Patent Application
20080259580
This invention relates generally to the field of assembling electrical packages by mounting electrical components on a substrate having electrically conductive traces that together with the components define electrical circuitry, and more particularly to improved mounting of an electrical component having a body and electrical leads projecting from the body and extending into through-holes defined in the substrate.
Electrical packages comprising electrical components that have a body and electrical leads extending from the body and into through-holes defined in the substrate typically require auxiliary securement structure to stabilize the component when the package is subject to excessive vibrations or other sources of acceleration that exert forces on the component which are transferred to the solder connections through the electrical leads of the component. The forces impose stress on the solder connection unless the component is otherwise stabilized or restrained. Repeated stress on the solder connection can cause fatigue failure (cracking) of the solder connection and the electrical package.
Methods for stabilizing an electrical component having projecting leads that are received in through-holes of a substrate have included dispensing a hot-melt adhesive onto the substrate and/or onto the component during the assembly process to adhere the body of the electrical component to the substrate, securing the component to the substrate with wire clamps, or securing the component to the substrate with auxiliary fasteners such as screws. All of these known techniques have disadvantages. Clamps are themselves subject to fatigue failure, complicate the assembly process, adding excessive cost to the completed product, and cannot be easily configured for use with differently shaped and/or sized electrical components. Similarly, the use of fasteners adds significantly to the cost of the assembled product, and requires that the electrical component be designed with an integral mounting bracket to accommodate the fastener.
The use of adhesives for securely mounting an electrical component to a substrate is typically a more effective and economical way of stabilizing a component against damage due to vibration or other acceleration induced forces as compared with clamps and/or fasteners. Unfortunately, the use of apparatus for dispensing a hot-melt adhesive during the assembly process is undesirable for several reasons. First, the apparatus needed for dispensing a hot-melt adhesive can require a substantial amount of floor space in the assembly area. This can be particularly undesirable when the assembly requires very few components having projecting leads received in through-holes of the substrate. Secondly, it is extremely difficult to precisely control the amount of hot-melt adhesive being dispensed onto the substrate and/or component. This can lead to waste and/or inadequate securement of the component to the substrate. It is also difficult to precisely control the location at which the hot-melt adhesive is dispensed. This can also result in waste and/or inadequate securement of the component to the substrate. Dispensing of hot-melt adhesives in an assembly process also tends to be a production rate limiting step. Finally, difficulties associated with precisely dispensing a hot-melt adhesive can lead to adhesive being deposited at locations where it is undesirable and/or detrimental to the assembled product.
The invention achieves the advantages of employing a hot-melt adhesive for stabilizing an electrical component having a body and electrical leads projecting from the body and received in through-holes defined in a substrate, while avoiding the disadvantages associated with dispensing a hot-melt adhesive during assembly of an electrical package.
The advantages of the invention are achieved by utilizing a process for stabilizing and/or securely restraining an electrical component having leads projecting from a body of the component and received in through-holes defined in a substrate by employing a preformed mass of a hot-melt adhesive which is disposed between the substrate and the body of the component. The preformed mass of hot-melt adhesive may be deposited on the substrate before the electrical component is positioned on the substrate, or the preformed mass of hot-melt adhesive may be placed on the electrical component to form a sub-assembly that is positioned on the substrate.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
The invention provides a process for securely attaching and stabilizing a leaded electrical component mounted to a substrate against vibration and other forces applied to the component leads by acceleration. The process involves using a preformed hot-melt adhesive that is positioned between and fills the gap between the body of the electrical component and the substrate.
The expression “leaded electrical component” as used herein refers to an electrical component such as an inductor, transformer, electrolytic can capacitor, battery, etc., having a body and at least two electrical leads that extend outwardly away from the body of the electrical component, with the distal ends of the leads (i.e., the ends furthest from the body of the electrical component) received in through-holes defined in a substrate.
The substrate can be generally any article to which the leaded electrical component is mounted, but is generally a circuit board or wiring board defining electrically conductive traces or pathways that together with components mounted on the substrate define electrical circuitry.
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During the assembly process, preform 16 is heated to a temperature above the softening or melting temperature of the hot-melt adhesive of which preform 16 is comprised. Heating occurs at a temperature and for a time sufficient to cause wetting of the upper surface 22 of substrate 18 by the molten hot-melt adhesive and subsequent adherence between the hot-melt adhesive preform 16 and substrate 18, and between the hot-melt adhesive preform 16 and electrical component 10 upon cooling to a temperature below the melting or softening temperature of the hot-melt adhesive. Soldering and activation (softening or melting) of the hot-melt adhesive preform may be done sequentially or concurrently, with soldering occurring either before or after establishment of adherence between the preform 16 and substrate 18.
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In any of the illustrated embodiments, hot-melt preforms 10, 30 and 40 may be preadhered to electrical component 10 before the preforms are adhered to substrate 18. Alternatively, hot-melt preforms may be first positioned on substrate 18, with component 10 being thereafter placed on the preform before activation (softening and/or melting) and subsequent re-solidification.
While the various embodiments of the invention have generally been described with respect to the use of preforms comprising a thermoplastic hot-melt adhesive, the various embodiments of the invention may alternatively employ a thermosetting hot-melt adhesive. In this case, activation refers to a step in which an irreversible chain reaction is initiated to form a cross-linked polymer network that will not melt or become thermoplastic upon subsequent heating.
Hot-melt adhesives are resin (synthetic polymer) compositions that can be activated (e.g., softened or melted) to develop tackiness and/or bond to a surface, and subsequently solidify to securely retain an electrical component on a substrate.
Suitable thermoplastic hot-melt adhesives are those that will melt and/or soften and wet electrical component and substrate surfaces that it contacts without developing a viscosity that is sufficiently low to cause substantial deformation or flow of the preform. Suitable thermoplastic hot-melt adhesives used for preparing the preforms 16, 30, 40 are comprised of 100% or near 100% solids at temperatures up to about at least 100° C., and have a softening or melting point temperatures in the range from about 120° C. to about 190° C., with suitable resin types including polyolefins, polyamides, ethylene-propylene copolymers, styrene-butadiene copolymers, ethylene-propene polymer, polyolefin waxes, ethylene-vinyl acetate copolymers, as well as various mixtures of the listed substances. Suitable thermosetting hot-melt adhesives include moisture-curable reactive urethane compositions.
In order to improve heat dissipation, it may be desirable to utilize a hot-melt adhesive for preforms 16, 30, 40 which comprise a thermally conductive filler (e.g. carbon black, alumina or zinc oxide) in an amount sufficient to provide a thermal conductivity in the range from about 0.5 to about 1.0 w/M-K.
The invention reduces solder cracks on large leaded components which are subject to excessive vibration or other forces induced by acceleration, eliminates the need for hot-melt dispensing apparatus on a production assembly line, and allows precise placement of a hot-melt adhesive in a precise amount, thereby reducing waste and/or component failure. The processes of this invention also allow greater control and repetitiveness, and eliminate the time required to warm-up hot-melt equipment. In addition, the use of preforms in accordance with the invention eliminates hot-melt strings that develop during dispensing of hot-melt adhesives. In general, the preforms of this invention can be custom designed and/or configured to conform to the geometry of generally any leaded component. Further, because the electrical component and the hot-melt adhesive are heated concurrently, the process imparts a reduced level of thermal shock to the electrical component, as compared with direct contact between a hot adhesive and a cold electrical component.
It will be understood by those who practice the invention and those skilled in the art that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.
