Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-06-21
2002-05-07
Cuneo, Kamand (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S256000, C174S262000, C174S263000
Reexamination Certificate
active
06384342
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of making improved electronic systems and circuit boards, and more specifically to methods of making improved electronic systems and circuit boards using heat-resistant composite materials. Various novel heat-resistant electronic systems, circuit boards, non-segregating solid reinforcing elements, and other products based on these methods are also disclosed.
2. Description of Related Art
Electronic systems with modern electronic circuits components or elements are used in almost every industry including manufacturing, servicing, banking, business, financial, medical, end weaponry, as well as in high-speed processors, cellular phones, satellite communication systems, deep-well equipment, jet engines, gas turbines, lap-top personal computers, nuclear reactors, and automobiles or other transportation vehicles. Users of these electronic systems continuously require larger, more powerful, and faster speeds, requiring such systems to posses and better and more processors, transistors, voltage regulators, memory, and other components.
Generally, electronic circuit components or elements are mounted e.g., by melting and solidifying a solder metal on plastic or ceramic circuits boards. Metallic lead wires or lines are provided on the circuit components for use in connecting these circuit components onto the circuit boards. These connecting lines must be as few in number and as short as possible to reduce the electrical resistances, which slow down the speed of the electronic systems. These metallic lines must also be rigid, strong, fatigue-resistant, creep-resistant, and thermally conductive to help dissipate heat. Excessive heat generation from, e.g., high electrical resistances, increases the system temperature, reduces the life of transistors, and lowers the mechanical strength and creep resistance of metallic lead wires, thereby causing run-away deterioration of electrical and thermal resistance, temperatures, transistor life, and lead wire mechanical strengths. The degraded electronic systems directly degrades the performance of any equipment containing such electronic systems.
In many electronic systems, thermal design already is the limiting factor. For example, to handle the heat of a high-power (15.4 W) TO-220 voltage regulator operating with a 233-MHz Pentium chip presents a formidable problem that requires a proper thermal solution without scraping the existing mother-circuit-board architecture. Pentium chips with even higher speeds are already in mass production.
An important consideration in the mechanical, thermal, and electrical design of a circuit board and an electronic system is the fact that many materials are used for the electronic circuit heat-resistant components, the plastic or ceramic circuit board, and the electronic system. In general, the electronic system has a metallic or plastic frame onto which the circuit board substrate is fixedly attached at a specific location thereon. The circuit board is used to electrically and physically connect a number of circuit components together. The circuit board substrate has a large number of through holes. Each electronic circuit component has a number of metallic lead wires embedded into and electrically separated by an encapsulant. All the metallic lead wires on each circuit component extend, and point in a common direction away, i.e., vertically downward as shown in
FIG. 7
, from the circuit component so that all the extending lead wires can be easily inserted simultaneously into selected through holes at given positions on the circuit board substrate. The inside surfaces of the through holes are coated with specific metal layers to facilitate the wetting and bonding of the inserted metallic lead wires.
Each of the many different materials on the circuit components, the circuit board substrate, and the system frame has a specific set of mechanical, electrical, and thermal properties, and a unique thermal expansion coefficient. At the contact area between any two different materials, there is an actual or a residual thermal expansion mismatch that generates thermal mismatch stresses. Specifically, when the electronic system changes in temperature, thermal mismatch stresses are generated between:
1) the metallic lead wires and their encapsulating plastic;
2) the metallic lead wires and the bonded metal layers on the through holes of the circuit board substrate; and
3) the circuit board substrate and the mounting frame of the electronic system.
The thermal mismatch stresses usually are highly localized and can be so very severe as to cause localized metal creep and fractures, or changes in electrical resistances and thermal conductances. Changes in these resistances and conductances are equally, if not more, damaging than other changes to the reliability and life of the circuit components, the circuit board, and the entire electronic system.
Hence, the circuit boards and the electronic systems must have radically improved lead wire connections, which may be fabricated by soldering, brazing, or welding methods. All these methods use molten metal alloys. Soldering metal alloys with melt temperatures below about 250° C. are employed so that low-cost plastic circuit boards may be used. Brazing and welding metal alloys require melt temperatures respectively below and above about 800° C. Such temperatures require ceramic circuit boards. Most such connections now are soldered joints that have:
1) high electrical resistance leading to wasteful heat generation, rise in temperatures, and reduced circuit component speed and life;
2) low thermal conductivity magnifying the problems in 1); and
3) inadequate mechanical strength of the lead wire connections particularly as to creep, fatigue, or shear, making all the bonded lead wires, the circuit board, or even the entire electronic system non heat-resistant, short-lived, and unreliable.
In this invention, the above-mentioned problems of prior-art composites are minimized by a unique, heat-resistant ceramic-reinforced composite material to be shown below.
For purposes of the present invention, a composite is any material that results when two or more materials, each having its own, usually different characteristics, are combined, giving useful properties for specific applications.
Further, when used in the present specification, a matrix is a material in which something is enclosed or embedded.
For purposes of the present invention uniform distribution of solid reinforcing elements in a composite matrix means that the concentration of the solid reinforcing elements in each unit of volume, e.g., cubic millimeter, of the solidified composite matrix is constant or substantially constant throughout the entire composite. In addition, a composite has a matrix component, the matrix component is generally characterized by the composite component that is in the majority. For example, a composite made from 20% by weight solid reinforcing elements and 80% by weight In is characterized as an In matrix composite.
Composites are important structural materials. Oftentimes composites are reinforced by suspending or embedding solid strengthening or reinforcing elements, such as, reinforcing powders, rods, sheets, weaves, or combinations thereof within the composite matrix. Generally, the solid reinforcing elements are rigid and temperature resistant and are thus used to make the entire composite matrix more rigid and temperature resistant. Many other benefits are achieved by reinforcing composites. For example, reinforced composites can be prepared which resist creep, fatigue, and tensile or shear fractures at temperatures which are close to the melting point of the composite matrix.
Reinforcing elements often segregate at corners, edges, and deep but narrow walls such as in a solder joint on a circuit board. Overcrowded reinforcing elements at certain segregated places, such as the bottom for heavier solid reinforcing elements or the top for lighter solid reinforcing elements, causes weakness in the composite ma
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