Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1999-12-16
2001-12-04
Gandhi, Jayprakash N. (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S752000, C361S758000, C361S796000, C361S797000, C361S800000, C361S816000
Reexamination Certificate
active
06327155
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to electronic equipment assemblies, and more particularly, to a printed circuit card assembly and system for preventing flame spread through an electronic equipment assembly.
BACKGROUND ART
Electronic equipment assemblies such as, for example, telecommunications equipment, generally comprise a cabinet that houses a number of printed circuit card assemblies within one or more shelves. Each printed circuit card assembly includes a printed circuit board with electronic components mounted thereon. In order to reduce equipment size, manufacturers must make maximum use of the space available on the printed circuit boards of the printed circuit card assemblies that comprise the electronic equipment assembly by increasing component density on each printed circuit board. Because a majority of the components found on the printed circuit boards are made of plastic or similar combustible material, the fuel load carried by each printed circuit card assembly within an electronic equipment assembly has increased tremendously. Moreover, these component-dense printed circuit card assemblies are inserted into the shelves of the electronic equipment assemblies at a tight card-to-card pitch in order to further reduce the size of the electronic equipment assembly. This decreased spacing between printed circuit card assemblies allows for an increased number of card assemblies per shelf/system and thus increases the amount of heat generated by the shelf/system, thereby increasing the risk of fire, and increasing the likelihood of adjacent printed circuit card assemblies igniting each other should one printed circuit card assembly catch fire.
Accordingly, flame spread through an electronic equipment assembly has become of great concern given these increased fuel loads resulting from increased component density on the printed circuit card assembly and increased printed circuit card assembly density within the shelves of the equipment.
Consider, for example, a telecommunications switching point that controls a regional calling area that might contain several electronic equipment assemblies. Should one printed circuit card assembly of one electronic equipment assembly catch fire, and that fire spreads unchecked through the electronic equipment assembly, the fire is likely to spread beyond that assembly to other electronic equipment assemblies resulting in loss of millions of dollars in telecommunications equipment, loss of telephone service for that area, as well as possible loss of life. As a result, guidelines that permit manufacturers to sell such electronic equipment assemblies have become increasingly more stringent.
In particular, GR-63-CORE, issued Oct. 1, 1995, sets forth the Telcordia (previously Bellcore)/NEBS standard which requires that all equipment assemblies pass a flame spread test based on the American National Standards Institute (ANSI) standard for fire propagation hazard testing in telecommunication equipment. The test requires that a burner be ignited and then placed within the electronic equipment assembly.
Specifically, sections 4.2.2 and 5.2.3 of GR-63-CORE, issued Oct. 1, 1995, call for a programmable methane line burner to be ignited and inserted into the equipment assembly at a location where it is anticipated that fire spread is most likely to occur. The burner is inserted into the vacated space created by removing one printed circuit card assembly from the shelf under test. The burner is ignited and a flow of methane is held at one liter per minute (l/m) for ten seconds. The flow is increased linearly to nine l/m during the next eighty seconds of the test at which point the flow is decreased linearly back to zero point zero one l/m during the following two-hundred-forty seconds. At this point, three-hundred-thirty seconds into the test, the burner is turned off and observations are made and recorded as to whether or not the fire is self-sustaining and whether or not there is a continued presence of observable smoke. Smoke, gas, temperature and heat release measurements are recorded throughout the test.
During the first ninety seconds of the GR-63-CORE test, a significant amount of heat is released in the burn area of the printed circuit board of the printed circuit card assembly, that is, the area on the printed circuit board that is most likely to catch fire. The principal modes of heat transfer are radiation and convection. Unprotected boards typically result in the components igniting during this first ninety seconds of the test. Moreover, these components can burn for up to and even beyond five and one half minutes of methane flow.
The current solution to meeting the flame spread test given the current, component-dense printed circuit card assemblies is to increase the printed circuit card assembly pitch in the electronic equipment assembly to provide more space between adjacent printed circuit card assemblies, or else to decrease the fuel load on the printed circuit card assemblies by reducing the number of components per printed circuit board (which requires more printed circuit card assemblies per electronic equipment assembly). Both of these solutions, however, result in undesirable increases in equipment assembly size.
DISCLOSURE OF THE INVENTION
This invention results from the realization that a truly effective printed circuit card assembly for preventing flame spread through an equipment assembly can be realized by providing at least one sacrificial heat absorbing flame resistant shield adjacent to and facing at least one side of the printed circuit board of the printed circuit card assembly. The flame resistant shield absorbs heat energy from an adjacent printed circuit card assembly which would otherwise be directed at the printed circuit board, thereby preventing heat transfer to the printed circuit board and the components mounted thereon so as to prevent ignition of the components.
The present invention features a printed circuit card assembly for preventing flame spread in an electronic equipment assembly. There is generally provided a printed circuit board having first and second sides, at least one of the first and second sides of the printed circuit board adapted for mounting a plurality of electronic components thereon. A sacrificial heat absorbing flame resistant shield is provided facing at least one of the first and second sides of the printed circuit board for absorbing heat energy, thereby reducing heat transfer to the printed circuit board from an adjacent printed circuit card assembly and preventing the electronic components from igniting.
The printed circuit card assembly can include a face plate disposed along an edge of the printed circuit board. There can also be a second flame resistant shield facing the other of the first and second sides of the printed circuit board. The flame resistant shield can include a sheet of synthetic aramid polyamide flame resistant fiber or a sheet of synthetic aromatic polyimide flame resistant film for absorbing heat energy. The flame resistant shield can face the component side of the printed circuit board. The flame resistant shield can be a laminate structure and can include an inner layer, an outer layer and an intermediate layer between the inner layer and outer layer. The inner layer and the outer layer can be a polyamide sheet such as NOMEX® calendered sheet and the intermediate layer can be a polyimide film such as KAPTON®, or the intermediate layer can include aluminum sheet metal.
The present invention also features a printed circuit card assembly for preventing flame spread in an equipment assembly including a printed circuit board having first and second sides, at least one of the first and second sides of the printed circuit board adapted for mounting a plurality of electronic components thereon. A first flame resistant shield faces the first side of the printed circuit board and a second flame resistant shield faces the second side of the printed circuit board, the first and second flame resistant shields absorbing heat energy from an adjacent print
Champion, Jr. Edward R.
Niepmann Mark Jeffrey
Wong Eric Hoyt
Gandhi Jayprakash N.
Nortel Networks Limited
Vigushin John B.
LandOfFree
Method and apparatus for preventing flamespread in an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for preventing flamespread in an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for preventing flamespread in an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2601423