Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-10-23
2001-10-23
Picard, Leo P. (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S707000, C361S715000, C361S719000, C361S728000, C257S687000, C174S050500, C174S050510, C174S050520, C174S050530
Reexamination Certificate
active
06307749
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to overmolded electronic modules that include circuit boards with surface-mount (SM) devices. More particularly, this invention relates to an overmolded electronic module in which a circuit board is not only protectively encased within an overmold material, but in which the surface-mount devices are underfilled with the overmold material.
BACKGROUND OF THE INVENTION
Circuit boards with semiconductor devices such as flip chips must often be protected from the environment in which the board is employed. One widely-practiced approach is to enclose a circuit board in an assembly composed of a pair of case halves that are secured together with fasteners to form an enclosure that supports and protects the circuit board within. Connectors attached to one of the case halves provide for electrical interconnection to the circuit board. The assembly typically includes a sealing material between the case halves to exclude moisture from the enclosure. The resulting package must then be tested for leakage to ensure the package was properly assembled and sealed.
Because the above-described assembly process is labor intensive, less complicated assembly processes are highly desirable. One solution is an overmolded assembly disclosed in U.S. patent application Ser. No. 09/081,498. The assembly includes a circuit board, a heatsink with pedestals that thermally contact one or more circuit components mounted to the circuit board, and an optional retainer that biases the components into contact with the heatsink pedestals. An overmolded body encases the circuit board and retainer such that, with the heatsink, the overmolded body forms a moisture-impermeable seal around the circuit board and its circuit components. The circuit board, heatsink and retainer define cavities into which the overmold material, such as a thermosetting epoxy, flows during molding to encase the circuit board.
While the above-noted overmolded electronic assembly provides an uncomplicated method for encasing a circuit board and its circuit devices in a protective enclosure, it does not provide or allow for simultaneously underfilling surface-mount devices, such as flip chips and ball grid array (BGA) packages that are physically and electrically connected to the board with numerous solder bump connections. Underfilling is well known for promoting the reliability of flip chips and BGAs on organic circuit boards, and entails completely filling the void between the component and the circuit board with a specially formulated underfill material, such as a thermosetting epoxy. The underfill process has conventionally entailed placement of the underfill material along the perimeter of the component so that capillary action draws the material beneath the component and completely encapsulates the solder bump connections. For optimum reliability, underfill process parameters must be carefully controlled so that voids are not present in the underfill material beneath the component, and to ensure that a uniform fillet is formed along the entire perimeter of the component.
Because of the above process demands, different materials have been employed to underfill surface-mount components and overmold circuit boards. For example, overmolded assemblies in accordance with U.S. patent application Ser. No. 09/081,498 have been manufactured with surface-mount components underfilled with a specially formulated underfill material, after which the circuit board is overmolded with a second and different material specially formulated for this purpose. Another restriction has been the conventional wisdom that, for reliability, the underfill material must not contact the backside of the die or flow between the die and any surface contacting its backside.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an overmolded electronic module and a method for forming the module. The method entails both overmolding a circuit board and underfilling one or more surface-mount circuit devices physically and electrically connected to the board with solder bump connections. The circuit board is supported on a heatsink that thermally contacts a surface of the device opposite the circuit board. The board may be enclosed on the heatsink with a retainer that includes means for biasing the circuit device against the heatsink. Finally, the circuit board is overmolded with a polymeric material to form an overmolded body that encases the circuit board and the circuit device with the heatsink. The overmold process is carried out so that the polymeric material completely underfills the circuit device and encapsulates its solder bump connections.
From the above, it can be seen that the present invention uses a single material to both overmold the circuit board and underfill its circuit devices. Therefore, the present invention is contrary to the conventional practice of using different materials to underfill a surface-mount device and overmold a circuit board, and is also contrary to the conventional wisdom that, for reliability, the material used to underfill a surface-mount device should not contact the backside of the device. An unexpected benefit of this invention is that the use of a single material having appropriate material properties, such as coefficient of thermal expansion (CTE) and shrinkage characteristics, to both overmold a circuit board and underfill its surface-mount circuit devices results in the circuit devices being in compression, such that the devices are mechanically locked to the board within the polymeric material and therefore less likely to experience solder joint failure.
In addition to the above, the present invention avoids the conventional practice of using individual case assemblies, fasteners and seals to form an electronic module. The electronic module of this invention can be manufactured on a completely automated final assembly line in which there is no assembly of seals to case halves and no fastening of case halves, and as a result no leak test requirement and reduced inventory of parts. Finally, the electronic module can exhibit improved heat transfer from the circuit devices, resistance to vibration and shock, and improved reliability due to the ability to more closely match the CTEs of the overmolding and circuit board materials.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
REFERENCES:
patent: 5311060 (1994-05-01), Rostoker et al.
patent: 5887435 (1999-03-01), Morton
patent: 5898224 (1999-04-01), Akram
patent: 5901044 (1999-05-01), Marro
patent: 6054337 (2000-04-01), Solberg
patent: 6180045 (2001-01-01), Brandenburg et al.
Brandenburg Scott David
Daanen Jeffery Ralph
Chervinsky Boris L.
Delphi Technologies Inc.
Funke Jimmy L.
Picard Leo P.
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