SnPb Solder Joints Research Articles

Gold in Flux-less Bonding: Noble or not Noble

ABSTRACTThis work highlights the solder joints reliability issues emerged during the development of a novel compliant contacting technology. The peculiar process in this technology is a mechanical lifting procedure in which a pulling force is exerted onto 63Sn-37Pb (eutectic) solder joints (realized by a flux-less thermo compression process), releasing metal traces from the substrate, to form free standing vertical structures. Since joints mechanical characteristics are critical for the successful fabrication of contacts, different bonding conditions (inert or forming atmosphere, temperature rates) and surface finishing (electroplated gold and preformed solder) have been tested. SEM and EDX analyses have been performed on failing joints to investigate failure causes and classify defect typologies. A constantly higher failure rate (percent number of failing joints) has been observed on gold finished surfaces. Analyses proved that such unusual rate was due to contamination of gold surface left by additives in the plating bath and to the embrittlement caused by gold diffusion into molten solder. Plating additives contamination reduces the wettability of gold surfaces. Concentration values of 3 wt.% for gold, considered safe for surface mount applications, caused embrittlement in solder bumps of 20-40 μm diameters.

Assessing the risk of “Kirkendall voiding” in Cu 3Sn

A common location of sporadic, usually unpredictable, premature failures of both SnPb and Pb-free solder joints is within the intermetallic bond to one of the contact pads. Cu pads tend to be less prone to such defects than Ni/Au coated pads, but one phenomenon is a cause of growing concern for high reliability applications. Sporadic intermetallic problems are notoriously difficult to screen for, but at least a number of them are detectable right after assembly. However, the occasional formation of a network of voids within the Cu 3Sn layer forming when soldering to Cu pads is rarely noticeable until much later. To make matters worse, efforts to establish practical engineering tests for purposes of identification and screening have usually not been successful. The root cause of the so-called ‘Kirkendall voiding’ has been shown to be associated with the quality of the electroplated Cu, apparently as determined by the incorporation of minute amounts of specific organic impurities. The present work describes and discusses a series of potential tests that allow the sorting of electroplated Cu in terms of its propensity for Cu 3Sn voiding during soldering and subsequent aging. The times and efforts involved in these tests differ greatly, as do their effectiveness and accuracy. Individual tests may therefore be useful at various stages from supplier qualification to monitoring of batch to batch variations.

Implementation and Characterization of a Medical Ultrasound Phased Array Probe With New Pb-Free Soldering Materials

The application of the European Directive on the Restriction of Hazardous Substances, named RoHS, and correlated documents concerning the restriction of the use of hazardous substances in electrical and electronic equipment is critical and expensive, above all when the use of new substances needs to review consolidated production processes. The use of innovative materials (Sn-Ag-Cu alloys or electrically conductive adhesives (ECAs), for instance) instead of the traditional Sn-Pb solder joints and the design review of the electronic equipment when “not-RoHS-compliant” components must not on sale in the market are typical examples. In this context, this paper takes into account a new soldering production process with a conductive adhesive. In particular, we focus on the soldering phase with the adhesive for the design and implementation of ultrasound transducers for an industrial biomedical application. The use of new Pb-free soldering materials, such as a silver conductive adhesive, is considered in this paper in order to implement an ultrasound array transducer. With this component being fundamental for the functionality of the equipment (echocardiography system), a measurement system for the electroacoustic probe characterization is proposed, and some results, in terms of both joint performance and electrical properties of the new electronic device with Pb-free solder joints, are presented. This paper shows the use of conductive adhesives for the implementation of a medical phased array probe, where the soldering material is located among a platelet of the piezoelectric and fingers. A comparison between electroacoustic measures on the elements soldered with an ECA and with the traditional technology Sn-Pb will be also shown. Moreover, in the field of biomedical application, it is also fundamental to assure that the performances of the transducer are maintained in the time. To this aim, some laboratory tests for the evaluation of the reliability performances of the new Pb-free soldering material are proposed, and the preliminary results are presented in this paper.

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.

Constitutive modeling on creep deformation for a SnPb-based composite solder reinforced with microsized Cu particles

In the present work, the creep strain of solder joints is measured using a stepped load creep test on a single specimen. Based on the creep strain tests, the constitutive modeling on the steady-state creep rate is determined for the Cu particle-reinforced Sn37Pb-based composite solder joint and the Sn37Pb solder joint, respectively. It is indicated that the activation energy of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of Sn37Pb solder joint. In addition, the stress exponent of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of the Sn37Pb solder joint. It is expected that the creep resistance of the Cu particle-reinforced Sn37Pb-based composite solder joint is superior to that of the Sn37Pb solder. Finally, the creep deformation mechanisms of the solder joint are discussed.

Investigation on Interfacial Reaction and Reliability for Sn-3Ag-0.5Cu, Sn-0.3Ag-0.7Cu and Sn-Pb solder joint under thermal shock

Investigation on Interfacial Reaction and Reliability for Sn-3Ag-0.5Cu, Sn-0.3Ag-0.7Cu and Sn-Pb solder joint under thermal shock

Failure mechanism of Pb-bearing and Pb-free solder joints under high-speed shear loading

The failure behaviors of ball grid array (BGA) solder ball joints under the various loading speeds of the high-speed shear test were investigated both experimentally and with non-linear, 3-dimensional finite element modeling. Conventional Sn-37Pb and Pb-free, Sn-3.5Ag solder alloys were used to compare the failure behaviors. Far greater shear forces were measured by the high-speed shear test than by the low-speed shear test. The shear force further increased with increasing shear speed, mainly due to the high strain-rate sensitivity of the solder alloys. Brittle interfacial fractures were more easily achieved by the high-speed shear test in the Sn-3.5Ag solder joints, especially at higher shear speed. This result was discussed in terms of the relationship between the strain-rate of the solder alloy, the work-hardening effect, and the resulting stress concentration in the interfacial regions. However, no transition of the failure mode was observed in the high-speed shear test of the Sn-37Pb solder joints.

Effects of reflow profile and thermal conditioning on intermetallic compound thickness for SnAgCu soldered joints

PurposeThe purpose of this paper is to investigate the effects of reflow time, reflow peak temperature, thermal shock and thermal aging on the intermetallic compound (IMC) thickness for Sn3.0Ag0.5Cu (SAC305) soldered joints.Design/methodology/approachA four‐factor factorial design with three replications is selected in the experiment. The input variables are the peak temperature, the duration of time above solder liquidus temperature (TAL), solder alloy and thermal shock. The peak temperature has three levels, 12, 22 and 32°C above solder liquidus temperatures (or 230, 240 and 250°C for SAC305 and 195, 205, and 215°C for SnPb). The TAL has two levels, 30 and 90 s. The thermally shocked test vehicles are subjected to air‐to‐air thermal shock conditioning from −40 to 125°C with 30 min dwell times (or 1 h/cycle) for 500 cycles. Samples both from the initial time zero and after thermal shock are cross‐sectioned. The IMC thickness is measured using scanning electron microscopy. Statistical analyses are conducted to compare the difference in IMC thickness growth between SAC305 solder joints and SnPb solder joints, and the difference in IMC thickness growth between after thermal shock and after thermal aging.FindingsThe IMC thickness increases with higher reflow peak temperature and longer time above liquidus. The IMC layer of SAC305 soldered joints is statistically significantly thicker than that of SnPb soldered joints when reflowed at comparable peak temperatures above liquidus and the same time above liquidus. Thermal conditioning leads to a smoother and thicker IMC layer. Thermal shock contributes to IMC growth merely through high‐temperature conditioning. The IMC thickness increases in SAC305 soldered joints after thermal shock or thermal aging are generally in agreement with prediction models such as that proposed by Hwang.Research limitations/implicationsIt is still unknown which thickness of IMC layer could result in damage to the solder.Practical implicationsThe IMC thickness of all samples is below 3 μm for both SnPb and SAC305 solder joints reflowed at the peak temperature ranging from 12 to 32°C above liquidus temperature and at times above liquidus ranging from 30 to 90 s. The IMC thickness is below 4 μm after subjecting to air‐to‐air thermal shock from −40 to 125°C with 30 min dwell time for 500 cycles or thermal aging at 125°C for 250 h.Originality/valueThe paper reports experimental results of IMC thickness at different thermal conditions. The application is useful for understanding the thickness growth of the IMC layer at various thermal conditions.

Critical Review of the Engelmaier Model for Solder Joint Creep Fatigue Reliability

A solder interconnect fatigue life model was developed by Werner Engelmaier in the early 1980s as an improvement upon the inelastic strain range-based Coffin-Manson model. As developed, the model provides a first-order estimate of cycles to failure for SnPb solder interconnects under power and thermal cycles. While the model has been widely adopted for SnPb solder joint reliability prediction, many issues that arise from simplifications in formulating input model parameters as well as from the complex physics of solder degradation challenge the model's ability to accurately estimate cycles to failure. Deficiencies with the model have been reported by a number of researchers. This paper reviews and summarizes the major issues with the Engelmaier model in its applicability to predict solder joint thermal fatigue life.

Effect of Thermal Aging on Impact Absorbed Energies of Solder Joints Under High-Strain-Rate Conditions

This study was concerned with the effect of thermal aging on the impact properties of solder joints. Three kinds of solders, conventional Sn-37Pb solder, Sn-3.8Ag-0.7Cu solder, and Sn-3.8Ag-0.7Cu doped with rare-earth (RE) elements, were selected to manufacture joint specimens for the Charpy impact test. U-notch specimens were adopted and isothermally aged at 150°C for 100 h and 1000 h, and then impacted by using a pendulum-type impact tester at room temperature. The Sn-37Pb solder joints exhibited higher performance in terms of impact absorbed energy in the as-soldered and 100 h thermally aged conditions. Interestingly, the Sn-3.8Ag-0.7Cu solder joints exhibited improved performance for the impact value after 1000 h of thermal aging. For the Sn-37Pb and Sn-3.8Ag-0.7Cu solder joints, the impact absorbed energies initially increased when the storage duration was limited to 100 h, and then gradually decreased with its further increase. For the Sn-3.8Ag-0.7Cu-RE specimens, impact performance decreased directly with increasing thermal aging. Furthermore, scanning electron microscopy (SEM) observation showed that the fracture paths were focused on the interface zone for the three kinds of joints in the aged conditions. For the Sn-37Pb joints, the fracture surfaces mainly presented a ductile fracture mode. For the Sn-3.8Ag-0.7Cu joints, with microstructure coarsening, crack propagation partly shifted towards the Sn/Cu6Sn5 interface. Compared with the 100 h aged joints, the area fraction of intergranular fracture of Sn grains on the Sn-3.8Ag-0.7Cu fracture surfaces was increased when the aging time was 1000 h. On the contrary, the fracture morphologies of Sn-3.8Ag-0.7Cu-RE were mainly brittle as thermal aging increased. Thus, an interrelationship between impact energy value and fracture morphology was observed.

Study of DC and AC Electromigration Behavior in Eutectic Pb-Sn Solder Joints

In this study, direct current (DC) and alternating current (AC) electromigration experiments were carried out using solder joints with a Cu/eutectic Pb-Sn/Cu joint configuration. During stressing using DC and AC, a fixed current density of 104 A/cm2 was applied to the joints at 150°C. In the joints stressed by DC, electromigration-induced damage occurred, and the corresponding microstructural changes mainly included valley and hillock formation in the solder region, pronounced phase segregation of Pb-rich and Sn-rich domains, asymmetric growth of Cu-Sn intermetallics at the interfaces, and excessive depletion of Cu at the cathode side. In contrast, no significant electromigration was observed after the AC treatments. This was especially true for the treatment with AC frequencies higher than 1 h−1. The dependence of the damage on AC frequency suggests that electromigration in solder joints can be inhibited to a large extent when a proper reverse current is delivered.

Study on creep characterization of nano-sized Ag particle-reinforced Sn–Pb composte solder joints

In the present work, the creep strain of solder joints is measured using a stepped load creep test on a single specimen. Based on the experimental results, the constitutive model on the steady-state creep strain is established by applying a linear curve fitting for the nano-sized Ag particle-reinforced Sn37Pb based composite solder joint and the Sn37Pb solder joint, respectively. It is indicated that the activation energy of the Ag particle-reinforced Sn37Pb based composite solder joints is higher than that of Sn37Pb solder joints. It is expected that the creep resistance of the Ag particle-reinforced Sn37Pb based composite solder joints is superior to that of Sn37Pb solder.

Polarity effect of electromigration on intermetallic compound formation in SnPb solder joints

The polarity effect of electromigration on the interfacial reactions of micro ball grid array (μBGA) solder joints was studied in terms of microstructural evolution. A dummy μBGA package with the same pair of solder joints was used to obtain reproducible results for a practical application. The μBGA package with Ni(P)/SnPb/Ni(P) structure showed a polarity effect on intermetallic compound (IMC) formation after stress by a current density of 3.0 × 10 3 A cm −2 at 120 °C, compared to the no-current case. Electric current enhanced the growth of Ni 3Sn 4 at the anode and retarded it at the cathode because of the change in Sn diffusion induced by electromigration. The IMC growth at the anode was about one order of magnitude faster than that at cathode. The growth of IMC at the anode had a parabolic dependence on time since the square of thickness of IMC increases linearly with time. The real μBGA package has unique and more complicated geometry compared to ordinary diffusion couples. So not only the polarity growth of IMC but also the fast dissolution of Cu and Ni at specific positions was found with the μBGA package. The unique geometry of μBGA solder joints caused heat generation at the upside interfaces due to Joule heating. Because of the heat generation and the high interfacial energy at the triple point of upside interfaces, the electroless Ni(P) finishes and Cu trace are consumed rapidly in the region where the current is crowded. Thus electromigration induced the rapid dissolution of Ni and Cu into solder and then formed (Cu,Ni) 6Sn 5 at the triple point of the current crowding region at high temperature. This is a rapid failure process because the localized fast dissolution of Ni and Cu accelerated the solid-state reaction between solder and electroless Ni(P) or Cu trace.

Finite-Element Simulation of Stress Intensity Factors in Solder Joint Intermetallic Compounds

The trend toward the miniaturization of electronic products leads to the need for very small sized solder joints, where the volume fraction of intermetallic compounds (IMCs) would be higher. In this paper, a fracture mechanics study of the IMC layer for SnPb and Pb-free solder joints has been carried out using a finite-element numerical computer modeling method. It is assumed that only one crack is present in the IMC layer. The linear elastic fracture mechanics approach is used for the parametric study of the stress intensity factors [(SIFs) K I and K II ] at the predefined crack in the IMC layer of the solder-butt-joint tensile sample. Contrary to intuition, it is revealed that a thicker IMC layer, in fact, increases the reliability of a solder joint for a cracked IMC-assuming that there is only a single crack that exists in the IMC layer. Even if the whole solder layer is replaced by the IMC in the solder joint, the fracture propagation possibility is greatly reduced. Values of K I and K II are found to decrease with the location of the crack farther away from the solder interfaces while other parameters are constant. Temperature and strain rate are also found to have a significant influence on the SIF values. It has been found that a soft solder matrix generates a nonuniform plastic deformation across the solder-IMC interface near the crack tip that is responsible for obtaining a wide range of K I and K II values.

Fracture mechanics analysis of solder joint intermetallic compounds in shear test

Abstract Solder joints in the electronics products are considered as critical reliability concerns. In this research, a quantitative evaluation of fracture mechanics parameters (such as Stress intensity factors (SIF, K I and K II ) and kink angle, θ ) in the IMC layer for SnPb and Pb-free solder joints has been carried out. Computation methods are based on finite element numerical modelling of stress analysis in a copper-IMC-solder-IMC-copper assembly under shearing condition. It is assumed that only one crack is present in one of the IMC layers. Linear Elastic Fracture Mechanics (LEFM) approach is used for the parametric study of SIFs and θ , at the predefined crack in the IMC layer of solder butt joint shear sample. Among different parameters studied in this research, the location of the crack from the solder interfaces has been found to be very sensitive. Crack near 1 micron distance from the interface has been found to be very prone to propagate. It is interesting to see that a thicker IMC layer reduces crack propagation propensity if there is only a single crack exists in the IMC layer and that crack is located at the middle of the IMC layer. Even if the whole solder layer is replaced by the IMC in the solder joint, the fracture propagation possibility is greatly reduced. Thickness of solder joints is also found to have a significant influence on the SIF values. It has been found that soft solder matrix generates non-uniform plastic deformation across the solder-IMC interface near the crack tip that is responsible to obtain a wide range of K I , K II and θ values.

Numerical Modelling of Cyclic Stress-Strain Behaviour of Sn Pb Solder Joint During Thermal Fatigue

This study examines the cyclic stress-strain response of solder joints in a surface mounted electronic assembly due to temperature cycles. For this purpose, a threedimensional model of an electronic test package is analyzed using finite element method. The model consists of 92 solder joints arranged along the peripheral of a 24x24 solder array. The various different materials considered in the simulation are Si-die, 60Sn-40Pb solder alloy, Cu-traces, Cu6Sn5 intermetallics, FR-4 substrate and PCB. The temperature- and strain-rate-dependent plastic stressstrain curves define the viscoplastic response of the near-eutectic solder alloys. Orthotropic behavior of the FR-4 substrate and PCB is modeled. Other materials are assumed to behave elastically with temperature-dependent material properties. Temperature loading of the package consists of an initial cooling down from the re-flow temperature at 183 oC to 25 oC followed by thermal cycling between -40 to 125 oC. Results of the analysis show that the package warps with a magnitude of 93 µm at 25 oC after re-flow. In this process, the critical solder joint accumulated an inelastic strain of 0.856 percent. Faster temperature ramp rate at 370 oC/min (load case TR1) versus 33 oC/min (load case TC1) resulted in 12 percent lower inelastic strain after completing 3 temperature cycles. However, the inelastic strain magnitude is achieved in a much shorter time. The shear stressstrain hysteresis loops display the largest strain ranges compared to other stressstrain components. The calculated shear strain range is 0.8 percent with the corresponding stress range of 34.0 MPa.

Effect of Thermal Aging on the Microstructure Evolution and Solder Joint Reliability in Hard Disk Drive Under Mechanical Shock

The effect of thermal aging on the microstructure evolution and solder joint reliability in hard disk drive (HDD) under mechanical shock was investigated. Significant coarsening of Ag3 Sn particles was found in SnAgCu solder, and AuSn4 intermetallic compound (IMC) changed from needle-type to layer-type during aging. For as-soldered SnAgCu solder joints after mechanical shock, the cracks were initiated in AuSn4 at the corner of the solder joints, and mainly propagated along the thin Ni3 Sn4 IMC layer. After aging at 150 degC for 21 days, the cracks were mainly propagated along the solder, Ni3 Sn4, Au-Sn-Ni-Cu, and Au-Cu-Sn. The significant coarsening of microstructure was found in SnPb solder joints, and only microcracks were found on the surfaces of as-soldered and aged solder joints after mechanical shock.

Comparison of Inelastic Behaviors of Lead-Free and Sn—Pb Solder Joints

In this study, finite element analysis is used to investigate the inelastic behaviors of lead-free solders, 96.5Sn—3.5Ag and 95.5Sn—3.9Ag—0.6Cu, and classical 63Sn—37Pb solder-bumped wafer level chip scale package (WLCSP) on printed circuit board (PCB) assemblies subjected to four different temperature cycle tests. The behaviors of equivalent strain range and strain energy density at the outmost corner solder joint adjoining to the upper pad are examined. It is found that: (1) the 95.5Sn—3.9Ag—0.6Cu solder has the best performance on the effect of plastic behavior among these three solder alloys, but it accumulates more plastic strain while the number of temperature cycling increased; (2) the 96.5Sn—3.5Ag solder alloy can sustain more creep than others; (3) the difference exists between using the equivalent total strain range and the strain energy density, respectively, to assess the thermo-mechanical behaviors of the solder-bumped WLCSP on PCB assemblies.

Mechanical reliability evaluation of Sn-37Pb solder joint using high speed lap-shear test

This study utilized a high speed lap-shear test to evaluate the mechanical behavior of Sn-37Pb/Cu under bump metallization (UBM) solder joints under high speed loading and hence the drop reliability. The samples were aged for 120 h at different temperatures (150 °С, 180 °С) and then tested at different displacement rates in the range of 0.01 mm/s to 500 mm/s to examine the effects of aging on the drop reliability. The combination of the stress-strain graphs captured from the shear tests and the fracture morphology analysis discloses that the aging at high temperatures has influenced critically the deformation behavior of the solder joints and the effects appears more significant at high strain rates. This study demonstrates a unique capability of a drop reliability evaluation method that utilizes a high speed lap-shear test.

Effect of laser input energy on AuSn x intermetallic compounds formation in solder joints with different thickness of Au surface finish on pads

Formation of AuSnx intermetallic compounds (IMCs) in laser reflowed solder joints was investigated. The results showed that few IMCs formed at the solder/0.1 μm Au interface. Needlelike AuSn4 IMCs were observed at the solder/0.5 μm Au interface. In Sn-2.0Ag-0.75Cu-3.0Bi and Sn-3.5Ag-0.75Cu solder joints, when the laser input energy was increased, AuSn4 IMCs changed from a layer to needlelike or dendritic distribution at the solder/0.9 μm Au interface. As for the solder joints with 4.0 μm thickness of Au surface finish on pads, AuSn4, AuSn2, AuSn IMCs, and Au2Sn phases formed at the interface. Moreover, the content of AuSnx IMCs, such as, AuSn4 and AuSn2, which contained high Sn concentration, would become larger as the laser input energy increased. In the Sn-37Pb solder joints with 0.9 μm or 4.0 μm thickness of the Au surface finish on pads, AuSn4 IMCs were in netlike distribution. The interspaces between them were filled with Pb-rich phases.