Semiconductor device manufacturing: process – Repair or restoration
Reexamination Certificate
2001-08-23
2002-08-06
Whitehead, Jr., Carl (Department: 2822)
Semiconductor device manufacturing: process
Repair or restoration
C438S015000, C438S112000, C438S127000
Reexamination Certificate
active
06429028
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of semiconductors and, more particularly, to a method and associated apparatus for removing semiconductor chips (die) from a plastic encapsulated package and reassembling the removed die in a high reliability hermetic package.
2. Description of Related Art
Semiconductor devices in the form of integrated circuits are used in virtually all electronic devices. A typical integrated circuit includes a die formed of silicon with transistors and other active and passive devices formed on the silicon. The integrated circuit is typically included in an integrated circuit package. The package typically provides for the die electrical interconnection, mechanical support, heat dissipation, and protection. Most integrated circuit packages include a leadframe which carries the external connection pins, wire bonds between the leadframe and the die, and an encapsulant. The encapsulant is typically a plastic which is molded around the other components and cured during manufacturing to thereby protect the encapsulated components from the ambient environment. Ceramics are used in some applications as the encapsulating material, but are more expensive and require higher energy processing as compared to plastics.
For new system designs, system engineers are often faced with using non-hermetic commercial temperature plastic encapsulated microcircuits in assemblies and environments for which they were not designed. This can create problems not previously encountered when high reliability devices were available. For example, a plastic device may outgas in the vacuum of space or absorb moisture in a high humidity environment. A commercial grade part may not function as designed when operating between the military temperature ranges of −55° to +125° C.
Even worse, a commercial-off-the-shelf device obsolescence issue may involve an expensive system redesign if a suitable alternative part is not a direct replacement in fit, form, and function. Additional screening and testing may also be necessary to achieve the desired level of confidence in the devices' long term reliability.
One solution to this growing problem is to procure the bare die, perform custom packaging, and test the devices to meet outer space or military requirements. However, many times the die is not available, and fabricating new wafers may be quite costly, especially when small quantities are desired. The present invention is a process for de-encapsulating plastic encapsulated microcircuits that provides a much needed solution to this problem. This process uses plastic encapsulated microcircuits as a source of die when the bare die is not available.
The concept behind the present invention is to remove the active circuit in die form from an already existing plastic encapsulated device. The die can then be reassembled into a high reliability hermetic package (such as ceramic or the like). To the military user, this provides the advantages of a larger source of die while providing the final product in the desired package and reliability level. This same process can also be used to re-encapsulate a recovered die into a different package configuration, thus providing much needed flexibility in achieving fit, form, and function.
The related art is represented by the following patents of interest.
U.S. Pat. No. 3,969,813, issued on Jul. 20, 1976 to Richard H. Minetti et al., describes a method and apparatus for removal of defective semiconductor chips from hybrid integrated circuits. U.S. Pat. No. 4,012,832, issued on Mar. 22, 1977 to John R. Crane, describes a method and apparatus for removing integrated circuit devices from their circuit environment in a manner which does not destroy or affect the circuit environment or the device being removed.
U.S. Pat. No. 5,424,254, issued on Jun. 13, 1995 to Pascale Damiot, describes a method for recovering bare semiconductor chips from plastic packaged modules which is not detrimental to the chip functionality and testability. U.S. Pat. No. 5,436,202, issued on Jul. 25, 1995 to Shinya Miura, describes a sealing method and apparatus which can hermetically seal a semiconductor package with excellent gas tightness.
U.S. Pat. No. 5,696,033, issued on Dec. 9, 1997 to Larry D. Kinsman, describes a method for packaging semiconductor dies. U.S. Pat. No. 5,700,697, issued on Dec. 23, 1997 to Joseph J. Diugokecki, describes a reconstructed plastic or other package, and a method of reconfiguring any prefabricated integrated circuit package (with or without an existing silicon chip and wires inside) so that an integrated circuit die can be installed and interconnected for normal use.
European Patent No. 413,451, published on Feb. 20, 1991, describes a semiconductor device which takes less volume than known packages and preferably can provide improved heat dissipation compared to the prior art. European Patent No. 548,603, published on Jun. 30, 1993, describes a method for replacing a semiconductor chip bonded face down to a substrate and encapsulated by a resin.
An article entitled ELECTRONIC CIRCUITS—PRESERVING TECHNIQUE FOR DECAPSULATING PLASTIC PACKAGES, published in November 1987 on pages 446 and 447 of IBM Technical Disclosure Bulletin, Volume 30, Number 6, describes a method for replacing a semiconductor chip bonded face down to a substrate and encapsulated by a resin.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.
SUMMARY OF THE INVENTION
The present invention is a method and associated apparatus in which a semiconductor chip (die) is removed from a plastic encapsulated package and reassembled into a high reliability hermetic package. The process is used to remove an already existing die using a unique disassembly and etching process and make the removed die more reliable by reattaching the die and rebonding the die wires into either a hermetic package or a different type of package. Also, the die can be used in other applications with different pin-outs or configurations.
The process begins by preparing an integrated circuit package for completely removing the die from the package. Such an integrated circuit package preferably includes a plastic encapsulant having a topside and a backside surrounding a leadframe carrying a bottom die-mount heatsink (or “paddle”), external pins, an integrated circuit die, and gold bondwires extending between the leadframe posts and the aluminum die bondpads. The integrated circuit package is prepared for a backside grind procedure by first removing the external leads. This can be done by cutting or grinding. The backside of the integrated circuit package plastic encapsulant is subsequently mechanically milled or ground away from backside of the device until the copper heatsink/paddle begins to appear. The remaining copper paddle from the backside of the die is then completely chemically dissolved by immersing the integrated circuit assembly in a beaker with 70% nitric acid and partially immersing the beaker in an ultrasonic agitation bath which greatly enhances the etching process. Any conventional ultrasonic cleaner can be used for this process. This process can be further accelerated with heat.
A backside selective chemical etch is then performed to remove die-attach medium/residue on the back surface of the die. Some die-attach adhesive or residue may remain after the copper is dissolved from the backside of the die. A backside etch, using a heated jet-stream of either 90% red fuming nitric acid or 90% yellow fuming nitric acid, for approximately ten seconds may be used to help clean the backside of the die. If a backside etch is used, care should be taken not to over-etch and thus undercut to the topside of the die surface and thereby expose the die bondpads to acid longer than necessary.
The device is then inverted and the topside of the plastic encapsulant is subjected to a localized etch at the die area, using a heated (+90° C.) jet-stream of either 90% red fumi
Bivins Gary
Ditto William S.
Lam Huong Kim
McDaniel Scott
Young Douglas
DPA Labs, Incorporated
Jr. Carl Whitehead
Litman Richard C.
Vockrodt Jeff
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