Eutectic Sn-Pb Solder Alloy Research Articles

Interrelation of wettability–microstructure–tensile strength of lead-free Sn–Ag and Sn–Bi solder alloys

A comparative investigation on the wettability and tensile strength of a Sn–2Ag, a Sn–40Bi and the traditional eutectic Sn–Pb solder alloys was carried out. The wettability is represented by thickness of covered layer (TCL) and spread area (SA) while the mechanical behaviour by the ultimate tensile strength (UTS). It is shown that the TCL of studied alloys decreased with the increase in the dipping temperature. It is also shown that TCL and SA have opposite behaviour with respect to the cooling rate. The Sn–Bi solder alloy has lower SA when compared with those of the Sn–Ag solder when similar cooling rates are considered. The Sn–Bi solder exhibits the best UTS/SA combination for dendritic spacings between 25 and 27 µm, associated with cooling rates ∼2°C s−1, 2× lower than those of the Sn–Ag alloy. Besides, the Sn–Bi alloy has shown SA >70∼80% associated with higher UTS (∼80 MPa) as compared with the other alloys examined.

Stress Relaxation and Fatigue Characterisation of Eutectic SnPb Solder Alloy

Soldering is the most important joining technology in the semiconductor industry, especially for IC chip packaging. The binary eutectic SnPb alloy had been in used for decades, before regulations and restriction on the usage of lead in solders was imposed. Thus, replacing SnPb solder alloy with Pb-free solders has also been an issue in the electronic industry. The previously used SnPb alloy was in used due to several reasons, namely (a) it has low melting temperature (183oC) and solidified at a single temperature to form eutectoid composition, (b) binary in composition and is readily available commercially and (c) many researches had been undertaken in the previous decades. Creep, stress relaxation and fatigue tests were conducted on eutectic SnPb solder alloy in order to study the alloy mechanical characteristics, and hence a suitable Pb-free solder alloy could be chosen as a replacement alloy. In this study, creep, stress relaxation and fatigue tests were conducted on eutectic SnPb solder alloy at 30oC and 50oC, respectively. The study showed that stress relaxation in the alloy decayed instantaneously to zero-value when cycling was done at R=-1 and that cycling was done at 600 cycle per minute (CPM) which enable fatigue test to be conducted on the bulk solder alloy. A non-zero stress relaxation value will result in the alloy to failed predominantly due to creep and fatigue failure will not be observed.

Electrochemical Migration Characteristics of Eutectic Sn-Pb Solder Alloy in NaCl and Na2SO4 Solutions

Anodic dissolution and the electrochemical migration characteristics of eutectic Sn-Pb solder alloy in deaerated 0.001% NaCl and Na2SO4 solutions were investigated using anodic polarization and water drop tests. Anodic polarization results revealed that a Pb-rich phase was preferentially ionized in deaerated 0.001% NaCl solution and an Sn-rich phase was predominantly ionized in deaerated 0.001% Na2SO4 solution, which coincides well with the composition of the dendrites formed during water drop tests. X-ray diffraction and photoelectron spectroscopy results showed that the surface oxide film formed on pure Sn in deaerated 0.001% NaCl solution is more stable than that formed on pure Sn in deaerated 0.001% Na2SO4 solution. The surface oxide film formed on pure Pb in deaerated 0.001% Na2SO4 solution is more stable than that formed on pure Sn in deaerated 0.001% NaCl solution. Therefore, the quality of the surface film of eutectic Sn-Pb solder in a chemical environment seems to be critical not only for corrosion resistance, but also for electrochemical migration resistance.

Effect of copper addition on the microstructure and mechanical properties of lead free solder alloy

The microstructure and shear strength of a transition joint consisting of Sn–Ag–Cu solder alloy and Cu substrate were evaluated in reflowed condition. The results were compared with those obtained from a similar joint made with conventional eutectic Sn–Pb solder alloy. The reaction products at the interface are mainly Cu 6Sn 5 and Cu 3Sn along with Cu 6Sn 5 for Sn–Pb and Sn–Ag–Cu to Cu joint, respectively. The shear strengths of the two systems are ∼55 and ∼68 MPa, respectively. The fracture takes place through the Cu 6Sn 5 intermetallic phase as well as through the solder alloy and the strength depends on the thickness of the reaction layer. The width of the brittle reaction layer at the interface for Sn–Ag–Cu to Cu transition joint is thinner with respect to Sn–Pb to Cu joint; hence the shear strength of the former is superior to the latter.

Effect of SO42- Ion on Corrosion and Electrochemical MigrationCharacteristics of Eutectic SnPb Solder Alloy

Electrochemical migration phenomenon is correlated with ionization of anode electrode, and ionization of anode metal has similar mechanism with corrosion phenomenon. In this work, in-situ water drop test and evaluation of corrosion characteristics for SnPb solder alloys in solutions were carried out to understand the fundamental electrochemical migration characteristics and to correlate each other. It was revealed that electrochemical migration behavior of SnPb solder alloys was closely related to the corrosion characteristics, and Sn Ivas primarily ionized in solutions. The quality of passive film formed at film surface seems to be critical not only for corrosion resistance but also for electrochemical migration resistance of solder alloys.ጊ吀Ѐ㘹〻Ԁ䭃䑎䴀

Electrochemical migration characteristics of eutectic SnPb solder alloy in printed circuit board

The smaller size and higher integration of electronic components result in smaller gaps between metal conducting layers in an electronic package and printed circuit board (PCB). Under high temperature and humidity, electronic components respond to applied voltages by electrochemical ionization of metal and the formation of a filament, which leads to short-circuit failure of an electronic component, which is termed electrochemical migration (ECM). In this study, a four-layered PCB with SnPb electroplated through-hole-via was used to determine the ECM characteristics of a eutectic SnPb (63Sn–37Pb, weight percent) solder at a temperature of 85 °C, a relative humidity of 85%, and DC bias voltages between 75 and 100 V. Additionally, polarization tests for the solder materials were carried out to determine ECM characteristics. Pb was found to be more susceptible to polarization than Sn, which matched the composition distribution of the conductive anodic filament on the PCB surface.

The Connection between Microstructural Damage Modeling and Continuum Damage Modeling for Eutectic Sn-Pb Solder Alloys

Researchers resort to a wide range of simplified representations at the continuum scale, to model creep-fatigue damage in viscoplastic heterogeneous materials, such as Sn-Pb eutectic solders, caused by thermomechanical and mechanical cyclic loading (e.g., due to power cycling, environmental temperature cycling, vibration, etc.). Typically, in macroscale phenomenological damage models, the cyclic damage is assumed to depend on some loading parameter, such as cyclic strain range, work dissipation per cycle, partitioned strain range, partitioned work dissipation per cycle, cyclic entropy changes, cyclic stress range, integrated matrix creep, etc. In many instances, some of these variables are weighted with a factor to account for rate-dependent effects. The task of finding the best damage metric is difficult because of complex microstructural interactions between cyclic creep and cyclic plasticity due to the high homologous temperature under operating conditions. In this study, we use insights obtained from microstructural and more mechanistic modeling to identify the most appropriate macroscale damage metrics. The microstructural models are based on such phenomena as grain boundary sliding, blocking of grain boundary sliding by second-phase particles, volumetric and grain boundary surface diffusion, void nucleation, void growth, and plastic collapse of cavitating grain boundaries. As has been demonstrated in the literature, micro-structural models suggest that fatigue damage caused by cyclic plasticity should correlate well with either of the two most commonly used damage indicators: cyclic strain range and plastic work dissipation per cycle. This study, however, demonstrates that in the case of damage dominated by cyclic creep, microstructural models developed by the authors indicate closer correlation of fatigue damage with creep work dissipation per cycle, than with cyclic creep strain range.

Solder joint reliability and characteristics of deformation and crack growth of Sn-Ag-Cu versus eutectic Sn-Pb on a WLP in a thermal cycling test

Sn-Ag-Cu (SAC) is now recognized as the standard lead free solder alloy for packaging interconnect in the electronics industry. This paper analyzes the performance of both SAC and eutectic Sn-Pb solder alloys on Kulicke & Soffa's (K&S') Ultra CSP/spl reg/ wafer level package (WLP) at a thermal cycling (TC) test. The Ultra CSP standard Al/Ni-V/Cu under bump metallurgy (UBM) system was used to analyze if this UBM system with SAC solder would produce acceptable reliability in the TC test. In this study, two TC tests were performed. In the first test, two parts were removed from the TC chamber about every 200 cycles to investigate the characteristics of deformation and crack growth in the SAC and eutectic Sn-Pb Ultra CSP solder joints during TC testing. These TC test results showed that both the SAC and eutectic Sn-Pb Ultra CSPs exhibited normal solder joint fatigue failures during the testing. The SAC Ultra CSP had an equal or 18% higher Weibull life than the eutectic Sn-Pb one. Based on these results it was concluded that the SAC Ultra CSP with the Al/Ni-V/Cu UBM system produces acceptable solder joint reliability in a TC test. The results also revealed that the deformation and crack growth characteristics of the SAC and eutectic Sn-Pb Ultra CSP solder joints were significantly different. The eutectic Sn-Pb solder joints showed significant inelastic shear deformation during the TC testing while the SAC solder joints did not display significant inelastic deformation even at the high temperature regime of the TC test.

Thermal stability of electroless-nickel/solder interface: Part A. interfacial chemistry and microstructure

The thermal stability of the interface between the Sn-Pb eutectic alloy and an electroless Ni-P coating was examined by cross-sectional transmission electron microscopy (TEM). The interface was formed by reflowing the eutectic Sn-Pb solder alloy between electroless Ni-P deposits. The microchemistry and microstructure of the interface were analyzed in the as-reflowed, mild-aged, and overaged conditions, by energy-dispersive spectroscopy (EDS), selected-area electron diffraction (SAD) convergentbeam electron diffraction (CBED), and bright- and dark-field imaging. In the as-reflowed condition, the interfacial microstructure consisted of a thin Ni3Sn4 intermetallic layer and a thin P-rich layer. The P-rich layer was composed of two phases, face-centered cubic (fcc) Ni and Ni3P, and the excess P was primarily due to the ejection of P from the electroless Ni-P when it reacted with Sn in the reflow process. Following the mild aging, a trilayer interfacial microstructure was found, including a coarsened Ni3Sn4 layer, a P-rich layer with increased P concentration, and a P-deficient layer. With overaging, a multilayer interfacial microstructure was developed, which consisted of two Ni-Sn intermetallic layers, Ni3Sn4 and Ni3Sn2, and three distinct P-rich layers, Ni12P5, Ni12P5 + Ni3P, and Ni3P + Ni.

Effect of cooling rate on interfacial fatigue-crack growth in Sn-Pb solder joints

The effect of the cooling rate after the reflow on the behavior of fatigue crack growth along Cu/Sn-Pb interfaces was examined in solder joints. Flexural peel specimens made from eutectic Sn-Pb solder alloy and Cu were cooled down from reflowing to room temperature using furnace cooling, air cooling, and water quenching. Kinetics of fatigue crack growth along the solder/Cu interface were measured from the flexural peel specimens as a function of strain energy release rate. The effect of cooling rate was found to depend on the level of strain energy release rate. Increasing the cooling rate from furnace cooling to water-quenching enhanced the fatigue crack growth resistance by up to more than 50% at low strain energy release rates, but reduced the fatigue crack growth resistance by up to 100% at high strain energy release rates. The enhanced fatigue crack growth resistance at the low strain energy release rates is shown to result from roughening of the interface with increasing cooling rate, while the reduction in the fatigue crack growth resistance at very high strain energy release rates followed the change in crack growth mechanisms from cohesive to interfacial.

Reactivity of no-clean pastes and fluxes for the surface mount technology process—part I: Corrosion behavior of Cu, Sn, and Pb

No-clean fluxes allow simplifications in the surface mount technology process, but introduce reliability problems for electronic assemblies during their service life. An electrochemical method is devised to study the anodic reactivity of Sn, Pb, eutectic Sn-Pb solder alloy and Cu in aqueous solutions containing no-clean paste residues obtained during a reflow process. The potential corrosion risk of residues from two different pastes is evaluated, and the corrosion behavior of the different metals assessed. Cu corrodes faster than Sn, Pb, and the solder alloy in presence of the residues, but corrosion starts at higher overvoltages. Pb corrodes at low overvoltage; Sn is prone to oxidation and passivation. The solder alloy has an intermediate behavior between those of the pure metals.