Effect of scale size, orientation type and dispensing method on void formation in the CUF encapsulation of BGA

Abas, Aizat; Ng, Fei Chong; Gan, Z. L.; Ishak, M. H. H.; Abdullah, M. Z.; Chong, Gean Yuen

doi:10.1007/s12046-018-0849-3

Cited by 20 publications

(19 citation statements)

References 19 publications

(39 reference statements)

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“…Ho et al (2007) reported that the reduction of void occurred can be achieved by increasing the plasma time and underfill dispense temperature, as well as selecting a smoother substrate surface. The more recent studies attempted to understand the void formations at various operating conditions, for instance bump arrangement (Abas et al , 2016) and dispensing type (Abas et al , 2018; Ng et al , 2017a, 2017b). Nonetheless, there is no detailed discussion on the mechanism of void formation during underfill process particularly from the microscopic aspect being proposed.…”

Section: Introductionmentioning

confidence: 99%

Spatial analysis of underfill flow in flip-chip encapsulation

Zawawi

Abas

2020

SSMT

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Purpose The 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/approach The 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. Findings Both 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 implications The 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/value Spatial 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.

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Section: Introductionmentioning

confidence: 99%

Spatial analysis of underfill flow in flip-chip encapsulation

Zawawi

Abas

2020

SSMT

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“…The functions of underfill encapsulation include to relief the residual stress due to thermal mismatch, to serve as a protective layer and to promote the package's reliability [2][3]. There are various researches conducted to optimize the underfill process through package design [4][5][6][7] as well as to resolve the issues of incomplete filling [8] and void defect [7,9]. The most common and straightforward approach in underfill research is through experiment [2][3][5][6][7][8][9][10][11].…”

Section: Introductionmentioning

confidence: 99%

“…However, the concerns of cost and limited versatility that arise from the underfill experiment prompt to the implementation of numerical simulation. There are variety of numerical schemes adopted in the underfill simulation, for instance, finite volume method (FVM) [6,[12][13], finite element method (FEM) [14], fluid/structure interaction (FSI) [8], discrete phase model (DPM) [10], lattice Boltzmann method (LBM) [7] and Petrov-Galerkin (PG) method [15]. As the underfill flow simulation usually based on the full flip-chip, therefore the numerical simulation took substantial long time to complete.…”

Section: Introductionmentioning

confidence: 99%

Symmetrical Unit-Cell Numerical Approach for Flip-Chip Underfill Flow Simulation

Chong¹,

Zawawi²,

Hao³

et al. 2020

CFDL

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Underfill process is a critical manufacturing process to safeguard and enhance the package reliability of flip-chip devices. In most underfill researches, the underfill flow was primarily investigated through numerical simulation. Nonetheless, the numerical simulation required a tremendously long time to complete. This paper presents a new symmetrical unit-cell approach to simulate the flip-chip underfill encapsulation process. The current numerical simulation is based on the finite volume method scheme. By exploiting the repetitive symmetry of bump array in flip-chip, the computational domain was simplified into a long array of unit-cells of one-pitch thick while the symmetrical walls between adjacent unit-cells were modelled using the periodic boundary condition. Accordingly, the computational costs can be greatly reduced. Alongside with the introduction of a new numerical approach of flip-chip underfill, the variation effect of bump pitch was studied by considering four flip-chip cases with bump pitches ranging from 0.08 mm to 0.16 mm. The numerical findings were found to in great consensus to the referencing experimental data, with the discrepancy, not more than 14.54%. Additional validation with the analytical filling time model revealed that both the numerical and analytical filling progressions are comparable. It is found that the increases in bump pitch can reduce the filling time at a particular filling distance, such that the filling time was halved within the investigated range of bump pitch. This new numerical approach is particularly useful for the simulation works of underfill process, especially the design and process optimization.

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“…As the underfill fluid is driven into the gap via capillary action, the underfill process usually takes a substantial amount of time to complete (Zhang and Wong, 2004). Technically, a slow underfill process is unfavorable, as it would incur additional manufacturing costs, while the package is also prone to void defects (Abas et al, 2018). It can be seen that the productivity and performance of the underfill process are strongly dependent on the filling time.…”

Section: Introductionmentioning

confidence: 99%

“…Such adoption of imitation chips was largely attributed to cost concerns, versatility in parameter variation and flow visualization. Moreover, Khor et al (2012c) pioneered the usage of a proportionally scaled-up imitation chip to experimentally visualize the underfill flow, which was also successfully being used in other subsequent underfill experiments (Khor et al, 2012b;Ng et al, 2017a;Abas et al, 2018;Ng et al, 2018). Among various scaled-up imitation underfill experiments, the scaling validity of the scaled-up imitation system was only being analyzed by Ng et al (2018) through the dimensionless number similarity.…”

Section: Introductionmentioning

confidence: 99%

Filling efficiency of flip-chip underfill encapsulation process

Abas

Abdullah

2019

SSMT

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Purpose This paper aims to introduce a new indicative parameter of filling efficiency to quantify the performance and productivity of the flip-chip underfill encapsulation process. Additionally, the variation effect of the bump pitch of flip-chip on the filling efficiency was demonstrated to provide insight for flip-chip design optimization. Design/methodology/approach The filling efficiency was formulated analytically based on the conceptual spatial and temporal perspectives. Subsequently, the effect of bump pitch on filling efficiency was studied based on the past actual-scaled and current scaled-up underfill experiments. The latter scaled-up experiment was validated with both the finite volume method-based numerical simulation and analytical filling time model. Moreover, the scaling validity of scaled-up experiment was justified based on the similarity analysis of dimensionless number. Findings Through the scaling analysis, the current scaled-up experimental system is justified to be valid since the adopted scaling factor 40 is less than the theoretical scaling limit of 270. Furthermore, the current experiment was qualitatively well validated with the numerical simulation and analytical filling time model. It is found that the filling efficiency increases with the bump pitch, such that doubling the bump pitch would triple the efficiency. Practical implications The new performance indicative index of filling efficiency enables the package designers to justify the variation effect of underfill parameter on the overall underfill process. Moreover, the upper limit of scaling factor for scaled-up package was derived to serve as the guideline for future scaled-up underfill experiments. Originality/value The performance of underfill process as highlighted in this paper was never being quantified before in the past literatures. Similarly, the scaling limit that is associated to the scaled-up underfill experiment was never being reported elsewhere too.

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Effect of scale size, orientation type and dispensing method on void formation in the CUF encapsulation of BGA

Cited by 20 publications

References 19 publications

Spatial analysis of underfill flow in flip-chip encapsulation

Spatial analysis of underfill flow in flip-chip encapsulation

Symmetrical Unit-Cell Numerical Approach for Flip-Chip Underfill Flow Simulation

Filling efficiency of flip-chip underfill encapsulation process

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