Eutectic SnPb Solder Research Articles

Interfacial reactions between eutectic Sn–Pb solder and Co substrate

This study examined the reaction couples of Co substrate with liquid and solid Sn-37 wt% Pb alloys, respectively. In the liquid/solid Sn–Pb/Co couples, one single-phase CoSn3 layer was found at the interface at temperatures from 210 to 250 °C. The layer thickness was measured as a function of the reaction time. The layer growth rate increased with increasing temperature. The CoSn3 layer growth was linear in the early stage and then became parabolic. In addition, the similar CoSn3 growth behavior results were observed in the solid/solid Sn–Pb/Co couples from 150 to 170 °C. Nevertheless, one Pb-rich phase layer accumulated at the solder/CoSn3 interface due to Sn consumption. Also, the reaction kinetics of Sn-37 wt% Pb/Co was symmetrically studied at various temperatures. The chemical reaction constants, diffusion constants, and the relevant activation energies were reported.

Computational Simulation of the Joint Shape after as-Reflowed Soldering

Using the interactive software, this study was mainly focused on developing a 3-D joint shape after the as-reflowed soldering which is the one of chief factors affecting the reliability. From the results, the soldering process can be simulated and observed directly. By changing the various conditions and constraints, such as surface tension, wetting angle and specific gravity, it makes a comparison between the simulated result and the experimental one. The simulated parameters are based on those of the traditional eutectic Sn-Pb solder. After slicing and comparing the different profiles of the simulated shape, the 3-D simulated geometry fits well with the experimental one.

Influence of electrochemical properties on electrochemical migration of SnPb and SnBi solders

SnPb solders have been widely used in many fields because of their excellent solderability. To respond to environmental and health concerns regarding SnPb solders, it is necessary to develop lead-free solder for electronic assembly. In this paper, the resistance to the electrochemical migration of eutectic SnPb and SnBi solder alloys was evaluated and analyzed to detect the difference in the electrochemical migration behavior between these solders by their electrochemical properties. Pb and Bi additions to a Sn-base solder improved the resistance to electrochemical migration because of enhanced polarization properties. The resistance was closely related to the cathodic deposition efficiency and the anodic dissolution behavior. In addition, the composition of the dendrite formed by the electrochemical migration was related to the standard electrode potential of the alloying element.

Measurement of electromigration activation energy in eutectic SnPb and SnAg flip-chip solder joints with Cu and Ni under-bump metallization

Electromigration activation energy is measured by a built-in sensor that detects the real temperature during current stressing. Activation energy can be accurately determined by calibrating the temperature using the temperature coefficient of resistivity of an Al trace. The activation energies for eutectic SnAg and SnPb solder bumps are measured on Cu under-bump metallization (UBM) as 1.06 and 0.87 eV, respectively. The activation energy mainly depends on the formation of Cu–Sn intermetallic compounds. On the other hand, the activation energy for eutectic SnAg solder bumps with Cu–Ni UBM is measured as 0.84 eV, which is mainly related to void formation in the solder.

Viscoplastic Creep Response and Microstructure of As-Fabricated Microscale Sn-3.0Ag-0.5Cu Solder Interconnects

The viscoplastic behavior of as-fabricated, undamaged, microscale Sn-3.0 Ag-0.5Cu (SAC305) Pb-free solder is investigated and compared with that of eutectic Sn-37Pb solder and near-eutectic Sn-3.8Ag-0.7Cu (SAC387) solder from prior studies. Creep measurements of microscale SAC305 solder shear specimens show significant piece-to-piece variability under identical loading. Orientation imaging microscopy reveals that these specimens contain only a few, highly anisotropic Sn grains across the entire joint. For the studied loads, the coarse-grained Sn microstructure has a more significant impact on the scatter in primary creep compared to that in the secondary creep. The observed lack of statistical homogeneity (microstructure) and joint-dependent mechanical behavior of microscale SAC305 joints are consistent with those observed for functional microelectronics interconnects. Compared with SAC305 joints, microscale Sn-37Pb shear specimens exhibit more homogenous behavior and microstructure with a large number of small Sn (and Pb) grains. Creep damage in the Pb-free joint is predominantly concentrated at highly misoriented Sn grain boundaries. The coarse-grained Sn microstructure recrystallizes into new grains with high misorientation angles under creep loading. In spite of the observed joint-dependent behavior, as-fabricated SAC305 is significantly more creep resistant than Sn-37Pb solder and slightly less creep resistant than near-eutectic SAC387 solder. Average model constants for primary and secondary creep of SAC305 are presented. Since the viscoplastic measurements are averaged over a wide range of grain configurations, the creep model constants represent the effective continuum behavior in an average sense. The average secondary creep behavior suggests that the dominant creep mechanism is dislocation climb assisted by dislocation pipe diffusion.

Modeling the failure of intermetallic/solder interfaces

Solder joint integrity is recognized as a key issue in the reliability of flip chip and ball grid arrays in integrated circuit packages. Significant reductions in the solder joint interconnect size results in both the increased volume fraction of brittle intermetallics in the joint and joule heating due to high current density. Previously, the prevalent failure mode in a solder joint was the ductile thermo-mechanical fracture of solder material due to repeated thermal cycling. In addition to this mode of failure the joints were also found to fail by brittle fracture near the solder–intermetallic interface. To predict and reduce the failure of solder joints, a model that incorporates both plastic damage in bulk solder and solder–intermetallic interface failure is timely and useful to the electronics industry. Based on cohesive fracture theory, a 3D finite element model has been developed to predict the interfacial damage of Sn–Ag–Cu eutectic lead-free solder interconnects. Unified creep-plasticity theory is incorporated in the model considering the creep and hysteresis effects in solder bulk. Using the unified creep-plasticity-cohesive finite element model, the intermetallic layer growth effect has been researched for different solders. The failure of solder is a complex, coupled thermo-electric-mechanical problem. To understand better the phenomenon, a thermo-electric finite element analysis has been conducted to predict the electrical concentration and joule heating effects on the failure of Sn–2.5Ag–0.8Cu–0.5Sb solder under different applied current densities. The temperature and current density distribution in a solder joint with a crack that propagates near the interface of the bulk solder and intermetallic layer has been predicted. Pronounced temperature and electrical current concentration are observed near the crack tip. Although the Pb-free solder is usually regarded as having a higher melting temperature compared with traditional Pb–Sn eutectic solder, the concentration of heat at the crack tip caused by joule heating may still melt the solder material under high current density stressing.

Ball Grid Array Lead-Free Solder Joint Strength under Monotonic Flexural Load

Abstract This work compared ball grid array (BGA) lead-free solder joint strengths to eutectic lead (Sn–Pb) solder joint strengths under monotonic bend load at room temperature. Flexural test methodologies for evaluating solder joint strength are presented. Various effects on solder joint strength were summarized systematically into three parts. The first part focused on the effect of solder joint geometries. BGAs with Sn-4Ag-0.5Cu and 63Sn-37Pb solders were tested, respectively. The effects of package side solder resist opening sizes, solder ball diameters, and board side metal defined/solder mask defined pads were investigated with 0.062 in printed circuit board (PCB). The results showed that the solder joint strength of Sn–Ag–Cu solder is lower than that of the traditional Sn–Pb solder under room temperature board flexural load and similar dynamic load. The second part investigated the effects of type of package (plastic BGA (PBGA) versus ceramic BGA (CBGA)), board thickness (0.093 in. versus 0.135 in.), and the effect of rework (reworked versus non-reworked) with Sn-3.9Ag-0.7Cu and 63Sn-37Pb solders. The joint strength of Sn–Ag–Cu solder is consistently lower than that of eutectic Sn–Pb solder for both board thicknesses, both CBGA and PBGA packages, and both non-reworked and reworked packages. The third part explored the feasibility of alternative low temperature solders as board-level interconnects. In addition to the traditional 63Sn-37Pb solder and the lead-free Sn-4Ag-0.5Cu solder, four other lead-free solders (Sn-52In, Sn-58Bi, Sn-57Bi-1Ag, and Sn-9Zn-0.006Al) were tested with 0.044 in PCB. Effects of board surface finishes with immersion silver (ImAg) or organic solderability preservatives, and pads with via-in-pad (VIP) or non-VIP pads were investigated. Test results showed that most of the BGAs with non-VIP pads performed better than those with VIP pads, except Sn–In solder with ImAg surface finish. The Sn–In solder showed the lowest performance, while Sn–Bi and Sn–Bi–Ag solder compositions showed better performance. Sn–Zn–Al solder joint strength performs better than others.

Interfacial Reaction Between 95Pb-5Sn Solder Bump and 37Pb-63Sn Presolder in Flip-Chip Solder Joints

Interdiffusion and interfacial reaction of 95Pb-5Sn solder bumps and 37Pb-63Sn presolder in flip-chip solder joints during high-temperature storage were studied. Reaction temperatures included 100°C, 130°C, 150°C, and 175°C. It was found that Cu6Sn5 and Cu3Sn formed on the board side and (Ni,Cu)3Sn4 formed on the chip side after 100 h of aging. After 2000 h of aging at 175°C, the Ni under-bump metallization (UBM) was exhausted. This caused the (Ni,Cu)3Sn4 layer at the chip-side interface to be gradually converted into (Cu0.6Ni0.4)6Sn5. It was also found that the consumption of the Ni UBM was faster than the case where eutectic Sn-Pb solder was used for the entire joint. Nevertheless, the consumption of the Cu on the substrate side was slower than the case where pure eutectic Sn-Pb solder was used for the entire joint.

Melting and solidification properties of the nanoparticles of Sn3.0Ag0.5Cu lead-free solder alloy

The Sn3.0Ag0.5Cu (wt.%) lead-free solder alloy is considered to be one of the most promising candidates to replace the traditionally used Sn–Pb solder. However, this alloy composition has some weaknesses, mainly as a result of its higher melting temperature compared to the eutectic Sn–Pb solder. In this paper, lead-free solder alloy nanoparticles of Sn3.0Ag0.5Cu were synthesized by chemical reduction with NaBH 4 as reducing agent. The experimental results indicated that the major particle size of Sn3.0Ag0.5Cu nanoparticles was smaller than 100 nm. The melting and solidification properties of the Sn3.0Ag0.5Cu nanoparticles were studied by differential scanning calorimetry at different scanning rates. It was evidenced by the differential scanning calorimetry curves that the melting temperature of Sn3.0Ag0.5Cu nanoparticles was lower than that of the bulk alloy. In addition, the undercooling of the Sn3.0Ag0.5Cu nanoparticles was in the range of 82.0–88.5 °C at different cooling rates, which was much larger than that of the Sn3.0Ag0.5Cu micro-sized particles, showing stronger cooling rate dependence.

SnPb 솔더에 대한 유한요소모델의 크리프 특성 검증

초록:본논문에서는네트워크서버용컴퓨터주기판내장착된열방열시스템지지구조물에대한유한요소모델의솔더크리프특성을검증하였다. 열방열시스템은앵커구조물로지지되며앵커구조물은솔더를이용하여인쇄회로기판에장착된다. 컴퓨터내발생하는지속적인고온환경하에서솔더의크리핑이발생하고이는궁극적으로지지구조물의파괴로이어진다. 유한요소모델은솔더에발생하는응력분석과수명예측을위해사용되며, 솔더크리프특성을모사하기위하여Anand 크리프모델을적용하였다. 모델을검증하고교정하기위하여크리프시험을수행하였다. 시험은인쇄회로기판의변형을제외한솔더변형만을측정하기위하여특별한지그를설계하여수행하였다. 크리프시험결과를유한요소해석결과와비교하여Anand 크리프모델을검증하고교정을수행하였다. 교정된유한요소모델을이용하여열방열시스템구조물의보다정확한수명예측을수행할수있다.Abstract: The heat sink system for a main board in a network server computer is built on printed circuit board byan anchor structure, mounted by eutectic SnPb solder. The solder creeping is caused by a constant hightemperature condition in the computer and it eventually makes fatal failures. The FE model is used to calculate thestress and predict the life of soldered anchor in the computer. In the model, Anand constitutive equation isemployed to simulate creep characteristics of solder. The creep test is conducted to verify and calibrate the soldermodel. A special jig is designed to mitigate the flexure of printed circuit board and to get the creep deformationof solder only in the test. Test results are compared with analysis and calibration is conducted on Anand model’sconstants. Precise life prediction of soldered anchor in creep condition can be performed by this model

Solder Jet Printhead for Deposition of Molten Metal Drops

We have developed a drop-on-demand printhead that can eject molten metal droplets. The capability of ejecting solder drops onto an X-Y stage controlled substrate makes this new technology ideal for fast prototyping of metallic traces on planar and three-dimensional (3D) objects. The ejection process uses the electromagnetic repulsion force between two parallel currents moving in opposite directions. One current path flows through the molten metal, such as solder, the other through a copper stripe. The electrical connection between the two conductors was accomplished by two Ni plated vias. By adding an appropriate nozzle in the solder channel, a current pulse can cause a drop to squirt out. The size of the ejected solder drop depends on the driving energy, which is controlled by both the pulse width and the drive voltage. For successful operation of the printhead, wettability of the solder to the printhead material needs to be taken into consideration. We have constructed the solder jet printhead in both a polyimide laminate stack and a ceramic form that can withstand a much high temperature. We have ejected molten PbSn eutectic solder as well as BiSn and InSn with precision on Si wafers, over substrates with different heights and connecting the traces on different levels, as well as creating freestanding 3D structures.

ALX Polymer for Wafer Level Packaging: Mechanical Property Evaluation After Multiple Lead-Free Solder Reflows

For bumping and wafer level packaging (WLP) applications, BCB and polyimide are the predominant spin-on dielectric materials used for re-passivation, redistribution, interlayer dielectric, and stress buffer layers. Each of these materials has their respective strengths and weaknesses. BCB has exceptionally low shrinkage on cure and moisture absorption, low curing temperature, and a low dielectric constant, but is a mechanically brittle material which limits its application in bump-on-polymer applications. Polyimides are superior mechanically to BCB and are utilized more in bump-on-polymer structures, but suffer from much higher shrinkage on cure and moisture absorption (which can lead to blistering if not carefully processed), have higher dielectric constants, and have much higher cure temperatures. ALX spin-on polymer dielectric was developed to combine low shrinkage, low moisture absorption, low dielectric constant, and excellent mechanical and stress buffering properties with a cure temperature between 190C to 250C, depending on the application(1-3). Previous papers have reported the mechanical properties at 190C and the application of ALX in eutectic SnPb solder bump structures. The use of lead-free solder requires reflow temperatures up to 260C. Although ALX polymer is curable at less than 200C, the influence of lead-free solder reflow wasnft investigated. In this presentation we evaluate mechanical property changes of ALX Polymer films after different cure schedules and multiple reflows at lead-free solder temperatures. The impact of these parameters on WLP will be discussed.

Microstructures formed from mixed solders on copper substrate

With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn–Ag–Cu (SAC) Pb free solders, as well as Sn–Zn–Al solder, melted together with eutectic Pb–Sn solder paste and also various SAC solder pastes, on a copper substrate. It was observed that using eutectic Pb–Sn solder paste with an SAC solder ball introduced some Pb–Sn eutectic microstructure and changed the ternary eutectic present from Ag3Sn–Cu6Sn5–Sn to Ag3Sn–Pb–Sn. Use of an SAC solder paste with Sn–Zn–Al solder introduced an apparent Ag–Cu–Zn ternary compound, replacing Zn lamellae of the Sn–Zn eutectic. With eutectic Pb–Sn solder paste, the Pb–Sn–Zn ternary eutectic was formed. It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.

Synthesis and DSC study on Sn3.5Ag alloy nanoparticles used for lower melting temperature solder

The traditional Sn–Pb eutectic solder alloys are being phased out from the electronics industry due to the toxicity of lead (Pb), leading to the development and implementation of lead-free solders. Sn3.5Ag lead-free solder alloy, considered to be one of the promising alternatives to replace the traditionally used Sn–Pb solder, however, still has some weaknesses, such as its higher melting temperature than that of the Sn–Pb solder alloy. A possible way to decrease the melting temperature of a solder alloy is to decrease the alloy particle size to the nanometer range. Sn3.5Ag nanoparticles with different size distribution were synthesized using chemical reduction method by applying NaBH4 as reduction agent. The melting properties of these synthesized nanoparticles were studied by differential scanning calorimetry (DSC), and size-dependent melting temperature depression of these nanoparticles has been observed. Gibbs–Thomson equation was used to analyze the size-dependent melting temperature property, giving a good prediction of the melting temperature depression for the Sn-based lead-free solder alloy nanoparticles.

Microstructure evolution in Cu pillar/eutectic SnPb solder system during isothermal annealing

The reaction between Cu pillar and eutectic SnPb solder during isothermal annealing was studied systematically. Intermetallic compounds (IMCs), such as Cu6Sn5 and Cu3Sn, were formed in between Cu and SnThe parabolic rate law was observed on IMC formation, which indicated that the growth of IMCs was controlled by atomic diffusion (a diffusion-limited process). Annealing at 165 °C for 160 h decreased the growth rate of Cu6Sn5, and at the same time increased the growth rate of Cu3Sn. This was when Sn in solder was exhausted completely. The activation energies for the growth of Cu3Sn and Cu6Sn5 were measured to be 1.77 eV and 0.72 eV, respectively. The Kirkendall void that formed at the interface between Cu pillar and solder obeyed the parabolic rate law. The growth rate of the Kirkendall void increased when the Sn in solder was consumed in its entirety.

Effect of Lead-Free Soldering on Key Material Properties of FR-4 Printed Circuit Board Laminates

The transition to lead-free soldering of printed circuit boards (PCBs) using solder alloys such as SnAgCu has resulted in higher temperature exposures during assembly compared with eutectic SnPb solders. The knowledge of PCB laminate material properties and their dependence on the material constituents, combined with their possible variations due to lead-free soldering temperature exposures, is an essential input in the laminate selection process. This paper provides laminate selection guidelines that were arrived at by assessing key material properties (glass transition temperature, coefficient of thermal expansion, decomposition temperature, and water absorption), and their responses to lead-free soldering assembly conditions. A range of commercially available FR-4 PCB laminate materials, classified on the basis of glass transition temperature (high, mid, and low), curing agents (dicyandiamide and phenolic), flame retardants (halogenated and halogen-free), and fillers (presence or absence) were studied. The laminate material properties under investigation were measured as per the IPC-TM-650 test methods before and after exposure to multiple lead-free soldering cycles. Combinatorial property analysis was conducted to investigate the causes behind variations in material properties.

Determination of Average Failure Time and Microstructural Analysis of Sn-Ag-Bi-In Solder Under Electromigration

Despite the extensive use of Sn-Ag-Cu as a Pb-free solder alloy, its melting point is significantly higher than that of eutectic Sn-Pb solder. Sn-Ag-Bi-In solder is an alternative Pb-free solder, with a melting temperature close to that of eutectic Sn-Pb. This study elucidates the electromigration behavior of Sn-Ag-Bi-In solder and then compares the results with those of the Sn-Ag-Bi system. The behavior of Pb-free Sn-Ag-Bi-In solder strips under electromigration is examined by preparing them in Si (001) U-grooves. The samples are then tested under various temperatures and current densities. Although the compounds thicken near both electrodes with current stressing, the thickness at the anode exceeds that at the cathode. Experimental results of the average failure time indicate that Sn-Ag-Bi-In solder has a longer lifetime than does Sn-Ag-Bi, which is attributed to the ζ phase. Additionally, the ζ phase dissolved by the current in the early stage replenishes the outgoing atomic flux. These atomic fluxes also enhance the growth of abnormally large particles in the middle of the strips. Field-emission electron probe microanalysis (FE-EPMA) results indicate that the amount of indium is reduced after the ζ phase near the cathode is exhausted for extended current stressing time.

Estimation Method for Liquidus Temperature of Lead-Free Solder Using Differential Scanning Calorimetry Profiles

It is usually complicated to analyze the liquidus temperature of lead-free solder, because unlike Sn-Pb eutectic solder, supercooling easily occurs during the cooling process of lead-free solder, and common lead-free solders contain only a small fraction of a primary phase. In order to determine the melting temperature range of lead-free solder easily, an estimation method using differential scanning calorimetry (DSC) profiles is proposed. The purpose of this study is to show the applicability of the newly proposed DSC-based approach. DSC profiles using several heating rates were measured and analyzed. As a result, it was found that the extrapolated onset temperature, the peak temperature, and the extrapolated end temperature of the endothermic peak were proportional to the square root of the DSC heating rate. For lower heating rate, the temperature–axis intercept of the relationship between the square root of the heating rate and the peak temperature can be regarded as the liquidus temperature under equilibrium conditions with better accuracy. For higher heating rate, the temperature–axis intercept of the relationship between the square root of the heating rate and the extrapolated end temperature can be approximately estimated as the liquidus temperature of noneutectic lead-free solder under equilibrium conditions.

Effect of Solder Thickness on Electromigration Behavior in Eutectic SnPb Solder Reaction Couples

The trend of miniaturization of electronic products induced the shrinking dimension of interconnects in the chip. When those interconnects are subjected to high current density (usually 103 to 104 A/cm2), electromigration (EM) could affect the reliability of the chip which would ultimately break the circuit. In this study, eutectic SnPb solders with thickness of 280, 128, and 50 μm were investigated under high current density (104 A/cm2) and high ambient temperature (120 °C). The EM-induced surface undulations were more prominent at the shorter thickness, demonstrating that the diffusion of metal atoms/ions was controlled by the actual temperature in the bulk solder instead of the back stress. Bamboo groove features were observed on the surface of solder extrusion at the anode side for the three solder thickness, which indicated the metal atoms/ions that migrated parallel to the direction of flow of electrons.

Constitutive models of creep for lead-free solders

The constitutive modeling of creep has been extensively studied due to the important of the creep failure mode in solder joints. However, there are very few studies that considered room temperature aging contributions in their creep modeling studies. This study investigated constitutive modeling of creep of solders by taking into account the possible contribution room temperature aging. Lead-free solder (Sn–4.0Ag–0.5Cu) was found to have a higher creep resistance than Sn–Pb solder at the same stress level and testing temperature. The higher creep resistance was contributed by the second phase intermetallic compounds, Ag3Sn and Cu6Sn5. The precipitation of these intermetallic compounds can significantly block the movement of dislocations and increase the creep resistance of the material. Constitutive models of creep for both lead-free and Sn–Pb eutectic solders were constructed based on the experimental data. The activation energy for SAC405 is much higher than that of Sn–Pb, which also indicates that SAC405 possesses higher creep resistance. The constitutive models can be used in finite element analysis of actual electronic packages to predict solder joint failure. The creep mechanisms of both lead-free and Sn–Pb eutectic solders were also extensively discussed in this dissertation. Dislocation gliding and climb is believed to be the major failure mode at high stresses, while lattice diffusion and grain boundary diffusion is believed to be the major failure mode at low stress levels. Grain boundary sliding is believed to contribute to creep deformation at both high-stresses and low-stresses. For eutectic Sn–Pb, superplastic deformation is a major the creep mechanism at low-stresses and high-temperatures.