U.S. patent application number 11/482335 was filed with the patent office on 2006-11-30 for method for modified bus bar with kapton.tm. tape or insulative material on loc packaged part.
Invention is credited to Robert W. Courtenay.
Application Number | 20060270117 11/482335 |
Document ID | / |
Family ID | 26751144 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060270117 |
Kind Code |
A1 |
Courtenay; Robert W. |
November 30, 2006 |
Method for modified bus bar with Kapton.TM. tape or insulative
material on LOC packaged part
Abstract
A semiconductor device is formed from a die and a lead frame
having one or more bus bars. Portions of the bus bars are overlain
with an electrically insulative material while leaving bonding
areas unobstructed, whereby bond wires which span the bus bar(s)
may be bonded with a shorter wire and a lower loop, without the
danger of shorting to the bus bar(s). The incidence of harmful wire
sweep in the encapsulation step is also reduced.
Inventors: |
Courtenay; Robert W.;
(Boise, ID) |
Correspondence
Address: |
TRASK BRITT, P.C.
P.O. BOX 2550
SALT LAKE CITY
UT
84110
US
|
Family ID: |
26751144 |
Appl. No.: |
11/482335 |
Filed: |
July 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
11099117 |
Apr 5, 2005 |
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|
11482335 |
Jul 6, 2006 |
|
|
|
10358572 |
Feb 5, 2003 |
6885087 |
|
|
11099117 |
Apr 5, 2005 |
|
|
|
09828379 |
Apr 6, 2001 |
6580157 |
|
|
10358572 |
Feb 5, 2003 |
|
|
|
09070464 |
Apr 30, 1998 |
6255720 |
|
|
09828379 |
Apr 6, 2001 |
|
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|
08872403 |
Jun 10, 1997 |
5780923 |
|
|
09070464 |
Apr 30, 1998 |
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Current U.S.
Class: |
438/123 ;
257/E23.039; 257/E23.04 |
Current CPC
Class: |
H01L 2224/45015
20130101; H01L 2224/85205 20130101; H01L 2224/45015 20130101; H01L
2224/48091 20130101; H01L 2224/48091 20130101; H01L 2224/4903
20130101; H01L 2924/01013 20130101; H01L 2224/4826 20130101; H01L
2224/48699 20130101; H01L 2224/4826 20130101; H01L 2924/01031
20130101; H01L 2924/01079 20130101; H01L 23/49513 20130101; H01L
24/06 20130101; H01L 2224/45124 20130101; H01L 2224/45144 20130101;
H01L 24/05 20130101; H01L 2224/05556 20130101; H01L 2224/48599
20130101; H01L 2224/48253 20130101; H01L 2224/49171 20130101; H01L
2224/04042 20130101; H01L 24/48 20130101; H01L 2224/48247 20130101;
H01L 2224/48599 20130101; H01L 2224/73215 20130101; H01L 2224/45124
20130101; H01L 24/49 20130101; H01L 2224/4903 20130101; H01L
2224/45144 20130101; H01L 2924/07802 20130101; H01L 2924/181
20130101; H01L 23/4951 20130101; H01L 2224/45144 20130101; H01L
2224/48465 20130101; H01L 2224/48465 20130101; H01L 2224/32245
20130101; H01L 2224/49051 20130101; H01L 2224/73215 20130101; H01L
2224/48699 20130101; H01L 2924/01005 20130101; H01L 2924/07802
20130101; H01L 2924/00015 20130101; H01L 2924/00 20130101; H01L
2924/00 20130101; H01L 2924/00 20130101; H01L 2924/00 20130101;
H01L 2924/00 20130101; H01L 2224/48247 20130101; H01L 2924/20752
20130101; H01L 2224/48091 20130101; H01L 2224/45144 20130101; H01L
2224/32245 20130101; H01L 2924/00 20130101; H01L 2224/48465
20130101; H01L 2924/00 20130101; H01L 2924/00 20130101; H01L
2224/48465 20130101; H01L 2224/45124 20130101; H01L 2924/00
20130101; H01L 2224/48465 20130101; H01L 2924/00 20130101; H01L
2224/05556 20130101; H01L 2924/00 20130101; H01L 2924/00 20130101;
H01L 2924/00014 20130101; H01L 2224/4826 20130101; H01L 2224/49171
20130101; H01L 2224/4826 20130101; H01L 2924/00 20130101; H01L
2924/00 20130101; H01L 2924/00014 20130101; H01L 2224/48465
20130101; H01L 2924/00 20130101; H01L 2924/20752 20130101; H01L
2924/00 20130101; H01L 2924/00014 20130101; H01L 2924/00 20130101;
H01L 24/45 20130101; H01L 2924/00014 20130101; H01L 2924/01014
20130101; H01L 2924/20752 20130101; H01L 2924/00014 20130101; H01L
2224/85205 20130101; H01L 2224/4826 20130101; H01L 2924/01029
20130101; H01L 2224/85205 20130101; H01L 2224/48247 20130101; H01L
2924/00014 20130101; H01L 2224/48247 20130101; H01L 2224/49171
20130101; H01L 2224/49171 20130101; H01L 2224/4903 20130101; H01L
2924/01006 20130101; H01L 2224/48465 20130101; H01L 2224/85205
20130101; H01L 2224/45015 20130101; H01L 2224/85201 20130101; H01L
2924/14 20130101; H01L 2924/181 20130101; H01L 2224/48465
20130101 |
Class at
Publication: |
438/123 |
International
Class: |
H01L 21/00 20060101
H01L021/00 |
Claims
1. A method of making a lead frame for a semiconductor device,
comprising: providing a conductive metal having an upper surface
and a lower surface; forming a lead pattern on the conductive
metal, the lead pattern including: a plurality of conductive leads
having inwardly directed lead fingers with wire bonding areas for
wire bonding to at least one bond pad of the semiconductor die; at
least one conductive lead of the plurality comprising a bus bar,
the bus bar for wire bonding to the at least one bond pad of the
semiconductor die; locating a layer of insulative material on a
portion of the upper surface of the conductive metal to form
insulation on at least a portion of at least one bus bar, the
insulative material including one of a layer of flowable insulation
material and a tape material; and removing the layer of insulative
material from at least one wire bonding area on an upper surface of
the at least one bus bar.
2. The method of claim 1, wherein the bonding a layer of insulative
material comprises applying a layer of flowable polymeric
insulative material onto the upper surface of the conductive
metal.
3. The method of claim 2, further comprising: preventing the layer
of flowable polymeric insulative material from flowing along the
upper surface of the at least one bus bar.
4. The method of claim 3, wherein the preventing the layer of
flowable polymeric insulative material from flowing along the upper
surface of the at least one bus bar comprises preventing the layer
of flowable polymeric insulative material from flowing along at
least a portion of the upper surface of the at least one bus bar.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of application Ser. No.
11/099,117, filed Apr. 5, 2005, pending, which is a continuation of
application Ser. No. 10/358,572, filed Feb. 5, 2003, now U.S. Pat.
No. 6,885,087, issued Apr. 26, 2005, which is a continuation of
application Ser. No. 09/828,379, filed Apr. 6, 2001, now U.S. Pat.
No. 6,580,157, issued Jun. 17, 2003, which is a continuation of
application Ser. No. 09/070,464, filed Apr. 30, 1998, now U.S. Pat.
No. 6,255,720, issued Jul. 3, 2001, which is a continuation of
application Ser. No. 08/872,403, filed Jun. 10, 1997, now U.S. Pat.
No. 5,780,923, issued Jun. 14, 1998.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to the fabrication of
semiconductor devices. More specifically, the invention relates to
a lead frame with power and/or ground buses for attachment to a
semiconductor chip in a leads-over-chip (LOC) configuration.
[0004] 2. State of the Art
[0005] Semiconductor devices combine an integrated circuit on a
silicon or gallium arsenide substrate, a die, with a series of
conductive leads which is connected to electrical circuits in an
electronic apparatus. The conductive leads are part of a thin lead
frame and are conventionally connected by wire bonds to
electrically conductive bond pads on the die.
[0006] A reduction in the package size of the semiconductor device
is enabled by a "leads-over-die" configuration as described in U.S.
Pat. No. 4,862,245 of Pashby et al. A die is described in which
bond pads are arrayed along a generally central axis of the die. A
lead frame may then be joined to the active surface of the die on
either or both sides of the row(s) of bond pads. An insulative
adhesive is used, typically in conjunction with an insulative
barrier such as Kapton.TM. tape, a trademarked product of duPont.
Thus, the lead frame will not become shorted to the active surface
of the die. The inner portions of the leads which project toward
the die bond pads are joined thereto by conventional wire bonding
techniques, typically using thin gold or alloy wires.
[0007] Efforts to improve the electronic characteristics of the
semiconductor device and reduce the package size have led to the
development of lead frames having power buses and/or ground buses,
as described in U.S. Pat. No. 5,233,220 of Lamson et al., U.S. Pat.
No. 5,227,661 of Heinen, U.S. Pat. No. 5,539,250 of Kitano et al.,
and U.S. Pat. No. 5,545,920 of Russell, for example. The buses are
positioned adjacent the row(s) of bond pads on the die. As shown in
these patent publications, bond wires extending from the die bond
pads to the inner portions of the leads, i.e. fingers of the lead
frame, must span a bus, significantly increasing the risk of
shorting. In order to avoid such shorting, the bonding technique
requires precise placement of the wire with a looping height high
enough to avoid shorting with the bus, yet low enough (with as
short a wire as possible) to avoid wire sweep during the
encapsulation of the die and a portion of the lead frame. In wire
sweep, the flow of viscous encapsulant over the device "sweeps"
wires laterally, causing shorting between wires and/or bond
breakage.
[0008] Several solutions have been proposed for avoiding the very
precise techniques required when such buses are used.
[0009] It is possible to configure the lead frame so that a bus is
on one side of the die bond pad row, and all other lead fingers
approach the bond pad row from the opposite side. This has several
distinct disadvantages: [0010] 1. If more than one bus bar is to be
used, e.g. power and ground, bond wires will still have to cross
one of the bus bars; and [0011] 2. The number of leads which may be
accommodated by the device is limited, unless the chip size is
significantly increased.
[0012] In U.S. Pat. No. 5,331,200 of Teo et al., bond wires are
avoided entirely by using lead frame fingers which extend to the
bond pads of the LOC die and are directly attached to them by inner
lead bonding (ILB). The bus bar is positioned away from the die
bond pads. The bus bar is elevated above the lead frame fingers and
crosses them. The bus has its own "fingers" which are directly
attached to the appropriate bond pads on the die. As disclosed, an
insulating tape may be inserted between the bus fingers and the
other lead frame fingers to prevent shorting therebetween.
[0013] The multilevel lead frame of Teo et al. introduces several
problems. First, the multilevel lead frame must be made in several
parts which are then joined with adhesive tape. This is an
additional critical step in the process, requiring special
equipment and precision techniques.
[0014] Second, a single tape automated bonding (TAB) step will not
accommodate placement of both the main lead frame and the second
level bus. A second TAB step would be required.
[0015] Third, the ILB bonding process may subject the die to
greater levels of thermal shock than desired in attempting to
compromise the bond formation requirements between the high
temperature, pressure, and bonding time required for improved joint
reliability and the need to limit exposure of the die to these
conditions to avoid die degradation. In ILB bonding, a hot
(300.degree. to 600.degree. C.) thermode applies pressure between
the TAB leads and the bond pads of the die, which may be excessive
and damage the die.
[0016] It is proposed in U.S. Pat. No. 5,252,853 of Michii that die
terminals to be bused, e.g. power or ground, have bond pads in an
inner row, and other bond pads be arrayed in rows outside of the
bused terminals. The bus bars are positioned to overlie the inner
rows and to be directly joined thereto. The signal leads are
directly joined to the outer rows of bond pads. Thus, no wires or
signal leads cross over (or under) a bus.
[0017] The completely successful bonding of a series of power
and/or ground bond pads to an overlying bus bar is difficult.
[0018] In another proposed solution to wire-to-bus grounding, U.S.
Pat. No. 5,550,401 of Maeda uses a bus bar remote from a row of die
bond pads, and elevated above the lead frame fingers. A series of
bus bar fingers includes a 180 degree downward bend to supportively
elevate the bus bar above the signal lead fingers. The bonding
wires do not cross the bus bars.
[0019] The bus of Maeda is complex and requires precision bending
of the associated bus fingers. High bending stresses are introduced
into each of the bus fingers, particularly in view of the very
tight radii required. If bent non-uniformly, the bus bar will not
be evenly supported, will have localized deformations, and will
have a number of differing bending stresses introduced into the bus
bar itself.
[0020] In each of the patent references cited above, an insulating
layer is applied as a tape to portions of the active surface of the
semiconductor die, electrically isolating it from the lead
frame.
[0021] In each of U.S. Pat. Nos. 5,331,200 and 5,550,401 (both
previously cited), the underside of the elevated bus bar is coated
with an insulating tape to prevent short circuiting with underlying
portions of the lead frame.
[0022] Of course, once the die has been encapsulated by transfer
molding, it is impossible to correct a wire which is broken,
shorted, or which has become unbonded. Even though the cost of
manufacturing a semiconductor device through the encapsulation step
is very expensive, it is rarely economical to attempt repair of a
defective wire or wire bond after transfer molding. Removal of the
encapsulant without destroying the interconnecting wires is
extremely difficult, particularly when the encapsulant is a filled
polymer.
[0023] Notwithstanding the effort by the semiconductor industry to
eliminate the possibility of causing wire short circuits during
device manufacturing, the problem has not been satisfactorily
resolved. The invention hereinafter described is directed toward
providing a simple, effective solution with numerous
advantages.
BRIEF SUMMARY OF THE INVENTION
[0024] In accordance with the invention, lead frames having
integral bus bars for use with leads-over-chip (LOC) and other
semiconductor die configurations are constructed with a layer of
electrical insulative tape or other coating applied as a liquid or
fluid and cured/hardened on the upper surface of the bus bars prior
to wire bonding. In the ensuing wire bonding step, the particular
height of wire loops is much less critical because inadvertent
shorting of the wires to the bus bars is avoided. Thus, the looping
heights and wire lengths may be reduced, avoiding subsequent wire
sweep during encapsulation.
[0025] Furthermore, no extra steps are introduced into the bonding
process. In addition, complex processes such as bending the bus bar
fingers is avoided.
[0026] The resulting proximity of wire loops to the insulated bus
bar(s) permits the insulation to limit lateral, i.e., "sweeping,"
movement of the crossing bond wires during subsequent
encapsulation.
[0027] Where lead wires are to be bonded to a bus bar, the
insulative tape or coating may be simply applied to areas where no
bonding is to be done. Alternatively, the insulation may be applied
to cover the entire bus bar and then etched to remove selected
portions thereof, exposing the bus bar at desired bonding
locations.
[0028] The insulative material applied to the upper surface of the
bus bar(s) may be any suitable well known polymer, such as a
polyimide, resin, etc. The preferred insulative material is a
polyimide which may be applied either in a flowable form, or more
preferably as a preformed tape.
[0029] The tape used for insulating an exposed surface of the bus
bars is preferably formed of a highly insulative material to which
a non-conductive adhesive is applied on one surface. Alternatively,
the tape may comprise a material which is self-adhesive to the bus
bars under an elevated temperature and/or pressure, or under other
controllable conditions.
[0030] Insulative tapes useful for the purposes of the invention
include a polymeric film having a melting temperature in excess of
175.degree. C. and having no ionizable species such as halides and
active metals. One such polyimide film used as an electrical
insulator and an .alpha. radiation barrier is duPont Kapton.TM.
tape. Other tapes and coatings having similar electrically
insulative and adhesion properties may also be used. Where the
applied material is to be etched at bond areas, the capability of
being readily etched or otherwise selectively removed is
required.
[0031] While the thickness of the insulative layer may be as much
as 8 mils or more, a minimal thickness is preferred, i.e. of the
order of about 1 to 4 mils, to minimize cost and device size. This
is particularly true when the (preferred) tape form is used. The
insulative layer may be applied to the bus bar(s) during
manufacture of the lead frame, or applied later prior to wire
bonding. As already indicated, the insulative layer may be applied
over the bus bar and then etched where wire connections are to be
made. Alternatively, the insulative layer may be applied to
selected areas to leave bonding areas uncovered. Where the number
of wire crossings is very limited, the insulative layer may be
applied only to broad areas underlying the planned wire crossing
paths.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0032] The invention is illustrated in the following figures,
wherein for the sake of clarity the elements are not necessarily
shown to scale:
[0033] FIG. 1 is a perspective view of a semiconductor device of
the invention;
[0034] FIG. 2 is a plan view of another embodiment of the
semiconductor device of the invention;
[0035] FIG. 3 is a plan view of a lead frame of the invention
corresponding to the semiconductor device of FIG. 2;
[0036] FIG. 4 is a plan view of a lead frame of the invention,
following application of a topside insulative layer and prior to
pattern formation; and
[0037] FIG. 5 is a perspective view of a semiconductor device of a
second embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] A method and apparatus are provided by the invention to
reduce the incidence of wire shorts in LOC and similar type
die-to-lead frame packaging where the lead frame includes one or
more buses.
[0039] With reference to the drawings, and particularly to FIG. 1,
a representative portion of a semiconductor device 10 is
illustrated. A die 12 is shown with bond pads 16 on the "active"
surface 14 thereof, for electrical connection via leads (signal
leads) 18 and one or more bus bars 20 to a circuit (not shown)
external to the die. The bond pads 16 are typically arrayed along
and proximate to one or more axes 54 on the active surface 14. The
axis or axes 54 may be oriented lengthwise and/or widthwise. The
leads 18 and bus bars 20 are integrally formed as part of a lead
frame as known in the art. Each of the leads 18 includes an inner
lead or lead finger 18A having a bonding area 30, the opposite end
of the lead 18 comprising an outer lead portion extending
externally of the device 10. A typical semiconductor device may
have two bus bars 20, one for power and the other for ground.
[0040] The leads 18 and bus bar 20 of a "leads-over-chip" (LOC)
type device of FIG. 1 are shown separated from the active surface
14 of the die 12 by a first insulative layer 24. The first
insulative layer 24 is generally known as an .alpha.-barrier and is
typically joined to the active surface 14 of the die 12 by a first
adhesive 26 and to the underside of the leads 18 and bus bar(s) 20
by a second adhesive 28. The first and second adhesives 26, 28 may
be the same or different adhesives of a thermosetting or
thermoplastic type.
[0041] The first insulative layer 24 is configured to leave
uncovered the areas of the active surface 14 having bond pads 16,
i.e. on either side of axis 54. Thus, the bond pads 16 are
unobstructed to enable access for wire bonding.
[0042] As shown, the bond pads 16 and bonding areas 30 of the leads
18 are conductively joined by corrosion resistant wires 34,
typically formed of gold and having a diameter of approximately 1
mil. The wires 34 may be joined to bond pads 16 of the die 12 and
bonding areas 30 of the signal leads 18 by any of thermocompression
bonding, ultrasonic bonding, thermosonic bonding or other wire
bonding methods well known in the art. The die bonds 36 and tail
bonds 38 in FIG. 1 are illustrated as ball bonds and wedge bonds,
respectively, formed by e.g. thermocompression.
[0043] In a similar manner, a bond wire 42 is shown bonded as a
wire-to-die bond 56 to a die bond pad 16A and as a tail bond 46 to
a bonding area 44 on the upper surface of the bus bar 20. The bonds
are shown as a thermocompression ball bond and wedge bond,
respectively. Because of the shorter required length of the
exemplary wire 42, wire 42 has a relatively low loop 48 in
comparison to loops 40 of the wires 34 crossing the bus bar 20.
[0044] A bus bar 20 is generally connected to a plurality of
separated die bond pads 16A and, thus generally, must traverse a
substantial portion or all of the length or width of a die 12.
Typically, at least one, and usually a considerably larger number
of wires 34, 42 must cross the bus bar(s) 20. This is particularly
true when the die 12 is a leads-over-chip (LOC) type die, i.e.
portions of the leads 18 and/or bus bar(s) 20 overlie an active or
major surface 14 of the die. As already indicated, the spanning of
a bus bar 20 by leads 18 may lead to difficulties, including:
[0045] a. inadvertent shorting of a bond wire 34 to an underlying
bus bar 20 or lead finger 18A during the bonding operation; and
[0046] b. to avoid shorting, the wire connections may be formed
with higher than usual wire loops 40. The long wires 34 are much
more subject to wire sweep during encapsulation of the device
10.
[0047] In the improvement represented by the present invention, a
layer 50 of polymeric electrical insulative material, such as an
{grave over (.alpha.)}-barrier type polymeric electrical insulative
material, is adhered to the upper surface 58 of bus bar 20 during
manufacture of the lead frame. Bonding areas 44 of the bus bar 20
to which wires 42 are to be bonded are left uncovered by layer 50,
or layer 50 is removed therefrom during lead frame manufacture. It
is not generally necessary to cover the entire bus bar(s) 20 with
insulative layer 50, as long as areas of the bus bar(s) generally
underlying crossing wires 34, 42 are covered to a width ensuring
that shorting will not occur. Such width will depend upon the
overall wire length, and may be, for example, about 6-15 mils wide
as a minimum. Of course, it is unnecessary to delete or remove
insulative material layer 50 from the bus bar(s) 20 where no wire
is to be bonded thereto.
[0048] As will be described further, the insulative material may be
applied as a fluid coating to the bus bar(s) 20 or more preferably,
applied as a polymeric tape such as Kapton.TM. polyimide tape with
a third adhesive 52 on one side.
[0049] Turning now to diagrammatic FIG. 2, a LOC type DRAM (dynamic
random access memory) semiconductor device 60 comprises a die 62
and an overlying lead frame 70 (FIG. 3) formed from a suitable
metal, such as copper, alloy 42, etc. In the figure, a perimetric
line 64 encloses and delineates each of the bond wires 66
connecting die bond pads 68 with members of the lead frame 70. Bond
wires which span one or both of the bus bars 76A, 76B are
identified by the numeral 66A. Portions of the lead frame 70 are
shown, including inner portions of leads 72 having bonding sites
74, and two bus bars 76A and 76B. The bus bars 76A, 76B are
attached to the active surface 78 of the die 62 between two rows
80A, 80B of die bond pads 68, and spaced apart from the two rows
82A, 82B of inner leads 72. Between the bus bars 76A, 76B and the
active surface 78 of the die are two intervening elongate
insulative members 86A and 86B, both adhesively joined to the die
62 and the overlying bus bars. The two members 86A, 86B are spaced
apart to leave one or more openings 88 for access to bond pads 68A
in a central portion of the active surface 78.
[0050] In this exemplary device 60, the die 62 is shown adhesively
attached to an underlying substrate 84 which may be a die paddle or
other package part.
[0051] Each of the bus bars 76A, 76B is shown with an upper surface
92 over which pass bond wires identified by indicia 66A. Each bus
bar 76A, 76B has its upper surface 92 partially covered with an
insulative layer 90 which prevents the shorting of crossing bond
wires 66A to the bus bar. The insulative layer 90 is shown by
diagonal lines in FIGS. 2 and 3. As shown, portions of the bus bars
76A, 76B comprise bus bonding areas 94A, 94B, respectively, which
are not covered by insulative layer 90, thus permitting access to
the upper surface 92 for bonding wires 66 between die bond pads 68
and a bonding area 94A, 94B of a bus bar 76A, 76B. In FIG. 2, bonds
to the bus bars 76A, 76B are identified by numeral 96. Any
appropriate bonding method may be used for attaching the die bond
pads 68 to the lead frame 70, including thermocompression,
ultrasonic and thermosonic techniques.
[0052] The insulative layer 90 may be configured to: [0053] a.
cover only those areas of the bus bars 76A, 76B over which a bond
wire 66A is to pass; [0054] b. cover all or most of the upper
surface 92, except where a bond wire 66 is to be attached to the
bus bar 76A, 76B; or [0055] c. be intermediate a. and b. in
coverage.
[0056] The particular configuration used will be a function of the
ease (and cost) of application of the insulative layer 90 to the
lead frame 70 to meet the design of the die 62 and specifications
of the final packaged product.
[0057] The insulative layer 90 may be formed of any material which
has sufficient electrically insulating properties to prevent
shorting between bond wires 66 and the bus bar(s) 76A, 76B, even
when applied in thin layers of about 1-4 mils. The material must
also be capable of convenient application to the bus bar(s) during
manufacture of the lead frame 70. Some of the insulative polymeric
materials which may be used include polyimide, being the preferred
material because of ease in handling and its low transmission of
.alpha.-radiation. The insulative layer 90 may be applied as a
hardenable fluid, as a paste, or preferably as a thin tape. The
tape may have an adhesive on one side, or an adhesive material may
be separately applied to the bus bar(s) prior to application of the
tape. A material which is readily formed as a tape and provides
excellent insulative properties is polyimide. One readily available
form of polyimide useful for this purpose is duPont Kapton.TM.
tape, typically available in thicknesses of 1 to 4 mils, and
greater. A preferred thickness is about 1 to about 2 mils.
[0058] Adhesives which may be used to join insulative layer 90 to
the bus bar(s) include epoxies, acrylics, silicones and polyimides.
The insulative layer 90 in tape or other form may comprise a
polymer which is itself bondable to the bus bars without an
additional adhesive material.
[0059] It should be noted that while the terms "upper," "lower,"
"top," "underside" and the like may be used herein, the usual
meanings may be reversed where the active surface 14 of the die 12
normally faces downwardly. Such may occur, for example, in a device
formed of two dies, one of which has a downwardly facing active
surface.
[0060] Looking now at FIG. 3, a major portion of the exemplary lead
frame 70 of FIG. 2 is depicted. As shown, the lead frame 70 is
ready to be joined to a die 62 (see FIG. 2) and wire-bonded
thereto. The lead frame 70 is produced by punching or
photolithographic techniques well known in the art. The particular
lead frame 70 is configured to overlay a semiconductor die 62 (not
shown) which has a plurality of bond pads 68 (see FIG. 2)
positioned generally along parallel axes 100A, 100B and 100C. Thus,
when applied to a die 62, the lead frame 70 has openings 102A, 102B
and 102C, respectively, along axes 100A, 100B and 100C for access
of a wire bonding tool to the die bond pads 68.
[0061] The lead frame 70 includes a first bus bar 76A and a second
bus bar 76B shown as extending through and across a central portion
of the die 62, between the longitudinal outer axes 100A and 100B.
The bus bars 76A, 76B are positioned generally on either side of
central axis 100C and largely spaced therefrom for accessing bond
pads located along and near central axis 100C. The exposed, upper
surfaces 92 of the bus bars 76A, 76B have bonding areas 94A, 94B to
which bond wires will be attached (compare FIGS. 2 and 3). The bus
bars 76A, 76B are shown with outer leads 110A, 110B which terminate
in external e.g. J terminals, not shown.
[0062] The lead frame 70 is shown as including a plurality of lead
fingers 104, each having a bonding site 74 to which a wire 66 is to
be bonded. In this exemplary semiconductor device, the lead fingers
104 are primarily on either side of axes 100A and 100B, and extend
outwardly as outer leads 106 which culminate in e.g. external J
terminals (not shown), as known in the art.
[0063] The leads 72 and bus bars 76A, 76B are joined to an outer
supporting frame, not shown, by temporary cross-members 108 for
maintaining the integrity of the entire lead frame 70 until wire
bonding is complete. The cross-members 108 are removed after
encapsulation to separate the lead frame 70 into a plurality of
separate conductive leads (including bus bars) having exterior
terminals.
[0064] As shown, the lead frame 70 includes two underlying members
86A, 86B of insulative material, separating (in this case) the bus
bars 76A, 76B from the active surface 78 of the die 62 (see FIG.
2). These members 86A, 86B are adhesively joined to the underside
of portions of the lead frame 70 overlying the die 62, and to the
active surface 78 of the die. An exemplary material from which
members 86A, 86B are constructed is a polyimide, although other
polymers have been found useful.
[0065] Overlying portions of the bus bars 76A, 76B and attached
thereto is a layer 90 of electrically insulative material as
previously described herein. Insulative layer 90 is identified in
FIG. 3 as hatched areas of the bus bars 76A, 76B. The insulative
layer 90 is adhesively joined to the bus bars 76A, 76B during the
lead frame manufacture process. Bus bar bonding areas, i.e. bonding
areas 94, comprise areas on the upper surface of the bus bars 76A,
76B which are not covered by insulative layer 90. In the example of
FIG. 3, bus bar 76A is shown with six wire bonding areas 94, and
bus bar 76B is shown as having three wire bonding areas 94.
[0066] The steps in producing the lead frame 70 of the invention
may vary, depending upon technical and economic factors.
[0067] The lead frame 70 may be produced from a single "plate" or
layer 120 of conductive metal. As shown in FIG. 4, a central
portion of lead frame 70 is indexed by index markings 114 for
precise removal of empty spaces 118 of the metal layer 120 to form
the pattern 116 of leads and bus bars. The metal layer 120 may be
applied to a polymeric backing, not shown, to form a sheet or
tape.
[0068] A continuous layer 90 of insulative material is adhesively
joined to a portion of the metal layer 120 which will become the
upper surfaces of underlying bus bars 76A, 76B. The insulative
layer 90 may be a polymeric material, such as polyimide, which is
applied as a fluid or paste to the metal layer 120. Preferably, the
insulative layer 90 is a tape formed of an insulative polymer such
as a polyimide. The bus bar portion of the pattern 116 underlies
insulative layer 90 and is not shown in FIG. 4. It is, however, as
depicted in FIGS. 2 and 3.
[0069] In one embodiment, the empty spaces 118 on the lead frame 70
are then excised from the lead frame 70 by e.g. using a punch to
cut the metal and attached insulative layer 90 to a desired pattern
116. The result is a pattern 116 of lead fingers 104 with bonding
sites 74, outer leads 106 with terminal portions, and one or more
bus bars, e.g., 76A, 76B, all interconnected by cross-members 108
which provide support for the pattern 116 within the lead frame 70.
Thus, the bus bars 76A, 76B are provided with an overlying layer 90
of insulative material.
[0070] Next, the insulative layer 90 is etched or otherwise removed
from selected locations which become wire bonding areas 94 on the
bus bars 76A, 76B.
[0071] The insulative members 86A, 86B may then be adhesively
applied to the underside of the lead frame 70 where it is to be
joined to the active surface 78 of the die 62.
[0072] In an alternative method of construction, photolithographic
techniques may be used to form the lead frame 70. Empty spaces 118
in the pattern 116, including those in the area covered by
insulative layer 90, are removed by etching. The etching step may
include those areas which are to become bonding sites 74 on the
lead fingers 104 and wire bonding areas 94A, 94B on the bus bars
76A and 76B (see FIG. 3).
[0073] In a further method of construction, the underside of the
metal layer 120 is first joined to a continuous polymeric
insulative layer 112 (not shown), e.g. a one-sided polymeric tape,
as known in the art. A layer 90 of an insulative polymer, such as
polyimide, is then bonded to the upper surface of the metal layer
120, followed by punching out empty spaces 118 to create the
desired lead frame pattern 116. An additional .alpha.-blocking
layer 86 (also not shown) between the die 62 and overlying portions
of the lead frame 70 may be used, if required, by the operating
specifications of the device 60.
[0074] After the completed lead frame 70 is adhesively attached to
the die 62, the next steps include wire bonding, removal of the
cross-members, testing, encapsulation, and formation of external
lead terminals.
[0075] With reference to the drawings, and particularly to FIG. 5,
another embodiment of the present invention is illustrated with
respect to a representative portion of a semiconductor device 10. A
die 12 is shown with bond pads 16 on the "active" surface 14
thereof, for electrical connection via leads (signal leads) 18 and
one or more bus bars 20 to a circuit (not shown) external to the
die. The bond pads 16 are typically arrayed along and proximate to
one or more axes 54 on the active surface 14. The axis or axes 54
may be oriented lengthwise and/or widthwise. The leads 18 and bus
bars 20 are integrally formed as part of a lead frame as known in
the art. Each of the leads 18 includes an inner lead or lead finger
18A having a bonding area 30, the opposite end of the lead 18
comprising an outer lead portion extending externally of the device
10. A typical semiconductor device may have two bus bars 20, one
for power and the other for ground. Each bus bar 20 is formed
having a plurality of notches or grooves 21 formed therein
extending across the width of the bus bar 20. The notches or
grooves 21 may be formed in the bus bar 20 by any suitable means,
such as stamping, etching, etc. The notches or grooves 21 are
formed on the bus bar 20 adjacent an area of the bus bar where a
wire 42 is to be bonded thereto.
[0076] The leads 18 and bus bar 20 of a "leads-over-chip" (LOC)
type device of FIG. 5 are shown separated from the active surface
14 of the die 12 by a first insulative layer 24. The first
insulative layer 24 is generally known as an .alpha.-barrier and is
typically joined to the active surface 14 of the die 12 by a first
adhesive 26 and to the underside of the leads 18 and bus bar(s) 20
by a second adhesive 28. The first and second adhesives 26, 28 may
be the same or different adhesives of a thermosetting or
thermoplastic type.
[0077] The first insulative layer 24 is configured to leave
uncovered the areas of the active surface 14 having bond pads 16,
i.e. on either side of axis 54. Thus, the bond pads 16 are
unobstructed to enable access for wire bonding.
[0078] As shown, the bond pads 16 and bonding areas 30 of the leads
18 are conductively joined by corrosion resistant wires 34,
typically formed of gold and having a diameter of approximately 1
mil. The wires 34 may be joined to bond pads 16 of the die 12 and
bonding areas 30 of the signal leads 18 by any of thermocompression
bonding, ultrasonic bonding, thermosonic bonding or other wire
bonding methods well known in the art. The die bonds 36 and tail
bonds 38 in FIG. 5 are illustrated as ball bonds and wedge bonds,
respectively, formed by e.g. thermocompression.
[0079] In a similar manner, a bond wire 42 is shown bonded as a
wire-to-die bond 56 to a die bond pad 16A and as a tail bond 46 to
a bonding area 44 on the upper surface of the bus bar 20 having
notches or grooves 21 located adjacent thereto. The bonds are shown
as a thermocompression ball bond and wedge bond, respectively.
Because of the shorter required length of the exemplary wire 42,
wire 42 has a relatively low loop 48 in comparison to loops 40 of
the wires 34 crossing the bus bar 20.
[0080] A bus bar 20 is generally connected to a plurality of
separated die bond pads 16A and thus generally must traverse a
substantial portion or all of the length or width of a die 12.
Typically, at least one and usually a considerably larger number of
wires 34, 42 must cross the bus bar(s) 20. This is particularly
true when the die 12 is a leads-over-chip (LOC) type die, i.e.
portions of the leads 18 and/or bus bar(s) 20 overlie an active or
major surface 14 of the die. As already indicated, the spanning of
a bus bar 20 by leads 18 may lead to difficulties, including:
[0081] a. inadvertent shorting of a bond wire 34 to an underlying
bus bar 20 or lead finger 18A during the bonding operation; and
[0082] b. to avoid shorting, the wire connections may be formed
with higher than usual wire loops 40. The long wires 34 are much
more subject to wire sweep during encapsulation of the device
10.
[0083] In the improvement represented by the present invention, a
layer 50 of polymeric electrical insulative material, such as an
{grave over (.alpha.)}-barrier type polymeric electrical insulative
material, is adhered to the upper surface 58 of bus bar 20 during
manufacture of the lead frame. Bonding areas 44 of the bus bar 20
to which wires 42 are to be bonded are left uncovered by layer 50,
or layer 50 is removed therefrom during lead frame manufacture. It
is not generally necessary to cover the entire bus bar(s) 20 with
insulative layer 50, as long as areas of the bus bar(s) generally
underlying crossing wires 34, 42 are covered to a width ensuring
that shorting will not occur. Such width will depend upon the
overall wire length, and may be, for example, about 6-15 mils wide
as a minimum. Of course, it is unnecessary to delete or remove
insulative material layer 50 from the bus bar(s) 20 where no wire
is to be bonded thereto.
[0084] As will be described further, the insulative material may be
applied as a fluid coating to the bus bar(s) 20, the insulative
layer 50 being applied through various well known techniques as
described herein. When the insulative layer 50 is applied as a
fluid coating, the notches or grooves 21 in the bus bar 20 act as
dams to prevent the fluid insulating material from flowing into the
bonding area 44 where tail bond 46 of wire 42 is bonded to the bus
bar. Suitable fluid insulating layer 50 includes polyimide, UV
curable epoxies, aminoproplytriethoxysilane, poly-siloxane, or
other known suitable materials.
[0085] As a result of the method described above, bonding of wires
to bonding sites on opposing sides of a bus bar is readily
accomplished without the danger of shorting between the wires and
the bus bar(s). All wire bonds may be completed without intervening
steps. Bonding of the spanning wires may be done with shorter wires
and lower loops. The incidence of destructive wire sweep in
encapsulation is reduced.
[0086] It is possible to approach the problem of wire-to-bus bar
shorting by (a) bonding those wires which are to be attached to a
bus bar, (b) removing the device from the bonding machine and
coating the upper surface of the bus bar with the insulative layer
between the wire bonds, and (c) returning the device to the bonding
machine to bond the remaining wire connections, including those
wires which are to span the bus bars. None of these approaches are
recommended because it requires additional steps and handling, both
of which introduce added possibilities for damage to the device. In
addition, some of the wires which span a first bus bar are attached
to another bus bar. Thus, not all of the wires spanning a bus bar
are underlain by an insulative coating, severely limiting the
method's effectiveness.
[0087] As described herein, the invention provides an improvement
in the construction of semiconductor devices. The method may be
applied to a wide variety of die and lead frame configurations
having at least one bus bond which is spanned by a bond wire, as
described herein, and includes typical and atypical LOC and
chip-on-board (COB) assemblies. The method is particularly
effective for LOC type packaging of dice having bond pads along die
edges, as well as within a central die area. The savings effected
by the invention are enhanced as the number of spanning wires in
the device becomes greater.
[0088] It is apparent to those skilled in the art that various
changes and modifications may be made in the semiconductor device
and methods as disclosed herein without departing from the spirit
and scope of the invention as defined in the appended claims. For
example, there may be variations in the number, location and
arrangement of bus bars, bond pads, and leads in the lead
frame.
* * * * *