U.S. patent application number 09/934647 was filed with the patent office on 2002-04-25 for process to remove semiconductor chips from a plastic package.
Invention is credited to Bivins, Gary, Ditto, William S., Lam, Huong Kim, McDaniel, Scott, Young, Douglas, Young, Phillip.
Application Number | 20020048825 09/934647 |
Document ID | / |
Family ID | 26922389 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048825 |
Kind Code |
A1 |
Young, Phillip ; et
al. |
April 25, 2002 |
Process to remove semiconductor chips from a plastic package
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.
Inventors: |
Young, Phillip; (Simi
Valley, CA) ; Young, Douglas; (Simi Valley, CA)
; McDaniel, Scott; (Simi Valley, CA) ; Bivins,
Gary; (Tujunga, CA) ; Ditto, William S.;
(Chatsworth, CA) ; Lam, Huong Kim; (Simi Valley,
CA) |
Correspondence
Address: |
Richard C. Litman
LITMAN LAW OFFICES, LTD.
P.O. Box 15035
Arlington
VA
22215
US
|
Family ID: |
26922389 |
Appl. No.: |
09/934647 |
Filed: |
August 23, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60228451 |
Aug 29, 2000 |
|
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|
Current U.S.
Class: |
438/4 ;
257/E21.502 |
Current CPC
Class: |
H01L 2224/04042
20130101; H01L 2224/05624 20130101; H01L 2224/32245 20130101; H01L
2924/01322 20130101; H01L 2924/01006 20130101; H01L 24/05 20130101;
H01L 2224/48482 20130101; H01L 2224/85947 20130101; H01L 2924/01028
20130101; H01L 24/98 20130101; H01L 2224/85205 20130101; H01L
2924/01005 20130101; H01L 2224/48465 20130101; H01L 24/48 20130101;
H01L 2224/48624 20130101; H01L 2224/48247 20130101; H01L 2224/73265
20130101; H01L 2924/10253 20130101; H01L 21/56 20130101; H01L
2224/85207 20130101; H01L 24/85 20130101; H01L 2224/45015 20130101;
H01L 2924/01014 20130101; H01L 2924/01013 20130101; H01L 2924/01079
20130101; H01L 2924/20752 20130101; H01L 2924/01029 20130101; H01L
2224/45144 20130101; H01L 2924/14 20130101; H01L 2224/48475
20130101; H01L 2224/85203 20130101; H01L 2224/48091 20130101; H01L
2224/48227 20130101; H01L 2924/01047 20130101; H01L 2924/01075
20130101; H01L 2924/181 20130101; H01L 2224/85051 20130101; H01L
2924/00014 20130101; H01L 24/45 20130101; H01L 2224/45144 20130101;
H01L 2924/00014 20130101; H01L 2224/48091 20130101; H01L 2924/00014
20130101; H01L 2224/48475 20130101; H01L 2924/01079 20130101; H01L
2224/45144 20130101; H01L 2924/01203 20130101; H01L 2224/45015
20130101; H01L 2924/20752 20130101; H01L 2224/48482 20130101; H01L
2224/48465 20130101; H01L 2924/00014 20130101; H01L 2224/78
20130101; H01L 2224/48465 20130101; H01L 2224/48227 20130101; H01L
2924/00 20130101; H01L 2224/48465 20130101; H01L 2224/48247
20130101; H01L 2924/00 20130101; H01L 2224/48465 20130101; H01L
2224/48091 20130101; H01L 2924/00 20130101; H01L 2924/00 20130101;
H01L 2224/73265 20130101; H01L 2224/32245 20130101; H01L 2224/48247
20130101; H01L 2924/00 20130101; H01L 2224/73265 20130101; H01L
2224/32245 20130101; H01L 2224/48227 20130101; H01L 2924/00
20130101; H01L 2924/10253 20130101; H01L 2924/00 20130101; H01L
2224/48624 20130101; H01L 2924/00 20130101; H01L 2224/85207
20130101; H01L 2924/00 20130101; H01L 2224/85205 20130101; H01L
2924/00 20130101; H01L 2224/85203 20130101; H01L 2924/00 20130101;
H01L 2924/181 20130101; H01L 2924/00012 20130101; H01L 2224/48465
20130101; H01L 2224/48091 20130101; H01L 2924/00 20130101; H01L
2924/00014 20130101; H01L 2224/4554 20130101; H01L 2224/73265
20130101; H01L 2224/32245 20130101; H01L 2224/48247 20130101; H01L
2924/00012 20130101 |
Class at
Publication: |
438/4 |
International
Class: |
H01L 021/00 |
Claims
We claim:
1. A process for removing a die from an integrated circuit plastic
package, the package including a plastic encapsulant having a
topside and a backside surrounding a leadframe carrying external
pins, an integrated circuit die, a copper die-mount paddle, and
bondwires extending between the leadframe and bondpads on the die,
the process comprising the steps of: (a) removing the pins; (b)
grinding away the backside of the integrated circuit package
plastic encapsulant until the copper of the die-mount heatsink
paddle begins to appear; (c) removing any remaining copper paddle
from the backside of the die by placing the die in a beaker with
heated 70% nitric acid while agitating the beaker in an ultrasonic
bath; (d) performing a backside etch to remove any die-attach
residue from the die; (e) etching the topside of the die with 90%
fuming nitric acid in order to remove the topside plastic; (f)
rinsing the die with acetone; (g) clean the die by placing the
integrated circuit with the exposed die in a beaker of
N-methyl-2-pyrrolidinone while subjecting the beaker to ultrasonic
agitation; and (h) rinsing the die with acetone.
2. The process for removing a die according to claim 1, further
comprising the step of twisting off each bondwire with tweezers at
the neckdown area without removing a bondfoot from the bondpad.
3. The process for removing a die according to claim 2, further
comprising the step of inserting the die into a new package after
the step of twisting off each bondwire.
4. The process for removing a die according to claim 3, further
comprising the step of attaching the die to a floor of a new
package.
5. The process for removing a die according to claim 4, wherein the
step of attaching the die further comprises epoxying the die to the
floor of the new package.
6. The process for removing a die according to claim 4, wherein the
step of attaching the die further comprises forming a eutectic
attachment of the die to the floor of the new package.
7. The process for removing a die according to claim 4, further
comprising the step of connecting new wires between the die and the
new package by bonding new die wires between the die and the new
package.
8. The process for removing a die according to claim 3, further
comprising the step of bonding new bondwires to the original
ball-bond with a "bond-on-top-of-bond" process, the
"bond-on-top-of-bond" process including forming a new ball-bond on
top of the original ball bond, in order to establish electrical
contact between the die and the new package.
9. The process to remove a die from a plastic package according to
claim 8, wherein the "bond-on-top-of-bond" process further
comprises the step of flattening the original ball-bond in order to
increase a targeting/bonding area and adhesion to both the die-pad
and the new ball-bond.
10. The process to remove a die from a plastic package according to
claim 9, wherein the step of flattening the original ball bond
further comprises inserting a flattening tool into a wirebonder
chuck for repeatable precision targeting and automation.
11. The process for removing a die according to claim 3, wherein
the new package is a hermetic package.
12. The process for removing a die according to claim 1, wherein
the step of etching the topside of the die is performed with a
fuming nitric acid selected from the group consisting of 90% red
fuming nitric acid and 90% yellow fuming nitric acid.
13. The process for removing a die according to claim 1, wherein
the step of etching the topside of the die further comprises
etching the topside of the die with a mixture of 20% sulfuric acid
and fuming nitric acid.
14. The process for removing a die according to claim 1, further
comprising the step of pre-thinning the topside plastic by grinding
or milling prior to performing step (e).
15. The process for removing a die according to claim 1, wherein
the pins are removed by cutting.
16. The process for removing a die according to claim 1, wherein
the pins are removed by grinding.
17. The process for removing a die according to claim 1, wherein
the step of performing a backside etch further comprises etching
the backside with an acid selected from the group consisting of 90%
red fuming nitric acid and 90% yellow fuming nitric acid for about
ten seconds.
18. The process for removing a die according to claim 1, wherein
the step of etching the topside plastic further comprises etching
the topside plastic with a jet-stream of the fuming nitric
19. The process for removing a die according to claim 18, wherein
the step of etching the topside plastic with the jet-stream is
performed for between about one to three minutes, the fuming nitric
acid being heated to about 90.degree. C.
20. The process for removing a die according to claim 19, further
comprising the step of rinsing the topside with a cold jet-stream
of the nitric acid at a temperature of about 25.degree. C. for
about twenty seconds immediately following the step of etching the
topside plastic.
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.degree.to +125.degree. 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' 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.degree. 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.degree. 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/thermocompress- ion
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.degree. 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] FIG. 1A is a cross-sectional view of a semiconductor chip
encapsulated in a plastic package.
[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] 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] 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] 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] 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] 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] 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] 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] 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] 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] 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'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.degree. 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.degree. 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.
1E, 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. 3A-3C, 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.degree. 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.
* * * * *