Underfill Flow Research Articles

Advancing packaging technology: computational fluid dynamics modeling for capillary underfill encapsulant in multi-chip heterogenous packages

Purpose The purpose of this study is to investigate the dynamics of capillary underfill flow (CUF) in flip-chip packaging, particularly in a multi-chip configuration. The study aims to understand how various parameters, such as chip-to-chip spacing (S12), chip thickness (tc) and others, affect the underfill flow process. By using computational fluid dynamics (CFD) simulations and experimental studies, the goal is to provide insights into understanding the dynamics of CUF in heterogeneous electronic packaging. Design/methodology/approach The paper introduces a CFD analysis and experimental study on CUF in a multi-chip configuration, aiming to understand underfill flow dynamics. A 3D geometry models of multi-chip arrangement are created using computer-aided design (CAD) software. After that, the CAD models are meshed and simulated in Ansys Fluent using incompressible and non-Newtonian fluid properties. The study maintains S12 of 2.86 and tc of 22.29 between experimental and simulation data for results validation. Next, a various of S12 values (1.14, 2.86, 5.71, 8.57, 14.29 and 20) which focus on tc of 22.29 have been investigated. Further studies have been conduct using S12 of 5.71 and tc of 8.00, 14.29 and 22.29. Findings Results show a strong correlation between simulation and experiment which validate the correctness and robustness of simulation. Further parameter’s studies using simulation for various of S12 indicated that higher S12 values lead to faster flow. This effect is due to large underfill weight from reservoir able to flow into S12 region which contributed to higher mass momentum movement. Furthermore, the effect of various of tc shows that the thicker the chip the faster the underfill to flow in S12 region. Research limitations/implications The intentional exclusion of solder bump pattern arrangements from the experiment and simulation may limit the study's ability to fully understand the impact of solder bump patterns on underfill flow. Therefore, more parameters can be investigated such as solder bump pattern, underfill weight and dispense pattern in the future using CFD. Practical implications The manuscript provides a comprehensive examination of the contributions of CFD to the advancement of knowledge regarding CUF phenomena in heterogeneous electronic packaging assemblies. Moreover, it delineates the utilization of CFD methodologies to assess the influence of chip-to-chip spacing (S12) and the thickness of the chip (tc) on the underfill flow characteristics. Originality/value This paper fulfills an identified need of computational fluid dynamics method to study capillary underfill flow dynamics in heterogenous electronic packaging.

Experimental and numerical study of solder bumps impact on the underfill fluid flow under electrohydrodynamic effect

Solder bumps can increase filling time, which is one of the main challenges in electronic packaging reliability. Here, we compare the capillary flow behavior between parallel plates with and without solder bumps to examine how solder bumps affect the length of the underfill fluid flow under the effects of an electric potential. We found that the solder bumps restrained the flow length, while the electric field enhanced it. By enhancing the voltage from 0 to 1000 V, in the case without solder bumps, the flow length increased by up to 30%, and it increased by up to 25% in the case of solder bumps. To determine the optimum bump design, we selected the diameter and pitch size of the solder bumps as the independent variables. The results revealed that larger pitch sizes and smaller diameters show longer fluid flow lengths. The effect of the electric field on varying nozzle positions was also investigated. We found that the fluid flow length increased when the nozzle was between the solder bumps compared to the top of the solder bumps. According to our observations, the nozzle position is also the main factor in determining the fluid flow length compared with the bump diameter and pitch sizes for the design of the underfill packaging process. Numerical simulations were also performed to compare the experimental results, and the average discrepancy between the experimental and numerical results at various time steps for different solder bump parameters was between 5 to 10%. Our findings demonstrate the potential of using electric potential in conjunction with solder bumps to control underfill flow, which can benefit flip-chip packaging applications. Numerical methods can accurately predict underfill fluid flow with solder bumps under the electric field effect.

Capillary Underfill Flow Simulation as a Design Tool for Flow-Optimized Encapsulation in Heterogenous Integration.

As the power electronics landscape evolves, pushing for greater vertical integration, capillary underfilling is considered a versatile encapsulation technique suited for iterative development cycles of innovative integration concepts. Since a defect-free application is critical, this study proposes a capillary two-phase flow simulation, predicting both the flow pattern and velocity with remarkable precision and efficiency. In a preliminary performance evaluation, Volume of Fluid (VOF) outperforms the Level-Set method in terms of accuracy and computation time. Strategies like HRIC blending, artificial viscosity, and implicit Multi-Stepping prove effective in optimizing the numerical VOF scheme. Digital mapping using physical experiments and virtual simulations validates transient flow predictions, achieving excellent agreement with deviations as low as 1.48-3.34%. The accuracy of flow predictions is thereby greatly influenced by non-Newtonian viscosity characteristics in the low shear range and time-dependent contact angle variations. The study further explores flow manipulation concepts, focusing on local flow speed adjustment, gap segmentation, and the use of arcuate shapes to influence interface confluence near the chip. Experimental validation corroborates the effectiveness of each design intervention. In conclusion, this research highlights the potential of predictive engineering to develop flow-optimized package designs that enhance reliability while supporting high manufacturing yields.

Electric field and viscous fluid polarity effects on capillary-driven flow dynamics between parallel plates

—Micropumps have attracted considerable interest in micro-electro-mechanical systems (MEMS), microfluidic devices, and biomedical engineering to transfer fluids through capillaries. However, improving the sluggish capillary-driven flow of highly viscous fluids is critical for commercializing MEMS devices, particularly in underfill applications. This study investigated the behavior of different viscous fluid flows under the influence of capillary and electric potential effects. We observed that upon increasing the electric potential to 500 V, the underfill flow length of viscous fluids increased by 45% compared to their capillary flow length. To explore the dynamics of underfill flow under the influence of an electric potential, the polarity of highly viscous fluids was altered by adding NaCl. The results indicated an increase of 20–41% in the underfill flow length of highly viscous conductive fluids (0.5–4% NaCl additives in glycerol) at 500 V compared to that at 0 V. The underfill viscous fluid flow length improved under the electric potential effect owing to the polarity across the substance and increased permittivity of the fluid. A time-dependent simulation, which included a quasi-electrostatic module, level set module, and laminar two-phase flow, was executed using the COMSOL Multiphysics software to analyze the effect of the external electric field on the capillary-driven flow. The numerical simulation results agreed well with the experimental data, with an average deviation of 4–7% at various time steps for different viscous fluids. Our findings demonstrate the potential of utilizing electric fields to control the capillary-driven flow of highly viscous fluids in underfill applications.

CFD Simulation Analysis of Capillary Underfill Flow in Heterogeneous Integration

CFD Simulation Analysis of Capillary Underfill Flow in Heterogeneous Integration

PACKAGE-ON-PACKAGE (POP) UNDERFILL PROCESS USING A MATERIAL DAM METHOD

Recent developments in the electronics industry have introduced a multi-stack ball grid array (BGA) to meet the growing consumer demand for both high-performance and smaller-sized chip packages. This study focused on the preliminary study of the Package-on-Package (PoP) underfill process using a material dam method. High viscosity type of underfill material is considered for the underfill process. In the current experimental work, L path-dispensing method was chosen due to its advantages, as reported in the previous work. The material dam method was used to prevent underfill from moving backwards and flowing out from the dispensing region. The material dam was built surrounding the PoP package. The effectiveness of the underfill process was analyzed based on the cycle time and lateral lapping, which are significant factors in material selection. The experimental results revealed that slow underfill flow may cause the quickly harden of material while the dispensing process is still running. This situation restricts the underfill flow and creates voids in the PoP package. The material dam method successfully enhanced the underfilling process for layers 3 and 4 stacked-package. This study is expected to provide the preliminary underfill process of stacking the PoP package and is useful as a reference for the engineer in the microelectronics industry.

Analytical and numerical analyses of filling progression and void formation in flip-chip underfill encapsulation process

PurposeThis paper aims to study the filling progression of underfill flow and void formation during the flip-chip encapsulation process.Design/methodology/approachA new parameter of filling progression that relates volume fraction filled to filling displacement was formulated analytically. Another indicative parameter of filling efficiency was also introduced to quantify the voiding fraction in filling progression. Additionally, the underfill process on different flip-chips based on the past experiments was numerically simulated.FindingsAll findings were well-validated with reference to the past experimental results, in terms of quantitative filling progression and qualitative flow profiles. The volume fraction filled increases monotonically with the filling displacement and thus the filling time. As the underfill fluid advances, the size of the void decreases while the filling efficiency increases. Furthermore, the void formed during the underfilling flow stage was caused by the accelerated contact line jump at the bump entrance.Practical implicationsThe filling progression enabled manufacturers to forecast the underfill flow front, as it advances through the flip-chip. Moreover, filling progression and filling efficiency could provide quantitative insights for the determination of void formations at any filling stages. The voiding formation mechanism enables the prompt formulation of countermeasures.Originality/valueBoth the filling progression and filling efficiency are new indicative parameters in quantifying the performance of the filling process while considering the reliability defects such as incomplete filling and voiding.

A Surface Energy Approach to Developing an Analytical Model for the Underfill Flow Process in Flip-Chip Packaging

Abstract This paper presents a new approach to formulating an analytical model for the underfill process in flip-chip packaging to predict the flow front and the filling time. The new approach is based on the concept of surface energy along with the energy conservation principle. This approach avoids the need of modeling the flow path to predict the flow front and the filling time, and thus it is suitable to different configurations of solder bumps, including different shapes and arrangements of solder bumps in flip-chip packaging. An experiment along with the computational fluid dynamics simulation was performed based on a proprietarily developed testbed to verify the effectiveness of this approach. Both the experimental and simulation results show that the proposed approach along with its model is accurate for flip-chip packages with different configurations besides the configuration of a regular triangle arrangement of solder bumps and a spherical shape of the solder bump.

Underfill Flow in Flip-Chip Encapsulation Process: A Review

Abstract The scope of review of this paper focused on the precuring underfilling flow stage of encapsulation process. A total of 80 related works has been reviewed and being classified into process type, method employed, and objective attained. Statistically showed that the conventional capillary is the most studied underfill process, while the numerical simulation was mainly adopted. Generally, the analyses on the flow dynamic and distribution of underfill fluids in the bump array aimed for the filling time determination as well as the predictions of void occurrence. Parametric design optimization was subsequently conducted to resolve the productivity issue of long filling time and reliability issue of void occurrence. The bump pitch was found to the most investigated parameter, consistent to the miniaturization demand. To enrich the design versatility and flow visualization aspects, experimental test vehicle was innovated using imitated chip and replacement fluid, or even being scaled-up. Nonetheless, the analytical filling time models became more accurate and sophiscasted over the years, despite still being scarce in number. With the technological advancement on analysis tools and further development of analytic skills, it was believed that the future researches on underfill flow will become more comprehensive, thereby leading to the production of better packages in terms of manufacturing feasibility, performances, and reliability. Finally, few potential future works were recommended, for instance, microscopic analysis on the bump–fluid interaction, consideration of filler particles, and incorporation of artificial intelligence.

No-flow underfill: Effect of chip placement speed on the void formation using numerical method

No-flow underfill process can enhance the production lead time since the curing and reflow of solder interconnect and underfill are achieved simultaneously. Additionally, no-flow underfill can produce near void-free flow during the assembly process. This paper investigates the effect of chip placement speed towards the void formation of no-flow underfill process using numerical simulation and experimental methods. The findings show that the increase in chip placement speed effectively increased the gauge pressure and velocity of the underfill flow. The highest chip placement speed of 14 ​mm/s produces 12,300 ​Pa of gauge pressure and 0.1 ​m/s of underfill flow velocity. The flow profiles revealed that finger-like profile was formed above 5 ​mm/s speed during the compression process of no-flow underfill that will lead to issue of void formation in the ball grid array (BGA) region. This study recommended the usage of below 5 ​mm/s of chip placement speed to minimize the issue of finger-like formation and subsequently void formation.

Surface energetic-based analytical filling time model for flip-chip underfill process

Purpose This paper aims to present new analytical model for the filling times prediction in flip-chip underfill encapsulation process that is based on the surface energetic for post-bump flow. Design/methodology/approach The current model was formulated based on the modified regional segregation approach that consists of bump and post-bump regions. Both the expansion flow and the subsequent bumpless flow as integrated in the post-bump region were modelled considering the surface energy–work balance. Findings Upon validated with the past underfill experiment, the current model has the lowest root mean square deviation of 4.94 s and maximum individual deviation of 26.07%, upon compared to the six other past analytical models. Additionally, the current analytically predicted flow isolines at post-bump region are in line with the experimental observation. Furthermore, the current analytical filling times in post-bump region are in better consensus with the experimental times as compared to the previous model. Therefore, this model is regarded as an improvised version of the past filling time models. Practical implications The proposed analytical model enables the filling time determination for flip-chip underfill process at higher accuracy, while providing more precise and realistic post-bump flow visualization. This model could benefit the future underfill process enhancement and package design optimization works, to resolve the productivity issue of prolonged filling process. Originality/value The analytical underfill studies are scarce, with only seven independent analytical filling time models being developed to date. In particular, the expansion flow of detachment jump was being considered in only two previous works. Nonetheless, to the best of the authors’ knowledge, there is no analytical model that considered the surface energies during the underfill flow or based on its energy–work balance. Instead, the previous modelling on post-bump flow was based on either kinematic or geometrical that is coupled with major assumptions.

Effect of different temperature distribution on multi-stack BGA package

PurposeThis paper aims to determine the optimum set of temperatures through correlation study to attain the most effective capillary flow of underfill in a multi-stack ball grid array (BGA) chip device.Design/methodology/approachFinite volume method is implemented in the simulation. A three-layer multi-stack BGA is modeled to simulate the underfill flow. The simulated models were well validated with the previous experimental work on underfill process.FindingsThe completion filling time shows high regression R-squared value of up to 0.9918, which indicates a substantial acceleration on the underfill process because of incorporation of thermal delta. An introduction of 11 °C thermal delta to the multi-stacks BGA managed to reduce the filling time by up to 16.4%.Practical implicationsTemperature-induced capillary flow is a relatively new type of driven underfill designed specifically for package on package BGA components. Its simple implementation can further improve the productivity of existing underfill process in the industry that is desirable in reducing the process lead time.Originality/valueThe effect of temperature-induced capillary flow in underfill encapsulation on multi-stacks BGA by means of statistical correlation study is a relatively new topic, which has never been reported in any other research according to the authors’ knowledge.

Study of Different Dispensing Patterns of No-Flow Underfill Using Numerical and Experimental Methods

Abstract The conventional capillary underfill process has been a common practice in the industry, somehow the process is costly and time-consuming. Thus, no-flow underfill process is developed to increase the effective lead time production since it integrates the simultaneous reflow and cure of the solder interconnect and underfill. This paper investigates the effect of different dispense patterns of no-flow underfill process by mean of numerical and experimental method. Finite volume method (FVM) was used for the three-dimensional (3D) simulation to simulate the compression flow of the no-flow underfill. Experiments were carried out to complement the simulation validity and the results from both studies have reached a good agreement. The findings show that of all three types of dispense patterns, the combined shape dispense pattern shows better chip filling capability. The dot pattern has the highest velocity and pressure distribution with values of 0.0172 m/s and 813 Pa, respectively. The high-pressure region is concentrated at the center of the chip and decreases out toward the edge. Low in pressure and velocity flow factor somehow lead to issue associated with possibility of incomplete filling or void formation. Dot dispense pattern shows less void formation since it produces high-pressure underfill flow within the ball grid array (BGA). This paper provides reliable insight into the industry to choose the best dispense pattern of recently favorable no-flow underfill process.

Spatial analysis of underfill flow in flip-chip encapsulation

PurposeThe purpose of the study is to investigate the spatial aspects of underfill flow during the flip-chip encapsulation process, for instance, meniscus evolution and contact line jump (CLJ). Furthermore, a spatial-based void formation mechanism during the underfill flow was formulated.Design/methodology/approachThe meniscus evolution of underfill fluid subtended between the bump array and the CLJ phenomenon were visualized numerically using the micro-mesh unit cell approach. Additionally, the meniscus evolution and CLJ phenomenon were modelled analytically based on the formulation of capillary physics. Meanwhile, the mechanism of void formation was explained numerically and analytically.FindingsBoth the proposed analytical and current numerical findings achieved great consensus and were well-validated experimentally. The variation effects of bump pitch on the spatial aspects were analyzed and found that the meniscus arc radius and filling distance increase with the pitch, while the subtended angle of meniscus arc is invariant with the pitch size. For larger pitch, the jump occurs further away from the bump entrance and takes longer time to attain the equilibrium meniscus. This inferred that the concavity of meniscus arc was influenced by the bump pitch. On the voiding mechanism, air void was formed from the air entrapment because of the fluid-bump interaction. Smaller voids tend to merge into a bigger void through necking and, subsequently, propagate along the underfill flow.Practical implicationsThe microscopic spatial analysis of underfill flow would explain fundamentally how the bump design will affect the macroscopic filling time. This not only provides alternative visualization tool to analyze flow pattern in the industry but also enables the development of accurate analytical filling time model. Moreover, the void formation mechanism gave substantial insights to understand the root causes of void defects and allow possible solutions to be formulated to tackle this issue. Additionally, the microfluidics sector could also benefit from these spatial analysis insights.Originality/valueSpatial analysis on underfill flow is scarcely conducted, as the past research studies mainly emphasized on the temporal aspects. Additionally, this work presented a new mechanism on the void formation based on the fluid-bump interaction, in which the formation and propagation of micro-voids were numerically visualized for the first time. The findings from current work provided fundamental information on the flow interaction between underfill fluid and solder bump to the package designers for optimization work and process enhancement.

Parametric Study on the Void Risk in FC-POP Molded Underfill Process Using a Novel Porous Media, Two-Phase, Compressible Flow Simulation Method

Molded underfill process provides many benefits for higher productivity and lower cost over the conventional capillary underfill process. However, the void defects in a molded underfill process could cause pop-corning effect and solder extrusion during the reflow process. This paper presents a highly efficient and accurate hybrid model that can be applied to diagnose the void risk in vacuum molded underfill processes of real flip-chip, package-on-package devices with complex ball arrays and large PCB strips. The model combines multi-zone porous media model, Hele-Shaw model and compressible two-phase computational fluid dynamics model. The model has been well validated in terms of the entrapped void size and its behavior with flow visualization experiments. Also, a set of parametric studies on a FC-POP molded underfill flow process were performed to pre-assess the future node’s molded underfill process. Not only the model provides a better understanding of the physics of void entrapment but also it is proven to be an effective tool for assessing the potential void risk through its application to 10nm flip-chip, package-on-package device manufactured in mass production line.

Effect of hourglass shape solder joints on underfill encapsulation process: numerical and experimental studies

PurposeIn line with the recent development of flip-chip reliability and underfill process, this paper aims to comprehensively investigate the effect of different hourglass shape solder joint on underfill encapsulation process by mean of experimental and numerical method.Design/methodology/approachLattice Boltzmann method (LBM) numerical was used for the three-dimensional simulation of underfill process. The effects of ball grid arrays (BGA) encapsulation process in terms of filling time of the fluid were investigated. Experiments were then carried out to validate the simulation results.FindingsHourglass shape solder joint has shown the shortest filling time for underfill process compared to truncated sphere. The underfill flow obtained from both simulation and experimental results are found to be in good agreement for the BGA model studied. The findings have also shown that the filling time of Hourglass 2 with parabolic shape gives faster filling time compared to the Hourglass 1 with hemisphere angle due to bigger cross-sectional area of void between the solder joints.Practical implicationsThis paper provides reliable insights to the effect of hourglass shape BGA on the encapsulation process that will benefit future development of BGA packages.Originality/valueLBM numerical method was implemented in this research to study the flow behaviour of an encapsulation process in term of filling time of hourglass shape BGA. To date, no research has been found to simulate the hourglass shape BGA using LBM.

Effect of adhesive force on underfill process based on lattice Boltzmann method

Purpose The purpose of this study is to investigate the effect of the adhesive force and density ratio using lattice Boltzmann method (LBM) during underfill process. Design/methodology/approach To deal with complex flow in underfill process, a framework is proposed to improve the lattice Boltzmann equation. The fluid flows with different density ratio and bump arrangement in underfill are simulated by the incorporated Carnahan–Starling (CS) equation of state (EOS). The numerical study conducted by finite volume method (FVM) and experimental results are also presented in each case at the different filling percentage for verification and validation purpose. Findings The numerical result is compared well with those acquired experimentally. Small discrepancy is detected in their flow profile. It was found that the adhesive force between fluid and solid was affected by the density ratio of the fluids and solder bump configuration. LBM has shown better adhesive force effect phenomenon on underfill process compared to FVM. LBM also demonstrated as a better tool to study the fluid flow in the underfill process. Practical implications This study provides a basis and insights into the impact of adhesive force and density ratio to the underfill process that will be advancing the future design of flip chip package. This study also provides superior guidelines, and the knowledge of how adhesive force is affected by flip chip package structure. Originality/value This study proposes the method to predict the adhesive force and density ratio effect for underfill flow in flip chip package. In addition, the proposed method has a good performance in representing the adhesive force during the underfill simulation for its natural physical basic. This study develops understanding of flow problems to attain high reliability for electronic assemblies.

Regional Segregation With Spatial Considerations-Based Analytical Filling Time Model for Non-Newtonian Power-Law Underfill Fluid in Flip-Chip Encapsulation

Abstract This paper presents a new analytical filling time model to predict the flow of non-Newtonian underfill fluid during flip-chip encapsulation process. The current model is formulated based on the regional segregation approach, instead of the conventional porous media approximation. In this approach, the filling times were computed separately at different filling stages, before being summed up till the required filling distance. The non-Newtonian property of underfill fluid is modeled using the conventional power-law constitutive equation. Additionally, the spatial aspects of the underfill flow were incorporated into the present analysis. For instance, the evolution of underfill menisci from convex to concave was analytically developed and the contact line jump (CLJ) criterion was improved using minimal flow assumption. Upon validated with three distinct past underfill experiments, the current analytical model is found to have the best performance as it predicted the filling times with the least discrepancy among other existing filling time models. Quantitatively, the discrepancies were averagely reduced by an absolute value of at least 8.68% and 4.90%, respectively, for the first two set of validation studies. Generally, this model is particularly useful in manufacturing lines to estimate the process time of flip-chip underfill, as well as for the optimizations of process and package design.

Finite Volume Method Study on Contact Line Jump Phenomena and Dynamic Contact Angle of Underfill Flow in Flip-Chip of Various Bump Pitches

Three distinct flip-chip underfill cases were considered in this work, with each possess a bump pitch of 0.8 mm, 1.0 mm and 1.2 mm. Contact line jump (CLJ) phenomenon during the underfill flow were successfully visualized with the aided of finite volume method (FVM) based simulation. The current simulated underfill flow fronts were qualitatively validated with the existing experimental data. Generally, the attachment process occurs much faster than the detachment process, by a factor of 14. Within the investigated pitches range, shorter bump pitch would yield a slower underfill flow but a faster-completion underfill process. The voiding mechanism during the underfill process were presented. Furthermore, the dynamic contact angle of underfill meniscus were analytically computed from the numerical data. The contact angle is found to be varies sinusoidally with the filling time, while the impact of bump pitch is minimal. Lastly, it is found that the bump pitch of flip-chip does affect the void formation and propagation.

An Analytical Model for Permeability of Underfill Flow in Flip-Chip Packaging With Consideration of the Actual Specific Surface and Tortuosity

Permeability is an important concept in understanding and modeling the flow or dynamic behavior of fluids in a porous medium (e.g., flip chip in electronics packaging). There are two factors (specific surface and tortuosity) that affect the permeability of a porous medium, but they are ignored by analytical models for permeability in the literature. In this paper, we present an analytical model for permeability of the underfill porous medium in flip-chip packaging by consideration of the actual specific surface and tortuosity. The proposed model and other models without consideration of the actual specific surface and tortuosity are compared with reference to a well-proven numerical model. The result of comparison has shown a significant improvement in accuracy with our model. This paper contributes not only to the underfill flip-chip package, but also to the field of porous mediums with regular arrangements of pores.