Board level solder joint reliability analysis of a fine pitch Cu post type wafer level package (WLP)

Wafer level packaging (WLP) has many advantages, such as ease of fabrication and reduced fabrication cost. However, solder joint reliability of traditional WLPs is the weakest point of the technology. In this paper, a 0.4 mm pitch Cu post type WLP has been developed for mobile computing application. The Cu post type WLP has 440 I/Os and 12 × 12 mm die size. The initial design WLP has been fabricated and subjected to a thermal cycling (TC) testing. The failure life of the original WLP under TC was 296 cycles. This paper also presents a nonlinear finite element analysis of the board level solder joint reliability and methods for enhancement of the WLP. A viscoplastic constitutive relation is adopted for the solder joints to account for its time and temperature dependence in TC. The fatigue life of the solder joint is estimated by the modified Coffin–Manson equation. The two coefficients in the modified Coffin–Manson equation are also determined. A series of parametric studies are performed by changing the passivation (PI) thickness, redistribution layer (RDL) thickness, polymer height (Cu post height accordingly varies), die thickness, PCB thickness, and PCB CTE. The results obtained from the modeling are useful to formulate design guidelines for board level reliability enhancement of the WLP.     

Rapid power cycling of flip-chip and CSP components on ceramic substrates

Power cycling has been done for flip-chip and CSP components solder joined onto ceramic substrates. Cycle periods as short as 1 min were applied in the experiments where the chip temperature varied between about 30°C in the power off-state and 100–150°C in the power on-state. Disconnections of the joints were found after 4000–17 000 power cycles. The flip-chip components joined onto low temperature cofired ceramic substrate showed slightly better reliability than the components joined onto alumina substrate. Most of the samples showed clear effects of deterioration of the joints seen as increasing chip temperature for power on-state. The experimental results are compared with calculations based on modified Coffin–Manson equation as well as with one-dimensional simulations.     

Impact of the number of chips on the reliability of the solder balls for wire-bonded stacked-chip ball grid array packages

This investigation examines how the number of chips affects the reliability of solder balls for wire-bonded stacked-chip ball grid array packages under thermal cycling tests. The studied objects were packages with one, two, three and four stacked chips. Three-dimensional finite element analysis was utilized to simulate the stress/strain behavior of all studied packages. Two kinds of properties of 63Sn/37Pb eutectic solder were employed individually in the finite element analyses. One property of the solder was assumed to exhibit the elastic–plastic–creep behavior. Temperature-dependent stress/strain curves and Norton’s steady creep equation were used in the analysis. Another property of the solder governed by the Anand’s viscoplastic model was also employed to describe the behavior of solder balls. The simulation results in the elastic–plastic–creep analyses and viscoplastic analyses reveal that the von Mises stress, the non-linear strain, and the inelastic strain energy density of the critical solder balls increase with the number of stacked chips, but the increments become gradually stable as the number of chips increases. Three fatigue life prediction models—Darveaux’s model, the modified Coffin–Manson model and the creep-fatigue model—were applied to evaluate the fatigue life of the studied packages. Prediction results indicate that the fatigue life of the solder balls decreases as the number of stacked chips increases, and the decrease in predicted life shows stable behavior as the number of chips increases. The stable trend is consistent with experimental observation in the thermal cycling tests. By comparing with the experimental data, it is shown that the Darveaux’s model gives better prediction than the other two models.     

Solder joint reliability of a low cost chip size package—NuCSP

The accumulated elasto-plastic strain range and the time-dependent elasto-plastic-creep deformation of a wire bondable land grid array (LGA) chip size package (NuCSP) assembly’s solder joint under thermal cycling condition are presented. The solder is assumed to be a temperature-dependent material. The thermal fatigue life of the solder joint is estimated based on two methods, namely, the accumulated plastic strain range and Coffin–Manson equation, and viscoplastic strain energy density and an empirical equation based on the assumption of linear fatigue crack growth.     

Fatigue analysis of miniaturized lead-free solder contacts based on a novel test concept

The paper presents the method of generating lifetime-prediction-laws on special prepared very stiff specimen. The combination of thin- and thick-film technology allows building up test samples on ceramic very similar to electronic packages including the measurement issues. Influences of pad surface metallurgy, microstructure of solder, ineutectic solder alloys and assembly process parameter are regarded now. The investigation objects provide monitoring of electrical and mechanical damage process of SnAgCu solder bump. Different thermo-mechanical loads will be applied in temperature ranges of 0 to +80 °C, −40 to +125 °C and −50 to +150 °C, where the temperature gradient and cycle frequency also vary. A Variation of four different chip sizes allows the determination of fatigue laws for each temperature profile, to be able to compare in between them. The results of these tests will give universal lifetime-prediction laws for SnAgCu base solder joints. Main goals are to find coefficients for lifetime prediction models such as Coffin–Manson- or Norris–Landzberg-relation, which are transferable in between different electronic packages.     

Reliability studies of two flip-chip BGA packages using power cycling test

Power cycling tests of the second level reliability of two flip-chip BGA packages are discussed in this paper. The first one is for a flip-chip on laminate package (FCPBGA) and the other for a flip-chip on ceramic package (FCCBGA). For the FCPBGA, test strategies will be first discussed and then focus will be given to a unique failure mode associated with this type of packages assembled back to back onto printed circuit board. Instead of anticipated failures of the corner solder joints under the die shadow, as in the case of wire-bonded packages, we found that solder joints failed first in the central region of the package and then failures of solder joints spread out in the radial direction from the center of the package. Explanation will be given to the physical mechanisms that caused this type of failure. For the FCCBGA, the improved test strategies based on what has been learned from the test of FCPBGA will be presented and focus will be given to the effect of different parameters on the second level reliability of the package. Here, because of the increased rigidity of the ceramic substrate solder joints failed as expected first at the corner(s) of the ceramic substrate. Based on the test results and the modified Coffin–Manson equation, predictions or the solder joint fatigue life will be shown.     

Thermomechanical reliability of a silver nano-colloid die attach for high temperature applications

The last several years have seen the advent of silicon carbide (SiC) power devices operating at temperatures well above 125 °C. These devices have the potential to provide higher switching speed and lower on-state losses with higher thermal conductivity. Developing reliable technologies for packaging is now the main hurdle to successful operation of SiC based power electronics at high temperature. This paper evaluates a novel silver nano-particle colloid material that has been suggested for use as a die attachment for high temperature environments. The material synthesis together with fundamental mechanical and electrical properties is presented relative to the low temperature sintering process. Using thermal fatigue data measured for this material, a low cycle fatigue curve for the silver nano-particle colloid was developed. A Coffin–Manson relationship was derived for the solder; which together with calculated strains in the joint, allows the low cycle fatigue life of the die attachment to be predicted.     

A microstructural study of the thermal fatigue failures of 60sn-40Pb solder joints

When an electronic package encounters thermal fluctuations, cyclical shear strain is imposed on the solder joint interconnections. The thermal cycling leads to a condition of thermal fatigue and eventual solder joint failure. This study was performed in order to understand the microstructural mechanisms that lead to solder joint failures in thermal fatigue. Thermal cycling tests were performed on 60Sn-40Pb joints using a -55° C to 125° C cycle and 19% imposed shear strain. A heterogeneously coarsened region of both Pb and Sn-rich phases develops within the 60Sn-40Pb solder joints. Cracks initiate in the heterogeneously coarsened Sn-rich phase at the Sn-Sn grain boundaries. Heterogeneous coarsening and failure occurs in both high (35 to 125° C) and low (-55 to 35° C) thermal cycles. The elevated temperature portion of the thermal cycle was found to be the most significant factor in the heterogeneous coarsening and failure of the solder joints.     

Influence of microstructure size on the plastic deformation kinetics, fatigue crack growth rate, and low-cycle fatigue of solder joints

The influence of microstructure size on the plastic deformation kinetics, fatigue crack growth rate and low-cycle fatigue of eutectic Sn-Pb solder joints is reviewed. The principal microstructure feature considered is the average eutectic phase size d=(d_Pb+d_Sn)/2. The effect of an increase in reflow cooling rate (which gave a decrease in d) on the flow stress and on fatigue life was irregular at 300K, depending on the stress or strain level and cooling rate. In contrast, a consistent increase in fatigue life with decrease in d occurred for thermomechanical cycling between −30° and 130°C. Constitutive equations for plastic deformation and fatigue crack growth rate are presented which include the microstructure size. It appears that the rate-controlling deformation mechanism is the intersection of forest dislocations in the Sn phase. The mechanism for both static and dynamic phase coarsening appears to be grain boundary diffusion with a t^1/4 time law. Some success has been achieved in predicting the cyclic stress-strain hysteresis loops and fatigue life, including the influence of the as-reflowed microstructure size and its coarsening. Additional definitive studies are however needed before we can accurately predict the fatigue life of solder joints over the wide temperature range and conditions experienced by electronic packages.     

Parameters affecting thermal fatigue behavior of 60Sn-40Pb solder joints

Solder joints in electronic packages experience cyclical thermally induced strain when temperature fluctuations are encountered in service. This study investigates three parameters that affect the microstructure and therefore the thermal fatigue behavior of 60Sn-40Pb solder joints. These parameters are: 1) the effect of a tensile component in thermal fatigue, 2) solder joint thickness variations, and 3) hold time variations at the elevated temperature portion of the thermal cycle. Solder joints were thermally fatigued in a tension/compression deformation mode. Cracks developed both in the interfacial intermetallic layer (early in thermal fatigue) and in the coarsened regions of the microstructure of the solder joint (after many more cycles). The effect of joint thickness on solder joints thermally fatigued in shear was also explored. Solder joint thickness was found not to significantly affect fatigue lifetimes. The effect of an increase in the hold time at the elevated temperature portion of the thermal fatigue cycle was also investigated. It was found that time spent at the high temperature end of the fatigue cycle does not determine solder joint lifetime, rather it is the combination of the amount of deformation induced during thermal fatigue in concert with the elevated temperature.     

Temperature profile effects in accelerated thermal cycling of SnPb and Pb-free solder joints

Accelerated thermal cycling (ATC) has been widely used in the microelectronics industry for reliability assessment. ATC testing decreases life cycle test time by one or more of the following means: increasing the heating and cooling rate, decreasing the hold time, or increasing the range of the applied temperature. The relative effect of each of these cycle parameters and the failure mechanisms they induce has been the subject of many studies; however uncertainty remains, particularly regarding the role of the heating and cooling rate. In this research, three conditions with two ramp rates (14 °C/min and 95 °C/min) and two temperature ranges (ΔT = 0–100 °C and −40 to 125 °C) were applied to resistor 2512 and PBGA 256 test vehicles assembled with SnPb and Pb-free solders. The test results showed that the higher ramp rate reduced the testing time while retaining the same failure modes, and that the damage per cycle increased with the temperature difference. For the resistors, the Pb-free solder joints lasted longer than the SnPb joints at the smaller ΔT, but were inferior at the larger ΔT. In contrast, the Pb-free solder joints in the PBGA test vehicles lasted longer than the SnPb solder under both conditions.     

Microstructural evolution of a lead-free solder alloy Sn-Bi-Ag-Cu prepared by mechanical alloying during thermal shock and aging

In a previous study, a lead-free solder, Sn-6Bi-2Ag-0.5Cu, was developed by mechanical alloying. The alloy shows great potential as a lead-free solder system. In the present work, the microstructural evolution during thermal shock and aging was examined. In the as-soldered joints small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles were finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ phase in the bulk of solder. During thermal shock and aging microstructural evolution occurred with Cu-Sn intermetallic compound (IMC) layer growth at interface, bismuth phase coarsening and Ag₃Sn phase coarsening. The microstructure of the solder appeared to be stable at high temperature. The shear strength of the present solder joint is higher than that of Sn-37Pb and Sn-3.5Ag solders. Shear failure occurred Cu-Sn IMC layer-solder interface and in the bulk of solder.     

Observations of microstructural coarsening in micro flip-chip solder joints

Coarsening of solder microstructures dramatically affects fatigue lifetimes. This paper presents a study of microstructural evolution due to thermal cycling and aging of small solder joints. The lead-tin solder joints in this study have a height of 55 5 m and a tin content of 65–70 wt.%, with a degenerate eutectic microstructure. The joint microstructure coarsens more rapidly during aging at 160°C than cycling from 0–160°C. No coarsened bands are observed. The cycling data scales with standard coarsening equations, while the aging data fits to an enhanced trend. The joints experiencing 2.8% strain during cycling fail by 1000 cycles.     

Effects of process conditions on reliability, microstructure evolution and failure modes of SnAgCu solder joints

In this study, microstructure evolution at intermetallic interfaces in SnAgCu solder joints of an area array component was investigated at various stages of a thermal cycling test. Failure modes of solder joints were analyzed to determine the effects of process conditions on crack propagation. Lead-free printed-circuit-board (PCB) assemblies were carried out using different foot print designs on PCBs, solder paste deposition volume and reflow profiles. Lead-free SnAgCu plastic-ball-grid-array (PBGA) components were assembled onto PCBs using SnAgCu solder paste. The assembled boards were subjected to the thermal cycling test (−40 °C/+125 °C), and crack initiation and crack propagation during the test were studied. Microstructure analysis and measurements of interface intermetallic growth were conducted using samples after 0, 1000, 2000 and 3000 thermal cycles. Failures were not found before 5700 thermal cycles and the characteristic lives of all solder joints produced using different process and design parameters were more than 7200 thermal cycles, indicating robust solder joints produced with a wide process window. In addition, the intermetallic interfaces were found to have Sn–Ni–Cu. The solder joints consisted of two Ag–Sn compounds exhibiting unique structures of Sn-rich and Ag-rich compounds. A crystalline star-shaped structure of Sn–Ni–Cu–P was also observed in a solder joint. The intermetallic thicknesses were less than 3 μm. The intermetallics growth was about 10% after 3000 thermal cycles. However, these compounds did not affect the reliability of the solder joints. Furthermore, findings in this study were compared with those in previous studies, and the comparison proved the validity of this study.     

倒装焊SnPb焊点热循环失效和底充胶的影响

采用实验方法 ,确定了倒装焊 Sn Pb焊点的热循环寿命 .采用粘塑性和粘弹性材料模式描述了 Sn Pb焊料和底充胶的力学行为 ,用有限元方法模拟了 Sn Pb焊点在热循环条件下的应力应变过程 .基于计算的塑性应变范围和实验的热循环寿命 ,确定了倒装焊 Sn Pb焊点热循环失效 Coffin- Manson经验方程的材料参数 .研究表明 ,有底充胶倒装焊 Sn Pb焊点的塑性应变范围比无底充胶时明显减小 ,热循环寿命可提高约 2 0倍 ,充胶后的焊点高度对可靠性的影响变得不明显     

Ball grid array reliability assessment for aerospace applications

Reliability of ball grid arrays (BGAs) was evaluated with special emphasis on space applications. This work was performed as part of a consortium led by the Jet Propulsion Laboratory (JPL) to help build the infrastructure necessary for implementing this technology. Nearly 200 test vehicles, each with four package types, were assembled and tested using an experiment design. The most critical variables incorporated in this experiment were package type, board material, surface finish, solder volume, and environmental condition. The packages used for this experiment were commercially available packages with over 250 I/Os including both plastic and ceramic BGA packages.The test vehicles were subjected to thermal and dynamic environments representative of aerospace applications. Two different thermal cycling conditions were used, the JPL cycle ranged from −30°C to 100°C and the Boeing cycle ranged from −55°C to 125°C. The test vehicles were monitored continuously to detect electrical failure and their failure mechanisms were characterized. They were removed periodically for optical inspection, scanning electron microscopy (SEM) evaluation, and cross-sectioning for crack propagation mapping. Data collected from both facilities were analyzed and fitted to distributions using the Weibull distribution and Coffin–Manson relationships for failure projection. This paper will describe experiment results as well as those analyses.     

Computational modeling of creep-based fatigue as a means of selecting lead-free solder alloys

The primary aim of this investigation was to understand the effect of temperature fluctuations on a number of various solder materials namely SAC105, SAC305, SAC405 and Sn–36Pb–2Ag. To achieve this objective, three different classic joint assemblies (a ball joint, a test specimen joint and finger lead joint) were modeled which provided the foundation for the creep and fatigue behaviors simulation. Anand’s viscoplasticity as a constitutive equation was employed to characterize the behavior of solders numerically under the influence of thermal power cycles (80–150 °C) and thermal shock cycles (−40 to 125 °C). To extend the research outcome for industrial use, two additional research activities were carried out. One of them was to obtain lifetime-predictions of solder joints based on Coffin Manson concept. The other one focused on parameterization to obtain the ideal solder thickness under the consideration of plastic strain and economic benefit.     

Metallurgical reactions in composite 90Pb10Sn/lead-free solder joints and their effect on reliability of LTCC/PWB assembly

Use of 90Pb10Sn solder as a noncollapsible sphere material with 95.5Sn 4Ag0.5Cu and SnInAgCu lead-free solders is investigated. Practical reflow conditions led to strong Pb dissolution into liquid solder, resulting in >20 at.% Pb content in the original lead-free solders. The failure mechanism of the test joints is solder cracking due to thermal fatigue, but the characteristic lifetime of 90Pb10Sn/SnInAgCu joints is almost double that of 90Pb10Sn/95.5Sn4Ag0.5Cu in a thermal cycling test (TCT) over the temperature range from −40°C to 125°C. It is predicted that this is mainly a consequence of the better fatigue resistance of the SnPbInAgCu alloy compared with the SnPbAgCu alloy. Indium accelerates the growth of the intermetallic compound (IMC) layer at the low temperature co-fired ceramic (LTCC) metallization/solder interface and causes coarsening of IMC particles during the TCT, but these phenomena do not have a major effect on the creep/fatigue endurance of the test joints.     

Numerical and experimental analysis of the Sn3.5Ag0.75Cu solder joint reliability under thermal cycling

The Sn3.5Ag0.75Cu (SAC) solder joint reliability under thermal cycling was investigated by experiment and finite element method (FEM) analysis. SAC solder balls were reflowed on three Au metallization thicknesses, which are 0.1, 0.9, and 4.0 μm, respectively, by laser soldering. Little Cu–Ni–Au–Sn intermetallic compound (IMC) was formed at the interface of solder joints with 0.1 μm Au metallization even after 1000 thermal cycles. The morphology of AuSn₄ IMC with a small amount of Ni and Cu changed gradually from needle- to chunky-type for the solder joints with 0.9 μm Au metallization during thermal cycling. For solder joints with 4 μm Au metallization, the interfacial morphology between AuSn₄ and solder bulk became smoother, and AuSn₄ grew at the expense of AuSn and AuSn₂. The cracks mainly occurred through solder near the interface of solder/IMC on the component side for solder joints with 0.1 μm Au metallization after thermal shock, and the failure was characterized by intergranular cracking. The cracks of solder joints with 0.9 μm Au metallization were also observed at the same location, but the crack was not so significant. Only micro-cracks were found on the AuSn₄ IMC surface for solder joints with 4.0 μm Au metallization. The responses of stress and strain were investigated with nonlinear FEM, and the results correlated well with the experimental results.     

Thermal Fatigue Endurance of Lead-Free Composite Solder Joints over a Temperature Range of −55°C to 150°C

The thermal fatigue endurance of two lead-free solder/plastic-core solder ball (PCSB) composite joint structures in low-temperature co-fired ceramic (LTCC) modules was investigated using a thermal cycling test over a temperature range of −55°C to 150°C. The investigated solder alloys were Sn-7In-4.1Ag-0.5Cu (SAC-In) and 95.5Sn-4Ag-0.5Cu (SAC). Three failure mechanisms were observed in the test joints. Transgranular (fatigue) cracking mixed with minor intergranular cracking was the dominant failure mechanism at the outer edge of the joints in both test assemblies, whereas separation of the solder/intermetallic compound (IMC) interface and creep cracking occurred in the other parts of the test joints. The propagation rate of the transgranular crack was lower in the SAC-In joints compared with in the SAC joints. Furthermore, the SAC solder seemed to be more prone to separation of the solder/IMC interface, and more severe intergranular (creep) cracking occurred in it compared with in the SAC-In solder. In the thermal cycling test conditions, the better thermal fatigue endurance of the SAC-In solder composite joints resulted in a 75% higher characteristic lifetime compared with the SAC composite joints.