Electronic Solder Research Articles

Finite element model updating of board-level electronic packages by factorial analysis and modal measurements

PurposeOne difficultly in building an effective finite element (FE) model of a board-level package is because of complex structure of the printed circuit board (PCB), as it contains copper layers, woven fabrics, plated-through holes and so forth. Therefore, it is often acceptable to obtain equivalent orthotropic material properties and use them in the simulation. This paper aims to provide a research methodology to produce equivalent FE models for board-level electronic packages.Design/methodology/approachIn this methodology, the FE models’ data were correlated with experimental modal analysis results in terms of natural frequencies and mode shapes. Statistical factorial analysis was used to examine the electronic assembly material properties effect on the structure’s resonant frequencies. The equivalent material properties of the PCB were adjusted using the optimization tool available in ANSYS software for free boundary conditions. The equivalent FE model was then validated for the fixed boundary conditions.FindingsThe resultant FE models were in great match with the measured data in terms of resonant frequencies and mode shapes. The so-developed models can be further used in the analysis of the dynamic response of the electronic packages and solder interconnects.Originality/valueThe current approach provides a sophisticated research methodology to provide high-accuracy FE models of electronic assemblies subjected to vibration. The main value of this approach is to first test the effect of each material property on the package dynamic characteristics before starting the correlation process, then to automate the correlation algorithm using the built-in FE model updating feature available in ANSYS software.

Nucleation and twinning in tin droplet solidification on single crystal intermetallic compounds

βSn nucleation is a key step in the formation of microstructure in electronic solder joints. Here, the heterogeneous nucleation of βSn is studied in undercooled tin droplets spread on the facets of various intermetallic compounds (IMCs). Nucleation undercoolings are measured in solidifying droplets and are linked to orientation relationships (ORs) measured by electron backscatter diffraction (EBSD). Preferred ORs developed on all IMCs studied. For the more potent nucleants (αCoSn3, IrSn4, PtSn4, PdSn4) the ORs represent relatively simple atomic matches. ORs with lower potency nucleants (Cu6Sn5, Ag3Sn, Ni3Sn4) had more complex atomic matches that are explored based on matching of the closest packed atomic rows. βSn solidification twinning is shown to be more complex than has been reported previously: both nucleation on an IMC facet and cyclic twinning of that grain occurred in many droplets on Cu6Sn5, Ag3Sn, Ni3Sn4; in all twinned droplets the <100>Sn twinning axis occurred along a direction on the IMC with the lowest linear atomic disregistry; and interrelated cyclic twins formed consisting of up to five rings of cyclic twins all related by shared <100>Sn axes.

Competition between stable and metastable eutectic growth in Sn-Ni alloys

Metastable Sn-NiSn4 eutectic can form in electronic solder joints and is a reliability concern. Here the competition between stable Sn-Ni3Sn4 and metastable Sn-NiSn4 eutectic growth is studied during unidirectional solidification. The stable and metastable eutectic points are measured, the dynamics of eutectic growth and the transition between the two eutectics is investigated by synchrotron radiography, and the crystallography of eutectic growth is measured by EBSD. It is shown that the Sn-Ni3Sn4 eutectic has a highly irregular morphology with diverging and converging rods because monoclinic Ni3Sn4 grows only along [010] without a reproducible orientation relationship (OR) with Sn. The metastable Sn-NiSn4 eutectic has a more regular broken-lamellar morphology and a reproducible OR with Sn. The critical velocity between the two eutectics is less than 1 μm/s which is discussed in terms of the small temperature difference between the two eutectic points and the kinetic growth advantages of the metastable eutectic.

Probabilistic fatigue damage estimation of embedded electronic solder joints under random vibration

The stress response to random vibrations is an important factor to be taken into account in designing embedded electronic devices. Several test specifications and qualifications use the random vibrations to increase the reliability of electronic products. Fatigue damage estimation of embedded electronic solder joints has not been addressed, especially under random vibration loading. In this paper numerical random vibration analysis with finite element method of an embedded electronic device is used to estimate the stresses Power Spectral Density (PSD). These PSD of stresses are then used in different probabilistic fatigue damage approaches to estimate the reliability of the solder joints regarding the vibration fatigue damage. Two different probabilistic approaches of random fatigue are employed; the time-domain approach based on Rainflow and Monte Carlo simulations and the frequency domain approach based on spectral techniques and statistical data. However, the fatigue damage estimation depends mainly on the accurate results of the stresses PSD. Thus, this work proposes to develop the sub-modelling technique to the random vibration analysis in order to provide accurate results of the stresses PSD. Moreover, this paper discusses the advantages of the approaches listed above to estimate the fatigue damage in solder joints of an electronic component and shows the effectiveness of sub-modelling techniques in the random vibration analysis.

Experimental and numerical investigations regarding laser drop on demand jetting of Cu alloys

Active noise reduction, structural health monitoring and energy harvesting are possible applications of active structure components. An integration of piezoceramic modules into casted Al-structures can be defined as a long term goal to achieve high functional integration and thus to address lightweight construction. Since the liquidus temperature of standard electronic solder is not sufficient to withstand the thermal loads during Al die casting processes, a suitable wire bonding process is an enabling technology to generate active Al structures. In the scope of this work, a laser based drop on demand joining process is introduced. The process consists of four main steps. First a spherical CuSn12 braze preform of 600 µm diameter with a liquidus temperature of 1233 K is inserted into a ceramic capillary and being molten by a laser pulse. The droplet is subsequently ejected by gas overpressure and impinges on the joining area, consisting of the electrode structure of the piezo element and the Cu wire, where it forms a firm joint of the Cu wire and the electrode structure. In order to evaluate time–temperature profiles of the capillary during melting and detachment of the braze preform a simulation model was set up, indicating heating and cooling rates of 22,100 K/s. Further, the impact of the capillary geometry on the velocity fields of a passing medium was evaluated. By changing the capillary geometry, the gas flow velocity could be reduced by about 10% according to the simulation model, which resulted in a reduction of droplet height deviation of 7.5% and a reduction of the droplet diameter deviation of 32.2% in the experiment.

Grain refinement of electronic solders: The potential of combining solute with nucleant particles

Sn-Ag-Cu electronic solder joints typically solidify from a single nucleation event producing either a single βSn grain or twinned grains. With so few and variable βSn orientations, each joint is mechanically unique due to the anisotropy of tetragonal βSn. Here we explore the potential of increasing the number of βSn orientations in solder joints by promoting heterogeneous nucleation during solidification. It is shown that large (60 g) samples can be grain refined effectively by combining growth restricting solute with potent heterogeneous nucleant particles introduced by alloying additions of Zn, Ti, Co, Ir, Pd, or Pt. These mechanisms are discussed with reference to the grain refinement of structural casting alloys. In 500 μm solder balls, these grain refinement approaches were less effective and a relatively high cooling rate of 17 K/s combined with solute and nucleant particles were required to trigger multiple nucleation events. With this approach, up to 12 independent grains formed in 500 μm balls of Sn-3Ag-0.5Cu alloyed with Pt, Co, or Ti, compared with one independent grain in balls without nucleant particle additions.

How Mitigation Techniques Affect Reliability Results for BGAs

Abstract Since 2006 RoHS requirements have required lead free solders to take the place of tin-lead solders in electronics. The problem is that in some environments the lead free solders are less reliable than the older tin-lead solders. One of the ways to solve this problem is to corner stake, edge bond or underfill the components. When considering what mitigation technique and material to use, the operating conditions must be characterized. The temperature range is important when selecting a material to use since the glass transition temperature (Tg) and coefficient of thermal expansion (CTE) are important properties. If improperly chosen, the mitigation material can cause more failures than an unmitigated component. This study focused on 208 I/O BGAs on a 4 layer FR4 board. There were three solders tested; two lead free (SAC305 and SN100C) and one leaded (SnPb). Three mitigation techniques were tested: corner staking, edge bonding, and underfilling. Each of these techniques had two mitigation materials tested. One material was reworkable and the other was not. The boards were subjected to mechanical shock testing and sinusoidal vibration testing until failure. The results of the testing show that no one mitigation technique is best for all of the conditions tested. The same is true for the mitigation material. The best choice of mitigation technique and material is application dependent.

Study of off-eutectic Zn–xMg high temperature solder alloys

Researchers have been working for some times to develop a Pb-free substitute for high-Pb containing solders in electronics. In this study the effect of Mg content on microstructure, melting behavior, thermal, electrical and mechanical properties of a new high-temperature Pb-free solder based on binary Zn–Mg alloy were investigated. The nominal compositions of Zn–xMg (x = 0.5–6.0) alloys were synthesized by conventional melting and casting route. The microstructures of the solders changed significantly on adding Mg content. Higher Mg containing alloys were found rich in intermetallic phases. The formation of the Mg2Zn11 and metastable MgZn2 intermetallic phases were identified and were also quantified by X-ray diffraction analysis. The presence of higher amount of intermetallic compounds in hypereutectic Zn–Mg systems deteriorated the tensile properties of the binary alloy. The results of electrical resistivity test indicated that the Mg could elevate the resistivity of newly developed Zn–Mg alloys. The melting behavior of the solder alloys studied by differential thermal analysis (DTA) analysis revealed that the melting temperature range of the solder alloys narrowed with Mg addition. Thermal mechanical analysis (TMA) analysis revealed that the co-efficient of thermal expansion (CTE) decreased on increasing Mg content.

How Mitigation Techniques Affect Reliability Results for BGAs

Since 2006 RoHS requirements have required lead free solders to take the place of tin-lead solders in electronics. The problem is that in some environments the lead free solders are less reliable than the older tin-lead solders. One of the ways to solve this problem is to corner stake, edge bond or underfill the components. When considering what mitigation technique and material to use, the operating conditions must be characterized. The temperature range is important when selecting a material to use since the glass transition temperature (Tg) and coefficient of thermal expansion (CTE) are important properties. If improperly chosen, the mitigation material can cause more failures than an unmitigated component. This study focused on 208 I/O BGAs on a 4 layer FR4 board. There were three solders tested; two lead free (SAC305 and SN100C) and one leaded (SnPb). Three mitigation techniques were tested: corner staking, edge bonding, and underfilling. Each of these techniques had two mitigation materials tested. One material was reworkable and the other was not. The boards were subjected to mechanical shock testing and sinusoidal vibration testing until failure. The results of the testing show that no one mitigation technique is best for all of the conditions tested. The same is true for the mitigation material. The best choice of mitigation technique and material is application dependent.

High temperature capacitors and transient liquid phase interconnects for Pb-solder replacement

Downhole oil and gas exploration, as well as military electronics have been the main markets for high temperature electronics above 150 °C. However, the use of wide band gap semiconductors, SiC and GaN, in power supplies and automotive applications, in addition to LED lighting, is increasing. These changes are driving the demand for improved capacitors and interconnect materials for packaging these. The development of high temperature multilayer ceramic capacitors (MLCC) with nickel inner electrodes is reviewed and recent developments highlighted. The application and properties of transient liquid phase sintered TLPS metal interconnects to replace solders in leaded MLCC are described in detail. The maximum shear stress capability of TLPS Cu–Sn and In–Ag surpasses Pb-based solders at higher temperatures up to 300 °C, so they are excellent candidates to replace Pb-based solders in future high temperature electronics.

Leaching behaviour and environmental risk assessment of heavy metals from electronic solder in acidified soil

The leaching behaviour of Sn and Pb elements from eutectic SnPb solder of electronic waste in acidic soil was investigated through acidification with HCl-H2SO4 solution and compared with saline solution. The amounts of Sn and Pb elements leached, when subjected to acidic soil, are higher than those with saline soil. Evidence for the significantly preferential release of Sn into the leachate is provided; the galvanic couple accelerated such preferential release. Surface product analysis reveals the slight damage of SnPb in saline soil. Serious dissolution due to electrochemical reaction and a thick, porous PbSO4 surface layer are observed in acidified soil, suggesting more severe toxicity potential of Pb in soil rather than in water.

Lead-Free Solders: Focus on Fundamentals, Reliability, and Applications

Although lead-free solders have been adopted by much of the commercial electronics industry, the implementation of lead-free processes still faces some challenges. The performance of lead-free solder joints in high-reliability aerospace, military, and medical applications is still a concern. There is a clear shortcoming in the ability to predict the lifetime of lead-free assemblies in service. Thus, a common objective of current researchers is to develop a fundamental understanding of the initial solder microstructure, its evolution under various reliability test conditions, and the failure mechanism of lead-free solder joints. Their research focus often includes control of the solidification microstructure by understanding the nucleation of Sn, investigating the role of precipitates and intermetallic compounds at the interface during the deformation of solders, and examining the effect of Sn grain morphology and orientation on the thermomechanical response of solder joints. Researchers are trying to better understand strain-enhanced coarsening, recrystallization, and crack initiation and propagation to develop models that can explain the failure mechanism in different reliability experiments. One goal of this research is to provide a microstructurally adaptive model that can accurately predict the life of soldered electronic devices. Concurrent with the transition to lead-free manufacturing, the electronics industry is experiencing a major revolution associated with the introduction of 2.5D and 3D packaging technologies. There, the lead-free challenges faced by materials scientists become even more complex as the size of electronic packages and solder interconnects shrink to enable deployment of thinner, faster devices. New challenges will be introduced as the entire solder joint may be transformed into intermetallic compounds during assembly or operation. Reducing the size of the solder bumps additionally results in higher current density and increased power dissipation, making electromigration reliability more of a concern. More efficient thermal interface materials are needed to increase the heat flux from die to thermal solution. Moreover, the electronic industry is seeking new solder alloys that are low cost but have acceptable reliability performance in both high-strain applications such as drop/shock and vibration and low-strain applications such as thermal and power cycling. The addition of various fourth and fifth alloying elements to SnAgCu solder alloys is being investigated. The effects of these additional elements on mechanical properties, failure mechanisms, and other reliability phenomena such as Sn whisker formation are not fully understood. In addition, there is a desire among the industrial electronics community to develop new solder alloys for high-temperature applications such as aviation engine controls and downhole drilling electronics as these industries anticipate a future transition to lead-free electronic assembly. Several of these topics were discussed during the Pb-Free Solders and Emerging Interconnect and Packaging Materials Symposium at the TMS 2014 Annual Meeting in San Diego, California. Researchers from industry and academia from different parts of the globe shared their latest research findings in this leading symposium on electronic packaging. More than 60 works were presented in 8 different sessions. A poster session was also organized to include the numerous additional contributions. Three of those technical presentations are presented in this issue of JOM, while others will be published in a special issue of the Journal of Electronic Materials as well as other journals published by TMS. The first article, by E. J. Cotts et al., discusses the effect of impurities on the solidification behavior Babak Arfaei is the guest editor for the Electronic Packaging & Interconnection Materials Committee of the TMS Functional Materials Division, and coordinator of the topic Progress with Lead-Free Solders in this issue. JOM, Vol. 66, No. 11, 2014

Effects of Electromigration on the Creep and Thermal Fatigue Behavior of Sn58Bi Solder Joints

Electromigration (EM), creep, and thermal fatigue (TF) are the most important aspects of the reliability of electronic solder joints, the failure mechanisms of which used to be investigated separately. However, current, mechanical loading, and temperature fluctuation usually co-exist under real service conditions, especially as the magnitude of current density is increasing with joint miniaturization. The importance of EM can no longer be simply ignored when analyzing the creep and TF behavior of a solder joint. The published literature reports that current density substantially changes creep rate, but the intrinsic mechanism is still unclear. Hence, the purpose of this study was to investigate the effects of EM on the creep and TF behavior of Sn58Bi solder joints by analyzing the evolution of electrical resistance and microstructure. The results indicated that EM shortens the lifetime of creep or TF of Sn58Bi solder joints. During creep, EM delays or suppresses the cracking and deforming process, so fracture occurs at the cathode interface. During TF, EM suppresses the cracking process and changes the interfacial structure.

Liquidus Projections of Sn-Co-Ni and Sn-Rich Sn-Ag-Co-Ni Systems

Alloys based on Sn and Sn-Ag are commonly used as Pb-free solders, and Ni is frequently used in barrier layers. Co has been studied as a possible alloying element in both solders and barrier layers. Thus, the Sn-Co-Ni and Sn-Ag-Co-Ni alloy systems are important for electronic soldering. Forty-nine Sn-Co-Ni alloys and 24 Sn-rich Sn-Ag-Co-Ni alloys were prepared. The primary solidification phases of these as-cast alloys were determined, and based on these results and the available phase diagrams of the constituent systems, the liquidus projections of Sn-Co-Ni ternary and Sn-Ag-Co-Ni quaternary systems at 90 at.% and 95 at.% Sn were determined. In the Sn-Co-Ni system, no ternary compound was found; (Ni,Co)3Sn2 and (Ni,Co) are continuous solid solutions, and there are eight kinds of primary solidification phases: Sn, CoSn3, CoSn2, CoSn, (Ni,Co)3Sn2, (Ni,Co), Ni3Sn, and Ni3Sn4. In the 90 at.% and 95 at.% Sn isoplethal sections of the Sn-Ag-Co-Ni liquidus projection, the primary solidification phases are CoSn2, CoSn, Ni3Sn4, and Ag3Sn.

Polytherms of angles of wetting by tin-strontium melts for aluminum films on silicon before and after photon annealing

Polytherms of the angles of wetting by Sn-Sr melts for aluminum films on silicon are investigated by the sessile drop method in helium. Such melts are promising for use as lead-free solders in electronics. It is found that the photon annealing of films enhances their wetting. Wetting thresholds are found at temperatures of >850 K for tin-strontium melts spreading over the surfaces of aluminum films on silicon that are preliminarily processed by photon annealing for 4 s.

Effects of POSS-Silanol Addition on Whisker Formation in Sn-Based Pb-Free Electronic Solders

Previous studies have indicated that silanol in the form of polyhedral oligomeric silsesquioxane (POSS) trisilanol could form strong bonds with solder matrix without agglomeration, and inhibit diffusion of metal atoms when subjected to high ambient temperature and/or high current density. Addition of POSS-trisilanol has also been shown to improve the comprehensive performance of Sn-based Pb-free solders, such as shear strength, resistance to electromigration, as well as thermal fatigue. The current study investigated the whisker formation/growth behaviors of Sn-based Pb-free solders (eutectic Sn-Bi) modified with 3 wt.% POSS-trisilanol. Solder films on Cu substrates were aged at ambient temperature of 125°C to accelerate whisker growth. The microstructural evolution of the solder films’ central and edge areas was examined periodically using scanning electron microscopy. Bi whiskers were observed to extrude from the surface due to stress/strain relief during growth of Sn-Cu intermetallic compounds (IMCs). Addition of POSS-trisilanol was shown to retard the growth of Bi whiskers. The IMCs formed between POSS-modified solders and the Cu substrate showed smoother surface morphology and slower thickness growth rate during reflow and aging. It was indicated that POSS particles located at the phase boundaries inhibited diffusion of Sn atoms at elevated temperatures, and thus limited the formation and growth of IMCs, which resulted in the observed inhibition of Bi whisker growth in POSS-modified solders.

Improvement of international standards for contact soldering in electronics

Proposals are made to improve international standards which can be used for correcting the currently valid and for developing new national standards.

Effects of In/Ce to Sn-3.5Ag Lead-Free Solder on Microstructures and Properties

The Sn-3.5Ag based lead-free electronic solder doped with element In /Ce were prepared respectively using a smelting method with protection by molten salt (SMPMS). The effects of In addition and Ce addition on microstructure, melting point and wettability of Sn-3.5Ag solder were investigated comparatively. And the effects of addition on the intermetallic compound (IMC) layer formed at solder/Cu substrate interface and the shear strength of solder joint were also studied. The results showed that appropriate In or Ce addition can refine Ag3Sn grains and improve the shear strength of solder joint. Element In can significantly reduce the melting point, narrow the melting range of the solder alloy and improve wettability on Cu substrate. When the content of In was 3%, the spreading area was the largest and the wettability was the best. The addition of Ce can efficiently reduce the thickness of the IMC layer grown at the solder/Cu substrate interface and improve the shear strength of solder joint. When the content of Ce was 0.5%, the solder joint had the minimum average IMC thickness and the maximum shear strength.

Reaction evolution and alternating layer formation in Sn/(Bi0.25Sb0.75)2Te3 and Sn/Sb2Te3 couples

(Bi1−xSbx)2Te3 is important for thermoelectric applications, and Sn is the major element for most electronic solders. This study examines interfacial reactions in the Sn/(Bi0.25Sb0.75)2Te3 and Sn/Sb2Te3 couples at 250°C. It has been observed that the interfacial reaction rates are extremely fast, and the growth rates of the layers are 27μm/min and 19μm/min, respectively. The initial reaction product is SnTe phase with nano-size pores which are filled with liquid. With prolonged reaction time, the nano-size liquid droplets connect together and form periodic liquid layer in the reaction zone. The liquid layer, which is primarily molten Sn, further reacts with Sb, and a self-assembled Sn3Sb2/SnTe alternating layer is found in the couples reacted for longer than 15min. Bi precipitates with feature size less than one micron are found as well in the reaction zone in the Sn/(Bi0.25Sb0.75)2Te3 couples in the later stage of reactions. The alternating layer and Bi precipitates result from supersaturation of Sb and Bi in the reaction layer caused by the fast in-flux of Sn.

Effects of Nano-Particles on Properties of Electronic Solders

It is analyzed that the mechanisms of nano-particles impact on melting temperature, wet ability, and mechanical properties of electronic solders, based on the characteristics of nano-particles, the crystal lattice structure and the interaction between particles and the matrix. The results indicates that if the interface between nano-particles and the matrix is low-energy state, it increases melting temperature of composite solders on ignoring the dissolution of nano-particles, conversely, the high-energy state reduces it. When nano-particles form appropriate frame structure in the liquid solder, the emergence of capillary adsorption can enhance the wettability, the strengthening mechanisms of nano-particles on solder include the Phase II enhancement, grain boundary strengthening and solid-solution strengthening. During the brazing process, nano-particles hinder the diffusion of atoms and reduce the dissolution rate of base materials in liquid solders, to inhibit the growth of the intermetallic compounds (IMC) of interface, thereby enhancing the strength and reliability of joints.