Process for preparing solderable integrated circuit lead frames by plating with tin and palladium

A process for preparing a solderable lead frame from a copper base lead frame is disclosed using plating of tin or tin alloys followed by plating of palladium. Preferably, the tin plating is a spot plating to deposit tin only on the external leads and the palladium plating is a flood plating to deposit palladium over the entire lead frame. A diffusion barrier, preferably of cobalt or nickel, can be applied by plating the base lead frame before tin plating.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the manufacture of integrated 
circuit lead frames. More particularly, it relates to a process for 
coating these lead frames to enhance the bonding of their internal leads 
to bonding wires connected to an integrated circuit (IC) and the 
solderability of their external leads to a circuit board. 
Lead frames are the standard means for connecting a microscopic integrated 
circuit to outside circuitry and are usually made of copper. Typically, a 
die or chip containing the integrated circuit is attached to a bonding pad 
of a lead frame. Once the chip is attached, wires are bonded to 
input/output pads of the integrated circuit and to internal leads of the 
lead frame. This arrangement of a chip, a lead frame, and bonding wires is 
then encapsulated in a plastic casing leaving external leads of the lead 
frame exposed (outside of the plastic casing). The packaged integrated 
circuit can then be connected to other electronic components on a 
conventional circuit board by its external leads. 
Integrated circuits have a multitude of uses. The packaged IC's themselves 
and the macroscopic circuit boards in which they are soldered are mass 
produced. Therefore, improvements in the manufacture and processing of the 
components of the packaged IC can have broad implications. 
Lead frames plated with solder can have distinct advantages over unplated 
ones. By providing the solder directly on the lead frame, soldering of the 
leads to circuit boards is more reliable. Similarly, the process of 
bonding wires to the internal leads can be improved by plating the bonding 
areas of the internal leads with appropriate materials. 
One lead frame technology has been described in European Patent Application 
89302939.7 and is diagrammed in FIG. 1. This process 200 stars with lead 
frames being payed out 202 by a reel. Subsequently, they are degreased and 
cleaned at a step 204. After a simple washing, they are electrocleaned in 
a caustic alkaline solution. By then bathing the frame in a pickling acid, 
the base solution is neutralized, non-bound copper is removed, and a rough 
clean surface suitable for plating is formed. Finally, the lead frame is 
rinsed before plating. 
The clean base frame is then plated overall with nickel at a step 206 by a 
conventional flood plating process. The initial nickel plating is followed 
by a palladium/nickel coating step 208. This in turn is followed by a 
nickel flood plating at a step 210 and then a palladium flood plating at a 
step 212 resulting in a lead frame having four coatings over the entire 
base copper frame. To complete the processing, the lead frames are 
cleaned, rinsed, and dried 214 before finally being wound 216 on a take-up 
reel. 
A variation of this four-layer process is described in European Patent 
Application 90300934.8. In that process, a two-layer lead frame is made by 
first applying nickel and then palladium. A further variation is described 
in U.S. Pat. No. 4,628,165. In that process, a three-layer frame is formed 
by plating with nickel, followed by palladium/nickel, followed by 
palladium. 
The above palladium layer approaches have been promoted as having good 
manufacturability. However, the solderability of the external leads of the 
processed frames, the primary reason for pre-plating, is quite 
questionable unless very corrosive fluxes are used. Texas Instruments has 
attempted the cumbersome technique outlined in European Patent Application 
903,009,34.8 of dipping in or plating with a tin/lead alloy to enhance 
solderability. This step is performed after the lead frames have been cut 
from the reel into strips, assembled as strips of IC devices, then hung on 
racks and dipped or plated with tin/lead. It is known in the art as a 
cut-strip plating process. Thus, this post-plating of tin/lead is not part 
of the continuous process and suffers from the disadvantages inherent 
therein. 
A state of the art technique for pre-plating lead frames is the Dyna-Craft 
PPF-3000, pre-plated frame, technology. An early version of PPF-3000 is 
described in U.S. Pat. No. 4,486,511. One version of PPF-3000 utilizes 
tin/lead platings on a copper lead frame base. A flow chart of the 
PPF-3000 manufacturing process 220 is shown in FIG. 2. 
In this process, as in most continuous processes, lead frames undergo much 
of their processing in reel to reel form. After being payed out at step 
222 by a reel, they must be degreased and cleaned at a step 224 just as in 
step 204 of the process described in FIG. 1. The clean base frame is then 
plated overall 226 with a nickel coating of about 30 to 60 microinches. 
This layer serves as a diffusion barrier to prevent diffusion of the 
copper in the lead frame base into a subsequently applied tin layer. 
Copper is known to rapidly diffuse into tin and tin alloys at standard 
integrated circuit packaging temperatures and thereby degrade 
solderability. 
Copper is subsequently spot plated in step 228 on the inner leads and die 
attach pad over the coat of nickel by a spot plating. Spot plating 
technology for lead frames appears in the three U.S. Pat. Nos. 4,405,432, 
4,404,079 and 4,404,080. Next, silver is spot plated 230 to aid the 
bonding of wires from the chip and improve the electrical conductivity and 
bondability at the wire bond junctions. The copper plating acts as a 
metallic glue to attach the silver to the nickel and to enhance the 
adhesion of the plastic encapsulation to the lead frame. To improve 
solderability to external circuitry, a tin/lead compound is spot plated 
232 on the external leads. The finished pre-plated frame is then rinsed 
and dried 234 and finally wound 236 onto a take-up reel. 
Although PPF-3000 represented a major advance in simplified IC packaging 
technology, and PPF-3000 frames sail through all solderability tests, the 
process has a few drawbacks. For example, its multiple spot platings 228, 
230, and 232 are equipment intensive and may require frequent adjustments 
and careful positional alignment of the locations of the edges or 
boundaries of the plated spots (sometimes referred to as registration 
requirements) of the processing apparatus. 
Also, a visible boundary of the tin/lead coat must be covered by the 
plastic encapsulation for cosmetic reasons. However, the tin/lead coat 
cannot reside too far inside the encapsulation, specifically within the 
areas where wirebonding will occur. Otherwise, the tin/lead may 
contaminate and disturb the delicate metallurgy of the wirebond to lead 
frame contact welds, rendering them unreliable. Therefore, a tight 
registration requirement is created for the tin/lead boundary to lie 
inside of the plastic encapsulation, primarily a cosmetic concern, yet far 
enough away from the wirebonding areas so as not to contaminate them. Many 
perfectly functioning packaged integrated circuits are discarded for 
merely failing the cosmetic requirement by having a visibly exposed 
tin/lead boundary. 
A new method for preparing solderable lead improving on the 
manufacturability of PPF-3000, while simultaneously overcoming the 
inherent solderability problems of the TI manufacturing scheme would be a 
great advance. Namely, the new method should be a continuous pre-plating 
process having good manufacturability. It should create lead frames 
without the cosmetic requirements of PPF-3000, provide good solderability 
of the external leads without the use of highly corrosive fluxes, and 
permit easy ultrasonic welding of bonding wires to the internal leads. 
This method should also be flexible enough to incorporate the use of 
Lead-free eletroplateable solders to permit removal of lead from the 
manufacturing process and the finished lead frames. 
SUMMARY OF THE INVENTION 
To achieve the foregoing and in accordance with the purpose of the present 
invention, a method for preparing a solderable lead frame having several 
leads is disclosed including the steps of first tin plating the leads of a 
base lead frame and then palladium plating all or a portion of the base 
lead frame after the tin plating. Preferably, both steps are performed as 
part of the same continuous process. The tin plating step forms tin-plated 
regions on the external leads of the base lead frame and is preferably 
performed in a spot plating reactor. The palladium plating step coats the 
tin-plated portions and is preferably performed in a flood plating 
reactor. In a preferred embodiment, the tin plating can include a tin 
alloy having a eutectic point between about 180 to 300 degrees centigrade. 
Examples of such alloys include tin alloyed with one or more of antimony, 
arsenic, bismuth, cadmium, gallium, gold, indium, iridium, lead, platinum, 
rhodium, ruthenium, silver, thallium, or zinc although other alloys are 
possible. In a further preferred embodiment, the tin or tin alloy is 
deposited only on the external leads of the base lead frame. In a still 
further preferred embodiment, a step of plating the base lead frame with a 
diffusion barrier occurs before the tin plating step such that the base 
lead frame material is prevented from diffusing into the tin. If the base 
lead frame material contains copper, the diffusion barrier preferably 
contains cobalt or nickel. 
In an aspect of the invention, a solderable lead frame having several leads 
is disclosed. Each lead possesses an internal region toward the center of 
the frame, closer to the die attach pad, and an external region away from 
the center of the frame. The solderable lead frame includes a base lead 
frame, a layer of tin or tin alloy, and a layer of palladium. The layer of 
tin or tin alloy coats the external regions of the leads and forms a 
boundary between the internal and external regions. The layer of palladium 
coats the tin-plated external leads, the boundary, and a part of the base 
lead frame. In a preferred embodiment, there is a diffusion barrier 
beneath the layer of tin or tin alloy. If the base lead frame material 
contains copper, the diffusion barrier preferably contains cobalt or 
nickel. The solderable lead frame preferably satisfies the MilSpec 883 
solderability standard. 
In another aspect of the invention, a packaged integrated circuit is 
disclosed including a solderable lead frame, an integrated circuit die, 
several bonding wires connecting the lead frame to the integrated circuit 
die, and an outer molding. The solderable lead frame has a layer of tin or 
tin alloy on its external regions and a layer of palladium on both its 
internal and external regions. The solderable lead frame of the packaged 
IC also preferably includes a diffusion barrier which prevents the base 
lead frame material from diffusing into the tin or tin alloy.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 3 shows a flowchart of a preferred embodiment of the continuous 
pre-plating process 100 of the present invention. Initially, a continuous 
lead frame ribbon, stamped or etched out of a copper base material, is 
payed out 102 from a lead frame reel. It must be degreased and cleaned in 
step 104 to remove a thin oil coat of stamping oils, antioxidants, and/or 
remnants of the photoresist. After a simple washing, the frames are 
connected to a direct current source and electrocleaned by immersion in 
caustic materials such as caustic soda, potash, soda ash and surfactant 
solutions. Hydrogen formed during the electrochemical reaction acts as a 
scrubbing agent and reduces copper oxides. By then bathing the frame in an 
acid pickling solution after rinsing, any adherent electrocleaning 
solution is neutralized, non-metallurgically bound base materials are 
removed, and a microscopically rough surface suitable for plating is 
formed. Finally, the lead frame is rinsed before plating. 
Although a base lead frame material will typically consist of solid copper 
or copper alloy, it need not. Indeed, the typical base lead frame may be 
formed from another substance such as alloy 42 or kovar and suitably 
prepared for plating. The preferred method of preparation is to coat these 
other substances with several microinches of copper or copper alloy to 
form copper surfaces. By describing base lead frames as substantially of 
copper, it is meant to include the case of a base frame having an exposed 
copper surface as well as those base frames whose primary or sole element 
is copper or a copper alloy. For illustrative purposes, only substantially 
copper base lead frames will be described below. 
After preparation for plating, an electroplated or clad plated layer which 
serves as a diffusion barrier layer is applied 106 in a flood plating 
reaction chamber. Flood plating is a relatively simple process used to 
apply a coating over an entire lead frame. Flood plating will be described 
in more detail during the discussion of FIG. 4 below. It is understood 
that the diffusion barrier layer does not have to be applied at this 
stage, it in fact may have been previously clad to the base frame at an 
earlier point in the base frame manufacturing process. 
It should be noted that the spot plating reactors described herein hold the 
lead frame ribbon in place between masks for a defined length of time 
while the spot is being plated, then release the lead frame ribbon, and 
the process controllers advance the ribbon to the next group of spots to 
be plated, where the process repeats. This is known as "step-and-repeat" 
operation. The flood plating reactors, however, can operate simultaneously 
with or without the lead frame ribbon being held in place during plating. 
Other readily available spot plating reactors are designed to operate 
without holding the lead frame ribbon in place for defined lengths of 
time. When operating the flood plating reactors continuously, and the spot 
plating reactors in step-and-repeat fashion, the lead frame ribbon is 
allowed to accumulate in between reactors. 
Cobalt, nickel and cobalt/nickel alloys are examples of suitable diffusion 
barriers used in step 106. They dramatically reduce the extensive 
diffusion of the copper of the base lead frame material into a tin or tin 
alloy layer which is later applied. Nickel also prevents the diffusion of 
iron from a kovar base lead frame material into the tin or tin alloy 
layer. Particularly at the temperatures used for curing adhesives used to 
attach an integrated circuit die to a bonding pad area of the lead frame, 
copper quickly diffuses into tin and forms copper/tin alloys via a solid 
state chemical reaction. This accumulation of copper/tin alloys at an 
interface of the base frame with the tin degrades solderability and 
defeats much of the purpose of pre-plating. An appropriate thickness of a 
nickel or cobalt diffusion barrier is about 15 to 100 microinches with a 
preferred range of about 30 to 60 microinches. The layer must be thick 
enough to effectively reduce the diffusion of the copper into tin to 
maintain solderability of the tin. Because of this solderability 
requirement, the lower limit is partially dependent upon the amount of tin 
or tin alloy applied at a later stage. Upper thickness limits depend 
partly on the spacing between the leads and on the size of spacings and 
openings of the lead frame. Additionally, excessively thick diffusion 
barrier layers may crack upon bending of the leads thereby reducing their 
effectiveness. 
After the application of the diffusion barrier, the base lead frame is fed 
or drawn into a spot plating reaction chamber to perform plating step 108. 
A spot plating reaction chamber is a particular type of step-and-repeat 
electroplating reactor so named because it divides a continuous process 
into repeated static steps as described above. Each step-and-repeat 
process requires relatively precise alignment of a mask to the lead frame. 
Spot plating will be described in more detail during the discussion of 
FIG. 5 below. 
State of the art preplating technologies, such as the PPF-3000 process 220, 
use spot plating. However, their use of spot plating is restricted by 
registration requirements defining alignment tolerances between the spot 
plating apparatus and portions of the lead frames. These registration 
requirements are partly dictated by the need to align successive spot 
platings. They are also dictated by the desire to cover the boundaries of 
plated portions with integrated circuit packaging material during final 
assembly of the packaged IC. 
Only one step-and-repeat reaction chamber, that used in step 108, is 
required in the process of the invention 100 unlike the three steps 228, 
230, and 232 used in PPF-3000. The tolerances arising solely from the 
alignment of the spot plating apparatus with the lead frames will thus be 
minimized. Additionally, since a single spot plating step 108 occurs in 
the process of the invention 100, the added restriction of electroplating 
boundary encroachment of successive spot platings will be completely 
eliminated. Together, these advantages afford a much higher degree of 
manufacturability of the processed lead frames. A subsequent palladium 
plating step will obviate the primarily cosmetic tin/lead registration 
requirement related to the final IC packaging. 
In the spot plating reaction chamber, tin or tin alloy is selectively 
deposited in step 108 onto an external region of the lead frame away from 
the center of the frame. If all of the external leads are coated, the edge 
of the applied tin or tin alloy essentially defines a boundary between the 
external and internal regions of the lead frame. A thickness of tin 
commensurate with good solderability of the external leads is about 50 to 
500 microinches with a preferred range of about 300 to 500 microinches. 
The spacing between leads and the bending of the external leads, along 
with the economic concerns over the time and materials required to plate 
unnecessary thick layers place upper limits on the thickness of the tin. 
Various electrolytes and processing conditions can be employed to plate the 
tin layers of this invention. For example, the electrolyte could contain 
salts of tin and salts of desired alloying elements in concentrations 
necessary to provide a desired alloy composition. The low reflow 
temperature of tin/lead solder of 183 degrees centigrade slows the curing 
process thus limiting the variety of adhesives for bonding the die to the 
pad. Therefore, alloys of tin having high eutectic points between 180 and 
300 degrees centigrade are particularly desirable. Such alloys can be 
obtained when tin is alloyed with one or more of antimony, arsenic, 
bismuth, cadmium, gallium, gold, indium, iridium, lead, platinum, rhodium, 
ruthenium, silver, thallium, or zinc. Of course, other tin alloys may also 
be used. 
Further referring to FIG. 3, the base lead frame (which is now partially 
coated with tin or tin alloy) is passed into a second flood plating 
reaction chamber during step 110 There the frame receives an overall coat 
of palladium while still on the lead frame ribbon and as part of the same 
continuous plating process 100. This palladium layer covers the tin coated 
portions and the boundary. The palladium layer should be between about 2 
to 20 microinches thick with a preferred range of about 3 to 6 
microinches. 
Those of skill in the art will appreciate that each electroplating step 
(i.e., diffusion barrier plating 106, tin/tin alloy plating 108, and 
palladium plating 110) can be conducted under many different conditions. 
For example, by varying the electrolyte composition, current density, lead 
frame residence time in the reactor, anode configuration, temperature, 
etc., the thickness and conditions of the coating can be controlled. The 
principles underlying those electroplating steps are described in standard 
treatises such as Electroplating by Lowenheim which is incorporated herein 
by reference for all purposes. It should be understood that the present 
invention is directed to a sequence of process steps and to the products 
of those process steps. 
The final steps 112 of the continuous pre-plating process 100 of the 
present invention are cleaning, rinsing, and drying the finished 
solderable lead frame. The lead frame is then wound 114 onto a take-up 
reel. Although the more efficient continuous reel method has been 
emphasized, the same flood and spot plating steps 106, 108, and 110 can be 
performed on individual lead frames or cut strips of lead frames moved 
from reactor to reactor. 
FIG. 4 is a diagrammatic representation of an exemplary flood plating 
reaction chamber or reactor 25. A tub 26 contains anodes 27 immersed in an 
electrolyte. The anodes 27 are typically either wire screens or baskets. 
An anode screen is generally titanium or platinized titanium. An anode 
basket is generally made of a titanium screen and contains chunks of the 
metal, such as nickel or palladium, which will be deposited. The anodes 
are connected to a pulsed or direct current source 28. 
Feed tubes 29 permit the electrolyte to be pumped into the tub 26 as 
indicated by the arrows. This pumping causes electrolyte to overflow as 
indicated by the arrows. The overflowing electrolyte will be captured by a 
reservoir (not shown) beneath the tub 26 and pumped through feed tubes 29 
back into the tub 26. 
A lead frame ribbon 30 is made cathodic by a connection to the pulsed or 
direct current source 28 and passed through the tub 26. This results in 
electrodeposition. Slits 31 may contain gaskets to prevent overflow of 
electrolyte through the slits 31. Clearly, many variations of the flood 
plating reactor 25 are possible. 
Metal ions from the electrolyte are deposited as metal layers onto the 
ribbon 30 by electrodeposition. Of course, the electrolytes used to plate 
different metals (e.g., cobalt/nickel as opposed to palladium) are 
different although the flood plating apparatus are generally similar. In 
the process of the invention 100, flood plating is performed in the 
diffusion barrier and palladium platings, invention steps 106 and 110. 
The same principles of plating are used in the spot plating reaction 
chamber or reactor 34 represented in FIG. 5. However, the plating regions 
are limited to certain "spots" on the lead frame. Inert perforated shells 
36 hold several anode wires 38. These shells 36 have resilient inert faces 
40 usually made from silicone. A pulsed or direct current source 42 is 
connected to these anode wires and lead frame ribbon 30. The lead frame 
ribbon 30 is clamped between the inert shells 36. Electroplating solution 
is jetted through the perforations in the shells 36 onto the ribbon 30 via 
a suitable pumping mechanism. In this way, the resilient faces 40 mask off 
areas of the ribbon where plating is not desired. But, the silicone rubber 
faces 40 only allow those portions of the lead frame exposed by the holes 
to react with the electrolyte. In the process of the invention 100, the 
tin spot plating step 108 selectively plates external leads of lead frames 
via this masking procedure as taught by the Kosowsky patent, U.S. Pat. No. 
4,405,432. 
The lead frames produced by the process of the present invention will now 
be described in more detail. FIG. 6 shows a top view of a conventional 
copper base lead frame 10 after intermediate invention step 108. After 
being payed out from a reel in step 102, the entire base frame 10 is flood 
plated during step 106 with a diffusion barrier of cobalt or nickel. The 
external regions including the external leads 12 are then spot plated 108 
with tin or tin alloy to enhance their solderability to external 
circuitry. This spot plating does not coat the die attach pad 14, nearby 
portions of the internal leads 16, or the inner portions of the fishtails 
18 supporting the die attach pad 14. A critical region containing the die 
attach pad 14 where tin should not reside is outlined in a dashed line in 
FIG. 6. Bonding wires will eventually be ultrasonically welded to the 
portions of the internal leads 16 which are near the die attach pad 14. It 
is undesirable to have even residual amounts of tin on areas where the 
wires will be bonded because non-precious metals are generally regarded as 
contaminants in wire bonding areas. This "partial" or "spot" plating of 
the frame will leave a boundary 20 which generally defines the boundary 
between internal and external regions subsequent to the formation of the 
tin coated portions. The entire base frame 10 is finally coated in 
invention step 110 with a layer of palladium to cover the tinplated 
portions of the external leads 12, the boundary 20, and the areas not 
plated with tin. Thereafter, the finished solderable pre-plated lead frame 
is cleansed, rinsed, dried, and gathered up on a take-up reel in the final 
processing steps 112 and 114. 
In FIG. 6, there are fourteen external leads 12 and fourteen internal leads 
16. Each external lead has a corresponding internal lead. Dam bars 22 
interconnecting the individual leads are eventually removed during 
packaging of the chip. Rails 24 touch sliding contacts supplying pulsed or 
direct current during the various plating stages. This electric connection 
through the rails 24 makes the lead frame ribbon 30 cathodic in the flood 
and spot reactors represented in FIGS. 4 and 5. The rails 24 are also 
eventually removed during IC packaging. 
Referring now to FIG. 7, a side cross section of a packaged integrated 
circuit 43 using a solderable lead frame of the present invention is 
shown. (FIG. 6 is a top view of the lead frame 10.) A lead having an 
external lead 12 and an internal lead 16 is shown. The external lead 12 
and internal lead 16 share an outer layer of palladium 44. The external 
lead 12 also has a layer of tin or tin alloy 46 immediately beneath the 
palladium layer 44. A cobalt or nickel diffusion barrier 48 coats both the 
internal lead 16 and the external lead 12. The die attach pad 14 has a 
base plating of the diffusion barrier 48 and an outer plating of palladium 
44. An integrated circuit die is electrically connected to the internal 
lead 16 by a bonding wire 52. An outer molding 54 encases the solderable 
lead frame 10, the die 50, and bonding wires 52. 
Solderable lead frames created by the above-mentioned process have various 
advantages over their prior art counterparts. For example, the resulting 
frame's die attach pad area 14 and portions of internal leads 16 near the 
pad have no significant levels of exposed tin. Further, the entire lead 
frame 10 has a palladium coat 44 to provide a pleasing appearance and good 
wire bonding characteristics. Because the palladium coat 44 is provided on 
a solderable tin 46 layer, it does not create the soldering problems 
encountered in prior art palladium plating processes such as the process 
shown in FIG. 1 and its variants. Specifically, the palladium coat 44 is 
thin enough to allow solderability of the external leads sufficient to 
satisfy the MilSpec 883 standard without the use of highly corrosive 
fluxes due to the soldering activity of the tin. The palladium 44 on the 
external leads 12 simply fuses into the tin layer 46 upon soldering. 
Further, the conventional ultrasonic welding technique for bonding wires 
52 to the internal leads 16 of the frame 10 will be unaffected by the thin 
palladium coat 44; the ultrasound will readily ram the wires into the 
copper. Additionally, the overall palladium coat 44 eliminates the 
cosmetic registration requirement of the packaged integrated circuit 43 by 
covering the external leads and the tin boundary 20. 
Although only one embodiment of the present invention has been described, 
it should be understood that the present invention may be embodied in many 
other specific forms without departing from the spirit or scope of the 
invention. Particularly, not all of the external leads need to be plated 
with tin. Tin alloys other than those explicitly specified may be used. 
The tin plating may be applied to the internal leads, but it must not 
cover areas where the bonding wires are to be attached. Although only 
substantially copper base lead frames have been described in detail, other 
base lead frame materials such as kovar or alloy 42 may be used. Other 
diffusion barriers besides nickel and cobalt may also be used. The 
continuous pre-plating method need not use reels. Although the continuous 
method is preferred, it can be replaced by a discontinuous method using 
the same spot and flood plating steps. Additionally, the thickness ranges 
specified for the diffusion barrier, the tin/tin alloy layer, and the 
palladium layer are not absolute. Similarly, the eutectic point 
temperature range for the tin alloys is not comprehensive. Finally, 
plating apparatus other than those described may be used, and spot plating 
may be performed by flood plating followed by etching or stripping 
portions of the substance deposited by flood plating. Therefore, the 
present examples are to be considered as illustrative and not restrictive, 
and the invention is not to be limited to the details given herein, but 
may be modified within the scope of the appended claims.