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.

中文翻译：

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倒装芯片封装中底部填充流动的空间分析

目的

该研究的目的是研究倒装芯片封装过程中底部填充流动的空间方面，例如弯月面演变和接触线跳跃（CLJ）。此外，制定了在底部填充过程中基于空间的空洞形成机理。

设计/方法/方法

使用微网孔单元法，可以直观地看到凸点阵列和CLJ现象之间的底部填充液的弯液面演变。此外，弯月面的演变和CLJ现象是建立在毛细管物理学基础上的解析模型。同时，对孔洞形成的机理进行了数值和分析解释。

发现

所提出的分析结果和当前的数值发现均取得了很大的共识，并在实验上得到了很好的验证。分析了凸点间距对空间方面的变化影响，发现弯月弧半径和填充距离随间距而增加，而弯月弧的对角随间距大小不变。对于较大的螺距，跳跃发生在远离凸块入口的位置，并且需要更长的时间才能达到平衡弯月面。这推断弯月面的凹面受凸块间距的影响。在排空机构上，由于流体-凸起的相互作用，空气的滞留形成了空气空隙。较小的空隙倾向于通过颈缩合并为较大的空隙，然后沿底部填充流传播。

实际影响

底部填充流动的微观空间分析将从根本上解释凸点设计将如何影响宏观填充时间。这不仅提供了可替代的可视化工具来分析行业中的流型，而且还可以开发准确的分析填充时间模型。此外，空洞形成机制为了解空洞缺陷的根本原因提供了实质性见识，并允许制定可能的解决方案来解决此问题。此外，微流体领域也可以从这些空间分析见解中受益。

创意/价值

由于过去的研究主要集中在时间方面，因此很少进行底部填充流动的空间分析。此外，这项工作提出了一种基于流体-凸点相互作用的空隙形成的新机制，其中首次以数字方式可视化了微孔的形成和传播。当前工作的发现为封装设计人员提供了有关底部填充液和焊料凸块之间的流动相互作用的基本信息，以进行优化工作和增强工艺。