Underfill Encapsulant Flow Measured using Capacitance Measurement Techniques
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Underfill Encapsulant Flow Measured using Capacitance Measurement Techniques T. E. Driscoll∗, G. L. Lehmann# and E. J. Cotts∗ ∗ Department of Physics, Binghamton University, Binghamton, N. Y. 13902 # Department of Mechanical Engineering, Binghamton University, Binghamton, N. Y. 13902 Abstract: Capillary flows of dense, model suspensions and industrial underfill encapsulants are investigated. Flow behavior is characterized by measuring the infiltration rate of an encapsulant or model suspension into a channel formed by parallel surfaces. A capacitance measurement technique is used to track the advancement of the front. Significantly this technique allows the channel surfaces to be formed from opaque materials such as those that found in the principal industrial application of electronics packaging. A scaling law to describe the functional dependence of the front position on channel spacing, surface tension and viscosity is presented. I. Introduction A critical step in the manufacture of a class of electronics components requires the capillary flow of an underfill material into a narrow passage (gap dimensions under a 100 µm). The underfill material or encapsulant is a mixture of epoxy resin with a high volume fraction loading of solid silica particles. Several authors have reported on techniques to measure and characterize the flow properties of encapsulants [1-5]. Typically this has been performed by measuring the nearly rectilinear motion of an encapsulant front (xf(t)) as it infiltrates a channel formed by parallel surfaces (c.f. Fig. 1). To observe the motion of the front optically requires that at least one channel wall be transparent. Also extracting the data (xf vs. t) from the record of the observed motion requires post-experiment image processing which can be costly in both analysis effort and digital storage capacity. In this paper, we shall describe a capacitance measurement technique, which provides simple, rapid post-experiment processing and allows one to use a variety of channel surfaces, including the opaque surfaces encountered in the industrial practice. This is particularly significant as the capillary motion is driven by the wetting characteristics of the encapsulant/surface pair. II. Experimental Experiments are conducted using a capillary flow cell, as depicted in Fig. 1. A liquidsolid suspension is dispensed at the entrance of a plane channel formed by two parallel plates. A precision spacer, located opposite to the entrance, defines a channel of spacing s and length L~400 mm. The channel width is W=25 mm and the side edges of the channel are open. The capacitance of the flow cell varies significantly with the motion of the encapsulant front, due the difference in the dielectric constants of air (κ1) and encapsulant (κ2) respectively ( 2 / 1 ≈ 2.5) . Figure 1: Schematic of a typical flow cell. The channel capacitance is a function of A fluid/suspension/encapsulant is dispensed the channel volume fraction occupied by along one edge and allowed to flow into the the encapsulant. Letting xf denote
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