U.S. patent application number 13/191731 was filed with the patent office on 2013-01-31 for mask-less selective plating of leadframes.
This patent application is currently assigned to TEXAS INSTRUMENTS INCORPORATED. The applicant listed for this patent is Donald C. Abbott, Kapil H. Sahasrabudhe. Invention is credited to Donald C. Abbott, Kapil H. Sahasrabudhe.
Application Number | 20130025745 13/191731 |
Document ID | / |
Family ID | 47596244 |
Filed Date | 2013-01-31 |
United States Patent
Application |
20130025745 |
Kind Code |
A1 |
Abbott; Donald C. ; et
al. |
January 31, 2013 |
Mask-Less Selective Plating of Leadframes
Abstract
A method for selectively plating a leadframe (1100) by oxidizing
selected areas (401, 402, 403, 404) of the leadframe made of a
first metal (102) and then depositing a layer (901) of a second
metal onto un-oxidized areas. The selective oxidations are achieved
by selective active marking
Inventors: |
Abbott; Donald C.; (Norton,
MA) ; Sahasrabudhe; Kapil H.; (Farmers Branch,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Abbott; Donald C.
Sahasrabudhe; Kapil H. |
Norton
Farmers Branch |
MA
TX |
US
US |
|
|
Assignee: |
TEXAS INSTRUMENTS
INCORPORATED
Dallas
TX
|
Family ID: |
47596244 |
Appl. No.: |
13/191731 |
Filed: |
July 27, 2011 |
Current U.S.
Class: |
148/277 |
Current CPC
Class: |
C23C 2/006 20130101;
H01L 21/4821 20130101; C23C 8/80 20130101; C23C 8/04 20130101; C23C
8/12 20130101; C23C 8/52 20130101; C23C 18/1633 20130101; C23C 8/02
20130101; C23C 14/04 20130101; C23C 8/72 20130101 |
Class at
Publication: |
148/277 |
International
Class: |
C23C 8/80 20060101
C23C008/80 |
Claims
1. A method for selectively plating a leadframe comprising the
steps of: oxidizing selected areas of a leadframe made of a first
metal; and depositing a layer of a second metal onto un-oxidized
areas.
2. The method of claim 1 wherein the second metal layers have
diffuse edges bordering the oxidized areas.
3. The method of claim 1 wherein the step of oxidizing includes the
step of contacting the selected leadframe areas with a chemical
agent suitable for oxidizing the first metal.
4. The method of claim 3 wherein the agent is sodium hypochlorite
(NaOCl).
5. The method of claim 3 wherein the step of contacting includes a
rubber stamp for pressing against the leadframe, the stamp having
elevated mesas matching the selected leadframe areas, the mesas
suitable to be dipped into the chemical agent.
6. The method of claim 3 wherein the step of contacting includes
the movable jet of a printer.
7. The method of claim 1 wherein the step of oxidizing includes the
step of transferring heat into the selected leadframe areas for
accelerating oxidation of the first metal.
8. The method of claim 7 wherein the step of transferring heat
includes a tool having elongated probes matching the selected
leadframe areas, the probes suitable to be electrically heated.
9. The method of claim 7 wherein the step of transferring heat
includes a movable laser beam.
10. The method of claim 1 wherein the first metal is selected from
a group including copper, copper alloy, aluminum, iron-nickel
alloy, and Kovar.TM..
11. The method of claim 1 wherein the step of depositing includes
the step of immersing the leadframe into a plating bath, the
deposited layer adhering to the un-oxidized leadframe areas while
not adhering to the oxidized areas.
12. The method of claim 11 wherein the step of depositing includes
the step of flood plating.
13. The method of claim 13 further including the step of peeling
away the deposited layer not adhering to the oxidized leadframe
areas.
14. The method of claim 13 wherein the second metal includes a
layer of nickel in contact with the first metal, a layer of
palladium in contact with the nickel, and a layer of gold in
contact with the palladium.
15. The method of claim 13 wherein the second metal includes tin.
Description
FIELD OF THE INVENTION
[0001] The present invention is related in general to the field of
semiconductor devices and processes, and more specifically to the
structure and fabrication process of metallic leadframes in
semiconductor packages having low-cost mask-less selective plating,
while oxidizing the un-plated leadframe portions to provide
improved adhesion to the polymeric compounds.
DESCRIPTION OF RELATED ART
[0002] In semiconductor devices, the chips are encapsulated in
packages to protect the enclosed parts against mechanical damage
and environmental influences, particularly against moisture and
light, while providing trouble-free electrical connections. Based
on their functions, the semiconductor packages include a variety of
different materials; metals are employed for electrical and thermal
conductance, and insulators, such as polymeric molding compounds,
are used for encapsulations and form factors. To ensure the unity
and coherence of the package, these different materials are
expected to adhere to each other during the lifetime of the package
while tolerating mechanical vibrations, temperature swings, and
moisture variations. Failing adhesion allows moisture ingress into
the package, causing device failure by electrical leakage and
chemical corrosion.
[0003] Leadframes for semiconductor devices provide a stable
support pad for firmly positioning the semiconductor chip, usually
an integrated circuit (IC) chip, within a package. The chips have
to be attached to the pad with reliable adhesion. It has been
common practice to manufacture single piece leadframes from thin
(about 120 to 250 .mu.m) sheets of metal. For reasons of easy
manufacturing, the commonly selected starting metals are copper,
copper alloys, iron-nickel alloys (for instance the so-called
"Alloy 42"), and aluminum. The desired shape of the leadframe is
stamped or etched from the original sheet.
[0004] In addition to the chip pad, the leadframe offers a
plurality of conductive leads to bring various electrical
conductors into close proximity of the chip. The remaining gaps
between the inner end of the leads and the contact pads on the IC
surface are bridged by connectors, typically thin metal wires of
gold or copper, individually bonded to the IC contact pads and the
leads. Consequently, the surface of the inner lead ends has to be
metallurgically suitable for stitch-attaching the connectors.
[0005] The end of the leads remote from the IC chip ("outer" ends)
need to be electrically and mechanically connected to external
circuitry such as printed circuit boards. This attachment is
customarily performed by soldering, conventionally with a tin alloy
solder at a reflow temperature above 200.degree. C. Consequently,
the surface of the outer lead ends needs to have a metallurgical
configuration suitable for reflow attachment to external parts.
[0006] Finally, the leadframe provides the framework for
encapsulating the sensitive chip and fragile connecting wires.
Encapsulation using plastic materials, rather than metal cans or
ceramic, has been the preferred method due to low cost. The
transfer molding process for epoxy-based thermoset compounds at
175.degree. C. has been practiced for many years. The temperature
of 175.degree. C. for molding and mold curing (polymerization) is
compatible with the temperature of >200.degree. C. for eutectic
solder reflow. The encapsulation compound has to adhere reliably to
leadframe, chip and wires.
[0007] Today's semiconductor technology employs a number of methods
to raise the level of adhesion between the diversified materials so
that the package passes accelerated tests and use conditions
without delamination. As an example, the adhesion between
copper-based leadframes and epoxy-based molding compounds and
chip-attach compounds can be improved by adding design features
such as indentations, grooves or protrusions to the leadframe
surface. A widely used technique is the mechanical "dimpling" of
the underside of the chip attach pad by producing patterns of
indentations in the leadframe metal, sized between about 500 and
1000 .mu.m. Another example to improve adhesion is the method to
chemically modify the leadframe surface by oxidizing the metal
surface, for instance creating copper oxide. Copper oxide is known
to adhere well to epoxy-based molding compounds.
[0008] Another example of known technology to increase adhesion
between leadframe, chip, and encapsulation compound in
semiconductor packages, is the roughening of the whole leadframe
surface by chemically etching the leadframe surface after stamping
or etching the pattern from a metal sheet. Chemical etching is a
subtractive process using an etchant. When, for some device types,
the roughening of the metal has to be selective, protective masks
have to be applied to restrict the chemical roughening to the
selected leadframe areas; the application of masks is
material-intensive and thus expensive. Chemical etching creates a
micro-crystalline metal surface with a roughness on the order of 1
.mu.m or less.
[0009] Yet another known method to achieve a rough surface is the
use of a specialized nickel plating bath to deposit a rough nickel
layer. This method is an additive process; it has to employ a
protective photomask when the deposition has to be restricted to
selected leadframe portions. The created surface roughness is on
the order of 1 to 10 .mu.m.
SUMMARY OF THE INVENTION
[0010] Applicant recognized that two major contributors to good
adhesion are the chemical affinity between the molding compound and
the metal finish of the leadframe, and the surface roughness of the
leadframe. In recent years, a number of technical trends have made
it more and more complicated to find a satisfactory solution for
the diverse requirements. First of all, package dimensions are
shrinking, offering less surface area for adhesion. Then, the
requirement to use lead-free solders pushes the reflow temperature
range into the neighborhood of about 260.degree. C., making it more
difficult to maintain mold compound adhesion to the leadframes.
This is especially true for the very small leadframe surface
available in QFN (Quad Flat No-lead) and SON (Small Outline
No-lead) devices.
[0011] Applicant further recognized that it is counterproductive
when contemporary leadframes have metal layers plated for enhanced
wire bonding or solderabililty and use flood plating as a low cost
plating method, resulting in plated metals in areas which are
superfluous for bonding or soldering but rather should be utilized
for enhancing adhesion. Improved definition of leadframe functions
calls for selective metal layer plating. Applicant saw that for
selective plating, traditional masks which just protect and are
otherwise inactive, are not practical because reusable rubber masks
are not suitable for slow plating processes or precision multilayer
plating, and photoimagible resist masks are too expensive,
especially for multilayer plating.
[0012] Applicant solved the problem of moisture-induced device
failures caused by insufficient adhesion by introducing the concept
of selective active marking. The marker, in contact with selected
areas of a first metal, actively oxidizes the areas so that a layer
of a second metal, deposited by a subsequent plating step, will
barely adhere and can thus be peeled away easily; the second metal
may not even deposit in the first place. As a result, the first
metal of the leadframe is plated only in un-marked areas with a
layer of a second metal, while the un-plated oxidized areas are
greatly improved for adhering to polymeric compounds.
[0013] In one method, the leadframe is contacted with a rubber
stamp patterned by mesas, which have been dipped in a strongly
oxidizing chemical agent such as sodium hypochlorite (NaOCl), which
can be easily cleaned away. Alternatively, any suitable bleach may
be used.
[0014] In an alternative method, applicant used an apparatus of
heated probes to locally contact and oxidize the leadframe. The
apparatus carrying the probes, patterned to match the leadframe
areas to be oxidized, may include electrically heated probes, where
the time needed for locally reaching elevated temperatures is
short; the spreading of thermal energy into adjacent leadframe
regions is thus short, causing only minor oxidation, which can be
removed by acid treatment before dipping the leadframe into the
plating station.
[0015] The preferred plating method is the low-cost flood plating.
The areas of plated metal may have diffuse or uneven edges, which,
however, do not affect functionality. Any traces of second metal
loosely deposited on the oxidized areas are easily peeled off.
[0016] It is a technical advantage that the methods of the
invention can be applied even to the fine geometries QFN/SON-type
leadframes (Quad Flat No-Lead, Small Outline No-Lead). It is
another advantage that the methods are low-cost and the employed
tools can be re-used.
[0017] The first metal may be copper or a copper alloy;
alternatively, the first metal may be aluminum, an iron-nickel
alloy (such as Alloy 42), or Kovar.TM.. The second metal may be
nickel; alternatively, the second metal may include a layer of
nickel in contact with the first metal, a layer of palladium in
contact with the nickel, and a layer of gold in contact with the
palladium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 depicts a schematic top view of a leadframe after
forming it from a starting sheet of a first metal.
[0019] FIG. 2 is a schematic cross section of the formed leadframe
of FIG. 1.
[0020] FIG. 3 illustrates a schematic cross section of a first
apparatus, suitable for oxidizing selected areas of the leadframe,
applied to the leadframe of FIG. 2.
[0021] FIG. 4 shows a schematic cross section of the leadframe of
FIG. 3 after removing the first apparatus used for the oxidation
step.
[0022] FIG. 5 shows a schematic cross section of a second
apparatus, suitable for oxidizing selected areas of the leadframe,
applied to the leadframe of FIG. 2.
[0023] FIG. 6 shows a schematic cross section of the leadframe of
FIG. 5 after removing the second apparatus used for the oxidation
step.
[0024] FIG. 7 depicts a schematic top view of the leadframes of
FIG. 4 and FIG. 6 after the oxidation step illustrated in FIG. 3
and FIG. 5.
[0025] FIG. 8 illustrates a schematic top view of the leadframe of
FIG. 7 after plating a second metal over the whole leadframe.
[0026] FIG. 9 shows a schematic cross section of the leadframe of
FIG. 4 after plating a second metal over the whole leadframe.
[0027] FIG. 10 shows a schematic cross section of the leadframe of
FIG. 6 after plating a second metal over the whole leadframe.
[0028] FIG. 11 illustrates a schematic top view of the leadframe of
FIG. 8 after removing the second metal layers plated over the
oxidized leadframe portions.
[0029] FIG. 12 depicts a schematic cross section of the leadframe
of FIG. 9 after removing the second metal layers plated over the
oxidized leadframe portions.
[0030] FIG. 13 depicts a schematic cross section of the leadframe
of FIG. 10 after removing the second metal layers plated over the
oxidized leadframe portions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] FIG. 1 illustrates schematically the top view of a metal
leadframe, generally designated 100, as used in a wide variety of
semiconductor devices, and FIG. 2 is a cross section of the
leadframe along the line indicated in FIG. 1. Not shown in FIGS. 1
and 2 are frame and tie bars of the leadframe. Leadframe 100 is
made of a first metal selected from a group including copper,
copper alloy, aluminum, iron-nickel alloy, and Kovar.TM.. The
leadframe originates with a sheet of the first metal in the
preferred thickness range from 100 to 300 .mu.m; thinner or thicker
sheets are possible. The pattern of the leadframe is stamped or
etched from the starting sheet; it includes a pad 101 for attaching
a semiconductor chip and a plurality 102 of leads.
[0032] FIG. 2 displays a cross section of the leadframe including
pad 101 and leads 102. As FIG. 2 indicates, all leadframe features
have a plurality of surfaces. For example, pad 101 has a first
surface 101a and an opposite second surface 101b. It should be
noted that herein first surface 101a is referred to as the top
surface since it serves as the attachment surface for a
semiconductor chip, and second surface 102 is referred to as the
bottom surface. Further, pad 101 has a plurality of side surfaces
101c. In analogous fashion, lead 102 has a top surface 102a, a
bottom surface 102b, and a plurality of side surfaces 102c. These
surfaces have to support a number of functions essential for
successful assembly, packaging, and operation of the semiconductor
device; examples of such functions include attaching a chip,
welding wire stitches, adhering to polymeric compounds, and
soldering to external parts. To enable these functions reliably,
the respective leadframe portions have to be given appropriate
surface constitutions. For example, reliable adhesion to adhesive
and molding compounds requires leadframe surfaces with affinity to
polymeric formulations; on the other hand, reliable soldering
requires leadframe surfaces, which can be wetted. The leadframe
zones of different constitutions are often located side by side, or
are restricted to selected areas.
[0033] As an example, in FIG. 2 surface 101a may have to be
prepared for attaching a semiconductor chip, surfaces 101b and 101c
for adhering to a molding compound. Even more complicated, a
portion of surfaces 102a may have to be prepared for stitch-welding
a bonding wire, another portion for adhering to a molding compound,
and yet another portion for wetting by a solder. The surfaces 102c
nearest pad 101 may have to be prepared for adhesion to molding
compounds, and surfaces 102c remote from pad 101 for wetting by
solder.
[0034] Among the popular methods to achieve surface constitutions
for reliable welding and soldering are the plating techniques for
depositing metal layers. However, when a plating technique involves
the use of masks, the ongoing trend to miniaturize semiconductor
devices and scale leadframes makes masking expensive, especially
when photomasks and alignments are employed. Repeated mask
applications, often required for consecutive plating baths, are
uneconomical. An example are the consecutive depositions of a
nickel layer on the first metal, followed by a palladium layer on
the nickel layer, followed by a gold layer on the palladium
layer.
[0035] Adhesion is the tendency due to intermolecular forces for
matter to cling to other matter. Among successful metal surface
constitutions for reliable adhesion to polymeric compounds are
metal oxides. Incidental exposure to ambient and operations, such
as clean-ups under environmental conditions, allows the interaction
of oxygen with surface-near metal atoms to form oxides, creating
thin and usually incomplete metal-oxide films. The readiness for
oxide-formation (release of electrons) increases with the
electronegative potential of the metal; for example, aluminum has
an electrochemical potential of -1.66 V (relative to a hydrogen
electrode), nickel -0.25 V, copper of +0.34 V, gold +1.5 V. The
more electronegative an element is, the stronger is its reducing
force, or the easier it can be oxidized.
[0036] In contrast to the observation that polymeric compounds
adhere well to metal oxides is the fact that it is difficult to
make a deposited layer of a second metal stick to an oxidized first
metal. After a layer of a second metal is deposited on an oxidized
first metal substrate, it is mechanically easy to peel or scratch
it off due to its lack of adherence to the oxidized first metal. As
stated, the second metal is chosen to promote welding of wire
stitch bonds and soldering to external parts.
[0037] Based on these facts, applicant reversed the conventional
way of selective plating. Rather than using masks to selectively
deposit a second metal layer on a first metal substrate, the new
method uses simple tools to selectively oxidize the first metal. A
layer of second metal is then deposited by a low-cost flood plating
technique, and followed by a peeling of those layer portions, which
do not adhere to the oxidized first metal areas. The result is a
substrate made of a first metal, which exhibits a second metal
layer in certain areas for promoting stitch bonding and solder
attaching, and further exhibits intentionally oxidized surfaces for
promoting adhesion to polymeric compounds.
[0038] FIGS. 3 and 5 illustrate schematically two exemplary methods
and tools for selectively oxidizing a leadframe. The method used in
FIG. 3 is based on a tool functioning as a stamp, which transfers a
strongly oxidizing agent and presses it against leadframe portions.
The method used in FIG. 5 is based on a tool functioning as a sort
of branding iron, which can be heated locally and transfers thermal
energy to leadframe portions for accelerating oxidation of
leadframe metal.
[0039] Shown in FIG. 3 are pad 101 and leads 102 of an exemplary
leadframe, clamped by a top portion 301 and a bottom portion 302 of
an exemplary stamp. The stamp may be made of rubber or any other
material suitable to be patterned in the design of the leadframe
areas to be oxidized and chemically inert enough for transporting a
strongly oxidizing agent such as sodium hypochlorite (NaOCl). In
FIG. 3, top portion 301 displays sections with the original surface
301a and other sections with a recessed surface 301b. The height
difference between surface 301a and surface 301b represents the
height 340 of a mesa; preferably, height 340 is the same for all
mesas. The mesas may have different width; FIG. 3 depicts width
310a for the mesa intended to create an oxidized area for leadframe
pad 101 suitable to attach a semiconductor chip; and width 310b for
leads 102 intended to create oxidized areas of the leads suitable
to enhance adhesion to encapsulation compounds. In FIG. 3, width
310b is the same for all mesas intended for leads 102, but in other
devices, the width 310b may be different for different leads. The
pattern of stamp portion 301 can thus be customized in accordance
with specific leadframe leads. Mesas 310a and 310b transport the
oxidizing agent 320. In FIG. 3, bottom portion 302 is shown with a
uniform original surface 302a, since in is not used with any
pattern in this example. Consequently, the uniform surface 302a
transports the oxidizing agent 320. It should be stressed, however,
that the patterning of top portion 301 and bottom portion 302 can
be changed at liberty from device to device as required by the need
of creating oxidized areas of the leadframe. As an example, no
agent 320 at all may be applied to surface 302a so that there will
be no oxide formed of the first metal surface 102b.
[0040] The arrows 330 in FIG. 3 indicate that at the start of the
process the stamp portions 301 and 302 do not yet touch the
leadframe; rather, they are dipped into an agent 320, or they
otherwise acquire the agent onto their surfaces (for instance by
brushing against it). Thereafter, the stamp portions are aligned
with the leadframe and moved along arrows 330 to touch the
leadframe's first metal (for instance, copper) surfaces. By
pressing stamp portions 301 and 302 against the leadframe metal,
the oxidizing agent becomes active to oxidize the touched first
metal (the result is shown in FIG. 5). When sodium hypochlorite is
the oxidizing agent, NaOCl gives off its oxygen to form the desired
metal oxide (for instance, copper oxide), while salt (NaCl) is left
over. Experience has shown that the edges of the oxidized first
metal areas, bordering the not-oxidized metal area, are somewhat
diffuse.
[0041] The result of the leadframe oxidation step using the stamp
in FIG. 3 is depicted in FIG. 4. The leadframe, in its oxidized
status generally designated 400, shows on its top surfaces 101a and
102a the oxidized surface layers as imprints of the stamp mesas:
Oxide layer 401 on the leadframe pad reflects the extent of the
mesa dimension 310a, and oxide layers 402 on the leads 102 reflect
the extent of the mesa dimensions 310b. On its bottom surfaces 101a
and 101b, leadframe 400 shows the oxidized surface layers as
imprints of surface 302a of the large stamp portion 302;
consequently, the oxide layers stretch across the leadframe pad 101
(oxide layer 403) and portions of the leads 102 (oxide layers
404).
[0042] It should be noted that other techniques, related to but
different from stamping, can produce similar oxidation results. As
an example, one such technique uses the movable jet of an oxidizing
liquid (technique related to ink jet).
[0043] FIG. 5 illustrates a tool for another method to selectively
oxidize leadframe surfaces. The method is based on the fact that
metal oxidation by oxygen of the ambient can be rapidly accelerated
when the temperature of the metal-to-be-oxidized is increased. To
exploit this fact, the exemplary tool of FIG. 5 is based on probes,
which can be individually heated, for instance by electrical
current.
[0044] The exemplary tool of FIG. 5 has a top half 510 and a bottom
half 511. The top half 510 has elongated probes of two distinct
widths 501 and 502, which can be heated. In the example of FIG. 5,
top half 510 with probes 501 and 502 is designed so that the probes
will create leadframe oxide areas of sized to match the oxide areas
produced by the top half 301 of the stamp tool described in FIG. 3.
The shaded cross sections 520 of the probe tips schematically
indicate the volumes of high temperature of the probes. Probe 501
is selected to oxidize a pad area suitable to attach a
semiconductor chip (in the example of FIG. 5 the same area as
created by mesa diameter 310a in FIG. 3).
[0045] The bottom tool half 511 also has probes, which may be
elongated and heatable. In the example of FIG. 5, the bottom half
includes an elongated probe 503 of a width similar to the width of
probe 501, and a plurality of probes 504 of a width similar to the
width of probes 502. Again, the shaded cross sections 520 of the
probe tips schematically indicate the volumes of high temperature
of the probes. In order to emphasize the flexibility of choices,
probe 503 is illustrated as not heatable, while probes 504 are
heatable. It should be pointed out that the probe widths may be
customized to oxidize any metal area desired.
[0046] The result of the leadframe oxidation step using the stamp
in FIG. 5 is depicted in FIG. 6. The leadframe, in its oxidized
status generally designated 600, shows on its top surfaces 101a and
102a the oxidized surface layers as imprints of the heated probes:
Oxide layer 601 on the leadframe pad reflects the extent of the
dimension of probe 501, and oxide layers 602 on the leads 102
reflect the extent of the dimensions of probes 502. On its bottom
surfaces 101a and 101b, leadframe 600 shows the oxidized surface
layers as imprints only of the surfaces of probes 504, since in
this example probe 503 is not heatable; consequently, only leads
102 will receive oxide layers 604.
[0047] It should be noted that other heating techniques can produce
similar oxidation results. As an example, one such technique
employs movable focused laser beams.
[0048] FIG. 7 displays the top surface of an exemplary leadframe
generally designated 700 after selectively oxidizing first metal
areas so that a semiconductor chip can be attached and wire-bonded.
Oxidized areas are marked by a first kind shading in FIG. 7. As
stated above, the exemplary oxidation tools described in FIGS. 3
and 5 had the capability to create identical oxide areas on the top
leadframe surface, as illustrated in the cross sections of FIGS. 4
and 6 and referred to by the cutaway line marked 4,6 in FIG. 7.
Consequently, the identical result of those oxidation techniques is
displayed in FIG. 7. Chip attach pad 101 includes the metal-oxide
area 701a for attaching the semiconductor chip, surrounded by
un-oxidized frame 701b intended for affixing a chip down-bond wire.
The surface of frame 701b displays the unchanged first metal shown
in FIG. 1. Metal-oxide 701a of FIG. 7 has been shown to greatly
improve the adhesion between epoxy-based chip attach compounds and
the leadframe pad 101. For other semiconductor devices, which do
not require wire down-bonds, it is preferred to extend the oxidized
area 701a across the whole chip pad area 101, thus incorporating
the metallic frame area 701b in the oxidized pad portion.
[0049] Of the plurality of leads 102, each lead has an oxidized
area 702a and left-over un-oxidized portions 702b displaying the
first metal of the leadframe. Due to their selectively oxidized
areas 702a, leads 102 offer greatly enhanced adhesion for polymeric
encapsulation compounds.
[0050] After the selective oxidation step of the invention, it is
advisable to clean the leadframe in a so-called reduction step. By
this quick-time clean-up step, any thin, unintentional, or
accidental oxide film can be removed from metal surfaces, which
have not been oxidized by the selective techniques described above.
Metal surface designated 701b and 702b in FIG. 7 will thus be clean
and ready for the deposition step of a second metal described in
FIG. 8.
[0051] Subsequent to the selective oxidation step of the invention,
a layer of a second metal is deposited on leadframe. The preferred
deposition method is a low-cost plating technique such as flood
plating. Alternatively, other deposition methods such as sputtering
or evaporation may be used. The deposited second metal adheres well
to the un-oxidized areas, but only poorly or not at all to the
oxidized areas. The resulting leadframe is shown in FIG. 8 and
generally designated 800; the deposited second metal is generally
indicated by a second kind shading. Since the whole leadframe has
been subjected to the deposition of the second metal, the second
kind shading covers all parts of the leadframe in the top view of
FIG. 8. The fact that the second metal is deposited as a layer
becomes evident by cutaways along the line marked 9,10 in FIG.
8.
[0052] If the leadframe had been selectively oxidized by the stamp
technique of FIG. 3, whereby the oxidized areas were formed in a
distribution as shown in the cross section of FIG. 4, the cutaway
line in FIG. 8 through the plated leadframe produces a cross
section of the plated leadframe as illustrated in FIG. 9. If the
leadframe had been selectively oxidized by the probe technique of
FIG. 5, whereby the oxidized areas were formed in a distribution as
shown in the cross section of FIG. 6, the cutaway line in FIG. 8
through the plated leadframe produces a cross section of the plated
leadframe as illustrated in FIG. 10. In both FIGS. 9 and 10, the
second metal is deposited as layer 901 on all surfaces of the pad
101 and the leads 102. Layer 901 is uniform in all leadframe areas
where the second metal is deposited on the first metal; the second
metal layer also adheres strongly to the first metal. However,
where FIGS. 9 and 10 show layer 901 over oxidized areas (401, 402,
403, 404, 601, 602, 604), the second metal is not adhering to the
first metal. For some deposition techniques and for some metals,
layer 901 may be much thinner over oxidized areas than over first
metal areas, or layer 901 may have been unable to stick to oxidized
surfaces, leaving the oxidized areas devoid of second metal and
looking un-covered and open.
[0053] As an example for leadframes with copper as first metal, a
frequently used plating step includes nickel as second metal; the
thickness of layer 901 may vary from submicron to several .mu.m. It
is a technical advantage of the invention to provide a patterned
layer of the relatively slowly deposited nickel without the help of
re-usable rubber masks, which are known to be cumbersome for metals
with slow plating rates.
[0054] Another frequently employed second metal includes a nickel
layer in the thickness range from about 0.5 to 2.0 .mu.m in contact
with the first metal copper, followed by a palladium layer in the
thickness range from about 0.01 to 0.1 .mu.m in contact with the
nickel layer, followed by a gold layer in the thickness range from
about 0.003 to 0.009 .mu.m in contact with the palladium layer. It
is a technical advantage of the selective oxidation approach that
these stacks of metal layers can be deposited, and are inherently
precisely aligned, without photomasks and without alignment; the
sequence of layers can be deposited just by moving from one plating
bath to the next. In contrast, it is known that conventional
selective plating of multilayer structures (NiPd, NiAu, NiPdAu,
etc.), which include nickel, is time consuming and costly because
of the need for photoimagible plating resist.
[0055] Yet another example of second metal is tin; the thickness of
layer 901 may vary over a wide range.
[0056] The fact that any second metal deposited on selectively
oxidized first metal areas adheres poorly or not at all to the
surface of the oxidized first metal, allows an easy process step of
removing any such deposited second metal from those oxidized areas.
For example, any second metal layer 901 plated on oxidized areas
can peeled by mechanical means from the oxidized metal. Preferred
methods include removing by air knife, water jet, bead blast, and
tape. It has been found that peeling can be promoted by briefly
heating the leadframe after the plating step. Further, it has been
found that the thickness of the metal oxide layer can be optimized
so that in the plating process, no or extremely low deposition
occurs on the oxidized metal surface. This phenomenon makes an
additional step of removing any plated second metal
superfluous.
[0057] FIG. 11 displays the top surface of an exemplary finished
leadframe of a first metal after selective oxidation of first metal
areas and subsequent deposition of a second metal on the
un-oxidized first metal areas. The leadframe, generally designated
1100, has the oxidized areas marked by a first kind shading (like
in FIG. 7) and the areas with a second metal layer by a second kind
shading (like in FIG. 8). Since any second metal deposited in
oxidized areas has been removed by a peeling process as described
above, the oxidized areas are clearly exposed and visible. Chip
attach pad 101 includes the metal-oxide area 701a for attaching the
semiconductor chip, surrounded by frame 1101 covered by second
metal and intended for affixing a chip down-bond wire. Metal-oxide
701a of FIG. 11 has been shown to greatly improve the adhesion
between epoxy-based chip attach compounds and the leadframe pad
101. For other semiconductor devices, which do not require wire
down-bonds, it is preferred to extend the oxidized area 701a across
the whole chip pad area 101 and eliminate metal frame 1101a.
[0058] Of the plurality of leads 102, each lead has an oxidized
area 702a and portions 1102 displaying the deposited second metal.
Due to their selectively oxidized areas 702a, leads 102 offer
greatly enhanced adhesion for polymeric encapsulation compounds,
and due to their deposited second metal 1102, leads 102 offer
greatly enhanced bondability for wire bonds and metal bumps, and
solderability for solder attachment.
[0059] The cutaway line marked 12,13 in FIG. 11 indicates the cross
sections of FIGS. 12 and 13 of the finished leadframe. If the
leadframe had been selectively oxidized by the stamp technique of
FIG. 3, whereby the oxidized areas were formed in a distribution as
shown in the cross section of FIG. 4, the cutaway line in FIG. 11
through the plated and cleaned leadframe produces a cross section
of the plated leadframe as illustrated in FIG. 12. If the leadframe
had been selectively oxidized by the probe technique of FIG. 5,
whereby the oxidized areas were formed in a distribution as shown
in the cross section of FIG. 6, the cutaway line in FIG. 11 through
the plated and cleaned leadframe produces a cross section of the
plated leadframe as illustrated in FIG. 13. In both FIGS. 12 and
13, the second metal is deposited as layer 901 on all surfaces of
the pad 101 and the leads 102, but has been removed and cleaned
from all oxidized first metal areas: Chip attach pads 401 and 601;
encapsulation adhesion areas 402, 403, 404, 602, and 604. Layer 901
is uniform in all leadframe areas where the second metal is
deposited on the first metal; the second metal layer also adheres
strongly to the first metal. As mentioned above, the deposited
second metal layers have diffuse edges bordering the oxidized areas
(in contrast to sharply defined edges in devices employing
photomasks).
[0060] While this invention has been described in reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various modifications and
combinations of the illustrative embodiments, as well as other
embodiments of the invention, will be apparent to persons skilled
in the art upon reference to the description. As an example, the
invention applies to products using any type of semiconductor chip,
discrete or integrated circuit, and the material of the
semiconductor chip may comprise silicon, silicon germanium, gallium
arsenide, or any other semiconductor or compound material used in
integrated circuit manufacturing.
[0061] As another example, the invention applies to all
leadframe-based semiconductor packages.
[0062] As another example, the oxidation process steps described
can be combined with other techniques to improve adhesion such as
surface roughening, forming dimples and other features for enhanced
grasping, and partial etching, including so-called half-etched
leadframes.
[0063] It is therefore intended that the appended claims encompass
any such modifications or embodiment.
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