Encapsulation Of Electronic Components Research Articles

Advances in high-field insulating polymeric materials over the past 50 years

The development of electrical insulation is closely associated with the development of electrical apparatus. Up to about 1925 only naturally occurring products such as asphalt, rubber, mica, cotton thread or fabric were generally used. Mineral oil-impregnated paper was used in transformers, capacitors, and underground cables at voltages up to 69 kV [1]. Then the first synthetic insulating materials, namely phenolic (Bakelite®) and alkyd (saturated polyester) resins, were used in insulating varnishes. Polyvinylchloride was introduced in 1936 to replace natural rubber in underground cable and house wiring, but in spite of tremendous advances in compounding polyvinylchloride to improve its properties, the electrical losses could not be reduced sufficiently. It was restricted to LV applications up to a few kilovolts. A variety of synthetic elastomers was used to replace natural rubber for HV applications, e.g., butyl rubber in the 1940s and ethylene-propylene rubber later on. Epoxy and polyester resins became available around 1945 and were first used for molding and encapsulation of electronic components, then mostly in laminated form as electrical insulation for LV applications [2]. Increased knowledge of the chemistry of insulating materials, and improved processing, then led to the availability of a large number of plastic materials such as polyethylene terephthalate, polycarbonate, polystyrene, low- and high-density polyethylene, polypropylene, polytetrafluoroethylene and fluorinated ethylene-propylene copolymer, suitable in thin film form for capacitor applications. These thin films could work at high electrical stresses or, put more practically, at minimal dielectric thickness appropriate to the capacitor working voltage. Thin films (<;10 μm) of these materials have intrinsic electric strengths in the range 500 to 1,100 V/μm at room temperature, probably 2 to 10 times higher than the average breakdown strength of plastic-film capacitors at the time [3]. Polyethylene-insulated of electrical energy, a statement that is still valid today and certainly will remain valid in the future. The objective of this paper is to give an overview of the research carried out over the last 50 years that has contributed to the development of high-field polymer dielectric materials in a broad sense. During this time thermoplastic polymers have been used in a large variety of electrical systems, e.g., cables, capacitors, transformers, bushings, electrical machines, and accessories operating in a large variety of environments (space, nuclear, thermal, etc.). Progress has been highly nonlinear in time, entangled with progress in material sciences, our understanding of the electrical properties of insulating polymers, and advances in material processing. We emphasize what are, in our view, the main features.

Time-Accurate, 3-D Computation of Wire Sweep During Plastic Encapsulation of Electronic Components

A computational technique is developed for accurate prediction of wire sweep during encapsulation of electronic components by transfer molding. The technique advances the current state-of-the-art by modeling the effects of the two-phase flow involved, the effects of the polymerization of the melt, and the effects of the time-dependence of the fluid and heat flows and of the wire stresses and deformations. The computational techniques used for separately computing the fluid and heat flows, for separately computing the structural-dynamic stresses and deformations, and for establishing two-way coupling between the separately computed “flow” and “structural” solutions are described. Validations comparing the computational predictions with the corresponding analytical solutions for reference cases with straight wires immersed in flowing liquids are presented and discussed. The validation results show close agreement with theory, and demonstrate the need for using large-deformation theory for computing wire deformations and stresses. Demonstrative results for the unsteady flowfield and wire sweep in a TO-220 package are also presented and discussed.

Unsteady Wire Sweep Due to Transfer Molding in a 160L QFP Package

Wire sweep has been recognized as one of the major defects in encapsulation of electronic components by transfer molding. The phenomenon is very complicated as it is sensitive to a large number of parameters. In this experimental work, where a 160L QFP package used as the test vehicle, the detailed time-dependent wire displacement is measured for the following two different flow initial conditions: (i) the wire is immersed in the liquid and is displaced due to the acceleration of the flow from rest to the steady-state velocity, and (ii) the wire is surrounded by the ambient air and is displaced first due to the passage of the liquid front and then due to the hydrodynamic load. Significant differences have been observed between the two cases, with important implications for analytical and numerical studies of wire sweep.