Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2000-03-27
2002-02-19
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S200000, C228S020100, C228S046000
Reexamination Certificate
active
06347734
ABSTRACT:
BACKGROUND OF THE INVENTION
A typical circuit board can include a variety of circuit board modules, i.e., components, such as integrated circuits (ICs), capacitors, resistors, connectors, and so on. These circuit board modules typically connect with a multi-layered board formed of conductive and non-conductive circuit board material (e.g., copper and fiberglass, respectively).
Some circuit boards include complex modules which have multiple components mounted on a miniature circuit board, i.e., a small section of circuit board material. Such a module is often called a multi-chip module (MCM) because it typically includes multiple ICs, i.e., multiple “chips”. Some MCMs include components mounted exclusively on a top surface of the miniature circuit board (hereinafter called an “MCM board” to distinguish it from the main circuit board onto which the MCM mounts). Other MCMs include components mounted on both a top surface and a bottom surface of the MCM board. Typically, contact members (e.g., pins, pads, etc.) of the MCM components connect with corresponding contact members (e.g., vias) of the MCM board at solder joints.
In a similar manner, modules (MCMs and non-MCMs) typically connect with circuit boards at solder joints. Some modules mount to circuit boards using ball grid array (BGA) technology. Mounting a module to a circuit board using BGA technology involves using a grid of solder balls (i.e., beads of solder) between pads of the module and corresponding pads of the circuit board. Applied heat melts the solder balls to form solder joints between the pads of the module and the circuit board.
In general, large circuit board fabrication facilities manufacture circuit boards on a large scale and under tightly controlled environmental conditions (i.e., temperature, humidity, etc.) in a highly automated manner (e.g., using large scale computer controlled automated equipment). Accordingly, manufacturing yields at such facilities are generally consistent and high.
Nevertheless, on occasion, a manufactured circuit board may operate improperly. In some cases, such improper operation may be due to a faulty module, i.e., a defective or improperly mounted module. A computerized circuit board analyzer may be able to test and identify the module causing the failure. In such a situation, a technician may be able to “rework” the circuit board by removing the faulty module and replacing it with a new one. To this end, the technician removes the faulty module using an assembly rework station. A typical assembly rework station includes a heated gas source, a vacuum source and a special nozzle that is adapted to fit over the faulty module which is mounted to the circuit board. In general, the technician lowers the nozzle over the module, applies heated gas through the nozzle to melt solder connections holding the module to the circuit board, and applies a vacuum (typically through a pipe in the middle of the nozzle) to lift the module from the circuit board once the solder connections have melted.
After the technician removes the failed module from the circuit board using the assembly rework station, the technician typically cleans out the installation location of the circuit board (e.g., removes any remaining solder debris), loads the nozzle with a new module (e.g., fastens the new module within the nozzle using the vacuum), and positions the nozzle holding the new module on the cleaned installation location. The technician then applies heated gas through the nozzle to thoroughly heat the module such that solder on contact members of the module (and perhaps additional solder placed at the installation location) melts to form new solder connections with the circuit board.
The nozzles of some assembly rework stations are configured to apply heated gas to a module, and apply cooler gas (e.g., room temperature gas) exclusively around a periphery of the module. In particular, as a technician operates the nozzle of such an assembly rework station to remove a faulty module from a circuit board or to install a new module onto a circuit board, the nozzle of the assembly rework station applies the heated gas to all parts of the module to melt solder between the module and the circuit board, and the cooler gas around the outside edges of the module to prevent the solder connections of the neighboring circuit board modules from re-melting or re-flowing.
SUMMARY OF THE INVENTION
Unfortunately, conventional assembly rework stations, which apply heated gas to install new modules on circuit boards, do not adequately protect the new modules against heat-related damage. For example, when a technician installs a new multi-chip module (MCM), i.e., a module formed by multiple components soldered to a miniature circuit board (an MCM board), onto a main circuit board, the application of heated gas to the new MCM can melt solder connections between MCM components and the MCM board. In some cases, the reflowing of solder can form unreliable cold solder joints between the MCM components and the MCM board. In more extreme cases, the reflowing of solder can result in components falling off the MCM board. Even if the module being installed is not an MCM, the module can sustain damage to internal circuitry due to the extreme temperatures of the heated gas. Such damage can be particularly costly when the new modules have already undergone thorough manufacturing and testing procedures prior to their installation on circuit boards during reworking of the circuit boards.
In contrast, the present invention is directed to techniques for installing a module on a circuit board by simultaneously heating a perimeter portion of the module, and bringing an inner portion of the module to a temperature that is lower than that of the perimeter portion. Heating the perimeter portion of the module melts solder disposed between contact members of the module and corresponding contact members of the circuit board in order to form solder connections. Bringing the inner portion of the module to a temperature that is lower than that of the perimeter portion reduces the likelihood of causing heat-related damage to the module itself.
One arrangement of the invention is directed to a module installation system for installing a module on a circuit board. The module has a perimeter portion and an inner portion. The module installation system includes a heating source, a cooling source, and a nozzle coupled to the heating and cooling sources. The nozzle is configured to simultaneously heat the perimeter portion of the module, and cool the inner portion of the module in order to install the module on the circuit board. Accordingly, contact members around the perimeter portion of the module can form solder connections with the circuit board while cooling of the inner portion of the module protects the inner portion from heat-related damage. For example, if the module is an MCM, solder joints connecting MCM components to the MCM board at the inner portion will be less likely to reflow and cause a failure.
In a preferred arrangement, the heating source provides a first fluid (a gas or a liquid), and the cooling source provides a second fluid. As such, the nozzle applies the first fluid to an area adjacent the perimeter portion of the module, and the second fluid to an area adjacent the inner portion of the module. In one arrangement, the first and second fluids are gases, e.g., nitrogen, which the nozzle preferably applies at the substantially the same pressure (i.e., +/−10%), e.g., each at four pounds of pressure per square inch (psi). In another arrangement, the first fluid is a gas and the second fluid is a liquid (e.g., a gel).
In one arrangement, the nozzle includes a housing that (i) contacts the inner portion of the module and (ii) defines a chamber through which the second fluid is capable of passing. In this arrangement, the housing operates as a thermal mass (or thermal capacitor) to keep the inner portion of the module cooler as the perimeter portion of the module is heated.
In one arrangement, the housing further defines multiple
Chapin & Huang , L.L.C.
EMC Corporation
Huang David E.
Stoner Kiley
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