Electric heating – Heating devices – Combined with diverse-type art device
Reexamination Certificate
2001-05-18
2002-12-10
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Combined with diverse-type art device
C361S782000, C347S210000
Reexamination Certificate
active
06492620
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to reworkable electronic semiconductor components, including multi-chip modules (“MCMs”), that incorporate electrical heaters integrally within the component structure to produce the heat necessary to soften or weaken the bond of the component to the printed wiring board to which the component is attach, allowing removal of the component from a printed wiring board for rework. More particularly, the invention relates to a new heater structure for the electronic semiconductor component that is fault tolerant to current-interrupting breaks as may be formed or produced in any of the heaters. The invention is applicable to substrate-to-printed wiring board attachments that employ adhesive bonds, such as found in the thermoset adhesive lead type components, or that employ reflow solder bonds, such as found in ball grid array lead-less type components.
BACKGROUND
The present invention improves upon the invention of Berkely et al presented in U.S. Pat. No. 6,031,729, granted Feb. 29, 2000 entitled “Integral Heater for Reworking MCMS and Other Semiconductor Components” (hereafter the “Berkely et al '729 patent”) assigned to TRW Inc., the assignee of the present invention. In a broader aspect, the invention improves upon electrical heater systems as may be applied in other ways than presented in the foregoing patent by incorporating circuits that provide fault tolerance to current-interrupting breaks in the electric heaters of an electric heater system for an electronic component that avoids disruption of heating.
A principal application of the present invention is with reworkable Multi-Chip Modules, such as described in the cited Berkley et al '729 patent. Multi-Chip Modules (“MCMs”) perform a variety of electronic functions, and are finding increasing use in sophisticated electronic applications, particularly airborne and space-borne application. By definition, an MCM contains two or more semiconductor die or chips, as variously termed, and ancillary electrical components, assembled in a single enclosed package, that together comprise an electronic circuit function. The semiconductor chips contain the micro-miniature integrated circuits, such as processors, amplifiers, memory, and the like.
In one type of MCM structure, the semiconductor chips and components are supported on a common base, consisting of an integral multi-layer printed wiring structure, referred to as the substrate. Often that substrate is formed of ceramic, an electrical insulator that is rigid, allows for plated-on conductors of the finest widths and spacing with the greatest accuracy and is able to maintain a hermetic seal. Metallic conductors printed on various layers of the substrate, and metallic vias through the layers, serve to electrically connect the semiconductor chips to each other and to the external interfaces of the MCMs.
The foregoing elements are contained together in a single enclosed four-sided package, often hermetically sealed, that serves as a protective housing for the semiconductor chips and associated components. The ceramic substrate, being hermetic, serves as the bottom wall to the module. A metal wall, or seal ring, is brazed to the substrate around the perimeter, encompassing the components and a lid welded to the top surface of this seal ring hermetically seals the components inside. A number of electrical contacts or leads extend out the four sides of the MCM to provide external electrical input-output connections to the MCM.
In practice MCMs are generally installed upon a printed wiring board, much larger in area than an MCM, that contains the electrical interconnections between the MCMs and other components thereon. The larger wiring board is typically constructed of a material such as glass-epoxy or glass-polyimide, a less expensive and lower quality material than the ceramic of the substrate. For airborne and space applications, MCMs are typically bonded to the printed wiring boards. Bonding enhances thermal conductivity to the MCM, and isolates mechanical loads from the input-output connections of the MCM, which promotes longer product life. A variety of adhesives, such as thermosetting epoxies or thermoplastics, and solder are available to provide the bonding.
To bond the MCM in place, as example, a layer of thermally sensitive adhesive is applied to either the underside surface of the MCM, or directly to the surface of the printed wiring board at the location to which that component is to be placed. With the MCMs and all other components for that circuit board properly positioned, the board is then placed in an oven and the temperature raised to cure or reflow the adhesive, attaching the MCMs and other components in place. When removed from the oven and cooled down to room temperature the MCMs are firmly attached to the printed wiring board.
Solder is another known thermally sensitive adhesive material used to fasten parts together. A second known technique for fastening the MCM to the circuit board is the solder ball grid array. Instead of incorporating electrical leads extending from the side of the MCM package and using a separate adhesive for fastening the MCM to the circuit board, as in the foregoing structure, the electrical leads are instead formed by electrical vias extending through the multiple layers of substrate to the underside surface of the MCM package. At the underside the terminal end of those vias typically appear by design arranged in regular rows and columns. Minute solder balls or solder columns, different geometry's for the dab of solder collectively referred to herein as solder balls, are formed at the terminal ends of those vias on the underside of the substrate.
In assembly, the MCM package is placed upon the printed wiring board, the latter of which contains solder pads that mate with the solder balls on the MCM package and the temperature is raised above the solder eutectic at which the solder reflows. When cooled, the solder solidifies and provides a firm mechanical connection that fastens the MCM package to the printed wiring board as well as completing the electrical connections to printed circuitry on that wiring board. The foregoing connection apparatus and technique is well known.
If failed components were detected during subsequent electrical testing of the assembled board, the failed components needed to be removed from the printed wiring board for repair or replacement. The problem in reworking MCM's, whether fastened to the circuit board by regular adhesives or with a solder ball grid array, is recognized as endemic to other large size electronic semiconductor components as well, even those that contain only a single physically large semiconductor chip. As those skilled in the art recognize, the more modern semiconductor chips are growing in physical size as more and more circuit functions are expected to be packed within a single die even in commercial devices, such as cellular telephones. As a consequence large numbers of very fine closely spaced wires are required to interface to the semiconductor die. Because the wires must all extend into the die they are necessarily physically small in width and must be packed closely together, typically one mil in diameter separated by a two mil space. However, conventional printed circuit board technology typically provides semiconductor die interface connections with no less than a four mil separation.
To resolve the apparent physical incompatibility in spacing requirements, the approach taken has been to mount the semiconductor chip onto an intermediate “interposer” substrate, which is often formed of ceramic material. The printed wiring formed on the substrate fans out from the microscopic spacing at the location of the semiconductor die or chip to the wider spacing and wider wiring required by the conventional printed circuit board.
That electronic semiconductor assembly is then mounted onto the printed circuit board. The electrical leads from the assembly substrate are soldered to the mating solder pads on the printed circuit board,
Fastovsky Leonid M
Goldman Ronald M.
TRW Inc.
Walberg Teresa
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