Pb-free Solder Joints Research Articles

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.

Three-dimensional (3D) visualization of reflow porosity and modeling of deformation in Pb-free solder joints

The volume, size, and dispersion of porosity in solder joints are known to affect mechanical performance and reliability. Most of the techniques used to characterize the three-dimensional (3D) nature of these defects are destructive. With the enhancements in high resolution computed tomography (CT), the detection limits of intrinsic microstructures have been significantly improved. Furthermore, the 3D microstructure of the material can be used in finite element models to understand their effect on microscopic deformation. In this paper we describe a technique utilizing high resolution (< 1 µm) X-ray tomography for the three-dimensional (3D) visualization of pores in Sn-3.9Ag-0.7Cu/Cu joints. The characteristics of reflow porosity, including volume fraction and distribution, were investigated for two reflow profiles. The size and distribution of porosity size were visualized in 3D for four different solder joints. In addition, the 3D virtual microstructure was incorporated into a finite element model to quantify the effect of voids on the lap shear behavior of a solder joint. The presence, size, and location of voids significantly increased the severity of strain localization at the solder/copper interface.

Investigation on the failure criterion of reliability testing for Pb-free BGA packages

Daisy-chained test vehicles are commonly used in board level reliability testing. By continuously monitoring the in-situ daisy chain resistance change over time, a failure could be captured during cycling and eventually the failure data could be used to establish the solder joints failure distribution under different testing conditions. One of the most debatable matters is that when should one to determine a failure to occur. Per IPC 9701A [1] a failure is defined as 10 1000-ohm events in 1 micro-second duration for event detector or 20% increase over the baseline resistance for data logger. Other threshold values such as 100, 300, or 500 ohms are also commonly used by packaging reliability community. Such a wide range of failure threshold values may introduce significant delta in terms of cycle numbers for Pb-free solder joints if different criteria would be used as reported by Henshall, etc [2]. Therefore a systematic study of the impact of using such diversified resistance values on the final failure distribution is necessary and important such that no big difference among reliability results from different sources. The purpose of this study is to investigate the impact of different failure thresholds on Pb-free solder joint failure distribution for most commonly used packages. The test vehicle, designed on an 8″×15″ double-sided printed circuit board (PCB) with multiple test sites, was populated on both sides with daisy-chained components. To reflect the real situation, the components were selected to include different package types (FCBGA, PBGA, CSP, QFN, etc), different pitches (0.4–1.0 mm), and different package size (6–50mm). The assembled test vehicles then went through 0C–100C thermal cycling, the cycle numbers corresponding to different resistance thresholds were recorded and compared. The test results showed that the failure threshold has significant impact on Pb-free solder joint failure distribution, thus it is important to unify the failure criterion such that the reliability results from different sources could be compared side by side. For some packages especially small wire-bond packages that have relatively low baseline resistance, the 20% failure criterion may be too sensitive to the resistivity changes caused not by solder joint failure but other events such as connection cable resistivity change over time or temperature.

Failure site transition in Pb-free solder joints caused by manufacturing variabilities in BGA and MLF components in low cycle fatigue analysis

A comprehensive three level design of experiment that is applied on the manufacturing process to compare the effect of variability in printing and reflow processes on quality and durability of Micro Lead Frame (MLF) and Ball Grid Array (BGA) components. The quality and durability of the BGA and MLF components were studied very closely by thermo-mechanical experiment and finite element simulation. Different populations of failures indicating different failure site and mechanisms were observed in both BGA and MLF components. Conflictions were observed when parameter variations caused durability improvement in one type and durability degradation in the other package.

Pb-free v/s Tin-Lead Reliability Comparison for Telecom/High Reliability Applications

With the European Union's (EU's) RoHS directives coming into force in July 2006 for consumer electronics products, the transition to lead-free (Pb-free) solder has occurred at a rapid pace. This push has driven many OEM suppliers/manufacturers to adopt Pb-free solder and End of Life many of their conventional Tin-Lead (Sn-Pb) components. This has forced telecom or high reliability applications to adopt Pb-free solder compositions with many reliability anomalies unanswered. While there have been many studies published on long term reliability of Pb-free solder joints at the component level, there have been few studies focused on the time zero reliability of the joints at the printed circuit board assembly (PCBA) level. The goal of this study is to help the OEM suppliers and their customers (like service providers) to come up with a common PCBA test methodology that will help identify and weed out early, marginal manufacturing and design defects that would crop up due to transition to the Pb-free solder. A normalized reliability data comparison and impact of the test on Pb-free and Tin-Lead solder alloys using test vehicles is presented in this study. The sequential PCBA level evaluation methodology involves a series of tests that include Thermal Aging, Mechanical Shock, Vibration, Functional test over elevated temperature and Destructive Analysis (Dye &amp; Pry and Cross-sectional analysis) . The solder joint reliability comparisons for different components are presented against this methodology using different PCBA constructions (test vehicles).

Correlations Between the Microstructure and Fatigue Life of Near-Eutectic Sn-Ag-Cu Pb-Free Solders

Relationships between the microstructure of near-eutectic Sn-Ag-Cu Pb-free solder joints and room-temperature fatigue lifetimes were studied. Correlations between the lifetimes of single Sn grained, SAC205 solder joints with the orientation of the Sn grain, and with differences in Ag3Sn and Cu6Sn5 precipitate microstructures were sought. Correlations between the number of Sn grains and fatigue life were observed. Surprisingly, it was found that Ag3Sn precipitates were highly segregated from Cu6Sn5 precipitates on a length scale of approximately 20 μm. Furthermore, large (factor of two) variations of the Sn dendrite arm size were observed within given samples. Such variations in values of dendrite arm size within a single sample were much larger than observed variations of this parameter between individual samples. Few significant differences were observed in the average size of precipitates in different samples. Although effects of average precipitate microstructure on lifetimes were not clearly delineated, one sample showed an anomalously high number of the smallest size (30 nm to 50 nm) Ag3Sn precipitates, and this sample also exhibited a much longer lifetime than all the other samples. Thus, some evidence was presented that samples of particular orientations and precipitate microstructures can exhibit anomalous fatigue lifetimes.

Effect of Electromigration on Interfacial Reactions in 90Sn-10Sb Pb-Free Solder Joints

The electromigration effect on interfacial reactions in Cu/90Sn-10Sb/Cu Pb-free solder joints was investigated under electric current stressing. The growth of the Cu(3)Sn and Cu(6)Sn(5) intermetallic compound (IMC) layers was enhanced at the anode but inhibited at the cathode, compared with the no-current case. The growth of the IMC at the anode followed a parabolic law. Upon increasing the temperature to about 140A degrees C, the thickness of the Cu(6)Sn(5) IMC at the anode increased significantly. Sn(3)Sb(2) IMC coarsened in the Cu(6)Sn(5) IMC at the anode and in the beta-Sn at the cathode. The possible mechanism of the electromigration effect is discussed.

Microstructure, Intermetallic Growth, and Reliability of Rapidly Solidified Pb-Free Solder Joints Formed via Solder Ball Jetting

The majority of research on Pb-free solders has been done with reflow times sufficient to allow significant intermetallic growth at the liquid solder/pad interface. With the need for small, fine-pitched solder joints and to avoid damage to heat-sensitive read/write sensors, the hard disk drive industry predominantly uses solder jet bonding for electrical connections of the heads. This technique does not use flux and has solidification times on the order of a millisecond, four orders of magnitude smaller than that of conventional solder technology. Therefore, the intermetallic formation is highly nonequilibrium and is localized near the pad interface. Surface finish thicknesses and composition have a significant influence on intermetallic growth and composition of solder joints. The intermetallic microstructure, along with voids, can significantly impact joint reliability. In this paper, results of wettability and mechanical solder ball shear measurements on as-plated and aged samples are reported and correlated with microstructure and growth of intermetallic compounds (IMCs) at the solder/pad interface. A wide range of Au-Sn IMCs are found at the pad interface, and Au-embrittlement effects are not found until Au exceeds 7 wt.%.

The Crystal Orientation of β-Sn Grains in Sn-Ag and Sn-Cu Solders Affected by Their Interfacial Reactions with Cu and Ni(P) Under Bump Metallurgy

Recently, it has been reported that the crystal orientation and grain size of the β-Sn phase in Sn-rich solders have profound effects on the reliabilities of Pb-free solder joints, such as thermo-mechanical fatigue and electromigration. Additionally, it is also known that the microstructure of the Sn-rich solders is strongly affected by their alloy composition. In this study the grain size and orientation of the β-Sn phase were investigated in terms of their alloy composition and interfacial reactions with two different under bump metallurgies (UBMs), Cu and Ni(P). Solder balls (380 μm in diameter) of pure Sn, Sn-0.5Cu, Sn-0.5Ag, and Sn-1.8Ag (in weight percent) were reflowed on Cu and Ni(P) UBMs. After the reflow at 250°C for 120 s, the microstructure of the solder joints was analyzed by cross-polarization light microscopy and electron backscatter diffraction. For the compositional analysis of solder joints, electron probe micro-analysis was used and thermodynamics calculations were also performed. During reflow on Cu or Ni(P) UBM, Cu and Ni atoms were dissolved quickly and were saturated to their solubility limits in the solders, causing changes in composition and β-Sn grain orientation.

In situ measurement of electromigration-induced transient stress in Pb-free Sn–Cu solder joints by synchrotron radiation based x-ray polychromatic microdiffraction

Electromigration-induced hydrostatic elastic stress in Pb-free SnCu solder joints was studied by in situ synchrotron x-ray white beam microdiffraction. The elastic stresses in two different grains with similar crystallographic orientation, one located at the anode end and the other at the cathode end, were analyzed based on the elastic anisotropy of the β-Sn crystal structure. The stress in the grain at the cathode end remained constant except for temperature fluctuations, while the compressive stress in the grain at the anode end was builtup as a function of time during electromigration until a steady state was reached. The measured compressive stress gradient between the cathode and the anode is much larger than what is needed to initiate Sn whisker growth. The effective charge number of β-Sn derived from the electromigration data is in good agreement with the calculated value.

Two-Dimensional Mapping of the Mechanical Properties of Pb-Free Solders for Reliability Optimization

The development of thermomechanical models of Pb-free solders is more complex than for Pb-Sn solders as a result of their higher reactivity and processing temperature that leads to the continuous evolution of stiff and angular intermetallics in the microstructure. In this paper, nanoindentation has been explored for its potential to characterize mechanically the complex microstructure of Pb-free solder joints. Hardness and Young's modulus of the various microphases in a Sn-Ag-Cu/Cu solder joint were characterized at 25°C and averages were obtained from nanoindentation maps composed of 100–200 indentations. The possibility to map mechanical property gradients across the various solder joint interfaces and to differentiate between different presentations of the same phase, including in situ at elevated temperatures, has been suggested to be useful in extending the available data for constitutive models used in reliability simulations of Pb-free solders.

Accelerated Thermo-Mechanical Fatigue Life of Pb-Free Solder Joints for PZT Ceramic Resonator

In this study, we optimized Pb-free Sn/Ni plating thickness and conditions were optimized to counteract the environmental regulations, such as RoHS and ELV(End-of Life Vehicles). The B life verification method was also suggested to have been successful when used with the accelerated life test(ALT) for assessing Pb-free solder joint life of piezoelectric (PZT) ceramic resonator. In order to evaluate the solder joint life, a modified Norris-Landzberg equation and a Coffin-Manson equation were utilized. Test vehicles that were composed of 2520 PZT ceramic resonator on FR-4 PCB with Sn-3.0Ag-0.5Cu for ALT were manufactured as well. Thermal shock test was conducted with 1,500 cycles from (-40 ± 2) C to (120 ± 2) C, and 30 minutes dwell time at each temperature, respectively. It was discovered that the thermal shock test is a very useful method in introducing the CTE mismatch caused by thermo-mechanical stress at the solder joints. The resonance frequency of test components was measured and observed the microsection views were also observed to confirm the crack generation of the solder joints.

Thermomigration in Pb-free SnAg solder joint under alternating current stressing

Lead-free solders have been adopted by the microelectronics industry. However, their thermomigration behaviors are unclear. Thermomigration in eutectic SnAg3.5 solder joints was investigated using an alternating current (ac) of 0.57 A at 100 °C. The ac eliminates the electromigration effect and creates a thermal gradient of 2829 °C/cm, facilitating the study of thermomigration. Arrays of tiny markers fabricated by a focused ion beam are employed to measure the thermomigration rate. It is found that Sn atoms migrated toward the hot end. The thermomigration flux and molar heat of transport are measured to be 5.0×1012 atoms/cm2 and 1.36 kJ/mole, respectively.

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.

Recent advances on kinetic analysis of electromigration enhanced intermetallic growth and damage formation in Pb-free solder joints

A comprehensive kinetic analysis was established to investigate the electromigration (EM) enhanced intermetallic compound (IMC) growth and void formation for Sn-based Pb-free solder joints with Cu under bump metallization (UBM). The kinetic model takes into account Cu–Sn interdiffusion and current stressing. Derivation of the diffusion coefficients and the effective charge numbers for the intermetallic compounds is an essential but challenging task for the study of this multi-phase multi-component intermetallic system. A new approach was developed to simultaneously derive atomic diffusivities and effective charge numbers based on simulated annealing (SA) in conjunction with the kinetic model. A consistent set of parameters were obtained, which provided important insight into the diffusion behaviors driving the IMC growth. The parameters were used in a finite difference model to numerically solve the IMC growth problem and the result accurately correlated with the experiment. EM reliability test revealed that the ultimate failure of the solder joints was caused by extensive void formation and subsequent crack propagation at the intermetallic interface. This damage formation mechanism was analyzed by first considering vacancy transport under current stressing. This was followed by a finite element analysis on the crack driving force induced by void formation. This paper is concluded with a future perspective on applying the kinetic analysis and damage mechanism developed to investigate the structural reliability of the through-Si-via in 3D interconnects.

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.

Blech effect in Pb-free flip chip solder joint

The Blech effect in electromigration is studied in flip-chip-like Pb-free solder joint structures. The results from two different studies indicate that the Blech limit (J×L)c (current density times solder length) is close to 30 A/cm in Sn1.8 Ag solders, where the dominating degradation mechanism is Sn self-diffusion. For Sn0.7 Cu solders, where the failure is driven by interstitial diffusion, the Blech effect is not observed. When Blech product is approaching the Blech limit, a steady increase in resistance is replaced by a near-zero resistance change. This saturation in resistance shift significantly extends the electromigration lifetime in SnAg solders.

Pb-Free High Temperature Solder Joints for Power Semiconductor Devices

Three types of inexpensive Pb-free solder joints, namely Zn-based and Bi-based solders, and a CuSn alloy were studied for application to the high-temperature operation of wide band-gap power semiconductor devices using GaN or SiC. Zn–Al solder sheets, whose melting point is 380°C were prepared, and then surface oxides were removed by RF plasma etching. Subsequently, Cu thin films were deposited by the DC sputtering method on the solder sheet. After that, joint samples were fabricated using a conventional electrical furnace with H2/N2 gas flow. That the joints were sufficiently dense was confirmed by scanning acoustic microscope. The wettability of the solder was improved due to contact between the pure Zn–Al solder without oxide and the Ni surface of the substrate, and the chemical reaction between the Zn–Al solder and the deposited Cu thin film. Bi with CuAlMn particle solders, whose melting point is 270°C, were fabricated and it was found that the solder has almost double the tensile strength of pure Bi. Excellent wettability was observed with the Ni or Pd thin film deposited on the Cu substrate. It was found that deposited Ni or Pd was not observed in the interface between the substrates after the joint samples were fabricated. CuSn alloy joints were fabricated using Sn thin films deposited and annealed on Cu substrates. The temperature during the annealing was kept at approximately 350°C; therefore, only the Sn film was melted and some chemical reaction occurred between the liquid Sn and solid Cu. Then, Cu3Sn alloy, whose melting point is 640°C, appeared in the joint and the composition was confirmed by X-ray diffraction analysis. Furthermore, it was found that the CuSn alloy is harder than Cu as measured by nano indentation analysis. In addition, the joints showed excellent thermal cycle reliability.Finally, the properties of these joints are summarized.

Anisotropic Crystal Plasticity Finite Element Modeling of the Effect of Crystal Orientation and Solder Joint Geometry on Deformation after Temperature Change

The crystal orientation of the tin phase in a Pb-free Sn solder joint has a significant effect on the stress state, and hence on the reliability of the solder joint. A set of crystal plasticity analyses was used to evaluate stress and strain resulting from a 165°C temperature change in a single-crystal joint using two simplified geometries used in practical solder joints. Phenomenological flow models for ten slip systems were estimated based upon semiquantitative information available in the literature, along with known anisotropic elastic property information. The results show that the internal energy of the system is a strong function of the tin crystal orientation and geometry of the solder joint. The internal energy (and presumably the likelihood of damage) is highest when the crystal c-axis lies in the plane of the substrate, leading to significant plastic deformation. When the a-axis is in the plane of the interface, deformation due to a 165°C temperature change is predominantly elastic. The texture of the copper substrate using isotropic Cu elastic properties, or anisotropic elastic properties with [001] $$ \parallel $$ substrate normal direction, does␣not have a significant effect on the stress or strain in the Sn phase of the joint.

Failure induced by thermomigration of interstitial Cu in Pb-free flip chip solder joints

Failure induced by thermomigration in Pb-free SnAg flip chip solder joints has been investigated by electromigration tests under 9.7×103 A/cm2 at 150 °C. The fast interstitial diffusion of Cu atoms from underbump metallization into Sn matrix caused void formation at the passivation opening on the chip side. The Cu diffusion was driven by a large thermal gradient and led to void formation even in the neighboring unpowered bumps. When the thermal gradient is above 400 °C/cm, theoretical calculation indicates that the thermomigration force is greater than the electromigration force at 9.7×103 A/cm2 stressing.