Reliability Of Lead-free Solder Joints Research Articles

Effects of voids on thermal-mechanical reliability of lead-free solder joints

Reliability of electronic packages has become a major issue, particularly in systems used in electrical or hybrid cars where severe operating conditions must be met. Many studies have shown that solder interconnects are critical elements since many failure mechanisms originate from their typical response under thermal cycles. In this study, effects of voids in solder interconnects on the electronic assembly lifetime are estimated based on finite element simulations.

Random vibration reliability of BGA lead-free solder joint

Solder joint fatigue failure under vibration loading has been a great concern in microelectronic industry. High-cycle fatigue failure of lead-free solder joints has not been adequately addressed, especially under random vibration loading. This study aims to understand the lead-free solder joint behavior of BGA packages under different random vibration loadings. At first, non-contact TV Laser holography technology was adopted to conduct experimental modal analysis of the test vehicle (printed circuit board assembly) in order to understand its dynamic characteristics. Then, its first order natural frequency was used as the center frequency and narrow-band random vibration fatigue tests with different kinds of acceleration power spectral density (PSD) amplitudes were respectively carried out. Electrical continuity through each BGA package is monitored during the vibration event in order to detect the failure of package-to-board interconnects. The typical dynamic voltage histories of failed solder joints were obtained simultaneously. Thirdly, failed solder joints were cross-sectioned and metallurgical analysis was applied to investigate the failure mechanisms of BGA lead-free solder joints under random vibration loading. The results show that the failure mechanisms of BGA lead-free solder joint vary as the acceleration PSD amplitude increases. Solder joint failure locations are changed from the solder bump body of the PCB side to the solder ball neck, finally to the Ni/intermetallic compound (IMC) interface of the package side. The corresponding failure modes are also converted from ductile fracture to brittle fracture with the increase of vibration intensity.

Effect of Reflow Profile on Intermetallic Compound Formation

Reflow soldering in a nitrogen atmosphere is a common process consideration in surface mount technology assembly. This is because the use of nitrogen in reflow equipment may benefit the process as well as the quality of the end product, where it can increase the reliability of the solder joint. So far, many papers have reported effects of cooling speed, type of solder pastes and solder fluxes on the reliability of lead-free solder joints. While the effects of reflow conditions on intermetallic compound (IMC) formation at the solder joint such as the atmosphere during the reflow process are still unclear. The present study investigated thoroughly the effect of different reflow soldering atmosphere, which is air and nitrogen on IMC formation and growth. Several techniques of materials characterization including optical, image analysis, scanning electron microscopy and energy dispersive X-ray analysis will be used to characterise the intermetallics in terms of composition, thickness and morphology. In addition, the effects of cooling rate and isothermal aging were also studied for the solder alloy Sn–4Ag–0.5Cu on electroless nickel/immersion gold (ENIG) surface finish. From the study, it was found that reflowing under nitrogen atmosphere had better effect on IMC formation and growth compared to reflowing under air. Besides, the cooling rate of solder during reflow also appears to have a significant effect on the final structure of the solder joint, and controlling the growth behaviour of the IMC during subsequent isothermal aging.

Effects of Board Design Variations on the Reliability of Lead-Free Solder Joints

Sn-Ag-Cu (SAC) solder alloys, such as Sn-3.0Ag-0.5 Cu (SAC305) are the popular choices of lead-free solders replacing SnPb solders. However, SAC solders are more brittle in nature due to stiffness and excessive intermetallic compounds growth at the solder joint to pad interface. This leads to higher risk of solder joints failures. Memory module-type smaller lead-free ball grid array (BGA) packages are constantly under dynamic stresses during handling and thermal stresses during operations. It is important to understand the dynamic performance and long-term reliability of memory module lead-free BGAs. It is believed that the printed circuit board (PCB) design variations cause dynamic and long-term failure discrepancies in the fields. In this paper, different pad and trace designs are introduced to evaluate the effects of PCB design variations on the bend and accelerated thermal cycling (ATC) performance of lead-free solder joints. Pad designs with nonsolder mask defined, solder mask defined (SMD), and a unique web design are assembled and tested. Different solder alloys, including SAC305, Sn-1.0Ag-0.5Cu (SAC105) SAC105, and SnPb solders, have been evaluated in this paper. Different PCB materials have also been evaluated in the test. Four-point monotonic bend tests are performed to characterize the bending performance variations with different PCB designs and compared with conventional Sn-Pb solder. The SMD pad is shown to have the best bend performance among all other types of designs in this paper. In addition, this design also shows improvement in mitigation of PCB pad cratering with lead-free solders. Wide trace width seems to degrade the strength and is not preferred. Just as it shows superior shock resistance when compared with SAC305, the SAC105 solder alloy also shows better bend performance. There is no significant improvement in bend performance with web design. After aging treatment, bend performance of both SAC305 and SAC105 degraded by up to 34% and 29%, respectively. However, the bend performance of eutectic SnPb is actually improved after aging. ATC tests are performed to investigate the effects of design variations on the long-term reliability of lead-free solder joints; SMD design shows less reliability life than others. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.

Effects of Multiple Reworks on the Accelerated Thermal Cycling and Shock Performance of Lead-Free BGA Assemblies

In this paper, the effect of multiple rework cycles (up to 5×) on reliability of lead-free assemblies is investigated. The test vehicle is designed to capture the reliability impact of rework processes on interconnects of ball grid array (BGA) devices, the solder joints of its adjacent components, and BGA devices of clamshelled devices. The effects on high aspect ratio plated through hole (PTH) vias and microvias are also considered. A variety of BGA packages, such as flip-chip BGA, plastic BGA, and chip array BGA are selected to represent different package technologies and stress levels. The effect of multiple reworks on the behavior of lead-free solder joints during accelerated thermal cycling (ATC) and mechanical shock is extensively explored in this paper. The experimental and analytical findings showed that multiple reworks significantly degraded the ATC performance of the reworked assemblies by up to 50% in terms of characteristic life. It is also found that multiple reworks affect the PTH vias significantly. The combination of increased lead-free rework temperature excursions and high-end printed circuit board (PCB) aspect ratios may incur higher stresses in the PTH copper barrel. Early failures are observed post multiple reworks and the subsequent ATC test due to PTH barrel cracking. However, the effects of multiple reworks are less severe for devices with microvias compared to those with PTH vias. Multiple reworks also cause severe degradation of long-term performance of devices with clamshell designs. With proper control in rework process, multiple reworks have minimum effects on the adjacent BGA solder joints. In addition, failure analysis is performed in order to study the impact of multiple rework on the microstructure of SnAgCu solder joints. It is found that the intermetallic compounds (IMC) layer grows thicker as the number of reworks increases; however, the IMC growth rate and morphology are dependent on the surface finish of the package substrate. The failure mode and failure mechanism after ATC are also compared for different number of rework cycles. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.

Wafer Bumping, Assembly, and Reliability of Fine-Pitch Lead-Free Micro Solder Joints for 3-D IC Integration

In this investigation, Cu-Sn lead-free solder microbumps on 10-μm pads with a 20-μm pitch are designed and fabricated. The chip size is 5 × 5 mm with thousands of microbumps. A daisy-chain feature is adopted for the characterization and reliability assessment. After pattern trace formation, the microbump is fabricated on the trace by an electroplating technique. A suitable barrier/seed layer thickness is designed and applied to minimize the undercut due to wet etching but to still achieve good plating uniformity. With the current process, the undercut is less than 1 μm and the bump height variation is less than 10%. In addition, the shear test is adopted to characterize the bump strength, which exceeds the specification. Also, the Cu-Sn lead-free solder microbumped chip is bonded on an Si wafer using chip-to-wafer bonding technique. Furthermore, the microgap between the bonded chips is filled with a special underfill. The shear strength of the bonded chips without the underfill is measured and it exceeds the specification. The bonding and filling integrity is further evaluated by open/short measurement, scanning acoustic tomography analysis, and cross-section with scanning electron microscopy analysis. The stacked ICs are evaluated by reliability (thermal cycling) test (-55 to 125°C). Finally, ultrafine-pitch (5-μm pads on a 10-μm pitch) lead-free solder microbumping is explored.

Critical studies to improve service reliability of Sn–Ag–Cu solder joints by thermal treatments

Lead-free electronic packages intended for use in applications such as aerospace, military, and other highly demanding service conditions, necessitate exceptional mechanical reliability of lead-free electronic solder joints under realistic service conditions. Most current design strategies employed for improving the reliability of lead-free electronic solder joints are aimed at developing suitable alloying additions and reinforcements to the solder itself. At present there exists no suitable methodology to minimize the effects of service conditions while the solder joint is in service. Since thermomechanical fatigue reliability of electronic solder joints is closely related to the crack nucleation that occurs during very early stages of repeated thermal excursions, this study is based on subjecting solder joints to a limited number of thermal shock (TS) cycles in a chosen temperature regime to nucleate cracks, then evaluating their effectiveness in improving reliability when the solder joints are subjected to additional TS cycles in a different temperature regime. This study is a preliminary investigation, aimed at developing suitable methodology to minimize the effects of damage to lead-free solder joint specimens subjected to repeated thermal excursions during service, by imposing appropriate thermal treatments. These thermal treatments can be automatically implemented at programmed intervals during the service life of the electronic packages. Methods employed in these studies may also be useful to enhance long-term service reliability and to obtain a conservative estimate of long-term service reliability.

Reliability behavior of lead-free solder joints in electronic components

With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.

Reliability of Lead-Free Solder Joints Under a Wide Range of Thermal Cycling Conditions

In this paper, we report a comprehensive set of accelerated thermal cycling (ATC) tests that were performed on test vehicles with different package types, sizes, pitches, and solder joint alloy metallurgies using four different thermal cycling profiles: 0 to 100, -40 to 125, -55 to 125, and -60 to 150°C. Samples from the tests were analyzed for their failure modes, and failure rates were calculated by using Weibull statistics. The characterized life for each test condition was determined and analyzed. The impact of solder alloy metallurgies, package types, sizes, and pitches on acceleration factors of the ATC tests to fatigue life was also analyzed and discussed. The quantified discrepancies among several acceleration factors from different studies compared to the experimental data presented in this paper are illustrated. The results provide valuable guidance on the effects of package types, size, pitches, and solder joints alloy metallurgies on various ATC test conditions. In addition, failure analysis was performed at different stages of the tests for each thermal cycling condition. Dramatic failure mode shifts at extreme ATC conditions were observed. The significance and the long-term impact of the failure modes and failure mechanism shift between various ATC test conditions to the life prediction of lead-free solders are extensively discussed.

Effect on lead-free solder joint reliability caused by solder volume

Sn–37Pb, which consists of lead (Pb) and tin (Sn), has long been used as solder. Since 2000, however, due to the toxicity of lead, this has been replaced by Sn–3.0Ag–0.5Cu, in which silver and copp...

High temperature reliability of lead-free solder joints in a flip chip assembly

The visco-plastic behaviour of solder joints of two models of a flip chip FC48D6.3C457DC mounted on a printed circuit board (PCB) via SnAgCu solder is investigated using Anand's model. While the bumps of one of the models are realistic with 6 μm thickness of intermetallic compound (IMC) at interconnects of solder and bond pads, the other are made up of conventional bumps without IMC at these interconnects. The solder bump profiles were created using a combination of analytical method and construction geometry. The assembled package on PCB was accelerated thermally cycled (ATC) using IEC standard 60749-25. It was found in the result of the simulation that IMC does not only impact solder joint reliability but also is a key factor of fatigue failure of solder joints. The IMC sandwiched between bond pad at chip side and solder bulk is the most critical and its interface with solder bulk is the most vulnerable site of damage. With reference to our results, it is proposed that non inclusion of IMC in solder joint models composed of Sn-based solder and metalized copper substrate is one of the major causes of the discrepancy on solder joint fatigue life predicted using finite element modelling and the one obtained through experimental investigation.

Application of Finite Element Method in Optimal Design of Flip-Chip Package

More and more solder joints in circuit boards and electronic products are changing to lead free solder, placing an emphasis on lead free solder joint reliability. Solder joint fatigue failure is a serious reliability concern in area array technologies. In this study, the effects of substrate materials on the solder joint thermal fatigue life were investigated by finite element model. Accelerated temperature cycling loading was imposed to evaluate the reliability of solder joints. The thermal strain/stress in solder joints of flip chip assemblies with different substrates was compared, and the fatigue life of solder joints were evaluated by Darveaux’s crack initiation and growth model. The results show the mechanisms of substrate flexibility on improving solder joint thermal fatigue.

鉛フリーソルダリングにおけるはんだ供給量の信頼性への影響

鉛フリーソルダリングにおけるはんだ供給量の信頼性への影響

Characteristic Life Based Acceleration Transforms for Lead-Free Solder Joint Reliability under Thermal Cycling Conditions

The conversion to Pb-free solders has already been implemented in the consumer market. However, the transition to Pb-free solders in the high performance, mission critical networking/server industry has not yet been fully implemented. A significant factor in the slow transition to Pb-free solders is the absence of industry accepted Pb-free acceleration transforms that can translate lab test results into field reliability life data. Without such industry accepted acceleration transforms, it is difficult to translate lab test data into reliable predictions of field life performance. In this two-phase study, a comprehensive set of test data have been generated over a wide spectrum of package types, solder joint metallurgies, joint size, pitch, die sizes, PCB thicknesses and thermal cycling conditions. The effect of aging on acceleration transforms was also included. The data has been analyzed and compared with existing Pb-free and SnPb acceleration transform models. The data helps explain contradictions in the existing literature, and offers more comprehensive acceleration transforms for Pb-free solders. Some publications indicate that the acceleration transforms for Pb-free solders are the same as SnPb acceleration transforms [1]. Comprehensive results of this study also show that in specific conditions, Pb-free acceleration transforms could even be lower than SnPb acceleration transforms.

Detrimental Effects of Excessive Gold Plating on Lead-Free Solder Joints

While most of the industries are striving very hard to produce totally lead-free electronic products, many concerns remain regarding lead-free solder joint reliability. One major concern is the robustness of gold metallization of the electronic components for lead-free soldering. Increasing gold content has been known to result in embrittlement and early failure in electronic assemblies. Therefore, information about the lead-free solder/gold metallization interdiffusion at high-temperature applications is very important for controlling the technological processes for the reliability of the electronic interconnects. The challenges of solder/gold metallization interdiffusion during high-temperature application/test are gold embrittlement, intermetallics growth, void formation, and also tin-whisker formation. This paper illustrates few case histories of such challenges. Importance of the thickness of the gold termination has been discussed and some parameters to optimizing the thickness of the gold termination have been suggested. Some remedial measures are suggested to control the lead-free solder/gold metallization diffusion in the electronics interconnects.

Development of a Novel Microimpact-Fatigue Tester and Its Application to Impact Reliability of Lead-Free Solder Joints

Drop-impact forces cause portable electronic devices to fail. Assessment of resistance against drop-impact force is required to predict life of an electronic package. Several methods have been used to evaluate impact-fatigue life and other advanced methods have been proposed. However, such conventional impact tests require excessive time and cost. In this paper, a novel micro-impact-fatigue tester is developed to overcome such drawbacks of conventional methods. A newly developed impact-fatigue apparatus directly applies impact force to solder joints and measures deformation of the solder joints. The impact-fatigue test apparatus consists of an electromagnetic actuator, an impact-pin, a load-cell, a displacement sensor, and a main frame. Electromagnetic actuator produces a repeatable impact force with a changeable amplitude and pulse duration. Impact-fatigue apparatus was used to test reliability of a lead-free solder (96.5Sn4.0Ag0.5Cu). An evaluation of impact-fatigue life was performed over a wide range of 1-10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cycles with various applied forces ranging from 40 N to 110 N. Two failure modes were observed in a section inspection. First type of failure was a mixed mode failure, where a bulk solder failure and interface failure coincide and the relation between the impact load and fatigue life is almost linear. Stress-based life-prediction model is proposed for the mixed mode failure. The second type of failure, interface failure between the Ni(P) layer and the solder, occurs under a high-load condition. Fatigue life is shorter in the second type of failure than in the mixed mode failure. Brittleness of the interface reduces the impact-fatigue life.

A Novel Mechanism of Embrittlement Affecting the Impact Reliability of Tin-Based Lead-Free Solder Joints

This work addresses a new mode of brittle failure that occurs in the bulk of tin-based lead-free solder joints, unlike the typical brittle failures that occur in the interfacial intermetallics. Brittle failures in the joint bulk result from the low-temperature ductile-to-brittle transition in the fracture behavior of β-tin. The bulk embrittlement of these joints is discussed by referring to the results of impact tests performed on both solder joints and bulk solder specimens. The mechanism of bulk embrittlement is largely explained based on the results of a fractography study performed on the bulk joint failures using scanning electron microscopy.

Experimental and numerical analysis of BGA lead-free solder joint reliability under board-level drop impact

Board-level solder joint reliability is very critical for handheld electronic products during drop impact. In this study, board-level drop test and finite element method (FEM) are adopted to investigate failure modes and failure mechanisms of lead-free solder joint under drop impact. In order to make all ball grid array (BGA) packages on the same test board subject to the uniform stress and strain level during drop impact, a test board in round shape is designed to conduct drop tests. During these drop tests, the round printed circuit board assembly (PCBA) is suffered from a specified half-sine acceleration pulse. The dynamic responses of the PCBA under drop impact loading are measured by strain gauges and accelerometers. Locations of the failed solder joints and failure modes are examined by the dye penetration test and cross section test. While in simulation, FEM in ABAQUS software is used to study transient dynamic responses. The peeling stress which is considered as the dominant factor affecting the solder joint reliability is used to identify location of the failed solder joints. Simulation results show very good correlation with experiment measurement in terms of acceleration response and strain histories in actual drop test. Solder joint failure mechanisms are analyzed based on observation of cross section of packages and dye and pry as well. Crack occurred at intermetallic composite (IMC) interface on the package side with some brittle features. The position of maximum peeling stress in finite element analysis (FEA) coincides with the crack position in the cross section of a failed package, which validated our FEA. The analysis approach combining experiment with simulation is helpful to understand and improve solder joint reliability.

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.

Reliability and Failure Analysis of Lead-Free Solder Joints for PBGA Package Under a Cyclic Bending Load

Nowadays, interest in the bending reliability of ball grid array packages has increased with the increase in mobile devices. Initially, bending tests were conducted to certify the safety of an electronic package during the manufacturing and shipping processes. But recently, the purpose of the bending test has changed: cyclic bending tests are being used to evaluate the electronic package's endurance against handling damage such as bending, twisting and key pressure. Furthermore, the bending test is being adopted as an alternative to a drop test. In this study, a four-point cyclic bending test was performed under various loading levels to investigate the fatigue behavior of solder joints with chemical compositions of 95.5Sn4.0Ag0.5Cu and 63Sn37Pb. It was found that the lead-free solder has a longer fatigue life than the lead-contained solder when the applied load is low. A finite element analysis (FEA) with plasticity and creep constitutive equations was conducted because there are no suitable sensors to measure stress and strain of solder ball joints directly. From the analysis results, it was found that the inelastic energy dissipation could be used as a good damage parameter. Also, from the inspection of the failure site and the FEA, it was found that the fatigue crack initiated at the exterior solder joints and propagated into the inner solder joints.