Electricity: conductors and insulators – Boxes and housings – Hermetic sealed envelope type
Reexamination Certificate
2000-06-30
2003-04-01
Ngo, Hung V. (Department: 2831)
Electricity: conductors and insulators
Boxes and housings
Hermetic sealed envelope type
C264S272110
Reexamination Certificate
active
06541701
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electronic circuit board manufacturing methods and apparatus. The invention relates more specifically to apparatus and methods for containing a hard-curing liquid material that is deposited over a circuit board after active components have been installed on the board.
BACKGROUND OF THE INVENTION
Electronic products such as circuit boards (“cards”) that are used in cryptographic computer equipment owned or operated by military and government agencies are required to conform to certain government standards. An example of such standards is the guidelines provided in Federal Information Processing Standards (FIPS) Publication 140-1. FIPS 140-1 defines four (4) increasingly stringent levels of security. One requirement of the FIPS 140-1 guidelines is that a manufacturer must cover a circuit board containing cryptographic circuitry with a hard opaque material that discourages tampering with, disabling or destroying the circuitry. The material also serves to show evidence of attempted tampering.
Covering circuit board components with such materials may also be desirable for reasons other than conformance to FIPS 140-1. For example, a board manufacturer may wish to preserve trade secret information by preventing competitors from determining the components and interconnections that are used in a board. Also, a manufacturer may wish to protect the components or prevent end users or consumers from tampering with or damaging the board or its components. To address any of these issues, in the manufacturing process a board may be covered with any of a variety of liquid materials that cure to a hard, non-conductive, opaque state. This process is known as “potting” the board, and the liquid is called a potting fluid.
A preferred material is an epoxy resin, which is poured over the circuit board in a liquid state. The epoxy then hardens, forming a solid, opaque barrier over and around the electronic components on the circuit board. The epoxy is electrically non-conductive, but is thermally conductive and aids in cooling of the components. Unfortunately, the potting process is difficult to accomplish when a circuit board has active components or other modules on both sides.
Accordingly, there is a need for an apparatus or method to consistently pot one or both sides of a board while minimizing the cost and complexity of the process. There is also a need for a method that can maintain a consistent minimal depth of epoxy over the tallest component on the board.
There is also a need to provide such an apparatus or method that does not result in an increase in the length or width of the circuit board.
One method used to solve these problems in the past is known as “dam and fill.” In this process, a first material is dispensed around the perimeter of the area to be coated. The first material hardens, and then acts as a dam or barrier to a second material, typically epoxy, that is used to fill the area within the first material. This process requires extra time, because the first material must cure before the potting material can be applied. Additional dispensing equipment and material is needed. Further, the finished product is not as “clean” looking. This method also is not well suited for applications where the depth of the epoxy may be significant.
Another alternative is to suspend the components to be epoxied in a cavity that is entirely filled. This method becomes complicated for boards that have holes or connectors that must not be covered. For example, there may be mounting holes that are used to affix the board in a larger system, or connectors that carry electrical signals from the board to other components of the system. Further, this method generally increases the overall size of the board.
Still another alternative potting method would be similar to injection molding. A mold could be created to completely contain the board and epoxy. The board would be placed inside the mold and the mold would be closed. Epoxy or other potting material would be introduced into the mold through gates or ports in the mold. When the potting material had cured, the mold would be opened and the potted board would be removed. However, this method would have numerous disadvantages. For example, special tooling would be required. Further, since hardened epoxy cannot be melted, keeping the gates or ports free of hardened epoxy would be extremely complicated. This might require flushing the gates or ports, or a mechanical design that somehow ensures that the gates or ports remain clean.
Based on the foregoing, there is a clear need in this field for an apparatus or method of containing hard-curing materials that are deposited on a board in a liquid state. There is a particular need for an apparatus or method to pot both sides of a board while minimizing the cost and complexity of the process, and that does not result in an increase in the length or width of the circuit board.
SUMMARY OF THE INVENTION
A fluid containment fence apparatus provides a border around the perimeter of an area to be potted on either or both sides of the board. First, or first and second (top and bottom), containment walls are secured in place using protrusions or interlocking elements integral to the walls such that no external fasteners are needed. The interlocking elements permit snug adhesion of the fence to the board even when the board has variations in thickness. The interlocking elements extend through and interlock through holes in the board. The size of the holes is minimized. As a result, a minimum amount of board space is occupied by the fence, and the overall size of the board is maintained.
According to one aspect of the invention, a potting fluid containment fence for a board having components to be encased in a potting fluid includes a first wall. The components to be encased are arranged on an upper face of the board, at least. The first wall has segments that form a closed first polygon and a protrusion for engaging a corresponding hole on the upper face of the board. The protrusion is on a lower face of the first wall. The first wall is formed of material that is rigid before the protrusion is engaged in the corresponding hole.
According to another aspect of the invention, a potting fluid containment fence is provided for a board which has components arranged on both an upper face and a lower face that are to be enveloped in a potting fluid. The fence includes a first wall having segments that form a closed first polygon. The first wall also has at least one element of a pair of interlocking elements. The element is disposed on a lower face of the first wall. The fence includes a second wall having segments that form a closed second polygon. The second wall has at least one complementary element of the pair of interlocking elements. The complementary element is disposed on an upper surface of the second wall. When the element engages the complementary element, space between the first wall and the second wall is adequate to interpose the board.
According to another aspect of the invention, a circuit card includes a containment fence. The circuit card includes a board having circuit components arranged on at least an upper face of the board to be encased in a cured potting fluid. The board also has a cavity on the upper face of the board. The fence includes a first wall with segments that form a closed first polygon providing containment for the cured potting fluid on the upper face of the board. The fence includes at least one protrusion for engaging the cavity in the board. The protrusion is disposed on a lower face of the first wall. The first wall is formed of material that is rigid before the protrusion is engaged in the cavity.
According to another aspect of the invention, a circuit card includes a fence with two walls. The circuit card includes a board having circuit components arranged on both an upper face and a lower face of the board, the components encased in a cured potting fluid. The board also includes a hole passing from the upper surface of the board
Eastman Scott
Loose Robert
Cisco Technology Inc.
Hickman Palermo & Truong & Becker LLP
Ngo Hung V.
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