Metal fusion bonding – Process – Plural joints
Reexamination Certificate
1998-08-19
2001-03-20
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S006100, C228S006200, C228S180100
Reexamination Certificate
active
06202917
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electronic assemblies and, more particularly, to a method of assembling components to circuit boards.
A common approach to the organization of electronic systems is to divide the system into a number of subsystems, and to place each subsystem onto a circuit card. For example, in a computer the central processor and its associated circuitry may be placed onto one card, the display graphics drivers may be placed onto another card, the driver for each input/output device may be on an individual card, and power-related functions may be on another card. On each of the cards, components are affixed to the surface of the card, and linked by wires or electrically conductive paths termed traces. The cards are linked by wires or by plug-in connectors. This architecture provides an organization which permits individual functions to be repaired, added, or removed easily.
To fabricate a system having this type of architecture, the components are individually fabricated, and the cards are separately fabricated with the appropriate connectors and traces. The components are attached to the card, either semipermanently with mechanical attachments or permanently with adhesives. Lastly, any permanent electrical connections are made by soldering. When the electronic system is to be used in applications which must remain reliable for extended periods and/or are subjected to mechanical loadings such as shocks, it is preferred to make all attachments permanent in nature, with the components adhesively bonded to their circuit boards and the electrical interconnects permanently soldered.
Thousands of circuit boards most be fabricated for use in complex systems such as communications satellites. In the conventional practice, the components are adhesively attached to the circuit cards, and this subassembly is cured. Electrical connections are thereafter made by soldering using pre-tinned bonding pads on the circuit card. This process is time-consuming and adds significantly to the cost of the electronic systems.
There is a need for an improved approach to the assembly stage of the fabrication of electronic circuit cards. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
This invention provides a method of assembling electronic circuit cards from subcomponents, and for accomplishing the electrical interconnection of the subcomponents. This approach substantially reduces the time for assembly and the use of expensive adhesive curing ovens, while producing acceptable quality of the final product.
In accordance with the invention, a method of fabricating an electronic assembly comprises the steps of furnishing a support having an electrical bonding pad thereon, furnishing metallic solder having a solder temperature/time reflow cycle, and tinning the solder to the electrical bonding pad of the support. The method further includes furnishing an electronic component having an electrical contact thereon, furnishing an adhesive having an adhesive temperature/time curing cycle permitting the adhesive to cure during the solder temperature/time reflow cycle, applying the adhesive to at least one of the component and a bonding location on the support to which the component is to be attached, and contacting the component to the bonding location and the electrical contact to the electrical bonding pad. Thereafter, the component and the support are heated to an elevated temperature following the solder temperature/time reflow cycle so that, in a single heating cycle, the adhesive is cured and the solder is simultaneously reflowed.
The selected adhesive must be properly curable during the processing used in the solder temperature/time reflow cycle. It must also not be damaged by the temperatures reached in the solder temperature/time reflow cycle, which are greater than those which are suitable for many adhesives. The adhesive must also exhibit the required mechanical and physical properties, thermal conductivity, and environmental properties for use in the electronic assembly. In the preferred approach, the solder is lead-tin eutectic-composition solder, and the adhesive is a mineral-filled (preferably aluminum oxide-filled) polyurethane produced from a polyol/carbonyl oligimer precursor.
The present approach achieves a simultaneous curing of the adhesive, to complete the mechanical connection of the components to the circuit board, and reflow of the solder, to complete the electrical connection of the components on the circuit board. In prior practice, the adhesive was first cured in a curing cycle that typically required two hours in a curing oven. The electrical interconnections were then individually soldered, a time-consuming operation. The present approach combines these two operations to require only a few minutes during the reflow cycle. Initial testing indicates that the quality of the finished product made by the present approach is at least as good as that achieved with the prior approach.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 4788767 (1988-12-01), Desai et al.
patent: 4951400 (1990-08-01), Elliott et al.
patent: 5385966 (1995-01-01), Hermansen et al.
patent: 5856022 (1999-01-01), McCormick et al.
patent: 5861678 (1999-01-01), Schrock
patent: 5985694 (1999-11-01), Cho
Long Lynn E.
Martinez Thomas
Shinaver Eric J.
Weaver Mark A.
Gudmestad T.
Hughes Electronics Corporation
Pittman Zidia T.
Ryan Patrick
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