Coating of Electronic Components by the RTV Dispersion—Part II: Determination of the Residual Yield Stress After Deposition, and Method to Produce a Drop of Required Cured Skin Thickness and Diameter

In this part of the paper the drop test method is used to show that RTV dispersions are in fact plastic “false body” materials, and to determine the magnitudes of the residual yield stresses after shearing for different RTV dispersion lots. With the aid of the equation for the wall shear stress derived in Part I of this paper [1], a correlation equation between the residual yield stress and the deposition variables is obtained, and analysis is made of the drop spreading phenomenon. It is shown that the drop spread, which is responsible for the run-over, or wicking, of external leads of electronic circuits, can be decreased by decreasing the deposition rate of the encapsulant. Finally, a method is developed to determine the required deposition flow rate and deposition time to produce a drop having required final diameter and cured skin thickness.

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Coating of Electronic Components by the RTV Dispersion—Part I: Physical Model of the Deposition Process Determined by Drop Tests

Journal of Electronic Packaging ◽

10.1115/1.2909323 ◽

1993 ◽

Vol 115 (3) ◽

pp. 233-239

Author(s):

J. A. Owczarek

Keyword(s):

Shear Stress ◽

Wall Shear Stress ◽

Yield Stress ◽

Physical Model ◽

Volumetric Flow Rate ◽

Deposition Process ◽

Test Method ◽

Electronic Components ◽

Drop Tests ◽

Dispersion Part

This paper describes a study of the process of deposition of RTV dispersion on electronic components placed on substrates. The objective was to develop a technique for the consistent manufacture of encapsulant coating of a desired thickness and extent. In addition, it was desired to obtain an understanding of the phenomenon of run-over, or wicking, of the RTV dispersion onto external leads of circuits being encapsulated, and of means to control it. In this paper physical properties of the RTV dispersion which influence the deposition process were determined using a novel drop test method. These properties allow building of a physical model of the deposition process, and its analysis. The results of drop tests show that the RTV dispersion behaves like a plastic “false body” material which possesses yield stress after a long rest, and which retains residual yield stress after shearing. Part I of this paper is concerned with building of the physical model of the encapsulant deposition process. It also deals with the derivation of an equation relating the wall shear stress to the encapsulant volumetric flow rate.

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