Advances in Vapor Pressure Modeling for Electronic Packaging

Two advanced techniques have been developed for modeling vapor pressure within the plastic IC packages during solder reflow. The first involves the extension of the "wetness" technique to delamination along multimaterial interface and during dynamic solder reflow. Despite its simplicity, this technique is capable of offering reliable and accurate prediction for packages with high flexural rigidity. For packages with low flexural rigidity, the new "decoupling" technique that integrates thermodynamics, moisture diffusion, and structural analysis into a unified procedure has been shown to be more useful. The rigorous technique has been validated on both leadframe-based as well as laminate-based packages. With high accuracy and computational efficiency, these dynamic modeling tools will be valuable for optimization of package construction, materials, and solder reflow profile against popcorn cracking for both SnPb and Pb-free solders     

Moisture absorption and diffusion characterisation of packaging materials––advanced treatment

Unlike thermo-mechanical properties, moisture properties of packaging materials are rarely reported, even though moisture has been known to be at least as damaging as temperature to plastic packaging. This is in part due to the lack of characterisation knowledge for such properties.This paper tries to address this issue through a comprehensive presentation of the characterisation techniques for moisture sorption and diffusion properties. Special focuses are given to advanced treatments on unique characteristics of polymeric packaging materials. The effect of aspect ratio of the test specimen on characterisation accuracy has been analysed and correction factor has been formulated. Two techniques for characterising anisotropic diffusivity are presented and illustrated with actual experimental data on organic laminates. The techniques and challenges of characterising moisture diffusivity at high temperature are presented. A new insight into the causes and physics of non-Fickian adsorption in polymeric packaging materials is presented along with a technique to alleviate the associated characterisation challenge.     

塑封芯片烘烤过程的有限元分析

含有湿气的塑封芯片在进行焊接时由于湿应力和蒸汽压力的作用,容易产生内部分层或“爆米花”效应,因此长期存放的塑封器件在回流焊前必须要进行烘烤以驱除内部的湿气。文章针对实际的PBGA和PQFP器件,利用有限元模型分析了烘烤过程中湿气扩散随时间变化的规律,以及温度对烘烤效果的影响。有限元模拟计算表明,随着烘烤时间的推移,湿气减少的速度越来越低;随着温度的降低,所需的烘烤时间迅速增加。PQFP器件的芯片粘接剂层由于空间狭窄,很难被烘干。     

An overview of solder bump shape prediction algorithms withvalidations

The trend to reduce the size of electronic packages and develop increasingly sophisticated electronic devices with more, higher density inputs/outputs (I/Os), leads to the use of area array packages using chip scale packaging (CSP), flip chip (FC), and wafer level packaging (WLP) technologies. Greater attention has been paid to the reliability of solder joints and the assembly yield of the surface mounting process as use of advanced electronic packaging technologies has increased. The solder joint reliability has been observed to be highly dependent on solder joint geometry as well as solder material properties, such that predicting solder reflow shape became a critical issue for the electronic research community. In general, the truncated sphere method, the analytical solution and the energy-based algorithm are the three major methods for solder reflow geometry prediction. This research develops solder joint reliability design guidelines to accurately predict both the solder bump geometry and the standoff height for reflow soldered joints in area array packages. Three simulation methods such as truncated-sphere theory force-balanced analytical solution and energy-based approach for prediction of the solder bump geometry are each examined in detail, and the thermal enhanced BGA (TBGA) and flip chip packages are selected as the benchmark models to compare the simulation and experimental results. The simulation results indicate that all three methods can accurately predict the solder reflow shape in an accurate range     

Evaluation of the Delamination in a Flip Chip Using Anisotropic Conductive Adhesive Films Under Moisture/Reflow Sensitivity Test

Anisotropic conductive adhesive films (ACFs) have been used for electronic assemblies such as the connection between a liquid crystal display panel and a flexible printed circuit board. ACF interconnection is expected to be a key technology for flip chip packaging, system-in-packaging, and chip size packaging. This paper presents a methodology for quantitative evaluation of the delamination in a flip chip interconnected by an ACF under moisture/reflow sensitivity tests. Moisture concentration after moisture absorption was obtained by the finite element method. Then, the vapor pressure in the flip chip during solder reflow process was estimated. Finally the delamination was predicted by comparing the stress intensity factor of an interface crack due to vapor pressure with the delamination toughness. It is found that the delamination is well predicted by the present methodology.     

Integrated vapor pressure, hygroswelling, and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis

In this paper, a comprehensive and integrated package stress model is established for quad flat non-lead package with detailed considerations of effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress and vapor pressure induced during reflow. The critical plastic materials, i.e., moldcompound and die attach are characterized for hygroswelling and moisture properties, which are not easily available from material suppliers. The moisture absorption during preconditioning at JEDEC Level 1, and moisture desorption at various high temperatures are characterized. The moisture diffusivity is a few orders higher at reflow temperature than moisture preconditioning temperature. Due to coefficient of moisture expansion mismatch among various materials, hygro-mechanical stress is induced. The concept is analogous to coefficient of thermal expansion mismatch which results in thermo-mechanical stress. Thermal diffusivity is much faster than the moisture diffusivity. During reflow, the internal package reaches uniform temperature within a few seconds. The vapor pressure can be calculated based on the local moisture concentration after preconditioning. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure introduces additional strain of the same order as the thermal strain and hygrostrain to the package. Subsequently, the interfacial fracture mechanics model is applied to study the effect of crack length on die/mold compound and die/die attach delamination.     

Interface delamination analysis of TQFP package during solder reflow

Interface delamination during solder reflow is a critical reliability problem for the plastic IC packages. The main objective of this study is to apply modified virtual crack closure method (MVCCM) for the analysis of interface delamination between the leadframe pad and the encapsulant during a lead-free solder reflow after the level 1 moisture preconditioning. In this study, the moisture diffusion parameters and the coefficient of moisture expansion (CME) of two different epoxy molding compounds (EMC) are characterized for moisture diffusion analysis and the deformation analysis due to hygroscopic swelling. At the same time, the entire thermal and moisture history of Thin Quad Flat Pack (TQFP) package is simulated from the start of level 1 moisture preconditioning (85 °C/85%RH for 168 h) to subsequent exposure to a lead-free solder reflow process. Finally, the transient development of the stress intensity factors due to thermal stress only K_t, hygrostress only K_h, vapor pressure only K_p and combined energy release rate G_tot are computed and studied by using MVCCM. Based on the calculated stress intensity factors and energy release rates, it seems that for the EMC, the Young’s modulus, moisture diffusion coefficient, CME and adhesion strength with leadframe at high temperature appear to be the most significant variables for the MSL performance of TQFP package and this matches well with the experimental finding.     

3D FEM Simulations of Drop Test Reliability on 3D-WLP: Effects of Solder Reflow Residual Stress and Molding Resin Parameters

Numerous three-dimensional (3D) packaging technologies are currently used for 3D integration. 3D-wafer level package (3D-WLP) appears to be a way to keep increasing the density of the microelectronic components. The reliability of 3D components has to be evaluated on mechanical demonstrators with daisy chains before real production. Numerical modeling is acknowledged as a very efficient tool for design optimization. In this paper, 3D finite-elements calculations are carried out to analyze the effects of molding resin’s mechanical properties and thickness on the 3D component’s dynamic response under drop loading conditions. Residual stress generated by solder reflow is also discussed. The influences of residual stresses on the numerical estimation of the component behavior during drop loading are studied. Solder reflow residual stresses have an impact on solder plastic strain and die equivalent stress calculations. We have compared the result of two numerical drop test models. Stress-free initial conduction is introduced for the first model. Solder reflow residual stresses are considered as the initial condition for the second drop test model. Quantitative and qualitative comparisons are carried out to show the effect of residual stress in drop test calculations. For the effect of molding resin thickness on the component behavior under drop loading, the stress-free initial condition is considered. The effect of the molding resin’s thickness on critical area location is discussed. The solder bump maximum plastic shear strain and the silicon die maximum equivalent stress are used as reliability criteria. Numerical submodeling techniques are used to increase calculation accuracy. Numerical results have contributed to the design optimization of the 3D-WLP component.     

Lead-Free Bumping Using an Alternating Electromagnetic Field

A novel lead-free bumping technique using an alternating electromagnetic field (AEF) was investigated. Lead-free solder bumps reflowed onto copper pads through AEF have been achieved. A comparison was conducted between the microstructures of the lead-free solder joints formed by the conventional thermal reflow and AEF reflow. Keeping the substrate temperature lower than that of the solder bumps, AEF reflow successfully created metallurgical bonding between the lead-free solders and metallizations through an interfacial intermetallic compound (IMC). The AEF reflow could be finished in several seconds, much faster than the conventional hot-air reflow. Considering the morphology of the interfacial Cu₆Sn₅ IMC, a shorter heating time above the melting point would be a better choice for solder joint reliability. The results show that AEF reflow is a promising localized heating soldering technique in electronic packaging.     

Reliability assessment and hygroswelling modeling of FCBGA with no-flow underfill

In the flip-chip ball grid array (FCBGA) assembly process, no-flow underfill has the advantage over traditional capillary-flow underfill on shorter cycle time. Reliability tests are performed on both unmolded and molded FCBGA with three different types of no-flow underfill materials. The JEDEC Level-3 (JL3) moisture preconditioning, followed by reflow and pressure cooker test (PCT) is found to be a critical test for failures of underbump metallization (UBM) opening and underfill/die delamination. In this paper, various types of modeling techniques are applied to analyze the FCBGA-8×8 mm on moisture distribution, hygroswelling behavior, and thermomechanical stress. For moisture diffusion modeling, thermal-moisture analogy is used to calculate the degree of moisture saturation in the multi-material system of FCBGA. The local moisture concentration along the critical interface, e.g. die/underfill, is critical for delamination, because the moisture weakens the interfacial adhesion strength, generates internal vapor pressure during reflow, and induces tensile hygroswelling stress on UBM during PCT. The results of moisture distribution can be used as loading input for the subsequent hygroswelling modeling. The magnitude of hygroswelling stress acting on UBM is found to be greater than the thermal stress induced during reflow, both in tensile mode which may cause the UBM-opening failure. Underfill with lower saturated moisture concentration (C_sat) and coefficient of moisture expansion (CME) are found to induce lower UBM stress and has better reliability results. Molded package generally has higher stress level than unmolded package. Parametric studies are performed to study the effects of no-flow underfill materials, package type (molded vs. unmolded), die thickness, and substrate size on the stresses of UBM during reflow and PCT.     

PBGA器件潮湿扩散和湿热应力的有限元分析

塑料封装器件暴露在一定的潮湿环境下将会吸收潮湿的现象已经得到广泛的认同。针对实际的PBGA器件,采用通用有限元软件分析和计算了器件在潮湿环境下的潮湿扩散。并且计算了由于吸潮使器件在高温下产生的湿热应力。有限元模拟计算表明,不同潮湿环境下器件的潮湿扩散状态是不一样的,这导致在其后的高温焊接过程中器件内部产生的湿热应力的不同。     

FEM modeling of temperature distribution of a flip-chip no-flow underfill package during solder reflow process

Flip chip on organic substrate has relied on underfill to redistribute the thermomechanical stress and to enhance the solder joint reliability. However, the conventional flip-chip underfill process involves multiple process steps and has become the bottleneck of the flip-chip process. The no-flow underfill is invented to simplify the flip-chip underfill process and to reduce the packaging cost. The no-flow underfill process requires the underfill to possess high curing latency to avoid gelation before solder reflow so to ensure the solder interconnect. Therefore, the temperature distribution of a no-flow flip-chip package during the solder reflow process is important for high assembly yield. This paper uses the finite-element method (FEM) to model the temperature distribution of a flip-chip no-flow underfill package during the solder reflow process. The kinetics of underfill curing is established using an autocatalytic reaction model obtained by DSC studies. Two approaches are developed in order to incorporate the curing kinetics of the underfill into the FEM model using iteration and a loop program. The temperature distribution across the package and across the underfill layer is studied. The effect of the presence of the underfill fillet and the influence of the chip dimension on the temperature difference in the underfill layer is discussed. The influence of the underfill curing kinetics on the modeling results is also evaluated.     

Novel micro-bump fabrication for flip-chip bonding

This paper describes a new bump-fabrication technique for flip-chip connection between a chip and substrate. We propose a novel idea of forming solder microbumps on the substrate and directly bonding bare chips to the substrate. We successfully achieved the new flip-chip connection by using a 0.05Au-0.95Sn solder bump and a hydrogen-plasma reflow technique. Because the method eliminates the need for any process on the chip wafer, it will be very useful in fabricating flip-chip connections for low-cost packaging.     

Study of interaction between Cu-Sn and Ni-Sn interfacial reactions by Ni-Sn3.5Ag-Cu sandwich structure

The interaction between Cu-Sn and Ni-Sn interfacial reactions in a soldering system has been studied using a Ni-Sn3.5Ag-Cu sandwich structure. A layer of Cu-Sn intermetallic compound was observed at the interface of the Ni foil after 30 sec of reflowing. Two stages of the Cu-Sn compound growth on the Ni side were observed: (1) in the first minute of reflow, the fast Cu-Sn compound formation was rate-limited by Cu diffusivity in the Cu-Sn compound layer of the opposite Cu side; and (2) after 1 min of reflow, the Cu-Sn compound growth was very sluggish and depended on the Ni diffusion in the Cu-Sn compound of the Ni side. Very little Ni can be detected in the Cu side. This implies that Cu diffused and dissolved in the molten Sn3.5Ag solder much faster than Ni. When the dissolved Cu arrived at the interface of the Ni foil, a Cu-Sn compound layer formed on the Ni interface to prevent the Ni foil from reacting with the solder. The driving force of the dissolved Cu atoms toward the Ni side attributed to the Cu solubility difference across the molten solder, which was established due to the reduction of the Cu solubility near the Ni interface. The reduction of Cu solubility was caused by the presence of dissolved Ni near the Ni interface. Knowing the experimental value of the Cu flux toward the Ni side and assuming the diffusion of Cu atoms in the molten solder following Fick’s first law, the diffusivity of Cu is found to be 10⁻⁵ cm²/s.     

Modeling of Micropitch Shift of a Magnetoelectrical Sensor During Laser Solder Ball Bonding Process

With the development of optoelectronics and microelectromechanical systems (MEMS) packaging, laser soldering has become an extensively used interconnection technique in electronic manufacturing industry. Postsolder shift in assembling of such components is the most challenging issue to affect the packaging yields. To maintain a high coupling efficiency or accuracy, tight control of postsolder shift is required. In the present work, a 3-D thermal-mechanical coupled finite element model was developed to investigate the time-dependent pitch shift induced by the laser solder ball bonding process. This model accounted for the laser interaction, the heat conduction, the thermal induced deformation and the phase change of the solder and could reflect the actual laser soldering process. The modeling results show different deformation mechanisms in the prebumping and reflow processes. The pitch shift is mainly induced by the thermal shrinkage of solder. The pitch angles obtained from the finite element analysis are in good agreement with those from the measurement using laser goniometer.     

Interaction Between Ni and Cu Across 95Pb-5Sn High-Lead Layer

Ni/95Pb-5Sn/Cu ternary diffusion couples were used to investigate the cross-interaction between Ni and Cu across a layer of 95Pb-5Sn solder. High-lead solder layers with a thickness of 100 μm or 400 μm were electroplated over Cu foils. A pure Ni layer (20 μm) was then deposited over the as-deposited high-lead solder surface. The diffusion couples were then aged at 150°C to 250°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high-temperature process such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that massive spalling also occurred during aging even though reflow was not used. The massive spalling began with the formation of microvoids. When the microvoids had congregated into large enough voids, intermetallic compounds (Cu₃Sn) started to spall from the interface. This spalling phenomenon occurred sooner with increasing temperature and decreasing solder volume.     

Drop impact reliability analysis of CSP packages at board and product levels through modeling approaches

Chip scale package (CSP) and fine pitch ball grid array (BGA) packages have been increasingly used in portable electronic products such as mobile cell phones and PDA, etc. Drop impact which is inevitable during its usage could cause not only housing crack but also package to board interconnect failure, such as BGA solder breaks. Various drop tests have been used to ensure high reliability performance of packaging to withstand such impact and shock load. Due to extreme difficulty in directly measuring responses in solder joint during drop shock event, computer simulation based modeling approach has been increasingly played an important role in evaluating product reliability performance during product development. An advanced modeling technique with a comprehensive failure criterion including high strain rate effect needs to be developed to quantitatively evaluate package reliability performance especially in cross comparisons between different board and system level designs. In this paper, three drop tests have been modeled, namely, bare board drop, board with fixture drop or shock, and system level phone drop. Submodeling and explicit-implicit sequential modeling techniques are used to characterize the dynamic responses of CSP/BGA packages in different board designs. Failure criteria and effects of strain rate and edge support on BGA in multicomponent boards are also investigated. A validation test with data acquisition is used to correlate the test results with numerical results.     

Plasma reflow bumping of Sn-3.5 Ag solder for flux-free flip chip package application

A new flux-free reflow process using Ar+10%H/sub 2/ plasma was investigated for application to solder bump flip chip packaging. The 100-/spl mu/m diameter Sn-3.5wt%Ag solder balls were bonded to 250-/spl mu/m pitch Cu/Ni under bump metallurgy (UBM) pattern by laser solder ball bonding method. Then, the Sn-Ag solder balls were reflowed in Ar+H/sub 2/ plasma. Without flux, the wetting between solder and UBM occurred in Ar+H/sub 2/ plasma. During plasma reflow, the solder bump reshaped and the crater on the top of bump disappeared. The bump shear strength increased as the Ni/sub 3/Sn/sub 4/ intermetallic compounds formed in the initial reflow stage but began to decrease as coarse (Cu,Ni)/sub 6/Sn/sub 5/ grew at the solder/UBM interface. As the plasma reflow time increased, the fracture mode changed from ductile fracture within the solder to brittle fracture at the solder/UBM interface. The off-centered bumps self-aligned to patterned UBM pad during plasma reflow. The micro-solder ball defects occurred at high power prolonged plasma reflow.     

Modeling nonlinear moisture diffusion in inhomogeneous media

While moisture diffusion in microelectronic device and packaging has been studied for decades, the problems involving complex nonlinear moisture diffusion in multi-material assembly have not been fully studied. This paper has developed a general nonlinear diffusion model by adopting water activity, a continuous state variable, as the field variable. The generalized solubility is introduced, which is temperature- and water activity-dependent. The effective diffusivity is defined and derived in terms of generalized solubility and water activity. By comparing the water activity-based model with the existing various normalized models, the present theory can unify and generalize the current approaches. More importantly, the present model can solve both linear and nonlinear moisture diffusion in inhomogeneous material system without normalization. The commercial finite element software has been applied to solve the nonlinear generalized moisture diffusion problem using the analogy of water activity and temperature. A source code of user-defined subroutines in ABAQUS has been provided in the Appendix of the paper. The mathematical formulation and the numerical implementation method presented in this paper can be applied to any nonlinear sorption or diffusion problems in inhomogeneous material system.     

The Influence of Substrate Enhancement on Moisture Sensitivity Level (MSL) Performance for Green PBGA Packages

As the electronics industry migrates to the lead- and halogen-free (green) packages, many of the materials used in plastic ball-grid array (PBGA) substrates, in particular the molding compounds and die attaches, will have to be improved. The moisture sensitivity level (MSL) performance of the large nongreen PBGA packages are typically reduced by at least one JEDEC/IPC level at the lead-free reflow temperature of 260$^circ$C. Common failure mechanisms of traditional large size PBGA packages include popcorning, as well as delamination and cracks between the solder mask/copper interface in the multiple layer substrates. In this paper, the interfacial adhesion of traditional and advanced substrate materials and processing technologies are presented based on reliability tests of various PBGA packages subject to moisture soaking followed by reflow soldering at 260$^circ$C. It was found that substrate failures with delamination at the solder mask/copper interface were dramatically improved by introducing advanced materials and processes for multiple-layer substrates. However, the partial or full delamination at the mold compound/solder mask interface could still be observed after lead-free reflow soldering. There is an urgent need to improve the adhesion between mold compound/solder mask in order to achieve high MSL performance of large size and green PBGA packages.