Abstract:
A process to remove a semiconductor die from a plastic package and then to reassemble the die in 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 all new die wires into either a hermetic package or a different type of package with a “bond-on-top-of-bond” technique. The original bondfoot on the removed die may be first preconditioned by a novel bond-flattening tool, which can be attached to the bond-head chuck of any wirebonder. Also, the die can be used in other applications with different pin-outs or configurations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/228,451, filed Aug. 29, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    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.  
           [0004]    2. Description of Related Art  
           [0005]    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.  
           [0006]    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.  
           [0007]    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&#39; long term reliability.  
           [0008]    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.  
           [0009]    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.  
           [0010]    The related art is represented by the following patents of interest.  
           [0011]    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.  
           [0012]    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.  
           [0013]    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.  
           [0014]    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.  
           [0015]    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.  
           [0016]    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  
         [0017]    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.  
           [0018]    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.  
           [0019]    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.  
           [0020]    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 fuming acid or 90% yellow fuming nitric acid for about one to three minutes, followed by a cold (+25° C.) jet-stream rinse of the same acid for about twenty seconds. A glass eye-dropper or pipette can be used to produce the directed jet-stream at the localized area of interest. 20% sulfuric acid may be added to the nitric acid for some types of plastics. The mix proportion and duration of the etch is determined by the compound and the thickness of the encapsulant. To reduce etch time, a topside grinding/milling operation may first be performed to thin down the encapsulation immediately above the die area prior to the localized acid etch. After the topside package etch is complete, there should be a cavity completely through the plastic package wherein the die is left hanging in place by only its bondwires connected to the leadframe posts. All edges of the die should be free of encapsulation. Immediately following the cold acid rinse, the suspended die is rinsed with a squirt of acetone, then the entire remaining integrated circuit is immersed in a beaker of acetone and subjected to ultrasonic agitation for about one minute.  
           [0021]    The die is finally cleaned to remove encapsulation residue by placing the device in a beaker of N-methyl-2-pyrrolidinone and subjecting the beaker to ultrasonic agitation for about ten to fifteen minutes. Subsequently, the die is rinsed with acetone.  
           [0022]    Removal of the gold bondwires is achieved by using a pair of tweezers to hold on to the mid-span of each wire and with a rotational motion, twist the wires in such a manner as to fatigue the wire at the “neckdown” area of the die ball-bond until the wire breaks off, leaving the ball-bond intact at the die-pad. Care must be taken to exert the least amount of vertical tension to the ball-bonds so as not to lift the balls off the delicate aluminum die-pads. Excessive vertical tension can result in lifting of the ball-bond and possibly tearing or lifting of the bondpad metallization. To speed up the wire removal process, all wires can be simultaneously pre-fatigued by holding the exposed die with tweezers by its edges and rotating the die both clockwise and counterclockwise repeatedly before twisting off the wires at the neckdown area. An alternate method to pre-fatigue the wires is to continuously hold the exposed die with tweezers by its edges while the rest of the integrated circuit assembly is immersed in an ultrasonic agitation bath for about thirty seconds. If this method is employed, the integrated circuit body should be in contact with the bottom of the beaker to minimize any vertical tension on the ball bonds while the wires are being pre-fatigued in the horizontal direction by the ultrasonic agitation. The complete removal of the die from the plastic package thus is achieved without inducing any electrostatic discharge, chemical, or mechanical damage.  
           [0023]    Once the die has been removed from the plastic encapsulation the die is inserted into a hermetic package or a different type of package and reattached to the new package by first re-attaching the die to the floor of the package and then rebonding new wires between the removed die and the new package. The re-attaching of the die may be accomplished by the use of conductive or non-conductive epoxy, silver glass epoxy, or eutectic attach methods. The re-connection of the wires between the die and the new package is achieved by bonding new thermosonic/thermocompression ball-bonds or ultrasonic wedge bonds on top of the original gold ball-bonds left at the bondpads of the die. This “bond-on-top-of-bond” technique can be either performed on an automated bonder or a manual wire bonder by readjusting the power, time, and force to obtain the maximum pull-strength without damaging the existing old ball-bonds or aluminum bondpads on the die. In order to facilitate the bonding targeting process and to increase adhesion, the original ball-bond on the die may be first flattened to increase the diameter of the ball, making it easier to target the new bond. A novel bondtool without a capillary may be used for this process. The ball-flattening process should be performed at a temperature of +175° C. to optimize malleability of the gold and to minimize the effects of Kirkendall voiding.  
           [0024]    Accordingly, it is a principal object of the invention to provide a process to remove a die from a plastic package.  
           [0025]    It is another object of the invention to provide a process to remove a die from a plastic package and then to reassemble the die in a high reliability hermetic package.  
           [0026]    It is further object of the invention to provide a process to remove all the wires from an exposed die in an integrated circuit without removing the bondfoot (gold ball) at any of the diepads. The removal of the wire without removing the bondfoot leaves the aluminum bondpad intact and undamaged and thus available for rebonding.  
           [0027]    Still another object of the invention is to provide a process to reconnect some or all of the pads of a removed die to the new package by a “bond-on-top-of-bond” technique utilizing the original bondfoot as the new “pad” upon which to bond a new wire.  
           [0028]    Another object of the invention to provide a process to flatten the original bondfoot with a new tool to increase the targeting area for the new bonds and to increase the adhesion between the new ball bond and the old ball bond. The flattened bondfoot, due to its increased bonding area and flatness, serves to both increase the adhesion between itself and the aluminum bondpad beneath as well as with the new ball bond above.  
           [0029]    Another object of the invention to provide a new bond-flattening tool made of ceramic or metal which can be inserted into capillary chucks of existing automatic or manual wirebonders for precision targeting and manual/automatic operation.  
           [0030]    A further object of the invention to provide a process to remove a die from a plastic package without inducing any electrostatic discharge, chemical, or mechanical damage.  
           [0031]    It is an object of the invention to provide improved elements and arrangements thereof in a process to remove semiconductor chips from a plastic package for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes. 
       
    
    
       [0032]    These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1A is a cross-sectional view of a semiconductor chip encapsulated in a plastic package.  
         [0034]    [0034]FIG. 1B is a cross-sectional view of a plastic encapsulated semiconductor chip wherein the back layer of the plastic package has been ground away exposing the bottom copper die mounting paddle according to the invention.  
         [0035]    [0035]FIG. 1C is a cross-sectional view of a plastic encapsulated semiconductor chip wherein the copper die mounting paddle has been chemically dissolved according to the invention.  
         [0036]    [0036]FIG. 1D is a cross-sectional view of a plastic encapsulated semiconductor chip wherein the top layer of the plastic package has been removed, leaving the die suspended only by its bondwires according to the invention.  
         [0037]    [0037]FIG. 1E is a cross-sectional view of a semiconductor chip wherein all its wires have been removed, leaving only the ball bonds on the diepads.  
         [0038]    [0038]FIG. 2A is a fragmented end view of a semiconductor die, bondwire, gold ball bond, and aluminum bondpad immediately prior to removal of bondwires according to the first example of the invention.  
         [0039]    [0039]FIG. 2B is a fragmented end view of a semiconductor die, and gold bondfoot, and aluminum bondpad immediately following removal of bondwires according to the first example the invention.  
         [0040]    [0040]FIG. 2C is a fragmented end view of a semiconductor die with “bond-on-top-of-bond” after re-attaching new wires in a new package according to the first example of the invention.  
         [0041]    [0041]FIG. 3A is a fragmented end view of a semiconductor die, bondwire, gold ball bond, and aluminum bondpad immediately prior to removal of bondwires according to a second example of the invention.  
         [0042]    [0042]FIG. 3B is a fragmented end view of a semiconductor die, and gold bondfoot, and aluminum bondpad immediately prior to flattening of the bondfoot according to the second example of the invention.  
         [0043]    [0043]FIG. 3C is a fragmented end view of a flattened bondfoot after being flattened with a bond-head tool according to the second example of the invention.  
         [0044]    [0044]FIG. 3D is a fragmented end view of a semiconductor die with “bond-on-top-of-bond” after re-attaching new wires in a new package according to the second example of the invention.  
         [0045]    Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0046]    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 invention disclosed herein is, of course, susceptible of embodiment in many different forms. Shown in the drawings and described hereinbelow in detail is a preferred embodiment of the invention. It is to be understood, however, that the present disclosure is an exemplification of the principles of the invention and does not limit the invention to the illustrated embodiment.  
         [0047]    The users of the product obtained using this process are those who require electronics in high reliability applications (i.e. military, space, etc.). Presently, many of these semiconductor chips only exist in encapsulated plastic packages, however, these available plastic encapsulated devices may not have the reliability or fit-form-function necessary for every end user&#39;s particular application (i.e. temperature, high humidity, corrosive atmosphere, mechanical stress, different pin-out, configuration, etc.). Although the present immediate need is for the removed die to be used in high reliability applications and hermetic packages, the die which has been removed by this process can also go into low reliability, non-hermetic packages or applications.  
         [0048]    As shown in FIG. 1A, an integrated circuit package  10  is shown which includes a plastic encapsulant having a topside  12  and a backside  14  surrounding a leadframe carrying external pins  28 , a copper die-mount heatsink (or “paddle”)  16 , an integrated circuit die  20 , and bond wires  26  extending between the leadframe posts and the aluminum bondpads  24  on the die  20 , as would be readily appreciated by those skilled in the art. The entire leadframe base material is usually made of nickel-plated copper for good heat and electrical conduction. The bondwires  26  (generally 1.0 millimeters in diameter and 99.9% gold) are usually made with thermosonic or thermocompression ball bonds  24  on the die pads  22  at one end and a crescent bond on the leadframe posts at the other end. The integrated circuit package  10  is prepared for a backside grind procedure by first removing the pins  28  external to the package. This can be done by cutting or grinding.  
         [0049]    Referring to Fig. 1B, the backside  14  of the integrated circuit package plastic encapsulant  10  is subsequently mechanically milled or ground away from back  14  of the device  10  until the copper of the die mount heatsink/paddle  16  begins to appear. The remaining copper paddle  16  from the backside of the die  20  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.  
         [0050]    Referring to FIG. 1C, a backside selective chemical etch is then performed to remove die-attach medium/residue  18  on the back. Some die-attach residue  18  may remain after the copper die-mount paddle is dissolved from the backside of die  20 . 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  20 . 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 bond pads  22  to acid longer than necessary.  
         [0051]    The topside plastic  12  of the device  10  is then pre-thinned down by grinding or milling. The device is then inverted and, as shown in FIG. 1D, 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 fuming nitric acid or 90% yellow fuming nitric acid for about one to three minutes, followed by a cold (+25° C.) jet-stream rinse of the same acid for about twenty seconds. A glass eye-dropper or pipette can be used to produce the directed jet-stream at the localized area of interest. 20% sulfuric acid may be added to the nitric acid for some types of plastics. The mix proportion and duration of the etch is determined by the compound and the thickness of the encapsulant. To reduce etch time, a topside grinding/milling operation may first be performed to thin down the encapsulation immediately above the die area prior to the localized acid etch. After the topside package etch is complete, there should be a cavity completely through the plastic package wherein the die  20  is left hanging in place by only its bondwires  26  connected to the leadframe posts. All edges of the die  20  should be free of encapsulation. Immediately following the cold acid rinse, the suspended die is rinsed with a squirt of acetone. The entire remaining integrated circuit is then immersed in a beaker of acetone and subjected to ultrasonic agitation for about one minute. The die  20  is finally cleaned to remove encapsulation residue by placing the device in a beaker of N-methyl-2-pyrrolidinone and subjecting the beaker to ultrasonic agitation for about ten to fifteen minutes. Subsequently, the die is rinsed with acetone. Thereafter the bondwires  26  are severed, as described below, leaving the die  20  as shown in FIG. 1E.  
         [0052]    Referring to FIGS. 2A and 2B, removal of the gold bondwires  26  is achieved by using a pair of tweezers to grab hold of the mid-span of each wire and with a rotational motion, twist the wires in such a manner as to fatigue the wire at the “neckdown” area of the die ball bond  24  until the wire  26  breaks off, leaving the ball bond  24  intact at the die-pad  22 . Care must be taken to exert the least amount of vertical tension to the ball bonds  24  so as not to lift the balls off the delicate aluminum die-pads  22 . Excessive vertical tension can result in lifting of the ball bond  24  and possibly tearing or lifting of the bondpad metallization. To speed up the wire removal process, all wires  26  can be simultaneously pre-fatigued by holding the exposed die  20  with tweezers by its edges and rotating the die  20  both clockwise and counterclockwise repeatedly before twisting off the wires  26  at the neckdown area. An alternate method to pre-fatigue the wires  26  is to continuously hold the exposed die  20  with tweezers by its edges while the rest of the integrated circuit assembly is immersed in an ultrasonic agitation bath for about thirty seconds. If this method is employed, the integrated circuit body should be in contact with the bottom of the beaker to minimize any vertical tension on the ball bonds  24  while the wires  26  are being pre-fatigued in the horizontal direction by the ultrasonic agitation. The complete removal of the die  20  from the plastic package, as shown in FIG.  1 E, thus is achieved without inducing any electrostatic discharge, or chemical or mechanical damage.  
         [0053]    Once the die  20  has been removed from the plastic encapsulation the die  20  is inserted into a hermetic package or a different type of package and reattached to the new package by first re-attaching the die  20  to the floor of the package and then rebonding new wires between the removed die  20  and the new package. The re-attaching of the die  20  may be accomplished by the use of conductive or non-conductive epoxy, silver glass epoxy, or eutectic attach methods. The re-connection of the wires between the die and the new package is achieved by bonding new thermosonic/thermocompression ball bonds  30  or ultrasonic wedge bonds on top of the original gold ball bonds  24  left at the bondpads  22  of the die  20 . This “bond-on-top-of-bond” technique can be either performed on an automated bonder or a manual wire bonder by readjusting the power, time, and force to obtain the maximum pull-strength without damaging the existing old ball bonds  24  or aluminum bondpads  22  on the die. As shown in FIGS. 2C and 3D, this rebonding is achieved by bonding the new bondwires  32  to the original ball bond  24 .  
         [0054]    Referring to FIGS.  3 A- 3 C, in order to facilitate the bonding targeting process and to increase adhesion, the original ball bond  24  on the die  20  may be first flattened to increase the diameter of the ball, making it easier to target the new bond. A novel bondtool  40  without a capillary may be used for this process. Such a bond-flattening tool is preferably made of ceramic or metal which can be inserted into capillary chucks of existing automatic or manual wirebonders for precision targeting and manual/automatic operation. The ball-flattening process should be performed at a temperature of +175° C. to optimize malleability of the gold and to minimize the effects of Kirkendall voiding.  
         [0055]    The invention provides the ability to remove a die  20  without damaging the delicate circuitry and reassembling the recovered die  20  into a different package than that available in plastic. The benefits of this process allow the end user to use an already encapsulated obsolete die and new technologies in a different type of package.  
         [0056]    It is to be understood that the present invention is not limited to the sole embodiment described above, but encompasses any and all embodiments within the scope of the following claims.