Effect of Size of Lid-Substrate Adhesive on Reliability of Solder Balls in Thermally Enhanced Flip Chip PBGA Packages

Six design cases of lid-substrate adhesive with various combinations of widths and heights were analyzed to investigate how the size of the adhesive affects the reliability of the solder balls of thermally enhanced flip chip plastic ball grid array (FC-PBGA) packages in thermal cycling tests. Analysis results were compared with data on the reliability of conventional FC-PBGA packages. Thermal-mechanical behavior was simulated by the finite element (FE) method and the eutectic solder was assumed to exhibit elastic-viscoplastic behavior. The temperature-dependent nonlinear stress/strain relationship of the adhesive was experimentally determined and used in the FE analysis. Darveaux's model was employed to obtain the predicted fatigue life of the solder ball. Simulation results reveal that the fatigue life of the solder balls in thermally enhanced FC-PBGA packages is much shorter than that in conventional FC-PBGA packages, and the life of solder balls increases with both the width and the height of the adhesive. However, the effect of the width of the adhesive on the reliability of the solder ball is stronger than that of the height. Moreover, increasing either the width or the height reduces the plastic strain in the adhesive at critical locations, indicating that the reliability of the adhesive can be improved by its size. The predicted results of the life of solder balls for some selected studied packages are also compared with experimental data from thermal cycling tests in the paper     

Impact of Electrical Current on the Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects

The interaction between electrical current and the long-term reliability of fine-pitch ball grid array packages with Sn-3.0Ag-0.5Cu (wt.%) solder ball interconnects is investigated. In this study, 0.4-mm fine-pitch packages with 300-μm-diameter Sn-Ag-Cu solder balls are used. Electrical current was applied under various conditions to two different package substrate surface finishes to compare the effects of chemically unmixed and mixed joint structures: a Cu/SAC305/Cu structure and a NiAu/SAC305/Cu structure, respectively. To study the thermal impact on the thermal fatigue performance and long-term reliability, the samples were thermally cycled from 0°C to 100°C with and without current stressing. Based on Weibull plots, the characteristic lifetime was degraded for the mixed joint structure, but little degradation was observed for the unmixed joint structure. The microstructure evolution was observed during constant current stressing and current stressing during thermal cycling. Accelerated intermetallic precipitation depletion at the package-side interface was observed in NiAu/SAC305/Cu structures due to current stressing, which was identified as the potential reason for the degradation in the thermal cycling performance.     

Impact of Isothermal Aging on Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects: Surface Finish Effects

The interaction between isothermal aging and the long-term reliability of fine-pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt.%) solder ball interconnects was investigated. In this study, 0.4-mm fine-pitch packages with 300-μm-diameter Sn-Ag-Cu solder balls were used. Two different package substrate surface finishes were selected to compare their effects on the final solder composition, especially the effect of Ni, during thermal cycling. To study the impact on thermal performance and long-term reliability, samples were isothermally aged and thermally cycled from 0°C to 100°C with 10 min dwell time. Based on Weibull plots for each aging condition, package lifetime was reduced by approximately 44% by aging at 150°C. Aging at 100°C showed a smaller impact but similar trend. The microstructure evolution was observed during thermal aging and thermal cycling with different phase microstructure transformations between electrolytic Ni/Au and organic solderability preservative (OSP) surface finishes, focusing on the microstructure evolution near the package-side interface. Different mechanisms after aging at various conditions were observed, and their impacts on the fatigue lifetime of solder joints are discussed.     

Finite element based fatigue life estimation of the solder joints with effect of intermetallic compound growth

This paper develops an analysis procedure to study the effects of intermetallic compound (IMC) growth on the fatigue life of 63Sn-37Pb (lead-rich)/96.5Sn-3.5Ag (lead-free) solder balls for flip-chip plastic ball grid array packages under thermal cycling test conditions. In this analysis procedure, the thickness of the IMC increased with the number of thermal cycles, and was determined using the growth rate equation. A series of non-linear finite element analyses was conducted to simulate the stress/strain history at the critical locations of the solder balls with various IMC thicknesses in thermal cycling tests. The simulated stress/strain results were then employed in a fatigue life prediction model to determine the relationship between the predicted fatigue life of the solder ball and the IMC thickness. Based on the concept of continuous damage accumulation and incorporated with the linear damage rule, this study defines the damage of each thermal cycle as the reciprocal of the predicted fatigue life of the solder joints with the corresponding IMC thickness. The final fatigue failure of the solder ball was determined as the number of cycles corresponding to the cumulative damage equal to unity. Results show that the solder joint fatigue life decreased as the IMC thickness increased. Moreover, the predicted thermal fatigue life of lead-rich solders based on the effects of IMC growth is apparently smaller than that without considering the IMC growth in the reliability analysis. Results also show that the influence of the IMC thickness on the fatigue life prediction of the lead-free solder joint can be ignored.     

High Cycle Cyclic Torsion Fatigue of PBGA Pb-Free Solder Joints

In this study, a comprehensive experimental and numerical approach was used to investigate high cycle cyclic torsion fatigue behavior of lead-free solder joints in a plastic ball grid array (PBGA) package. The test vehicle was a commercial laptop motherboard. The motherboard was subjected to torsional loading and life tests were conducted. Using finite element analysis (FEA), the test assembly was simulated as a global model and the BGA component was simulated as a local model. Strains measured on the motherboard surface near by the BGA were used to calibrate the FEA models. By combining the life test results and FEA simulations, a high cycle fatigue model for the lead-free solder joints was generated based on the Coffin-Manson strain-range fatigue damage model. This model can now be used to predict the cycles to failure of BGA interconnects for new electronic product design under cyclic torsion loading.     

Random vibration analysis of 3-Arc-Fan compliant interconnects

Microelectronic packaging compliant interconnects offer increased reliability when compared to traditional rigid solder ball interconnects. These interconnects are subject to various forms of mechanical damage including thermal cycle fatigue, drop impact shock, and vibration environments that often lead to mechanical or electrical failure. Second-level compliant interconnects seek to alleviate this issue by decoupling the substrate and board, facilitating independent deformation while experiencing lower stresses and strains. In order to develop compliant interconnects as an effective alternative to rigid solder balls, various design optimization, thermal cycling test, and drop impact studies have been performed. However, the area of vibration characterization and analysis is lacking for microelectronic packaging and nonexistent for compliant interconnects. Therefore, this paper will present a complete vibration analysis of a particular multi-path compliant interconnect design, the 3-Arc-Fan compliant interconnect. This design features three electroplated copper arcuate beams that provide a spring-like effect to increase compliance and mechanical reliability. Experimental vibration characterization was performed and used to validate the simulation model. Following which a random vibration analysis method wais established, and the samples were tested at various conditions. Finally, both experimental and simulation results were integrated to develop a preliminary fatigue life prediction model to demonstrate the increased reliability.     

Laser moiré interferometry for fatigue life prediction of lead-free solders

A laser moiré interferometry based technique to predict the fatigue life of Sn4.0Ag0.5Cu solder joints in a plastic ball grid array (PBGA) package when subjected to a typical accelerated thermal cycling (ATC) loading has been presented in this paper. The fatigue life is estimated by measuring the in-plane strains in the solder joints at various temperatures. The methodology can be used to predict the reliability of the package in a matter of few days as opposed to many months taken by an actual ATC test. The technique can be extended to include multiple temperature regimes in a single experimental setup thereby making it possible to estimate the fatigue life of solder joints when subjected to various ATC loading profiles in a very short duration.     

Reliability Investigation of Mixed BGA Assemblies

The effect of different reflow profiles on the reliability of lead-free (LF) Sn-3.0 Ag-0.5 Cu (wt.%) (SAC 305) ball grid array (BGA) devices assembled with a SnPb eutectic paste was investigated. The memory modules in a back-to-back configuration were reflowed on standard graphic cards finished with immersion silver (IAg) or hot air solder leveling (HASL) coatings. The reflow peak temperatures ranged from 209$^circ$C to 227$^circ$C, while the time above liquidus (TAL) varied from 45 to 80 s. Depending on the reflow conditions, the solder interconnects displayed varied degrees of SnPb and LF solders intermixing. It was established that in order to receive a homogeneous solder alloy, the reflow peak temperature had to be in the 218$^circ$C–222$^circ$C range. The reliability of solder interconnects of memory modules was assessed by subjecting the cards to 1500 cycles of accelerated thermal-cycling with a profile from 0$^circ$C to 100$^circ$C. It was found that the control SnPb/SnPb assemblies displayed superior reliability to that of the mixed assemblies. Regardless of the degree of homogeneity of the BGA balls, the predominant failure mode of the mixed solder joints was interfacial cracking through a Pb-rich phase near the intermetallic layer. In contrast, only partial cracks propagating diagonally through the bulk solder were present on the control boards. It was concluded that a combination of state of stress and segregation of the Pb-rich phase at the interface was responsible for the shortened thermal–mechanical fatigue life of the mixed solder interconnects.     

Effect of Isothermal Aging on the Long-Term Reliability of Fine-Pitch Sn–Ag–Cu and Sn–Ag Solder Interconnects With and Without Board-Side Ni Surface Finish

The combined effects on long-term reliability of isothermal aging and chemically balanced or unbalanced surface finish have been investigated for fine-pitch ball grid array packages with Sn–3.0Ag–0.5Cu (SAC305) (wt.%) and Sn–3.5Ag (SnAg) (wt.%) solder ball interconnects. Two different printed circuit board surface finishes were selected to compare the effects of chemically balanced and unbalanced structure interconnects with and without board-side Ni surface finish. NiAu/solder/Cu and NiAu/solder/NiAu interconnects were isothermally aged and thermally cycled to evaluate long-term thermal fatigue reliability. Weibull plots of the combined effects of each aging condition and each surface finish revealed lifetime for NiAu/SAC305/Cu was reduced by approximately 40% by aging at 150°C; less degradation was observed for NiAu/SAC305/NiAu. Further reduction of characteristic life-cycle number was observed for NiAu/SnAg/NiAu joints. Microstructure was studied, focusing on its evolution near the board and package-side interfaces. Different mechanisms of aging were apparent under the different joint configurations. Their effects on the fatigue life of solder joints are discussed.     

Solder joint reliability of TFBGA assemblies with fresh and reworked solder balls

In this article, the solder joint reliability of thin and fine-pitch BGA (TFBGA) with fresh and reworked solder balls is investigated. Both package and board level reliability tests are conducted to compare the solder joint performance of test vehicle with fresh and reworked solder balls. For package level reliability test, ball shear test is performed to evaluate the joint strength of fresh and reworked solder balls. The results show that solder balls with rework process exhibit higher shear strength than the ones without any rework process. The results also exhibit that the different intermetallic compound (IMC) formation at solder joints of fresh and reworked solder balls is the key to degradation of shear strength. For board level reliability tests, temperature cycling and bending cyclic tests are both applied to investigate the fatigue life of solder joint with fresh and reworked solder balls. It is observed that package with reworked solder ball has better fatigue life than the one with fresh solder ball after temperature cyclic test. As for bending cyclic test, in addition to test on as-assembled packages, reworked and fresh samples are subjected to heat treatment at 150 °C for 100 h prior to the bending cyclic test. The purpose is to let Au–Ni–Sn IMC resettle at solder joints of fresh solder ball and examine the influence of Au–Ni–Sn IMC on the fatigue life of solder joints (Au embrittlement effect). The final results confirm that reworked solder balls have better reliability performance than fresh one since Au embrittlement dose exist at fresh solder ball.     

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.     

Effect of cooling rate on the isothermal fatigue behavior of CBGAsolder joints in shear

This paper investigates the distribution characteristics of the isothermal fatigue lifetime of ceramic ball grid array (CBGA) solder joints in shear. Placement direction of the board-level assembly on the oven conveyor during reflow critically influences the fatigue lifetime of solder joints in shear: the front or outer solder joints have a longer shear lifetime than the rear or inner ones. The solder joints that moved diagonally during reflow have a longer fatigue lifetime and a tighter distribution. Cracks initiated in the eutectic solder region on the card and package side and tend to propagated in that region, while final failure occurred mainly on the card-said eutectic solder region. This phenomenon can be explained that the front or outer solder bumps have a resistant effect to the gas fluid which passes through the rear or inner solder bumps, and lower these solder joints' cooling rate during solidification. Fast cooling rate can cause a more fine-grained and homogeneous microstructure in eutectic solder alloy, which can delay crack initiation and slow crack growth. When the board-level assembly moves diagonally during reflow, the resistant effect of front solder bumps to the gas fluid reduces markedly. So the fatigue lifetime of solder joints and its distribution characteristic enhance substantially. The theories of fluid dynamics and heat transmission are used to calculate the decrease of gas fluid velocity and the corresponding reduction of mean coefficient of heat transfer (h_m)     

Thermomechanical reliability of underfilled BGA packages

The effect of underfill on various thermomechanical reliability issues in super ball grid array (SBGA) packages is studied in this paper. Nonlinear finite element models with underfill and no underfill are developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperature-dependent, while other materials are modeled temperature and direction-dependent, as appropriate. The stress/strain variations in the package due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, time-dependent creep strain and total inelastic strain in solder balls. The effect of copper core on the solder ball strains is presented. The possibility of delamination at the interposer-underfill interface as well as substrate-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA packages. The predicted number of thermal cycles to solder joint fatigue are compared with the existing experimental data on similar nonunderfilled BGA packages.     

Board level solder joint reliability modeling and testing of TFBGA packages for telecommunication applications

For thin-profile fine-pitch BGA (TFBGA) packages, board level solder joint reliability during the thermal cycling test is a critical issue. In this paper, both global and local parametric 3D FEA fatigue models are established for TFBGA on board with considerations of detailed pad design, realistic shape of solder joint, and nonlinear material properties. They have the capability to predict the fatigue life of solder joint during the thermal cycling test within ±13% error. The fatigue model applied is based on a modified Darveaux’s approach with nonlinear viscoplastic analysis of solder joints. A solder joint damage model is used to establish a connection between the strain energy density (SED) per cycle obtained from the FEA model and the actual characteristic life during the thermal cycling test. For the test vehicles studied, the maximum SED is observed at the top corner of outermost diagonal solder ball. The modeling predicted fatigue life is first correlated to the thermal cycling test results using modified correlation constants, curve-fitted from in-house BGA thermal cycling test data. Subsequently, design analysis is performed to study the effects of 14 key package dimensions, material properties, and thermal cycling test condition. In general, smaller die size, higher solder ball standoff, smaller maximum solder ball diameter, bigger solder mask opening, thinner board, higher mold compound CTE, smaller thermal cycling temperature range, and depopulated array type of ball layout pattern contribute to longer fatigue life.     

Finite element modeling of electronic packages subjected to drop impact

As more electronic products become portable, many product manufacturers have started to pay more attention to the robustness of their products. Finite element (FE) simulation has become increasingly popular in the analysis of products subjected to impact loading. The need for details in a FE mesh is always balanced by considerations of simulation time and available computational resources. In this paper, three commonly used approaches to FE modeling of a ball grid array (BGA) package subjected to drop impact are evaluated. The first model comprises a detailed mesh of the printed circuit board (PCB), integrated circuit (IC) package and interconnecting solder balls using solid three-dimensional (3-D) elements. The degrees of freedom is reduced for the second mesh by using shell elements for the PCB and IC package while retaining the detailed mesh of the solder balls using solid 3-D elements. The third mesh is a further simplification of the second mesh whereby the solder balls are replaced by a single beam element each. The stresses within the solder balls are then obtained in a separate FE analysis of a detailed solder ball mesh using the displacement history of nodes around the beam elements from the previous analysis as inputs. Solder ball stresses from all three meshes were found to differ by as much as 40% although PCB deflection compared favorably.     

A Study on the Thermal Fatigue Behavior of Solder Joints Under Power Cycling Conditions

Failure mechanisms exposed by environmental accelerating testing methods such as thermal cycling or thermal shock test, may differ from those at service operating conditions. While the device is heated up or cooled down evenly on its external surface during environmental testing, real operating powered devices experience temperature gradients caused by internal local heating, components' different heat dissipation capability, and ambient temperature variation, etc. In this study, a power cycling technique is introduced to better approximate the field operating conditions so as to activate the field failure modes. Power cycling thermal fatigue test is performed with different ball grid array solder joints, that is, lead contained [Sn/37 Pb (SP)] and lead free [Sn/4.0Ag/0.5 Cu (SAC)], and the result is compared. In order to account for the thermal fatigue life behavior discrepancy for different solder joint composition, real time Moire interferometry is applied to measure the global/local thermo-mechanical behavior during power cycling excursion. Effective damage parameter, the total average shear strain, is extracted from the experiment and applied to account for the difference in fatigue life result of two different solders. In addition, amount of experimentally measured total average shear strain is mutually verified with finite element method analysis. It is clear that total average shear strain of a solder joint can be an effective damage parameter to predict thermo-mechanical fatigue life. A physical mechanism in terms of thermal material property of solder joints' is proposed to offer some thoughts to abnormal shear strain behavior that leads to discrepancies in fatigue life of two solders. An importance of power cycling testing method is emphasized for certain package designs.     

Impact of temperature cycle profile on fatigue life of solder joints

In this paper the influence of the temperature cycle time history profile on the fatigue life of ball grid array (BGA) solder joints is studied. Temperature time history in a Pentium processor laptop computer was measured for a three-month period by means of thermocouples placed inside the computer. In addition, Pentium BGA packages were subjected to industry standard temperature cycles and also to in-situ measured temperature cycle profiles. Inelastic strain accumulation in each solder joint during thermal cycling was measured by high sensitivity Moire interferometry technique. Results indicate that fatigue life of the solder joint is not independent of the temperature cycle profile used. Industry standard temperature cycle profile leads to conservative fatigue life observations by underestimating the actual number of cycles to failure.     

基于埋置式基板的3D-MCM封装结构的研制

研制一种用于无线传感网的多芯片组件（3D-MCM) . 采用层压、开槽等工艺获得埋置式高密度多层有机（FR-4）基板,通过板上芯片（COB) 、板上倒装芯片（FCOB) 、球栅阵列（BGA）等技术,并通过引线键合、倒装焊等多种互连方式将不同类型的半导体芯片三维封装于一种由叠层模块所形成的立体封装结构中;通过封装表层的植球工艺形成与表面组装技术（SMT）兼容的BGA器件输出端子;利用不同熔点焊球实现了工艺兼容的封装体内各级BGA的垂直互连,形成了融合多种互连方式3D-MCM封装结构. 埋置式基板的应用解决了BGA与引线键合芯片同面组装情况下芯片封装面高出焊球高度的关键问题. 对封装结构的散热特性进行了数值模拟和测试,结果表明组件具有高的热机械可靠性. 电学测试结果表明组件实现了电功能,从而满足了无线传感网小型化、高可靠性和低成本的设计要求.     

A Probabilistic Approach to Predict Thermal Fatigue Life for Ball Grid Array Solder Joints

Numerous studies of the reliability of solder joints have been performed. Most life prediction models are limited to a deterministic approach. However, manufacturing induces uncertainty in the geometry parameters of solder joints, and the environmental temperature varies widely due to end-user diversity, creating uncertainties in the reliability of solder joints. In this study, a methodology for accounting for variation in the lifetime prediction for lead-free solder joints of ball grid array packages (PBGA) is demonstrated. The key aspects of the solder joint parameters and the cyclic temperature range related to reliability are involved. Probabilistic solutions of the inelastic strain range and thermal fatigue life based on the Engelmaier model are developed to determine the probability of solder joint failure. The results indicate that the standard deviation increases significantly when more random variations are involved. Using the probabilistic method, the influence of each variable on the thermal fatigue life is quantified. This information can be used to optimize product design and process validation acceptance criteria. The probabilistic approach creates the opportunity to identify the root causes of failed samples from product fatigue tests and field returns. The method can be applied to better understand how variation affects parameters of interest in an electronic package design with area array interconnections.     

Finite element analysis for solder ball failures in chip scale package

The failure mechanism of solder ball connect in chip scale package (CSP) utilizing wire-bonded ball grid array was elucidated using finite element analysis in this study. The macro-micro-coupling technique was used in the current model. There exist two factors which contribute to solder ball cracking: shear stress due to thermal expansion mismatch between the package and the PCB and warpage of the package itself. This study revealed that shear stress due to the thermal expansion mismatch prevailed over warpage of the package in causing the solder ball cracking in the present type of CSP.