Conference Paper

Three dimensional numerical prediction of epoxy flow during the underfill process in flip chip packaging

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In the present paper, a 3D numerical prediction has been made to study the flip chip underfill process using the epoxy molding compound (EMC). The prediction considered the EMC filling behavior for the flow induced between the tiny gap of silicon die and substrate. Three different arrangements of the solder bump have been tested in this work. The EMC is treated as a generalized Newtonian fluid (GNF). The developed methodology combines the Kawamura and Kawahara technique, and the melt front volume tracking method to solve the two-phase flow field around the solder bumps. The Castro-Macosko rheology model with Arrhenius temperature dependence is adopted in the viscosity model. The predictions are made to investigate the filling patterns at several time intervals. The results show that the underfill process for solder bump with Type A gives minimum filling time and better filling yield. The effect of gap height between the plate and substrate on the underfill process also has been considered. The close agreement between prediction and experimental results from the previous work illustrates the applicability of the present numerical model.

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... Process engineers can use transparent dies [18,19], and different coloured underfills if a flow analysis is required. The industry used the 'I' and 'L' path techniques [20], followed by a seal pass. The underfill process was brought with a new level of robustness when jetting technology was introduced. ...
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.
... For protecting the package from adverse environmental affect, encapsulation is required. Generally, thermoset material is used for encapsulation and the process involved is transfer molding [1][2]. This molding process is preferred since it offers high accuracy of transfer molding tooling and at the same time, low cycle time of the process. ...
Conference Paper
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3-D packaging is a technology that offers high density packaging and high performance. It enables chips to be stack in a single package and widely adopted in multi-media products. Thermosetting material is used for their encapsulation, is flowed through their thin space and wide filling area during package encapsulation process has become vital concern i.e. void formation. In this paper, such issue has been numerical studied due to the effect of transfer speed of the plunger head during encapsulation process. There are five transfer speeds of 1.6, 2.0, 3.4, 6.8, 9.04 mm/s have been chosen in order to investigate the quality of air entrapped (hereafter namely as void), where it degraded the package's reliability. It is found that the longest transfer speed delivered to the best encapsulation process, which had lowest volume of air trap or void formation in the package.
This paper studies the three-dimensional simulation of pressurized underfill process for I-type dispensing method on a 10 by 10, middle empty ball-grid-array (BGA) using lattice-Boltzmann method. Pressurized underfill process is introduced to overcome the drawbacks of capillary underfill process that include incomplete filling, void formation and extended filling time. The effects of capillary and pressurized underfill methods of BGA encapsulation process on the flow front, filling time and pressure of the fluid are investigated. Four different pressure values are used as the input pressure in the lattice-Boltzmann simulation. Experiments are carried out as well to validate the simulation results. In general, pressurized underfill method results in a dramatically shorter filling time compared to capillary underfill. The flow front obtained from both the simulation and experimental results are quite similar for the BGA model studied. The pressure inside the flow domain is significantly higher than its inlet pressure and thus, caution needs to be taken by the design engineer such that the printed circuit board (PCB) and flip chip is capable to withstand this pressure to prevent from the occurrence of breakage.
Available at: Salt stress can affect alfalfa growth directly by adversely affecting metabolism, or indirectly by its effect on Rhizobium capacity for symbiotic N2 fixation. Growth and carbohydrate metabolism in leaves, roots and nodules of two alfalfa cultivars (Medicago sativa cv Apica and salt-tolerant cv Halo) in association with two rhizobial strains (A2 and salt-tolerant Rm1521) exposed to different levels of NaCl (0, 20, 40, 80 or 160 mM NaCl) were assessed under controlled conditions. For both cultivars, shoot and root biomasses and shoot to root ratio significantly declined with increasing NaCl concentrations. Under 80 mM NaCl, Halo plants yielded 20% more fresh shoot biomass than Apica while plants inoculated with Rm1521 allocated more biomass to the roots than to the shoots compared to A2. Halo plants maintained a steady shoot water content (about 80%) under the entire range of NaCl concentrations. Shoot water content was more variable in Apica. Apica in association with salt-tolerant strain Rm1521 maintained a better water status than with strain A2, as indicated by the higher shoot water content at 80 mM NaCl. Under salt stress, two major compatible sugars involved in plant osmoregulation, sucrose and pinitol, increased in leaves while a large accumulation of starch was observed in roots. In nodules, pinitol, sucrose and starch increased under salt stress and were much more abundant with strain Rm1521 than with A2. This suggests that there could be an active transport from the shoot to the nodules to help maintain nodule activity under NaCl stress and that strain Rm1521 increases the sink strength toward nodules. Our results show that combining cultivars and rhizobial strains with superior salt tolerance is an effective strategy to improve alfalfa productivity in salinity affected areas.
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Underfill is used to improve the reliability of the flip-chip interconnect systems. A conventional underfill is filled to the gap between the chip and substrate around the solder joints by capillary flow. A modified Hele-Shaw flow, that considered the flow resistance in both the thickness direction and the restrictions between solder bumps, was used. This model estimates the flow resistance induced by the chip and substrate as well as the solder bumps, and provides a reasonable flow front prediction as compared to the experimental observations. A method to simulate the edge effects during the underfill process was also proposed, which gave a close flow front prediction in a chip assembly with full array solders.
The stacked-Chip Scale Package (S-CSP) is a new technology that provides high density of the package. It enables to stack the die in a single package. The S-CSP is widely adopted in portable multi-media products. However, the resin flow through a thin surface and wide filling area is of concern. Therefore, this paper presents a study of flow visualization during encapsulation process in S-CSP. The Navier–Stokes equation has been solved by the finite different method. For non-linear terms, the Kawamura and Kuwahara technique has been adopted in the flow analysis. Pseudo-concentration based on the volume of fluid (VOF) technique was used to track a melt fronts for each time step. The numerical model has been verified by comparing the prediction with the experimental results. The numerical results show good agreement with the experimental results. The prediction also shows that the short shot problem that occurred for the die top clearance is lower than 0.25 mm.
The plastic packaging process for integrated circuits is subject to several fabrication defects. For packages containing leadframes, three major defects may occur in the molding process alone, namely, incomplete filling and void formation, wire sweep, and paddle shift. Paddle shift is the deflection of the leadframe pad and die. Excessive paddle shift reduces the encapsulation protection for the components and may result in failures due to excessive wire sweep. Computer-aided analysis is one of the tools that could be used to simulate and predict the occurrence of such molding-process-induced defects, even prior to the commencement of mass production of a component. This paper presents a methodology for computational modeling and prediction of paddle shift during the molding process. The methodology is based on modeling the flow of the polymer melt around the leadframe and paddle during the filling process, and extracting the pressure loading induced by the flow on the paddle. The pressure loading at different times during the filling process is then supplied to a three-dimensional, static, structural analysis module to determine the corresponding paddle deflections at those times. The paper outlines the procedures used to define the relevant geometries and to generate the meshes in the "fluid" and "structural" subdomains, and to ensure the compatibility of these meshes for the transfer of pressure loadings. Results are shown for a full paddle shift simulation. The effect on the overall model performance of different element types for the mold-filling analysis and the structural analysis is also investigated and discussed. In order to obtain more accurate results and in a shorter computational time for the combined (fluid and structural) paddle shift analysis, it was found that higher-order elements, such as hexahedra or prisms, are more suitable than tetrahedra.
Incompressible high-Reynolds-number flows around a circular cylinder are analyzed by direct integration of the Navier-Stokes equations using finite-difference method. A generalized coordinate system is used so that a sufficient number of grid points are distributed in the boundary layer and the wake. A numerical scheme which suppresses non-linear instability for calculations of high-Reynolds-number flows is developed. The computation of an impulsively started flow at Re = 1200 is compared with corresponding experimental observations, and excellent agreements are obtained. A series of computations are carried out on the flow around a circular cylinder with surface roughness. The height of the roughness in these computations is 0.5% of the diameter. The range of Reynolds numbers is from 103 to 105; no turbulence model is employed. Sharp reduction of drag coefficient is observed near Re = 2 × 104, which indicates that the critical Reynolds number is captured in the present computation.
Electronic packaging protects the integrated circuit chip from environmental and mechanical damages. Underfilling encapsulation is an electronic packaging technology used to reinforce the solder joints between chip and the substrate. For better mould design and optimization of the process, flow analysis during the encapsulation process is the first necessary step. This paper focuses on the study of fluid flow in underfilling encapsulation process as used in electronics industry. A two-dimensional numerical model was developed to simulate the mould filling behaviour in underfilling encapsulation process. The analysis was carried out by writing down the conservation equations for mass, momentum and energy for a two-dimensional flow in an underfilling area. The governing equations are solved using characteristic based split (CBS) method in conjunction with finite element method to get the velocity and pressure fields. The velocity field was used in pseudo-concentration approach to track the flow front. Pseudo-concentration is based on the volume of fluid (VOF) technique and was used to track fluid front for each time step. A particular value of the pseudo-concentration variable was chosen to represent the free fluid surface which demarcates mould compound region and air region. Simulation has been carried out for a particular geometry of a flip-chip package. The results obtained are in good agreement with the available numerical and experimental values and thus demonstrate the application of the present numerical model for practical underfilling encapsulation simulations. Copyright
We developed a numerical method for simulating the underfill flow of conventional capillary flow and no-flow types in flip-chip packaging. The analytical models for the two types of underfill encapsulation processes are proposed. In the capillary flow type, the underfill material is driven into the cavity with solder bump by the surface tension with an effect of contact angle as the capillary action. In the no-flow type, the movement of IC chip during the reflow attachment is controlled by an appropriate loading to get the proper interconnect between IC chip and substrate. In both types, the flow behavior and filling time of underfill material in underfilling encapsulation process are investigated, taking the fluid dynamic force acting on the solder bump into account. It is found that the proposed analytical models have a considerable potential for predicting the underfill flow.
In the prediction of underfill flow in a flip-chip package, numerical methods are usually used for flow analysis and simulation since analytical methods cannot meet the requirement for predicting fluid distribution in a planar analysis. At present, there appears to be no simulation software commercially available that is able to provide adequate prediction for the underfill flow process driven by capillary force in a micro-cavity situation. In the study presented in this paper, a numerical model was proposed for the prediction of flip-chip underfill flow. In this model, the power-law constitutive equation was used to describe the non-Newtonian behavior of encapsulant fluids and a time-dependent velocity boundary condition was used instead of the pressure boundary condition commonly used. The comparison between the model-predicted and experimental results indicated that this model can give a good prediction for the underfill flow in a micro-cavity. This model was implemented by a general-purpose commercially available software program ANSYS, which has a high reliability and wide accessibility.
The viscosity of the underfill encapsulant may be different under the conditions of different shear rate, filler content, and temperature. Most of the encapsulant is epoxy containing silica fillers. It exhibits non-Newtonian behavior in the underfill flow. The effect of the viscosity variations on the underfill filling flow was investigated in this study. An analytical model of the filling flow is proposed to accomplish the shear-rate depending viscosity. Due to the addition of fillers in the encapsulant, the viscosity may exhibit both shear thinning and thickening behaviors depending on the temperature and filler content. This study proposes a model of the viscosity considering both effects. In the situations demonstrated in the results, the shear thinning and thickening effects may have major influence on the velocity profile and the filling speed.
Flip chip package is the most important technology in IC package necessary for scale, velocity and cost by the development of semiconductor technology and the innovation of computer product. It has the advantages of low cost, low interface and small volume in IC package. This paper emphasizes the analysis for underfill encapsulation between the solder ball and chip. The finite element simulation in a three-dimensional inertia-free, incompressible flow is presented. A control volume scheme with a fixed finite element mesh is employed to predict fluid front advancement. The epoxy is used for the underfill material. The injection situation is used for central point, one line, L line and U line injection. The underfill process is used for different parameters (mold temperature, melt temperature, injection pressure, injection time and injection situation). The results show that the injection situation is the most important factor for processing parameters. The result indicates that the L line injection is the best injection situation for underfill encapsulation of flip chip.
In this paper, the finite volume method (FVM) based numerical simulation is used for the flow visualization of capillary driven underfill process for different solder bump arrangements of flip chip packages is presented. Three different 3D flip chip package models are developed and simulated using computational fluid dynamic (CFD) code, FLUENT 6.3. Capillary action and cross viscosity model are taken into account in the simulation. One-line dispensing method is applied in the analysis and the volume of fluid (VOF) technique is used to track the flow front. The effect of solder balls arrangement on flow behavior and filling time is studied and the solder balls arrangement is found to affect the flow behavior and filling time. The flow patterns of simulation are observed for three flip chip packages and compared. The ability of the proposed model and FLUENT in handling flip chip underfill problems is proved to be excellent.
This paper presents the simulation of pressurized underfill encapsulation process for high I/O flip chip package. 3D model of flip chip packages is built using GAMBIT and simulated using FLUENT software. Injection methods such as central point, one line, L-type and U-type are studied. Cross-viscosity model and volume of fluid (VOF) technique are applied for melt front tracking of the encapsulant. The melt front profiles and pressure field for all injection types are analyzed and presented. The pressure distribution within the flip-chip, fill volume versus filling time and viscosity versus shear rate are also plotted. The U-type injection is found to be faster in filling. The numerical results are compared with the previous experimental results and found in good conformity. The strength of CFD software in handling underfill encapsulation problems is proved to be excellent.
Flip-chip underfill process is a very important step in the flip-chip packaging technology because of its great impact on the reliability of the electronic devices. In this technology, underfill is used to redistribute the thermo-mechanical stress generated from the mismatch of the coefficient of thermal expansion between silicon die and organic substrate for increasing the reliability of flip-chip packaging. In this article, the models which have been used to describe the properties of underfill flow driven by capillary action are discussed. The models included apply to Newtonian and non-Newtonian behavior with and without the solder bump resistance for the purpose of understanding the behavior of underfill flow in flip-chip packaging.
The underfill flow process is one of the important steps in Microsystems technology. One of the best known examples of such a process is with the flip-chip packaging technology which has great impact on the reliability of electronic devices. For optimization of the design and process parameters or real-time feedback control, it is necessary to have a dynamic model of the process that is computationally efficient yet reasonably accurate. The development of such a model involves identifying any factors that can be neglected with negligible loss of accuracy. In this paper, we present a study of flow transient behavior and flow resistance due to the presence of an array of solder bumps in the gap. We conclude (1) that the assumption of steady flow in the modeling of the flow behavior of fluids in the flip-chip packaging technology is reasonable, and (2) the solder bump resistance to the flow can not be neglected when the clearance between any two solder bumps is less than 60–70 μm. We subsequently present a new model, which extends the one proposed by Han and Wang in 1997 by considering the solder bump resistance to the flow.
Underfill process is a very important step in the flip-chip packaging because of its great impact on the reliability of electronic devices. In the control of the underfill dispensing in flip-chip packaging, an analytical model for the underfill flow behavior is required to perform the control action. Traditionally, the Washburn model is used for predicting the viscous flow behavior in the flip-chip underfill process driven by capillary forces. Unfortunately, some studies in the literature have shown that the model does not match the measured results well due to the neglect of the characteristics such as solder bump resistance and non-Newtonian behavior of underfills. Although some underfill flow models have been developed for considering these characteristics, there is no sufficient account for such a mismatch from the literature. In this article, we present an experimental investigation aimed to understand the possible causes responsible for the observed mismatch with the Washburn model. The experimental investigation confirmed that the underfill fluid used in flip-chip packaging shows a complex non-Newtonian behavior and that the Washburn model is, indeed, only applicable to the Newtonian fluid in this setting. Another contribution of the work reported in this article is the provision of measured data on a test bed which was built upon using the off-the-shelf components; as such the data can be used by other researchers to validate their theoretical findings.
Thesis (Ph.D.)--The University of Saskatchewan, 2005. Includes bibliographical references.
The capillary rise of liquid in a cylinder bank is examined in order to study the capillary pressure variation perpendicular to the direction of the cylinders. The calculations consider the local geometric variation of the flow channel and the position-dependent capillary pressure. The capillary flow around each cylinder is calculated by balancing the capillary pressure and the viscous drag along the flow path. The rate of filling for several layers of cylinders is used to estimate the equivalent capillary pressure. The method is also applied to the underfill of a flip chip system, which is modeled as a cylinder bank between parallel plates.
This paper presents recent results on underfill flow characterization. The flow properties of a number of commercial and experimental underfills were recorded and analyzed using quartz test chips with specially designed bump patterns (e.g., peripheral, full array, and mixed designs). Each was bonded onto an organic laminate substrate to form a flip chip package. Underfill was then applied to the packages and flow time, filler settling, and air entrapment were evaluated. Good flow can be described in terms of three measurable parameters, namely, viscosity, contact angle, and more importantly, filler size and distribution. Viscosity and contact angle are commonly used in Hele Shaw and Washburn models. However, these models do not take filler properties into consideration. In general, underfills with particles less than 5 μm exhibited faster and more uniform flow fronts than materials with larger particles. The best flowing materials worked well with standoff heights between 50 and 75 μm, while the poorer flowing materials showed streaking, voiding, and fingering at these heights. At gaps of 25 μm, however, nearly all the materials exhibited pronounced and reproducible streaking
The flow characteristics of a number of underfills were evaluated with quartz dies of different patterns and pitches bonded on different substrate surfaces. Perimeter, mixed array, and full array patterns were tested. Observations on the flow front uniformity, streaking, voiding, and filler segregation were collected, The information was compared with the results predicted by a new simulation code, plastic integrated circuit encapsulation-computer aided design (PLICE-CAD) under DARPA-funded development. The two-phase model of the combined resin and air takes into account geometrical factors such as bumps and die edges, together with boundary conditions in order to track accurately the propagation of the flow fronts, The two-phase flow field is based on the volume-of-fluid (VOF) methodology embedded in a general-purpose three-dimensional (3-D) flow solver
This article describes an analytical model for the prediction of the underfill flow characteristics in a flip-chip package driven by capillary action. In this model, we consider non-Newtonian fluid properties of the encapsulant as opposed to most other studies where Newtonian fluid properties were assumed for the underfill flow. The power-law constitutive equation was applied in our study. The simulation based on this model agreed well with the measurement obtained from the experiments available in literature. It was further shown that this model performs better than the Washburn model traditionally used for the prediction of underfill flow characteristics in the flip-chip packaging. Based on this model, the effects of the solder bump pattern (including bump pitch, solder bump diameter, and gap height) on the process variables (i.e., flow front and filling time) were studied, which facilitated both the package design and the process optimization.
In this paper, the flow of encapsulant during the underfill encapsulation of flip-chips has been studied. Analytical as well as numerical methods have been developed to analyze the flow. For capillary-driven encapsulation (by dispensing), the capillary force at the melt-front has been calculated based on a model for the melt-front shape. A model has also been developed for the analysis of forced-injection encapsulation. The numerical analysis uses a finite-element method based on a generalized Hele-Shaw method for solving the flow field. Experiments have been performed to investigate the flow behavior using actual chips and encapsulants. Short-shot runs have been performed to observe the melt-front advancement at different flow times. In addition, measurements have been made of the material properties of the encapsulant, namely its viscosity, curing kinetics and surface-tension coefficient. The experimental and simulation results have been compared in terms of the flow-front shapes and times at different fill fractions. Such comparisons indicate that the model developed in this study is adequate to approximately simulate the flow during encapsulation of flip chips
Computation of high Reynolds number flow around a circular cylinder with surface roughness Three-dimensional paddle shift modeling for IC packaging [22] C. Pozrikidis. 1997. Introduction to theoretical and computational fluid dynamics
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Ani. 2010. Finite volume based CFD simulation of pressurized flip chip underfill encapsulation process. Microelectronics Reliability. Vol. 50, Issue 1, pp.98-105. [20] T. Kawamura and K. Kuwuhara. 1984. Computation of high Reynolds number flow around a circular cylinder with surface roughness, AIAA 22 nd Aerospace Science Meeting, NV, pp. 1-11. [21] C. C. Pei and S. J. Hwang. 2005. Three-dimensional paddle shift modeling for IC packaging, Transactions ASME J. Electronic Packaging, Vol. 127, pp. 324-334. [22] C. Pozrikidis. 1997. Introduction to theoretical and computational fluid dynamics, New York: Oxford Univ. Press, pp. 324-334. [23] C. Y. Khor, M. Z. Abdullah, M. Abdul Mujeebu, F. Che Ani. 2010. FVM based numerical study on the effect of solder bump arrangement on capillary driven flip chip underfill process. International Communications in Heat and Mass Transfer. Vol. 37, pp.281-286.
Influence of transient flow and solder bump resistance on underfill process Modelling of a non-Newtonian flow between parallel plates in a flip chip encapsulation Study on flow visualization of flip chip encapsulation process for numerical simulation
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Yang and S. A. Bistrup-Allen, 1999. Underfill of flip chip on laminates: Simulation and Validation. IEEE Transactions on Component and Packaging Technology, Vol. 22, No. 2, pp. 168- 176. [11] J. W. Wan, W.J. Zhang, D. Bergstrom, 2005. Influence of transient flow and solder bump resistance on underfill process. Microelectronics Journal, Vol. 36, pp. 687-693. [12] W. B. Young, 2010, Modelling of a non-Newtonian flow between parallel plates in a flip chip encapsulation. Microelectronics Reliability, doi:10.1016/jmicroel.2010.03.008. [13] Y. K. Shen, S.T. Huang, C.J. Chen, S. Yu, 2006. Study on flow visualization of flip chip encapsulation process for numerical simulation. International Communications in Heat and Mass Transfer, Vol.33, pp.151-157. [14] T. Hashimoto, T. Shin-ichiro, K. Morinishi, N. Satofuka, 2008. Numerical simulation of conventional capillary flow and noflow underfill in flip chip packaging. Computers and Fluids, Vol. 37, pp. 520-523. [15] V. M. Kulkarni, K.N. Seetharamu, I. Abdul Azid, P.A.
Numericla simulation of underfill encapsulation process based on characteristic split method FVM based numerical study on the solder bump arrangement on capillary driven flip chip underfill process
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Aswatha Narayana, G. Abdul Quadir, 2006. Numericla simulation of underfill encapsulation process based on characteristic split method. International Journal for Numerical Methods in Engineering. Vol. 66, pp. 1658-1671. [16] C. Y. Khor, M. Abdul Mujeebu, M. Z. Abdullah, F. Che Ani. 2010. FVM based numerical study on the solder bump arrangement on capillary driven flip chip underfill process. International Communications in Heat and Mass Transfer. Vol. 37, Issue 3, pp.281-286. [17] M. K. Abdullah, M. Z. Abdullah, S. Kamaruddin, and Z.
Modelling of a non-Newtonian flow between parallel plates in a flip chip encapsulation
  • W B Young
W. B. Young, 2010, Modelling of a non-Newtonian flow between parallel plates in a flip chip encapsulation. Microelectronics Reliability, doi:10.1016/jmicroel.2010.03.008.
Computation of high Reynolds number flow around a circular cylinder with surface roughness
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T. Kawamura and K. Kuwuhara. 1984. Computation of high Reynolds number flow around a circular cylinder with surface roughness, AIAA 22 nd Aerospace Science Meeting, NV, pp. 1-11.