High-current circuit trace and composition and method therefor

Details High-current circuit trace and composition and method therefor High-current circuit trace and composition and method therefor

1999-08-30

2001-04-24

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C148S023000, C148S522000, C148S527000, C174S126100, C174S126200, C228S223000, C228S037000, C228S039000, C428S647000, C428S546000, C428S901000, C428S931000, C420S558000

Reexamination Certificate

active

06221514

ABSTRACT:

TECHNICAL FIELD
The present invention generally relates to electrical circuits that require high-current routing. More particularly, this invention relates to a method of modifying a circuit trace with a layer of high-conductivity solder alloy to yield a high-current circuit trace for high-current routing.
BACKGROUND OF THE INVENTION
Conductor traces for laminate circuit boards are often formed of thin copper plating or foil, often in the range about 0.0015 to about 0.0028 inches (about 0.038 to about 0.071 mm) in thickness. Such thicknesses are well suited for carrying relatively low currents, e.g., up to about six amps for traces with widths of about 0.1 inch (about 2.5 mm), but are susceptible to overheating if higher currents are carried. In the development of bussed electrical modules (BEM), both high-current and low-current circuits are required in the same module. In the past, surface-mount bus bars have been employed to form high-current paths where needed. However, bus bars are manufactured separately and must then be assembled and soldered to the circuit board, incurring additional material and assembly costs. Accordingly, it would be desirable if another method were available for incorporating high-current routing on a circuit board, preferably in a manner that does not require the placement of a discrete component and is compatible with the processing of laminate circuit boards with thin copper traces.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a method by which a conventional circuit trace is modified with a layer of high-conductivity solder alloy to yield a high-current circuit trace for high-current routing on a circuit board. The method generally entails the use of a solder composition that contains a dispersion of metal particles in a solder alloy. The metal particles are selected on the basis of having a higher electrical conductivity than the solder alloy, and are present in sufficient amounts so that the electrical conductivity of the solder composition is significantly higher than that of the solder alloy. The solder composition is deposited on a circuit conductor, and then reflowed to form a conductive layer on the conductor. The metal particles remain as a discrete dispersion within the conductive layer in order to suitably promote the conductivity of the layer. The solder composition is preferably deposited in an amount sufficient to increase the current-carrying capacity of the resulting circuit trace for high-current routing on the circuit substrate, e.g., in excess of six amps.
According to the invention, the amount, size and geometry of the metal particles have been determined to affect the conductivity of the conductive layer, as well as influence the surface tension and rheology flow properties of the solder composition during deposition on the conductor. Tests have further shown that particles appropriately chosen for size and geometry can, when present in suitable quantities, inhibit the flow of the molten solder during reflow to maintain a more uniform cross-section for the conductive layer. Deposition of the solder composition is preferably performed with a stencil whose configuration is specially adapted to the rheology of the solder composition, as well as to the process of selectively and uniformly depositing the composition on narrow parallel conductors to achieve uniform coverage and thickness of the resulting conductive layers without shorting between adjacent conductors.
In view of the above, a significant advantage of the method and solder composition described above is the elimination of the need for discrete bus bars that must be selectively mounted to a circuit to provide high-current routing. Instead, the present invention is compatible with conventional laminate circuit board structures, and involves modifying an otherwise conventional low-current conductor trace to yield a high-current circuit trace of consistent quality. Accordingly, high-current circuit traces can be readily formed essentially anywhere high-current routing is required on a substrate, and at a cost that can be significantly lower than possible with the prior art use of bus bars.
Other objects and advantages of this invention will be better appreciated from the following detailed description.


REFERENCES:
patent: 5011658 (1991-04-01), Niedrich
patent: 5390845 (1995-02-01), McDonald

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