Bulk Solder Research Articles

Effect of Design Parameters on Drop Test Performance of Wafer Level Chip Scale Packages

Today’s consumer market demands electronics that are smaller, faster, and cheaper. To cater to these demands, novel materials, new designs, and new packaging technologies are introduced frequently. Wafer level chip scale package (WLCSP) is one of the emerging package technologies that have the key advantages of reduced cost and smaller footprint. The portable consumer electronics are frequently dropped; hence, the emphasis of reliability is shifting toward the study of effects of mechanical shock loading increasingly. Mechanical loading typically induces brittle fractures (also known as intermetallic failures) between the solder bumps and the bond pads at the silicon die side. This type of failure mechanism is typically characterized by the board level drop test. WLCSP is a variant of the flip-chip interconnection technique. In WLCSPs, the active side of the die is inverted and connected to the printed circuit board (PCB) by solder balls. The size of these solder balls is typically large enough (300 μm pre-reflow for 0.5-mm pitch and 250 μm pre-reflow for 0.4-mm pitch) to avoid the use of underfill that is required for the flip-chip interconnects. Several variations are incorporated in the package design parameters to meet the performance, reliability, and footprint requirements of the package assembly. The design parameters investigated in this effort are solder ball compositions with different silver (Ag) contents, backside lamination with different thicknesses, WLCSP type—direct and redistribution layer (RDL), bond pad thickness, and sputtered versus electroplated under bump metallurgy (UBM) deposition methods for 8 × 8, 9 × 9, and 10 × 10 array sizes. The test vehicles built using these design parameters were drop tested using Joint Electron Devices Engineering Council (JEDEC) recommended test boards and conditions as per JESD22-B11. Cross-sectional analysis was used to identify, confirm, and segregate the intermetallic and bulk solder failures. The objective of this research was to quantify the effects and interactions of WLCSP design parameters through drop test. The drop test data were collected and treated as a right censored data. Further, it was analyzed by fitting empirical distributions using the grouped and ungrouped data approach. Data analysis showed that design parameters had a significant effect on the drop performance and played a vital role in influencing the package reliability.

Rate-dependent behavior of Sn alloy–Cu couples: Effects of microstructure and composition on mechanical shock resistance

Solders serve as electrical and mechanical interconnects in electronic packaging. The mechanical shock behavior of a Pb-free solder joint is quite complex, since the influences of solder microstructure, intermetallic compound (IMC) layer thickness, and strain rate on the overall dynamic solder joint strength need to be quantified. Dynamic solder joint strength is hypothesized to be controlled by two factors. At low strain rates it should be controlled by the bulk solder, whereas at high strain rates it may be controlled by the brittle intermetallic compound layer. In this paper, the dynamic solder joint strength of Sn–3.9 Ag–0.7 Cu solder joints was experimentally measured over the strain rate range 10−3–12s−1. The influences of changes in solder microstructure and IMC layer on dynamic solder joint strength were quantified, and visualized in three dimensions. Fracture mechanisms operating in the solder-controlled and IMC layer-controlled dynamic strength regimes are discussed. Finally, qualitative numerical simulations were conducted, which accurately depict the experimentally observed fracture behaviors.

Prediction of damage and fatigue life of high-temperature flip chip assembly interconnections at operations

The determination of the real value of damage/plastic work density in solder joints from computer numerical modelling and its usage in fatigue life prediction models based on accumulated energy density is critical to improving the accuracy of predicted life of solder joints. Commercial ANSYS software based on three-dimensional finite element analysis (FEA) was employed to investigate damage of bonded materials of lead-free solder joints in a flip chip (FC48D6.3C457) mounted on a printed circuit board (PCB). The trend behaviour of accumulated damage and fatigue life per cycle over many accelerated thermal cycles (ATCs) are also studied. The solder bumps deformation is modelled using ANAND’s visco-plasticity and the performances of all other materials in the assembly were captured with appropriate material models. It was observed that the difference in stress magnitude and amplitude between inter-metallic compounds (IMCs) at the die side and solder bulk was highest and the presence of IMC in the joints increases bump damage which occurs in three stages during temperature cycle loading. These results demonstrate that while IMC impacts solder joint reliability, the bond at interconnect between IMC at the die side and solder bulk is most vulnerable to fatigue crack initiation and propagation. A new methodology to find accurate solder joint damage is presented. The findings show that average damage from cycle of hysteresis loop stabilisation to cycle of onset of tertiary damage demonstrates potential of being adequate in determining magnitude of the solder joint damage. However, considering that damage evolution is in three-phase, we propose the use of polynomial function to estimate plastic work damage in FC solder joints.

The bulk alloy microstructure and tensile properties of Sn‐1Ag‐0.5Cu‐xAl lead‐free solder alloys (x=0, 1, 1.5 and 2 wt.%)

PurposeThe purpose of this paper is to investigate the effect of Al addition on the bulk alloy microstructure and tensile properties of the low Ag‐content Sn‐1Ag‐0.5Cu (SAC105) solder alloy.Design/methodology/approachThe Sn‐1Ag‐0.5Cu‐xAl (x=0, 1, 1.5 and 2 wt.%) bulk solder specimens with flat dog‐bone shape were used for tensile testing in this work. The specimens were prepared by melting purity ingots of Sn, Ag, Cu and Al in an induction furnace. Subsequently, the molten alloys were poured into pre‐heated stainless steel molds, and the molds were naturally air‐cooled to room temperature. Finally, the molds were disassembled, and the dog‐bone samples were removed. The solder specimens were subjected to tensile testing on an INSTRON tester with loading rate 10−3 s−1. The microstructural analysis was carried out using scanning electron microscopy/Energy dispersive X‐ray spectroscopy. Electron Backscatter Diffraction (EBSD) analysis was used to identify the IMC phases. To obtain the microstructure, the solder samples were prepared by dicing, molding, grinding and polishing processes.FindingsThe addition of Al to the SAC105 solder alloy suppresses the formation of Ag3Sn and Cu6Sn5 IMC particles and leads to the formation of larger Al‐rich and Al‐Cu IMC particles and a large amount of fine Al‐Ag IMC particles. The addition of Al also leads to refining of the primary β‐Sn grains. The addition of Al results in a significant increase on the elastic modulus and yield strength. On the other hand, the addition of Al drastically deteriorates the total elongation.Originality/valueThe addition of Al to the low Ag‐content SAC105 solder alloy has been discussed for the first time. This work provides a starting‐point to study the effect of Al addition on the drop impact and thermal cycling reliability of the SAC105 alloy.

Novel Fe-containing Sn–1Ag–0.5Cu lead-free solder alloy with further enhanced elastic compliance and plastic energy dissipation ability for mobile products

This work investigates the effects of small additions (0.1 and 0.3wt.%) of Fe on the microstructure, mechanical, and thermal properties of the Sn–1Ag–0.5Cu (SAC105) solder alloy. The addition of Fe leads to the formation of large FeSn2 intermetallic compound (IMC) particles located in the eutectic regions besides the small Ag3Sn and Cu6Sn5 IMC particles. Fe-bearing solders have also been shown to form large primary β-Sn grains. The formation of large FeSn2 IMC particles, together with the presence of large primary β-Sn grains have resulted in a significant reduction on the elastic modulus and yield strength. Moreover, the presence of large primary β-Sn grains causes the Fe-bearing solders to maintain the total elongation at the SAC105 level. This effect can increase the elastic compliance and plastic energy dissipation ability of the bulk solder, which play an important role in drop impact performance enhancement. The addition of Fe did not produce any significant effect on the melting behavior. As a result, the use conditions of the Fe-bearing solders are consistent with the conditions for conventional Sn–Ag–Cu solder alloys.

Board-Level Solder Joint Reliability of Edge- and Corner-Bonded Lead-Free Chip Scale Package Assemblies Subjected to Thermal Cycling

In this paper, we present the results of thermal cycling test for edge- and corner-bonded lead-free chip scale packages (CSPs), which was carried out on the basis of the IPC-9701 test standard. Six materials were used in this study: four edge-bond adhesives and two corner-bond adhesives. These adhesives were compared with CSPs with full capillary flow underfill (FCFU) and without adhesives. The thermal cycling test results show that corner-bond adhesive has comparable solder joint reliability performance with CSP without adhesive, and is better than edge-bond adhesive, followed by CSPs with FCFU. In addition, the adhesives with a low coefficient of thermal expansion, a high glass transition temperature and a intermediate storage modulus yielded good performance. Results of detailed failure analysis indicate that the dominant failure mode is solder bulk fatigue cracking near package and/or printed circuit board (PCB) pads, and that the location of critical solder joints change from die edges to package corners with the introduction of adhesives.

Board-Level Solder Joint Reliability of Edge- and Corner-Bonded Lead-Free Chip Scale Package Assemblies Subjected to Thermal Cycling

In this paper, we present the results of thermal cycling test for edge- and corner-bonded lead-free chip scale packages (CSPs), which was carried out on the basis of the IPC-9701 test standard. Six materials were used in this study: four edge-bond adhesives and two corner-bond adhesives. These adhesives were compared with CSPs with full capillary flow underfill (FCFU) and without adhesives. The thermal cycling test results show that corner-bond adhesive has comparable solder joint reliability performance with CSP without adhesive, and is better than edge-bond adhesive, followed by CSPs with FCFU. In addition, the adhesives with a low coefficient of thermal expansion, a high glass transition temperature and a intermediate storage modulus yielded good performance. Results of detailed failure analysis indicate that the dominant failure mode is solder bulk fatigue cracking near package and/or printed circuit board (PCB) pads, and that the location of critical solder joints change from die edges to package corners with the introduction of adhesives.

Microstructure, orientation and damage evolution in SnPb, SnAgCu, and mixed solder interconnects under thermomechanical stress

In this study, the microstructure, orientation evolution and failure modes of SnPb, SnAgCu, and mixed (SnAgCu bump+SnPb paste) solder interconnects were studied by thermal cycling between 0^oC and 100^oC (with 15min dwell times and 15min ramps). The cross-sectioned ball grid array (BGA) solder interconnects were characterized by scanning electron microscopy (SEM) equipped with an electron backscattered diffraction (EBSD) detector to observe the orientation, microstructure and damage evolution under thermal cycling. The results show that the as-reflowed SnPb solder interconnect is composed of a limited number of grains with similar crystallographic orientations, while the mixed and SnAgCu solder interconnects are composed of a few @b-Sn grains or even only one grain. Significant recrystallization behavior was observed in strain concentration regions in all these interconnects after thermal cycling. Small subgrains with low angle grain boundaries could finally evolve into recrystallized grains by rotation and coalescence as thermomechanical fatigue continues. Due to strain enhanced diffusion, dramatic coarsening of Pb phase was observed in the recrystallized regions of SnPb and mixed solder interconnects; similarly, the coarsening of Ag3Sn intermetallic compound (IMC) particulates occurred in the recrystallized microstructures of mixed and SnAgCu solder interconnects after thermal cycling. Cracks were initiated and propagated mainly within the recrystallized microstructures by intergranular cracking in the solder bulk. Interphase sliding was frequently observed along the crack path of SnPb solder interconnects. Vickers microhardness results show that SnAgCu and mixed solder interconnects degrade significantly as thermal cycling continues, and hardness correlates well with the accumulated damage in these solder interconnects.

Drop impact reliability of Sn–1.0Ag–0.5Cu BGA interconnects with different mounting methods

The poor drop-shock resistance of near-eutectic Sn–Ag–Cu (SAC) solder interconnects drives the research and application low-Ag SAC solder alloys, especially for Sn–1.0Ag–0.5Cu (SAC105). In this work, by dynamic four-point bend testing, we investigate the drop impact reliability of SAC105 alloy ball grid array (BGA) interconnects with two different surface mounting methods: near-eutectic solder paste printing and flux dipping. The results indicate that the flux dipping method improves the interconnects failure strain by 44.7% over paste printing. Further mechanism studies show the fine interfacial intermetallic compounds (IMCs) at the printed circuit board side and a reduced Ag content inside solder bulk are the main beneficial factors overcoming other negative factors. The flux dipping SAC105 BGA solder joints possess fine Cu6Sn5 IMCs at the interface of solder/Cu pads, which increases the bonding strength between the solder/IMCs and the fracture resistance of the IMC grains themselves. Short soldering time of flux dipping joints above the solder alloy liquidus mitigates the growth of interfacial IMCs in size. In addition, a reduced Ag content in flux dipping joint bulk causes a low hardness and high compliance, thus increasing fracture resistance under higher-strain rate conditions.

Solderability, IMC evolution, and shear behavior of low-Ag Sn0.7Ag0.5Cu-BiNi/Cu solder joint

The solderability, intermetallic compounds (IMC) evolution, and shear behavior of the low-Ag Sn0.7Ag0.5Cu-3.5Bi-0.05Ni (SAC0705-BiNi)/Cu solder joint was investigated by comparing with Sn0.7Ag0.5Cu (SAC0705)/Cu and Sn3.0Ag0.5Cu (SAC305)/Cu solder joints. Experimental results demonstrated that the melting temperature of Sn0.7Ag0.5Cu-BiNi solder alloy was lower than that of SAC0705 and SAC305 solder. But the melting range of Sn0.7Ag0.5Cu-BiNi was wider. Compared with the other two kinds of alloys, SAC0705-BiNi showed the best wettability. SAC0705/Cu, SAC0705-BiNi/Cu, and SAC305 solder joints appeared similar IMC morphologies and grain size at the beginning of soldering, but evolved to different appearance as the soldering process proceeded. The growth rate of the IMC grains in SAC0705-BiNi/Cu solder joint was the lowest because of the refinement of Ni. SAC0705-BiNi/Cu solder joint showed the highest shear strength before and after being aged, mainly due to the enhancement of solid solution strengthening and dispersion strengthening of Bi and Ni in the bulk solder, as well as the refinement of Ni at the soldering interface.

A novel high speed impact testing method for evaluating the low temperature effects of eutectic and lead‐free solder joints

PurposeThe purpose of this paper is to present a novel high speed impact testing method for evaluating the effects of low temperatures on eutectic and lead‐free solder joints. Interfacial cracking failure of Sn‐based and Pb‐free solders at subzero temperatures is of significant concern for electronic assemblies that operate in harsh environments.Design/methodology/approachThis paper presents a newly designed low temperature control system coupled with an Instron micro‐impact testing machine, which offers a package level test for solder bumps, and that is used at subzero temperature ranges as low as −40°C. This study examined the failure characteristics of 63Sn‐37Pb (Sn37Pb) and 96.5Sn‐3Ag‐0.5Cu (SAC305) solder joints at temperatures ranging from room temperature (R.T.) to −40°C, and at impact speeds of 1 m/s.FindingsThree types of failure mode were identified: M1 interfacial fracture with no residual solder remaining on the pad (interfacial cracking); M2 interfacial fracture with residual solder persisting on the pad (mixed mode failure); and M3 solder ball fracture (bulk solder cracking). The experimental results indicated that the energy to peak load for both types of solders decreased significantly, by approximately 35 percent to 38 percent when the test temperature was reduced from R.T. to −40°C. In addition, the peak load of the Sn37Pb solder joint increased noticeably with a decreasing test temperature. However, the peak load of the SAC305 specimen remained virtually unchanged with a reduction in the temperature. The Sn37Pb solder joints failed in an M3 failure mode under all the considered testing temperatures. The SAC305 solder joints displayed both M1 and M2 failure modes at R.T.; however, they failed almost exclusively in M1 mode at the lowest test temperature of −40°C.Originality/valueThis paper presents a novel technique for evaluating high‐speed impact strength and energy absorbance of Sn‐based and Pb‐free solders at the chip level within a low temperature control system. To overcome the drawbacks experienced in other studies, this study focused specifically on cryo‐impact testing systems and the performed experimental steps to improve the accuracy of post‐test analysis.

Bismuth-Antimony as an Alternative for High Temperature Lead Free Solder

The development works on high temperature lead free solder are mostly discussed nowadays. To replace the current high temperature lead free solders, further research need to be done. A great deal of effort has been put into the development of lead free solder alloys. Bi (Bismuth) and Sb (Antimony) solder system proved as one of the promising candidates for electronic assembly. Melting temperature of three Bi-Sb solder alloys studied in this research enhanced their potential as the alternative solder candidates for high temperature lead free solder. At interface, Cu3Sb IMC layer was formed for 95Bi-5Sb solder alloy. Spallation of Cu3Sb IMC layer took placed with the results of Cu3Sb IMC also found in the solder bulk. Analysis of 97.5Bi-2.5Sb solder alloy classified as no metallurgical reaction at the interface and only the mechanical joining existed at the interface. The dissolution of Cu from subtrate affected the formation of Cu rich phase and the unstable Bi-Cu rich phase phenomena act as the isothermal product found in solder bulk. Mechanical grain boundary grooving observed in 98.5Bi-1.5Sb solder alloys at interface. Different compositions of Bi-Sb solder alloys resulted in different types of microstructures at interface and in solder bulk after reflow.

A review on effect of minor alloying elements on thermal cycling and drop impact reliability of low‐Ag Sn‐Ag‐Cu solder joints

PurposeThe purpose of this paper is to discuss the reliability of board level Sn‐Ag‐Cu (SAC) solder joints in terms of both thermal cycling and drop impact loading conditions, and further modification of the characteristics of low Ag‐content SAC solder joints using minor alloying elements to withstand both thermal cycle and drop impact loads.Design/methodology/approachThe thermal cycling and drop impact reliability of different Ag‐content SAC bulk solder will be discussed from the viewpoints of mechanical and micro‐structural properties.FindingsThe best SAC composition for drop performance is not necessarily the best composition for optimum thermal cycling reliability. The content level of silver in SAC solder alloys can be an advantage or a disadvantage depending on the application, package and reliability requirements. The low Ag‐content SAC alloys with different minor alloying elements such as Mn, Ce, Bi, Ni and Ti display good performance in terms of both thermal cycling and drop impact loading conditions.Originality/valueThe paper details the mechanical and micro‐structural properties requirements to design a robust bulk SAC solder joint. These properties provide design and manufacturing engineers with the necessary information when deciding on a solder alloy for their specific application.

On the relationship between solder-controlled and intermetallic compound (IMC)-controlled fracture in Sn-based solder joints

The strain-rate-dependent mechanical behavior of Sn-rich solder is of fundamental importance. Dynamic solder joint strength is hypothesized to be controlled by two factors. At low strain rates it is believed to be controlled by the bulk solder and at high strain rates by the brittle intermetallic layer. In this paper, the dynamic solder joint strength of Sn–3.9Ag–0.7Cu solder joints was experimentally quantified to verify the solder and intermetallic compound-controlled behavior hypothesis.

STUDY ON LEAD-FREE SOLDER JOINT RELIABILITY BASED ON GRAIN ORIENTATION

During service, coefficient of thermal expansion (CTE) mismatch between different materials in electronic devices can lead to stress and strain concentration, and the creep and fatigue damage will accumulate, leading to final failure of solder joints. The main constituent of Pb-free solder joint is β-Sn, which is body-centered tetragonal metal. There is big difference in CTE and elastic modulus along different directions of β-Sn, showing strong anisotropy. Therefore, solder joints with different orientations show quite different thermo-mechanical responses. In this study, ball grid array (BGA) assemblies were subjected to thermal cycling, and the orientation of the solder joints was characterized by EBSD to track the orientation evolution in different solder joints. Surface Evolver was adopted to simulate the three-dimensional shape of the solder joint. Based on the shape and grain structure of real lead-free solder joints, the thermal stress and strain distribution in BGA assemblies under thermal loading were computed. Sub-model based on grain numbers and orientation distribution is solved to get the strain distribution of the three typical solder joints. The experimental and simulated results show that grain orientation significantly influences the solder joint reliability and failure mode. For single-grained solder joints, stress and strain concentration is located in the solder bulk near the interface, where recrystallization accompanied with initiation and propagation of cracks. However, for

Isothermal Aging Effects on the Thermal reliability Performance of Lead-Free Solder Joints

Electronics assemblies containing solder joints are often exposed to elevated temperatures for prolonged periods of time. The time-at-temperature stress impacts the overall package reliability of the assembled circuitry due to evolving materials, microstructural, and mechanical properties. It is especially important to understand the impact of isothermal aging on the long term behavior of lead (Pb)-free solder joints which operate in harsh environments. In this study, we have explored the effects of elevated temperature isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies on board level packages. As the isothermal aging temperature increases, the Weybull characteristic lifetime for SAC 105 and 305 solder joints is drastically reduced compared to Sn-37Pb.In parallel mechanical studies on bulk solder specimens, the creep rate for SAC105, 305 rapidly increases with aging. A full test matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch μBGAs. The test structures were built on three different board finishes (ImSn, ImAg and SnPb). Storage condition temperatures were 25°C, 55°C, 85°C and 125°C with aging over time periods of 0, 6, and 12 months. Subsequently, the specimens were thermally cycled from −40°C to 125°C with 15 min dwell times at the high temperature. It was found that the thermal performance of lead-free fine-pitch packages significantly degrades up to 55–60% after aging at elevated temperature. The dominant failure mode can be associated with the growth of Cu6Sn5 intermetallic compounds (IMC) during the aging, particularly on the pad side.

Stability of molybdenum nanoparticles in Sn–3.8Ag–0.7Cu solder during multiple reflow and their influence on interfacial intermetallic compounds

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. ► Mo nanoparticles do not dissolve or react with the SAC solder during reflow. ► Addition of Mo nanoparticles results smaller IMC thickness and scallop diameter. ► Mo nanoparticles influence the interfacial IMC through discrete particle effect.

High temperature reliability of lead-free solder joints in a flip chip assembly

The visco-plastic behaviour of solder joints of two models of a flip chip FC48D6.3C457DC mounted on a printed circuit board (PCB) via SnAgCu solder is investigated using Anand's model. While the bumps of one of the models are realistic with 6 μm thickness of intermetallic compound (IMC) at interconnects of solder and bond pads, the other are made up of conventional bumps without IMC at these interconnects. The solder bump profiles were created using a combination of analytical method and construction geometry. The assembled package on PCB was accelerated thermally cycled (ATC) using IEC standard 60749-25. It was found in the result of the simulation that IMC does not only impact solder joint reliability but also is a key factor of fatigue failure of solder joints. The IMC sandwiched between bond pad at chip side and solder bulk is the most critical and its interface with solder bulk is the most vulnerable site of damage. With reference to our results, it is proposed that non inclusion of IMC in solder joint models composed of Sn-based solder and metalized copper substrate is one of the major causes of the discrepancy on solder joint fatigue life predicted using finite element modelling and the one obtained through experimental investigation.

Effects of bump size on deformation and fracture behavior of Sn3.0Ag0.5Cu/Cu solder joints during shear testing

Abstract To elaborate deformation and fracture behavior of Sn3.0Ag0.5Cu/Cu solder joint, dynamic shear testing was designed and performed on as-reflowed and aged solder joints which contained limited numbers of grains. Polarized light microscopy (PLM) and electron backscatter diffraction (EBSD) were used to reveal the grain morphology and the grain distribution. Scanning electron Microscopy (SEM) was applied to observe the influences of microstructures and intermetallic compounds (IMCs) on the solder joints fracture behavior. The experimental results suggested that the morphology and distribution of IMCs (Cu 6 Sn 5 and Ag 3 Sn) were the main reasons for the size effect in Sn3.0Ag0.5Cu solder joints. The shear strength of as-reflowed and aged solder joints decreased with the increased joint sizes. The nano-scale particle-like Ag 3 Sn dispersed in the small solder joints had a strengthen effect on their mechanical property. While the dendritic and feather-like Ag 3 Sn brought the big solder joints the brittleness. Furthermore, obvious plastic deformation in company with dynamic recovery and recrystallization was observed in small solder joints during dynamic shear testing. The fracture occurred in the bulk solder. While the brittle fracture occurred in big solder joints and the fracture positions were at the areas close to the soldering interface. The cracks propagated by the way of transgranular fracture. After aging, dynamic recovery and recrystallization occurred in all solder joints. While the plasticity of big solder joint was enhanced and the transgranular fracture as well as the intergranular fracture occurred during dynamic shear testing.

Die attach properties of Zn–Al–Mg–Ga based high-temperature lead-free solder on Cu lead-frame

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, CuZn4 ,C u 5Zn8 and CuZn intermetallic compound (IMC) layers were formed. At the interface with Si, Al3Ni2 IMC formed when 200 nm Ag layer was used at the die back and AgZn and AgZn3 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%.