Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With housing mount
Reexamination Certificate
1999-02-17
2001-05-08
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With housing mount
C257S704000, C257S710000, C257S730000, C257S731000
Reexamination Certificate
active
06229208
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to large hermetically sealed electronic packages and, more particularly, to an improved lid construction for such packages.
BACKGROUND
Two or more semiconductor chips and associated circuits may be assembled together in a single package, referred to as a hybrid or multi-chip module (MCM). The package serves as a protective container for the semiconductor chips, ancillary electrical components, and associated electronic circuits, that are typically supported on a common base. The base generally supports, or consists directly of, a printed circuit element which includes the electrical interconnection paths for the various components and associated connections to the outside of the package.
In order to provide an hermetic seal for the package a lid is generally attached by an appropriate metallurgical bond around the perimeter such that the various components are enclosed within. A seal ring is typically incorporated in the package base to facilitate forming the metallugical bond to the lid. This seal ring may take various forms such as metal side walls on a metal package, a metallized ring surrounding the enclosed component area on a ceramic package, or a separate raised metal ring metallurgically bonded to a ceramic package.
A desirable feature of MCM packages is that they be “reworkable”, in the event one or more of the semiconductor devices in the MCM fail, and require replacement. Whereas an individually packaged semiconductor chip cannot be repaired and if it fails is simply discarded, it is not economical to discard an entire MCM should one of its many semiconductor chips fail. Thus it is desirable that the MCM package be sealed in such a manner that it is possible to remove the lid, remove and replace the failed device(s), and thereafter replace the lid and reseal the package.
In certain instances benefits such as size, weight, and power reductions, and improved electrical performance of an electronic device may be realized by increasing the overall size of MCM packages to envelope a greater number of circuit elements, elements that would otherwise be housed in separate packages. As the length or width dimensions of the associated large MCM lids are driven beyond about 2 inches, forces on the package lids can become problematic.
Significant forces on large hermetic MCM lids arise as a result of differential pressure between the inside and outside of the package. As example, such pressure differential occurs during post-seal leak testing when an external overpressure is used to drive a detectable leak tagging gas into the package and also occurs later when high altitude or space applications reduce the external ambient pressure to levels far below the gas pressure entrapped within the package's interior.
Such large forces tend to deflect the lid inward, possibly contacting and damaging enclosed components, or outward, where undesirable contact may occur with structure or other parts located in close proximity to the package. In addition to the possibility of such undesirable physical contact, the deflection creates large forces on the metallurgical bond between the lid and seal ring, enough to exceed elastic stress limits, with an associated tendency to lose hermeticity.
It may be shown by approximating a section of the lid with beam equations that lid deflection is proportional to PL
4
/t
3
, where P is differential pressure, L is the unsupported beam length, and t is the beam thickness. From this it may be appreciated that lid deflection for a 2.5 inch span is approximately 2.6 times that of a 2.0 inch span. It may also be recognized that to minimize deflection simply by increasing lid thickness, the lid thickness must increase geometrically relative to the increase in span. For large MCMs, this leads to prohibitively heavy lids, especially for avionics and spacecraft applications.
Other solutions have been offered to protect internal components from inward deflection, and the seal from large stresses, but each solution poses an offsetting disadvantage. Internal support posts or other intermediate internal supports, upstanding from the base, have been used to buttress and strengthen the lid. However, those posts and other supports require space within the package reducing the area available for component placement and routing. And with reduced space, another package might be required to house a complete electronic circuit.
Others have recently identified and publicized the foregoing problems in an article entitled “Lid Deflection for MCMs in Airborne Application”, Huey et al., ICEMCM '96 Proceedings of the 1996 EHM Conference on Multichip Modules, Apr. 7, 1996, pp. 167-172, which the reader is invited to review. In that article, the authors recognize the foregoing solutions and explore the disadvantages of those solutions at greater length than here. The solution the article's authors think best is to hermetically seal the MCM at a lower gas pressure of one-half atmosphere, instead of the normal one atmosphere pressure. The present invention also seeks to ensure continued integrity of the lid in such high pressure differential situations and offers a different and better solution than proposed in that article.
With hindsight, one seeking cure to the pressure differential based deflection problem might first briefly think to make reference for same to the individual semiconductor packaging art, that is, the packaging semiconductors individually, and quickly dismiss the thought.
As is well recognized, a semiconductor chip is a physically small device, perhaps no greater in size than one-quarter to one-half inch square. One such chip package is known as the ceramic side brazed dual in-line package. The package, that is the base, is formed of ceramic material, is of rectangular shape, a Kovar™ metal seal ring forms a rectangular wall of short height on the base enclosing the semiconductor, and two rows of metal pins are brazed to the edges of the base, providing terminals for connection to other electrical circuits. And means are included for connecting the installed semiconductor device's bonding pads to metal pathways incorporated into the package that connect to those metal pins. The semiconductor package is also hermetically sealed by welding or soldering a closure lid atop the seal ring. The closure lids for those packages are formed either of a compatible metal such as Kovar™, or of the same ceramic material that forms the package. Thus the package for the smaller individual chips are recognized to share a generic similarity with the physically much larger MCMs.
The reader is reminded that, notwithstanding the fact that the package seal ring and/or lid is formed of a metal alloy, MCM individual chip packages are generally referred to by technicians in the industry as ceramic packages, however imprecise that reference may be, so long as the package contains a base that is formed of ceramic material.
In looking to the individual chip packaging art, however, one is quickly reminded that reworkability is not a requirement for those semiconductor packages as it is for MCM. The devices cannot be reworked. If a semiconductor chip fails to perform, it is simply discarded and replaced by a like component. If it fails in operation in a circuit, the package is unsoldered from its place on the printed circuit board and is replaced with another like chip.
As stated by Byrne in U.S. Pat. No. 4,769,272, a patent that describes a ceramic package for an individual chip, and alluded to in his divisional U.S. Pat. No. 4,833,102, metal lids, such as nickel/iron alloy lids, can be “popped off” a ceramic package, viewed in the single chip package art as a disadvantage, but the ceramic lids cannot. To effect removal of a sealed ceramic lid, according to Byrne, the lid ordinarily must be fractured and removed in pieces. Hence, putting higher cost of the ceramic aside, the ceramic lid is believed more reliable than the Kovar™ for packaging individual semiconductor chips because it is permanent and cannot be reworked.
Were the l
Berkely Ryan S.
Massey Mary C.
VanLiew Steven F.
Chambliss Alonzo
Chaudhuri Olik
Goldman Ronald M.
TRW Inc.
Yatsko Michael S.
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