Ni-rich IMC is found. Therefore, it is highly suspected that the presence of Cu-rich IMC posed a detrimental effect on the joint strength and tends to cause brittle joint failure. Both of the effect is then showed in ball pull result that at Time Zero after assembly and after 6× reflow, SnAgCu solder has 90% and 100% brittle mode failure, where SnAgNiCo solder has 0% and 5% for the respective read points. This result correlates with missing ball responses after packing drop tests which shows no failure for SnAgNiCo alloy. Thus, SnAgNiCo lead-free solder is a potential candidate for lead-free solder joint improvement for overall lead-free package robustness.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=4763497&tag=1

Nowadays, nanoelectroctronic, electro-nanotechnologies environmental friendly products become the roadmap in industry. Hence, nanoscale of reinforcement into lead-free solder becomes more popular rather than the micro size of reinforcement. In this paper, Cobalt nanoparticles-reinforced Sn-Ag-Cu composite solders were prepared by thoroughly blending 0.75 wt% of cobalt nanoparticles with near eutectic cement tended to suppress the growth of Cu 3Sn intermetallic layer. However, upon addition of Co nanoparticles, the growth of Cu6Sn5 was enhanced. The distribution of the Co nanoparticles was observed by the elemental mapping which was carried out by using electron micro probe analysis (EPMA) and transmission electron microscopy which equipped together with electron-dispersive X-ray spectroscopy (TEM-EDX). There was no Co detected in the Cu3Sn, it only presented in Cu 6Sn5. The interdiffusion coefficient was increased with the ageing temperature. Upon additionof Co nanoparticles, the interdiffusion coefficient for the formation of Cu3Sn intermetallic layer was reduced, but the interdiffusion coefficient for the formation of Cu 6Sn5 layer was increased. The activation energy for the formation of Cu3Sn was also increased with the Co addition.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5702678&tag=1

A study was conducted on BGA lead-free C5 solder joint system to compare SnAgNiCo versus conventional Sn3.8Ag0.7Cu solder alloy. This study showed that SnAgNiCo C5 solder system performed better than Sn3.8Ag0.7Cu in terms of joint strength and brittle mode failure. Shear and pull strength was measured by Dage which is representative of the intermetallic (IMC) strength between the C5 solder sphere and Cu/Ni/Au pad finishing. Tray drop test and packing drop test were done to gauge solder joint performance against handling and impact force. A comprehensive study was done to study the effect of microstructure and interface intermetallics of both solder system after assembly, after test, after high temperature storage (HTS) at 150°C for 168 hours and 504 hours and after 6x reflow towards the joint integrity. Microstructure studies on SnAgNiCo solder reveals that formation of rod shape Ag 3Sn IMC distributed across the solder surface helps to act as dispersion hardening that increases the mechanical strength for the SnAgNiCo solder after thermal aging. EDX analysis confirmed that in SnAgCu solder/Ni interface, Cu-rich IMC formed on top of the Ni-rich IMC. For SnAgNiCo system, only Ni-rich IMC is found. Therefore, it is highly suspected that the presence of Cu-rich IMC posed a detrimental effect on the joint strength and tends to cause brittle joint failure. Both of the effect is then showed in ball pull result that after 6x reflow, SnAgCu solder has 100% brittle mode failure, where SnAgNiCo solder has only 5%. This result correlates with missing ball responses after packing drop tests. Thus, SnAgNiCo lead-free solder is a potential candidate for lead-free solder joint improvement for overall lead-free package robustness.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5507840&tag=1

Environmental and health concerns have resulted in significant activities to find substitutes for lead-contained solders for microelectronics. The potential candidates such as Sn-Ag1 and Sn-Ag-Cu1 eutectic solders with melting temperatures of 221°C and 217°C, respectively are the most prominent solders because of their excellent mechanical properties as compared with that of eutectic Sn-Pb solder2. Other candidates as drop in replacements for eutectic Pb-Sn solder, such as Sn-In-Zn alloys, may have melting point close to 185°C, though not eutectic, and an acceptable solidification range but have received only limited attention due to various reasons & concerns1. In semiconductor packaging industry, lead-free solders such as Sn-Ag1 and Sn-Ag-Cu1 have been widely applied in mass production of ball grid array products. Such alloys are often used for C5 solder system. However, one of the major challenges is oxidation after thermal processing such as reflow, burn-in, data retention bake and hot temperature testing. A study was conducted on BGA lead-free C5 solder joint system to assess the effect of Germanium (Ge) addition to Sn3.5Ag and Sn3.8Ag0.7Cu solder alloy. The main objective of this study is to find a way to resolve solder surface oxidation after thermal processes, while determining if there's any adverse effect on the solder joint by Ge addition. Experimental works were carried out to observe the melting properties and solder surface morphology by Differential Scanning Calorimetry (DSC) and SEM. Solder surface oxidation was measured by EDX. Shear and pull strength was measured by Dage which is representative of the intermetallic (IMC) strength between the C5 solder sphere and Cu/Ni/Au pad finishing. Solderability test was conducted per Jedec standard. Tray and Packaging Drop Tests were done to gauge solder joint performance against impact force. A comprehensive study was done to study the effect of microstructure and interface intermetallic of both solder system at ambient, high temperature storage (HTS) at 150°C for 24, 48, 96, 168, 504 and 2000 hours and multiple reflow of 1x, 2x, 3x, 6x towards the joint integrity. Overall, Ge doped alloys had significantly higher ball shear and ball pull strength. EPMA microstructure analysis after cross-sectioning on bulk solder and IMC revealed traces of Ge that contributed to the significant increase in ball shear and ball pull strength, while did not cause any bulk solder and IMC morphology changes. Solderability test passed. Drop tests had comparable performance as non Ge doped alloys. In conclusion, addition of Ge in Sn3.5Ag and Sn3.8Ag0.7Cu lead-free solder alloys is able to resolve surface oxidation problem after thermal processing, with improvement in solder joint strength for overall lead-free package robustness.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5416568&tag=1

This paper investigates the growth of intermetallics in Sn-3.8Ag-0.7Cu (SAC) solder with and without Co nanoparticle reinforcement. Co reinforced composite was prepared by mechanically mixing 0.5 wt% and 1.5 wt% Co nanoparticles respectively into Sn-3.8Ag-0.7Cu solder paste. The formation and growth of the intermetallic compounds (IMC) at the solder joint interface were evaluated after thermal aging at 150°C for up to 1008 hours. In the solder joint, Cu6Sn5 intermetallic was observed on Cu substrate, followed by Cu3Sn intermetallic formation between Cu 6Sn5 and Cu after prolonged aging. The thickness of both IMC increased linearly as a function of square root of aging time. The Co nanoparticle reinforcement tended to suppress the growth of Cu3Sn intermetallic layer. However, the total of IMC thickeness was almost the same for the specimen with or without Co nanoparticle reinforcement.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5270667

The effects of adding silver and indium to lead-free solders was investigated through an experiment. The solder alloys used for this study included Sn-0.7Cu-1.01n-xAg, where x = 0, 0.1, 0.3, and 0.5. Thermal analysis of the solder alloys was performed using a DSC-50 differential scanning calorimetry. The heating rate was 5°C/min from room temperature to 350°C in 50mL/min argon gas flow. The wetting balance test was conducted using a SAT5100 Rhesca, and the parameters used for wetting balance test were 5 mm/s of dipping speed with a dipping depth of 10 mm. The Vickers hardness number of the solder alloy was determined by conducting a microhardness test. The curve obtained from DSC analysis shows the onset temperature of Sn-0.7Cu is 227.4°C. Indium was observed distributed in primary grain because of its similar atomic size. Higher Ag contents produced better wettability by lowering wetting time and increasing maximum wetting force.

www.aws.org/bsmc/wj0409-45.pdf

Much literature has been written and debated on the effects of reflow profiles on IMC {Inter-metallic compound) formation for solder alloys and the diffusion of elements during this process. This data has been concerning primarily assembly operations. However, not much data has been acquired on the effects of thermal aging on solders at back end pre-pack operations. The thermal aging conducted at these operations is to dispose of any possible trapped moisture which could lead to severe failures at the SMT customer's site during board level reflow operations, or even on the field of usage due to overheating. The purpose of this study is to present a quantitative analysis to understand the effects of heat and time on the behavior and diffusion of elements in these solders just before they are shipped to the customers. By understanding the behavior of these elements, it would help to correlate towards brittle and ductile fractures much better through CBP tests {Cold ball pull). EDX spot analysis was performed at 3 main spots on cross-sectioned units, i.e. the bulk solder itself, the formed IMC layer and the subsequent layer just below the IMC's. Through this study, it has been conclusive that Copper diffuses more actively than Nickel. It has also been seen that both Copper and Nickel cannot co-exist, and the "cancelling effect" of either element is obvious. It has been conclusive that temperature and baking hours at the pre-pack operations do influence brittle failures in Sn-Pb36-Ag2 more acutely than Sn-Ag3.8-Cu0.7 due to Gold-embrittlement from the formation of AuSn 4. The precipitation of Ag 3Sn is restrained in Sn-Ag3.8-Cu0.7 due to the presence of Gold, thus no obvious Ag 3Sn IMC is observed. This study also confirmed the formation of Ni 3Sn 4 as a primary IMC layer for Sn-Pb36-Ag2. It was seen on the other hand that Sn-Ag3.8-Cu0.7 showcased the formation of two IMC layer's which were defined as (Cu, Ni) 6 Sn 5 and (Ni, Cu) 3 So, where (Cu, Ni) 6 Sn 5 formed on top of the latter.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=4763493&tag=1

In this research, the effects of Mn nanoparticles on wettability and interfacial intermetallic compounds in between Sn-3.8Ag-0.7Cu (SAC) solder and copper (Cu) substrate was investigated. The nanocomposite solders were fabricated by mechanical mixing of SAC solder paste with Mn nanoparticles. The melting characteristic of the solders was characterized by differential scanning calorimeter (DSC). The solder pastes were reflowed in a reflow oven at 250°C for 60 seconds. The spreading rate and contact angle of the solders was calculated to measure the wettability. The solder joints were characterized by field emission scanning electron microscope (FESEM) and energy dispersive X-Ray (EDX). It was found that with the addition of Mn nanoparticles the total IMC thickness decreased after first and six times reflow. The Cu 3 Sn layer was not affected with the addition of Mn nanoparticles. However, some probable mechanism is suggested to explain the effect of Mn nanoparticles on SAC solder.

Interfacial reactions during Si die attachment with Zn-Al-Mg-Ga high-temperature lead (Pb)-free solder on bare Cu lead-frame (Tamac 4) and Ni metallized Cu lead-frame were investigated using an optical microscope, scanning electron microscope (SEM) and energy dispersive x-ray (EDX). Die attachment was performed in an automatic die attach machine in a forming gas environment at temperature 380C. The back side of the die was metallized with Ti/Ni/Ag layers. Comparative studies of die attach properties such as wetting, void and die shear strength on bare and Ni metallized Cu lead-frame was made. Cross sectional microstructural investigation revealed that as many as three intermetallic compound (IMCs) layers form at the solder/lead-frame interface for bare Cu lead-frame. A CuZn intermetallic layer forms close to copper, a scallop shaped CuZn4 forms at the solder side, while Cu5Zn8 forms at the middle. At the interface with Si die, IMC layer could not be detected by SEM. With Ni metallized Cu lead-frame, no IMC layer was observed at the solder/lead-frame as well as Si die/solder interface by SEM. Wetting on Ni metallized Cu lead-frame was found to be lower as compared to that at bare Cu lead-frame. Die shear strength was found to be higher on bare Cu lead-frame (24.2 MPa) as compared to Ni metallized Cu lead-frame (20.5 MPa). Die shear strength of standard Pb-5Sn solder was also measured for comparison and found to be 28.2 MPa. © 2010 IEEE.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5746721&tag=1

In this study, copper particles with different sizes 20-30 nm, 3 and 10 μm were incorporated into Sn-3.5Ag solder paste to form Sn-Ag-Cu composite solder. The Cu particles were added at 0.7 and 3% by paste mixing for 30 min. The composite solder samples were prepared on copper substrate at 240°C for 60 s. Differential scanning calorimetry was conducted to measure the melting point of the composite solder. The wetting angle and microstructure of the composite solder were studied using optical microscope and scanning electron microscope. Micro hardness was measured using a 10 gf load. It was reported that the lowest melting point was obtained at 216.3°C when Cu nanoparticles was added at 3% to Sn-3.5Ag. The microstructure of Sn-3.5Ag solder structure was dendritic in nature. With the addition of Cu nanoparticles, the microstructures were modified with more refined Sn structures. The existence of sunflower morphology of un-melted copper was observed when Cu microparticles were added.

http://link.springer.com/content/pdf/10.1007%2Fs10854-011-0441-7.pdf

Purpose - The purpose of this paper is to enhance the understanding on the electrodeposition of various lead (Pb)-free solder alloys, so that new studies can be carried out to solve processing issues. Design/methodology/approach - The paper reviews the available reports on the electrodeposition of tin (Sn)-based solder systems and identifies the challenges in this area. Findings - Compositional control remains a major challenge in this area, where the achievement of desired composition for binary and ternary alloys is subjected to uncertainties. The use of chelating agents in the bath and optimization of parameters can assist the achievement of near-desired alloy composition. Acidic plating baths are preferred due to their compatibility with photoresists but oxidation of stannous ions causes poor bath stability. Antioxidants, reducing agents and low oxygen overpotential anodes can suppress the oxidation rate and increase the lifespan of plating baths. Apart from chelating agents and antioxidants, various categories of additives can be added to improve quality of deposits. Surfactants, grain refiners and brighteners are routinely used to obtain smooth, fine-grained and bright deposits with good thermo-mechanical properties. Originality/value - The paper provides information on the key issues in electrodeposition of Pb-free solder alloys. Possible measures to alleviate the issues are suggested so that the electrodeposition technique can be established for mass production of a wider range of solder alloys.

http://www.emeraldinsight.com/journals.htm?issn=0954-0911&volume=25&issue=2&articleid=17084668&show=pdf

Zn-Al based alloys have been suggested as high temperature lead-free solder for Si die attach by different researchers. But, so far, these alloys have not been tested in the industrial environment. In this study, Zn-Al-Ge alloy solder wire was used for Si die attachment on Cu substrate in an automatic die attach machine. The back side of the die was metalized with Ti/Ni/Ag layers. Die attachment was performed in a forming gas environment at temperatures ranging from 370°C to 390°C. Interfacial structure at the Si die and Cu substrate interfaces, and the bulk solder microstructure were investigated using an optical microscope, scanning electron microscope (SEM), energy dispersive x-ray (EDX) and electron probe microanalyzer (EPMA). Wetting characteristics and die shear strength were also studied. Cross sectional microstructural investigation revealed that as many as three intermetallic (IMCs) layers form at the solder/copper interface. A CuZn intermetallic layer forms close to copper, a scallop shaped CuZn 4 forms at the solder side, while Cu 5Zn 8 forms at the middle. At the Si die-solder interface, with Ti/Ni/Ag metallization, no IMC layer was detected. Microstructural evidence suggests that Ni layer did not react with the Zn-Al-Ge solder. The microstructure of the bulk solder consists of mainly two phases: one with a lighter contrast and the other one being darker under the SEM. The phase with lighter contrast is a zinc rich phase containing an average of about 70.9% Zn. The darker one is found to be an aluminum rich phase containing an average of about 54.9% Al. In addition, some particles with brighter appearance are observed at different places in the bulk solder. These particles are rich in zinc and their composition closely resembles that of scallop type CuZn 4 IMC which forms at the solder side of the copper-solder interface. It is suggested that these particles are CuZn 4 IMC which have been spalled from the interface during soldering. Wetting of Zn-Al-Ge solder on Cu substrate was found to be lower at 370°C as compared with that at 390°C. Voids were found in the solder which were more in number at higher die attach temperatures. Die shear strength was found to be higher at the die attached temperature of 390°C (22.3 MPa) as compared with those obtained at 370-380°C (15.5-17.4 MPa). Die shear strength of standard Pb-Sn solder was also measured for comparison and was found to be 28.2 MPa. The microstructure and properties of the solder joint are discussed.

ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5643009

This paper reports on the effects of adding Ni nanoparticles to a Sn-3.8Ag-0.7Cu solder. The nanocomposite was prepared by manual blending of SAC solder paste with various percentages of Ni particles. Results showed that the addition of Ni nanoparticles did not bring any significant change in the onset melting temperature of the solder. An increase in the weight percentage of nanoparticles in the solder caused an increase of the wetting angle and a decrease of spreading rate. Moreover, the addition of Ni nanoparticles changed the interfacial intermetallic compound morphology from a scalloped structure into a planar type structure, enhanced the growth of (Cu,Ni)6Sn 5 and suppressed that of Cu3Sn. The concentration of Ni in (Cu,Ni)6Sn5 was higher at the solder side compared with the substrate side. No nickel was detected in the Cu3Sn phase. Ni nanoparticle additions caused an increase in the interdiffusion coefficient in (Cu,Ni)6Sn5, but a reduction in Cu3Sn. All these effects found in the Ni nanoparticle doped solder are similar to the case when Ni is added as an alloying element. Hence, it is suggested that Ni nanoparticles dissolve into the molten solder and influence the intermetallic compound formation through conventional alloying effects.

http://www.sciencedirect.com/science/article/pii/S096697951200369X

Purpose - The purpose of this paper is to investigate the effects of addition Co nanoparticles on the characteristic properties of Sn-3.8Ag-0.7Cu solder. Design/methodology/approach - Cobalt (Co) nanoparticles were added to Sn-Ag-Cu solders by thoroughly blending various weight percentages (0-2.0 wt%) of Co nanoparticles with near eutectic SAC387 solder paste. Blending was done mechanically for 30 min to ensure a homogeneous mixture. The paste mixture was then reflowed on a hot plate at 250 degrees C for 45s. The melting points of nanocomposite solder were determined by differential scanning calorimetry. Spreading rate of nanocomposite was calculated following the JIS Z3198-3 standard. The wetting angle was measured after cross-sectional metallographic preparation. Findings - No significant change in melting point of the solder was observed as a result of Co nanoparticle addition. The wetting angles of the solder increased with the addition of nanoparticles, while the spreading rate decreased. Although the wetting angle increased, the values were still within the acceptable range. Scanning micrograph observations revealed that the as-solidified microstructure of the composite solder was altered by the addition of Co nanoparticles. Microhardness of the solders slightly increased upon Co nanoparticles addition to SAC387. Originality/value - The paper demonstrates that a simple process like paste mixing can be used to incorporate nanoparticles into solder.

http://www.emeraldinsight.com/journals.htm?issn=0954-0911&volume=23&issue=1&articleid=1905669&show=pdf

This study uses modified compact tension specimens to evaluate the fracture toughness of solder joints under a combination of tensile and shear loads. Copper substrates were coated with electroless nickel plating before they were soldered together. The samples were reflowed to peak temperatures of 215 and 240°C for Sn–37Pb and Sn–3.5Ag, respectively, with appropriate preheating temperatures, holding time and cooling

This work investigates the effects of molybdenum nanoparticles on the growth of interfacial intermetallic compound between Sn-3.8Ag-0.7Cu solder and copper substrate during multiple reflow. Molybdenum nanoparticles were mixed with Sn-3.8Ag-0.7Cu solder paste by manual mixing. Solder samples were reflowed on a copper substrate in a 250 °C reflow oven up to six times. The molybdenum content of the bulk solder was determined by inductive coupled plasma-optical emission spectrometry. It is found that upon the addition of molybdenum nanoparticles to Sn-3.8Ag-0.7Cu solder, the interfacial intermetallic compound thickness and scallop diameter decreases under all reflow conditions. Molybdenum nanoparticles do not appear to dissolve or react with the solder. They tend to adsorb preferentially at the interface between solder and the intermetallic compound scallops. It is suggested that molybdenum nanoparticles impart their influence on the interfacial intermetallic compound as discrete particles. The intact, discrete nanoparticles, by absorbing preferentially at the interface, hinder the diffusion flux of the substrate and thereby suppress the intermetallic compound growth.

http://www.sciencedirect.com/science/article/pii/S1044580311002452

Interfacial reactions during Si die attachment with Zn-Al-Mg-Ga high-temperature lead (Pb)-free solder on bare Cu lead-frame (Tamac 4) and Ni metallized Cu lead-frame were investigated using an optical microscope, scanning electron microscope (SEM) and energy dispersive x-ray (EDX). Die attachment was performed in an automatic die attach machine in a forming gas environment at temperature 380C. The back side of the die was metallized with Ti/Ni/Ag layers. Comparative studies of die attach properties such as wetting, void and die shear strength on bare and Ni metallized Cu lead-frame was made. Cross sectional microstructural investigation revealed that as many as three intermetallic compound (IMCs) layers form at the solder/lead-frame interface for bare Cu lead-frame. A CuZn intermetallic layer forms close to copper, a scallop shaped CuZn 4 forms at the solder side, while Cu 5Zn 8 forms at the middle. At the interface with Si die, IMC layer could not be detected by SEM. With Ni metallized Cu lead-frame, no IMC layer was observed at the solder/lead-frame as well as Si die/solder interface by SEM. Wetting on Ni metallized Cu lead-frame was found to be lower as compared to that at bare Cu lead-frame. Die shear strength was found to be higher on bare Cu lead-frame (24.2 MPa) as compared to Ni metallized Cu lead-frame (20.5 MPa). Die shear strength of standard Pb-5Sn solder was also measured for comparison and found to be 28.2 MPa.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5746721

This study uses modified compact tension specimens to evaluate the fracture toughness of solder joints under a combination of tensile and shear loads. Copper substrates were coated with electroless nickel plating before they were soldered together. The samples were reflowed to peak temperatures of 215 and 240 • C for Sn–37Pb and Sn–3.5Ag, respectively, with appropriate preheating temperatures, holding time and cooling rates. Fracture toughness tests were conducted as per ASTM E399-90. Cross-sectional microstructural and fractography studies were carried out on the tested samples. Earlier fracture studies which were based on J integral as per ASTM E813-89 did not produce valid results because of brittleness of the solder joints. The current results showed that the fracture of the solder joints follows the general principles of a mixed mode fracture mechanism map and shear is the preferred mode of failure regardless of different mode of loadings, type of solders and substrate coatings.

Several candidate alloys have been suggested as high-temperature lead-free solder for Si die attachment by different researchers. Among them, Zn-Al based alloys have proper melting range and excellent thermal/electrical properties. In this study, Zn-Al-Mg-Ga solder wire was used to attach Ti/Ni/Ag metallized Si die on Cu lead-frame in an automatic die attach machine. Die attachment was performed in a forming gas environment at temperature ranging from 370 to 400 °C. At the interface with Cu lead-frame, CuZn 4, Cu 5Zn 8 and CuZn intermetallic compound (IMC) layers were formed. At the interface with Si, Al 3Ni 2 IMC formed when 200 nm Ag layer was used at the die back and AgZn and AgZn 3 IMC layers when the Ag layer was 2,000 nm thick. Microstructure of the bulk solder consists of mainly two phases: one with a brighter contrast (about 80.9 wt% Zn) and the other one is a mixture of light (about 73.7 wt% Zn) and dark phases (about 45 wt% Al). Zn-Al-Mg-Ga solder wetted well on Cu lead-frame, covered entire die area and flowed in all directions under the Si die. Less than 10% voids were found in the die attach samples at die attach temperatures of 380 and 390 °C. Die shear strength was found within the acceptable limit (21.8-29.4 MPa) for all the die attach temperatures. Die shear strength of standard Pb-Sn solder was also measured for comparison and was found to be 29.3 MPa. In electrical test, maximum deviation of output voltage after 1,000 thermal cycles was found 12.1%. © 2011 Springer Science+Business Media, LLC.

www.springerlink.com/index/C875964628105GR6.pdf 
http://download.springer.com/static/pdf/369/art%253A10.1007%252Fs10854-011-0511-x.pdf?auth66=1394788287_f84ea5525783d8b26583f58a4b9dba96&ext=.pdf

In this research, the effects of Mn nanoparticles on wettability and interfacial intermetallic compounds in between Sn-3.8Ag-0.7Cu (SAC) solder and copper (Cu) substrate was investigated. The nanocomposite solders were fabricated by mechanical mixing of SAC solder paste with Mn nanoparticles. The melting characteristic of the solders was characterized by differential scanning calorimeter (DSC). The solder pastes were reflowed in a reflow oven at 250°C for 60 seconds. The spreading rate and contact angle of the solders was calculated to measure the wettability. The solder joints were characterized by field emission scanning electron microscope (FESEM) and energy dispersive X-Ray (EDX). It was found that with the addition of Mn nanoparticles the total IMC thickness decreased after first and six times reflow. The Cu3Sn layer was not affected with the addition of Mn nanoparticles. However, some probable mechanism is suggested to explain the effect of Mn nanoparticles on SAC solder.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6320519&tag=1

Several candidate alloys have been suggested as high-temperature lead-free solder for Si die attachment by different researchers. Among them, Zn-Al based alloys have proper melting range and excellent thermal/electrical properties. In this study, Zn-Al-Mg-Ga solder wire was used to attach Ti/Ni/Ag metallized Si die on Cu lead-frame in an automatic die attach machine. Die attachment was performed in a forming gas environment at temperature ranging from 370 to 400 °C. At the interface with Cu lead-frame, CuZn 4, Cu 5Zn 8 and CuZn intermetallic compound (IMC) layers were formed. At the interface with Si, Al 3Ni 2 IMC formed when 200 nm Ag layer was used at the die back and AgZn and AgZn 3 IMC layers when the Ag layer was 2,000 nm thick. Microstructure of the bulk solder consists of mainly two phases: one with a brighter contrast (about 80.9 wt% Zn) and the other one is a mixture of light (about 73.7 wt% Zn) and dark phases (about 45 wt% Al). Zn-Al-Mg-Ga solder wetted well on Cu lead-frame, covered entire die area and flowed in all directions under the Si die. Less than 10% voids were found in the die attach samples at die attach temperatures of 380 and 390 °C. Die shear strength was found within the acceptable limit (21.8-29.4 MPa) for all the die attach temperatures. Die shear strength of standard Pb-Sn solder was also measured for comparison and was found to be 29.3 MPa. In electrical test, maximum deviation of output voltage after 1,000 thermal cycles was found 12.1%.

http://link.springer.com/content/pdf/10.1007%2Fs10854-011-0511-x.pdf