Protected conductive foil assemblage and procedure for preparing same using static electrical forces

A conductive copper foil for use in preparing printed circuit boards is protected from damage during storage, shipment and further processing by covering at least one side of the foil with a sheet of plastic film. The film is removably joined with the foil as a result of the essential absence of gaseous material between the film and the foil. Thus, the film conforms intimately to the contours and shape of the foil surface and clings tightly thereto to permit movement and further processing of the foil with the film adhering tightly thereto. The absence of air between the film and the foil is produced by oppositely statically electrically charging the film and the foil such that they are forced together sufficiently to squeeze the air out from therebetween. The film is selected to be sufficiently resistant to laminating temperature and pressure conditions so as to remain in its covering, protecting relationship to the foil and avoid sticking to the laminating press plate and retain its removability from the foil after lamination.

FIELD OF THE INVENTION 
The present invention relates to etchable conductive foils useful in 
preparing printed circuit boards and particularly to a means and a method 
for protecting such foils from damage during the handling and further 
processing of the foil leading up to the etching procedures whereby the 
foil is converted into the lines and other components making up the 
conductive pathways for printed circuit board circuitry. In particular the 
invention provides a highly efficient and effective procedure for 
removably joining a protective film to a piece of foil using static 
electrical forces. 
DESCRIPTION OF THE PRIOR ART 
Printed circuit components have become widely used in a variety of 
applications for radios, televisions, computers, etc. Of particular 
interest are multi-layer printed circuit board laminates which have been 
developed to meet the demand for miniaturization of electronic components 
and the need for printed circuit boards having a high density of 
electrical interconnections and circuitry. In the manufacture of printed 
circuit boards, raw materials, including conductive foils, which are 
usually copper foils, and dielectric supports comprising organic resins 
and suitable reinforcements, are packaged together and processed under 
temperature and pressure conditions to produce products known as 
laminates. The laminates are then used in the manufacture of printed 
circuit board components. In this endeavor the laminates are processed by 
etching away portions of the conductive foil from the laminate surface to 
leave a distinct pattern of conductive lines and formed elements on the 
surface of the etched laminate. Further, laminates and/or laminate 
materials may then be packaged together with the etched product to form a 
multi-layer circuit board package. Additional processing, such as hole 
drilling and component attaching, will eventually complete the printed 
circuit board product. As printed circuit board technology has advanced to 
provide higher density boards with smaller printed circuit lines, surface 
contamination of the raw material products has become a significant 
problem which generally cannot be tolerated in a successful commercial 
application. 
Although many of the printed circuit board manufacturing and handling 
procedures are potential sources of surface contamination, one significant 
problem area involves the construction and lamination procedures by which 
the laminated products are prepared. Generally speaking, in this 
connection, the construction of the laminates involves stacking (or 
lay-up) of conductive foil pieces and dielectric substrates (prepregs) 
which will then be bonded to one another in a laminating press. Residual 
particles, particularly those emanating from the prepreg raw materials, 
are known to be present in the lay-up room environment as a result of the 
handling of such materials. Eventually such particles may contaminate the 
surfaces of the conductive foil pieces bonded to the laminates. 
Presently such contamination problems are dealt with by attempting to 
maintain a clean environment in the lay-up area by air filtration, 
intensified housekeeping techniques, etc. Additionally, the conductive 
foil surfaces of laminates may often simply be spot cleaned. However, spot 
cleaning is not necessarily an acceptable procedure for laminates clad 
with so-called double-treated, electrodeposited copper foils. 
Double-treated copper foils are those which have been treated on both the 
matte side and the shiny (drum) side for enhanced bonding ability and 
improved peel strength when bound to a prepreg. In this regard, it should 
be noted that while double-treated copper foils are theoretically more 
desirable than single-treated foils for preparing multi-layer laminates, 
because such prior treatment enables the elimination of the chemical oxide 
roughening step, i.e., black oxide treatment, that is otherwise required 
to enhance bonding and peel strength of the shiny side of the lines after 
etching, double-treated copper foils cannot be subjected to spot cleaning 
for removal of contamination because such cleaning would also remove the 
treatment and prevent, or at least interfere with, subsequent bonding of 
the conductive lines to another prepreg during fabrication of a multiple 
layer laminate. 
Thus, in the past, surface contamination has been a major economical 
disadvantage in the production of printed circuit boards, and particularly 
in the production of multiple layer printed circuit board products having 
high component densities. Moreover, even through the use of such measures 
as the environmental control of the lay-up room area, those involved in 
the manufacture of printed circuit boards have not been able to achieve 
acceptable prevention of surface contamination without incurring major 
costs and losses of efficiency in manufacturing procedures. Spot cleaning 
of laminate surfaces is simply expensive and inefficient, and as indicated 
above, cannot be tolerated in applications which employ double-treated 
foils. 
A number of proposals are presented in said co-pending '841 application for 
releasably or removably joining film layers to pieces of foil in a 
removable, overlying, covering, protecting relationship relative to at 
least one side of the foil. The present invention provides a highly 
efficient and effective improved procedure for removably joining the film 
and the foil. 
SUMMARY OF THE INVENTION 
The problems and shortcomings inherent in the prior art procedures and 
applications discussed above are minimized, if not eliminated entirely, by 
applying the concepts and principles of the present invention. Thus, the 
invention provides an effective means for protecting the surfaces of 
conductive foils throughout those procedures involved in the manufacture 
of printed circuit boards where surface contamination may potentially 
create problems and difficulties. Such protection results from the 
provision of a protected conductive foil assemblage comprising a 
conductive metallic foil which has two sides. One side of the foil is 
adapted, generally by chemical treatment, for bonding to a dielectric 
support during a lamination process involving pressing between plates of a 
laminating press. The protected assemblage further includes a dielectric 
plastic film layer which overlies the other side of the foil in covering, 
protecting relationship thereto. The plastic film layer is removably 
united or joined with the foil sufficiently to permit movement and further 
processing of the foil with the film layer remaining in said covering, 
protecting relationship relative to the other side of the foil. In 
accordance with the invention, the plastic film is removably united or 
joined with the foil as a result of the provision of an essential absence 
of gaseous material between the plastic film layer and said other side of 
the foil, whereby the plastic film layer conforms intimately to the 
contours and shape of the other side of the foil and clings tightly 
thereto sufficiently to permit movement and further processing of the foil 
with the film layer remaining in said covering, protecting relationship to 
said other side of the foil. The plastic film layer is removable from the 
foil by peeling. The plastic film is sufficiently resistant to the 
temperature and pressure conditions encountered during a lamination 
process to avoid sticking to a laminating press plate and to retain its 
removability from the foil after lamination of the foil to a dielectric 
support. 
In a particularly preferred commercial aspect of the invention, the 
protected foil assemblage may include an electrodeposited foil having a 
matte side and a shiny side. The foil further may be a double-treated 
conductive metallic foil wherein both the matte side and the shiny side of 
the foil have been treated for enhanced lamination bonding and peel 
strength between the surface of the foil and a dielectric support. In 
accordance with this aspect of the invention, the plastic film is 
preferably removable after laminating without disturbing the treatment on 
the shiny side. 
In another important aspect of the invention, the plastic film layer may be 
sufficiently transparent to permit visual inspection of the shiny side of 
the metallic foil while the film layer is in said covering, protecting 
relationship thereto. This is a particularly valuable aspect of the 
invention in connection with double-treated foils so that the treatment on 
the shiny side of the metallic foil may be visually inspected while the 
film layer is in said covering, protective relationship thereto. 
In accordance with the invention, the assemblage may include a foil and a 
film layer which are each in the form of an elongated web, and the foil 
and the film layer webs may be wound up together in the form of a roll. In 
another aspect of the invention, the foil and the plastic film layer may 
be configured as sheets which are coextensive in size and the assemblage 
may include a coextensively sized dielectric support layer which contains 
a curable laminating resin disposed against the matte side of the foil. 
In a preferable and more detailed aspect of the invention, the plastic film 
layer may have a thickness in the range of from about 0.5 to about 5.0 
mils and which preferably is about 2.0 mils or less. In a commercially 
desirable form the thickness of the plastic film may be about 0.92 mil. 
Additionally, the plastic film layer should preferably be formed from a 
dielectric material that is capable of being exposed to the conditions 
encountered in a laminating press without releasing chemicals which might 
contaminate the foil. 
In a particularly preferred aspect of the invention, the foil may comprise 
copper and the plastic film layer may comprise a polyester, preferably a 
polyester such as a polyethylene terphthalate. 
In a different but related aspect of the invention, a procedure is provided 
for protecting a conductive metallic foil during further handling and use 
in the preparation of laminates and printed circuit board components 
therefrom. The procedure comprises the steps of providing a piece of 
conductive foil having two sides, one side being adapted for bonding to a 
dielectric support through the use of a lamination process involving 
pressing between plates of a laminating press. A layer of a dielectric 
plastic film is also provided and the latter is placed in overlying, 
covering and protecting relationship to the other side of said piece of 
metallic foil. The layer of film is then removably joined with the 
metallic foil sufficiently to permit movement and further processing of 
the foil with the film layer remaining in said covering relationship to 
said other side of the foil. In accordance with the invention, the layer 
of film and the foil are removable joined by forcing said layer of film 
and said foil together by applying thereto an evenly distributed force 
pattern to drive or squeeze gaseous material out from between the film and 
the other side of the foil and produce an essential absence of gaseous 
material (or vacuum) between the film and the other side of the foil, 
whereby the plastic film layer conforms intimately to the contours and 
shape of the other side of the foil and clings tightly thereto to permit 
movement and further processing of the foil with the film layer remaining 
in said covering, protecting relationship to said other side of the foil. 
The plastic film layer is removable from the foil by peeling. The plastic 
film should be sufficiently resistant to the temperature and pressure 
conditions encountered during the lamination process to avoid sticking to 
a laminating press plate and to retain its removability from the foil 
after lamination of the foil to a dielectric support. In accordance with a 
preferred aspect of the invention, the evenly distributed force pattern is 
created by oppositely static electrically charging said film and said 
foil. 
Clearly the procedure of the invention is applicable in conjunction with 
conductive foils generally and in particular with electrodeposited 
conductive metallic foils and plastic film layers of the sort outlined 
above. Further, the invention is applicable in conjunction with those 
instances wherein the shiny side of an electrodeposited foil has been 
treated for enhanced lamination bond strength between the shiny side and a 
dielectric support; wherein the plastic film is removable after laminating 
Without disturbing such treatment; wherein the plastic film layer is 
sufficiently transparent to permit visual inspection of the shiny side of 
the metallic foil and/or the treatment thereon while the film layer 
remains in its covering, protective relationship to the foil; wherein the 
foil and the film layer are in the form of elongated webs that are wound 
up together in the form of a roll; wherein the foil and the plastic film 
layer are coextensive in size and are laid-up with a coextensively sized 
dielectric support layer containing a curable laminating resin disposed 
against the matte side of the foil; wherein the plastic film layer has a 
thickness in the range from about 0.5 to about 5.0 mils and which 
preferably is about 2.0 mils or less; wherein the plastic film layer is 
capable of being exposed to the conditions encountered in the laminating 
press without releasing chemicals which might contaminate the foil; 
wherein the foil comprises copper; and/or wherein the plastic film layer 
comprises a polyester, and preferably a polyethylene terphthalate 
polyester. 
The invention also provides a procedure for releasably joining a 
protective, covering layer of dielectric film to one side of a conductive 
metallic foil adapted for being bonded to a dielectric support during a 
lamination process. In this aspect of the invention, the procedure 
includes a step of providing a piece of conductive foil having two sides, 
with one side of the foil being adapted for bonding to a dielectric 
support by a lamination process involving pressing between plates of a 
laminating press. A layer of dielectric film is provided and which is 
sufficiently resistant to the temperature and pressure conditions 
encountered during a lamination process to avoid sticking to a laminating 
press plate or to a piece of foil after exposure to laminating conditions. 
The layer of film is placed over the other side of said piece of foil in a 
covering, protecting relationship thereto. In accordance with the 
invention, the procedure includes the step of oppositely static 
electrically charging the film and the foil to cause the film and the foil 
to be attracted to one another with sufficient force to thereby drive 
gaseous material out from between the film and the other side of the foil 
and produce an essential absence of gaseous material between the film and 
the other side of the foil. Thus, the plastic film layer conforms 
intimately to the contours and shape of the other side of the foil and 
clings tightly thereto sufficiently to permit movement and further 
processing of the foil with the film layer remaining in its covering, 
protecting relationship to said other side of the foil. The plastic film 
layer is removable from the foil by peeling. 
In accordance with the procedure of the invention, the static electrical 
charges on the film and on the foil are evenly distributed over the 
respective surfaces thereof whereby the film and the foil are attracted 
together by an evenly distributed force pattern. In a preferred aspect of 
the invention the static electrically charging step includes passing the 
foil with the film thereon through an electrical gradient field. 
In a particularly preferred form of the invention, an electrical gradient 
field is established by providing a positively charged, generally straight 
conductive wire electrode and causing the foil to be a negatively charged 
electrode. The wire is positioned in laterally spaced, generally parallel 
relationship relative to the major plane of the foil, and the foil with 
the film thereon are moved in a direction which is generally parallel to 
the plane of the foil and which is generally transverse relative to the 
longitudinal axis of said wire. The film is positioned between the wire 
and the foil during such movement thereof. 
Preferably the foil is in conductive contact with the surface of a 
rotating, conductive, negatively charged roller during the movement of the 
foil with the film thereon and the roller is arranged for rotation about 
an axis that is generally parallel to the longitudinal axis of said wire. 
In this preferred form of the invention the wire may be formed from 
stainless steel and may have a diameter of about 0.008 inches, the gap 
between the wire and the roller may be approximately 3/8 inches, and a 
voltage potential of about 15 kV may be applied between said roller and 
said wire. 
In another aspect of the invention the procedure may include the step of 
causing the film to be brought into contact with a grounding element after 
the foil with the film thereon have been passed through the electrical 
gradient field. In this regard an elongated static grounding element may 
be provided that is disposed in laterally spaced, generally parallel 
relationship relative to said wire, and which element is located in a 
position for contacting and grounding the foil after the foil and the film 
have been moved through said field. Preferably the grounding element may 
be positioned for simultaneously contacting the entire lateral extent of 
said film as the latter is moved in said direction. 
In yet another aspect of the invention, the same provides a protected 
conductive foil laminate which comprises a conductive metallic foil as 
specified above, a plastic film layer overlying one side of the foil in 
removable covering, protecting relationship thereto and a dielectric 
support. In accordance with the invention there is an essential absence of 
air or other gaseous material between the plastic layer and said other 
side of the foil, whereby the plastic film layer conforms intimately to 
the contours and shape of the other side of the foil and clings tightly 
thereto sufficiently to permit movement and further processing of the foil 
with the film layer remaining in said covering, protecting relationship to 
said other side of the foil. The plastic film layer is removable from the 
foil by peeling. In this aspect of the invention the other side of the 
foil is bonded to the support through the utilization of a lamination 
process involving pressing between the plates of a laminating press, and 
the plastic film layer remains removably joined to the foil sufficiently 
to permit movement and further processing of the laminate with the film 
layer remaining in said covering, protecting relationship to the foil. In 
this connection, the plastic film is characterized as having initially 
been, prior to the lamination process, sufficiently resistant to the 
temperature and pressure conditions generally encountered during such 
process to avoid sticking to a laminating press plate and to retain its 
removability from the foil after lamination of the foil to the dielectric 
support.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As outlined above, the present invention relates to the protection of foil 
materials which are eventually to be processed to form printed circuits. 
In this regard, the invention is applicable generally to all kinds of 
conductive foils and the methodology for preparing and/or producing the 
foil is not a critical feature of the present invention. For example, the 
invention may be used to provide protection for electrodeposited foils, 
for rolled (wrought) foils, for multi-layer layer foils, for clad foils, 
etc. Additionally, the specific conductive metal which is utilized for 
forming the foil is not a feature of the invention, and the invention is 
applicable generally whether the foil is of copper, nickel, chromium, or 
other conductive metal. Generally speaking, however, the invention has 
particular advantages when applied to copper foils prepared using 
conventional electrodeposition methodology whereby the foil has a smooth 
shiny side and a rough or matte side. Conventionally such foils are then 
bonded to dielectric substrates to provide dimensional and structural 
stability, and in this regard, it is conventional to bond the matte side 
of the electrodeposited foil to the substrate so that the shiny side of 
the foil faces outwardly from the laminate. 
The specific features of the substrate are also not critical features of 
the invention and those skilled in the art are familiar with a large 
variety of such substrates. In a commercial sense, useful dielectric 
substrates may be prepared by impregnating woven glass reinforcement 
materials with partially cured resins, usually epoxy resins. Such 
dielectric substrates are commonly referred to as prepregs. 
In preparing laminates, it is conventional for both the prepreg material 
and an electrodeposited copper foil material to be provided in the form of 
long webs of material rolled up in rolls. The rolled materials are drawn 
off the rolls and cut into rectangular sheets. The rectangular sheets are 
laid-up or assembled in stacks of assemblages. Each assemblage may 
comprise a prepreg sheet with a sheet of foil on either side thereof, and 
in each instance, the matte side of the copper foil sheet is positioned 
adjacent the prepreg so that the shiny sides of the sheets of foil face 
outwardly on each side of the assemblage. 
The assemblages may be subjected to conventional laminating temperatures 
and pressures between the plates of laminating presses to prepare 
laminates comprising a sandwich of a sheet of prepreg between sheets of 
copper foil. Although it is commercially conventional to provide laminates 
with copper foil on each side of the prepreg, it is also within the 
contemplation of the present invention that the laminate might be made up 
of a single sheet of copper foil bonded to one side only of the prepreg. 
As indicated above, the prepregs used conventionally in the art generally 
consist of a woven glass reinforcement fabric impregnated with a partially 
cured two-stage resin. By application of heat and pressure, the matte side 
of the copper foil is pressed tightly against the prepreg and the 
temperature to which the assemblage is subjected activates the resin to 
cause curing, that is cross linking of the resin and thus tight bonding of 
the foil to the prepreg dielectric substrate. Generally speaking, the 
laminating operation will involve pressures in the range of from about 250 
to about 750 psi, temperatures in the range of from about 350.degree. to 
450.degree. F., and a laminating cycle of from about 40 minutes to about 2 
hours. The finished laminate may then be utilized to prepare printed 
circuit boards. 
As set forth above in the description of the prior art, printed circuit 
board manufacturers seek methodology for producing smaller circuit lines 
and to otherwise densify the circuitry. As smaller and smaller lines are 
attempted, the toleration for contamination on the surface of the copper 
foil decreases rapidly. Such contamination may come from any one of a 
number of different sources. 
In order to densify the circuitry of the printed circuit board, the single 
substrate laminates described above are married or joined together to 
present multi-layer printed circuit board structures. Such marriage or 
joinder is accomplished after the etching of the foil to thereby remove 
unwanted copper and present defined lines, etc. To then join a laminate 
with an etched pattern (inner layer) to another laminate with an etched 
pattern and thus present a multi-layer laminate, the outer surface of the 
copper inner layer which comprises the shiny side of the foil must be 
roughened and/or treated to enhance lamination bonding and improve peel 
strength in the finished structure. It has been traditional to treat the 
shiny surfaces of the printed copper circuit with a chemical oxide 
treatment to enhance subsequent bonding. To avoid such treatment, which 
generally must be accomplished after the printed circuit has been 
prepared, it has become desirable, at least for some applications, to 
provide a foil which has been treated to enhance lamination bonding and 
improve peel strength on both sides thereof. Such foils are known as 
double-treated foils. And in this regard, it should be pointed out that 
conductive foils are conventionally treated on the matte side for enhanced 
lamination bonding and peel strength between the matte side and the 
prepreg. 
Typically the foil treatment involves treatment with a copper/copper oxide 
bonding material to increase surface area and thus enhance bonding and 
increase peel strength. The foil may also be treated to provide a thermal 
barrier, which may be brass, to prevent peel strength from decreasing with 
temperature. Finally, the foil may be treated with a stabilizer to prevent 
oxidation of the foil. These treatments are well known and the description 
thereof is not necessary at this point. 
When the foil is double-treated, the treatments for enhanced bonding, 
improved peel strength and stabilization are applied to both sides of the 
foil. Through the use of double treated foil, the chemical oxide 
roughening step may be eliminated. However, the treatment on the shiny 
side of the double-treated foil is fragile and the treatment can be 
dislodged if the treated shiny side is not protected. This will result, at 
least at the points of injury to the treatment, in loss of bond strength, 
loss of peel strength and increased susceptibility to oxidation. 
The present invention provides protection for either treated or untreated 
foil surfaces by providing an assemblage which includes a plastic film 
layer that overlies the foil in covering, protecting relationship thereto. 
In accordance with the concepts and principles of the invention, the 
plastic film layer is removably (or releasably) joined to the foil 
sufficiently to permit movement and further processing of the foil with 
the film layer remaining in its covering, protecting relationship relative 
to one side of the foil. The plastic film must be sufficiently resistant 
to the temperature and pressure conditions encountered during a lamination 
process so as to avoid sticking to the laminating press plate and so as to 
retain its ability to be removed from the foil after lamination of the 
foil to the dielectric support. The foil layer may then be etched using 
conventional methodology to present a printed circuit. 
A procedure for preparing a protected conductive foil assemblage which 
embodies the concepts and principles of the invention is illustrated in 
FIG. 1. In FIG. 1 a web of conductive metallic foil material 20 is pulled 
from a roll 26 thereof and a web of plastic film material 24 is pulled 
from a roll 22 thereof. The foil web 20 pulled from roll 26 and the 
plastic film web 24 pulled from roll 22 are caused to merge at point 28, 
and the interleafed assemblage is wound, for storage purposes, onto a roll 
30. 
Foil 20 may consist of any conductive material that is useful in the 
preparation of printed circuit board materials. Foil 20 may comprise, for 
example, conductive copper, or nickel, or chromium, or any other material 
which may be formed into a foil and then subsequently etched to present 
the PCB circuitry. Foil 20 may, for example, consist of a wrought foil 
prepared by rolling or a clad metal foil comprising two or more layers of 
metal. All such conductive foils may have at least one side that is 
specially treated for enhanced bonding to a dielectric support and another 
side that may be exposed to contamination and potential damage during 
storage, shipment and further processing. The present invention provides 
protection for such exposed side of the foil. Preferably, however, foil 20 
may be an electrodeposited, double-treated, conductive copper foil. 
Although the thickness of the foil is not critical for purposes of the 
present invention, typically such foils are available commercially in 
thicknesses ranging from 3/8 ounce to 10 ounces. In this regard, the 
required thickness of the foil is generally a function of the eventual 
commercial application which may be known only to the final printed 
circuit board fabricator. 
With regard to the plastic film layer 24, the same must simply operate to 
keep contamination off of the outer, otherwise exposed surface of the foil 
layer. As described above, such outer surface generally comprises either 
the treated or untreated shiny side of an electrodeposited foil. The 
plastic film must be capable of protecting the foil and must also be 
removable after the need for protection no longer exists. Additionally, 
the film must be sufficiently resistant to the temperature and pressure 
conditions encountered during lamination so that it does not stick to the 
laminating press plate and so that it retains its ability to be removed 
from the foil after the completion of the laminating process. Thus, the 
film must have sufficient temperature resistance to keep from melting at 
the temperatures of the laminating process since melting might cause 
fusing of the plastic material and the resultant sticking of the film to 
either the plate or the foil. Moreover, the plastic film should preferably 
be resistant to shrinking and capable of being exposed to the conditions 
encountered in the laminating press without releasing chemicals which 
might contaminate the foil. 
It is desirable that the plastic film be capable of withstanding exposure 
to a temperature of about 475.degree. F. for about 2 hours and to a 
temperature of about 325.degree. F. for about 10 hours, without 
substantial dimensional change. Such characteristics are determinable 
using conventional testing methodology. Moreover, the plastic film should 
generally be free of volatile or organic material which might be released 
during laminating to contaminate the copper. Preferably, the plastic 
material utilized to protect the conductive foil should be transparent or 
translucent after exposure to laminating conditions to facilitate 
inspection of the underlying foil and/or the treatment thereon while 
protection of the foil is continued. For purposes of the present 
invention, the film should have dielectric characteristics. 
For commercial purposes, the preferred film material should be readily 
available in widths of up to 80 inches. And in this connection, the 
present commercially available foils are generally available in nominal 
widths exceeding 40 inches. The plastic layers may have thicknesses 
ranging from about 0.5 to about 5.0 mils and the thickness preferably 
should be about 2.0 mils or less to avoid the necessity for enlarged 
packaging. The film should also be sufficiently flexible and soft at room 
temperatures to facilitate wrapping of the same around the roll up 
mandrels used in an interleafing process such as the one illustrated in 
FIG. 1. Finally, the plastic material should remain useful when subjected 
to temperatures ranging from about -50.degree. F. to about 500.degree. F. 
to facilitate storage in winter and utilization under laminating press 
conditions. Needless to say, many of the foregoing desirable 
characteristics are variable depending on process and end use parameters. 
And generally speaking, as will be appreciated by those skilled in the art 
to which the present invention pertains, empirical procedures will need to 
be employed in order to determine an optimum material for any given 
application. 
A Commercially preferred double-treated copper foil material for use in 
connection with the invention, is presently available from Gould Inc., 
Foil Division under the designation TC/TC. Such material is useful in 
multi-layer printed circuit board applications and is uniformly treated on 
both its shiny and matte sides with treatments to enhance bond strength, 
increase peel strength, and provide resistance to thermal, chemical and 
oxidative degradation. 
A preferred plastic film material for protecting the treated shiny side of 
the TC/TC foil is a polyester film available commercially from DuPont 
under the designation Mylar 92 DB. Such material has a nominal thickness 
of about 0.92 mils; a density of about 1.395 g/cc, as determined by ASTM 
Spec D-1505; a tensile strength of 30,000 MD psi, as determined by ASTM 
Spec D-862, Method A; a tensile elongation of 100 MD%, as determined by 
ASTM Spec D-882, Method A; a tensile modulus of 547,000 MD psi, as 
determined by ASTM Spec D-882, Method A; an F-5 of 15,500 MD psi, as 
determined by ASTM Spec D-882, Method A; a melting point of 255.degree. 
C., as determined by ASTM Spec D-3417; a shrinkage coefficient at 
105.degree. C. of 0.5 MD% and 0.5 TD%, each as determined by ASTM Spec 
D-1204; a coefficient of thermal expansion of 1.7.times.10.sup.-5 
inch/inch/.degree.C., and Talysurf surface characteristics as determined 
by Taylor Hobson Talysurf Spec 5 of 0.031 Talysurf RA, 0.264 Talysurf RZ 
and 38 Talysurf NP. The preferred material is clear and contains only 
about 0.5 inclusions greater than 1.47 microns (.mu.) per 100 sq/cm. In 
this latter regard, it is desirable that the film be sufficiently 
transparent to permit visual inspection of the underlying shiny side of 
the foil in compliance with Mil Spec 13949 for Mil Class B materials. 
With further reference to FIG. 1, webs 20 and 24 may desirably have widths 
of about 42 inches. And since a popular commercial width useful in 
preparing printed circuit board materials is 39 inches, a one and one half 
inch marginal area is thus presented at each side of the web. 
With reference to FIG. 1, the apparatus includes a mechanism 32 for 
removably joining film 24 to foil 20 at their merger point 28. Mechanism 
32 is illustrated in greater detail in FIGS. 3, 4 and 5 and is operable 
for causing film 24 and foil 20 to be attracted to one another by an 
evenly distributed static electric force pattern of sufficient strength 
such that air and other gaseous materials are driven by squeezing out from 
between the film 24 and the foil 20. Thus, an essential absence of gaseous 
material (or vacuum) is produced between the foil 20 and the film 24. As a 
result, the plastic film 24 conforms intimately to the contours and shape 
of the surface of foil 20 and clings tightly thereto to permit movement 
and further processing of the foil 20 with the film 24 remaining in its 
covering protecting relationship to the foil 20. On the other hand, the 
film 24 is removable from foil 20 by peeling. 
The webs 20 and 24 merge at point 28 and form an assemblage which is 
illustrated in FIG. 2. In this regard, it should be noted that FIG. 2 is a 
cross-sectional view taken along the line 2--2 in FIG. 1 to illustrate the 
assemblage which comprises the foil 20 and the plastic film 24, the latter 
now being removably joined to the foil as a result of the action of 
mechanism 32 s that it is in intimate covering, protecting relationship 
relative to the shiny side 20a of the foil. 
The mechanism 32 is illustrated schematically in FIG. 3 and includes a wire 
electrode 34 supported by insulators 36, a roller 38 mounted for rotation 
about an axis 46, a static grounding ribbon 48 and a dc power supply 50. 
Wire electrode 34 may have a diameter ranging from about 0.004 to about 
0.025 inches and may be formed from any suitable electrically conductive 
material. Preferably, however, electrode 34 may be formed from a stainless 
steel wire having a diameter of about 0.008 inches. 
Roller 38 is a conventional roller of the sort utilized widely in the foil 
processing art. Such rollers are illustrated, for example, in U.S. Pat. 
Nos. 4,022,648 and 3,589,975. The only really important criteria for 
roller 38 is that the surface thereof should preferably be formed from an 
electrically conductive material. As shown in FIG. 3, roller 38 is mounted 
for rotation about axis 46 in the direction of the arrow. Preferably 
roller 38 may have a metallic conductive surface, a length of about 65 
inches, and a diameter of about 3 inches. 
Electrode 34 is preferably mounted so that it extends between insulators 36 
in essential parallelism with axis 46 of roller 38. Electrode 34 is also 
mounted in spaced relationship relative to the surface of roller 38 so as 
to present a gap g therebetween. The gap g may vary from about 1/4 to 
about 1/2 inch; however, in the preferred embodiment illustrated herein, 
gap g should be about 3/8 inch. In this regard, gap g simply needs to be 
small enough so that a gradient field is established in film 24 and large 
enough to avoid sparking. 
Power supply 50 is arranged to provide a dc voltage potential between 
roller 38 and electrode wire 34. For this purpose, power supply 50 may 
preferably be in the form of a Universal Voltronics Model BPA-22-10-D3 dc 
generator which is adjustable to provide 0 to 22 kV dc power. As shown in 
FIG. 3, wire 34 is connected to the positive terminal 52 of source 50 
while roller 38 is connected to the negative terminal 54 of source 50. A 
ground 56 may be provided as is conventionally understood. 
In operation, film 24 and foil 20 move together in the direction of the 
arrows in FIG. 3 and they converge at point 28 in the vicinity of roller 
38. This is shown in more detail in FIG. 4. Power source 50 may ideally be 
adjusted to provide a dc electrical potential of about 15 kV, and under 
the dimensions and conditions outlined above, the operating current will 
be in the order of 2.5 milliamps. As will be appreciated by those skilled 
in the art, since foil 20 is formed from a conductive material, when it 
moves into contact with the surface of roller 38, foil 20 becomes the 
negatively charged electrode. In a general sense the voltage simply needs 
to be sufficient to establish a static electrical gradient field in the 
film 24 and should preferably be in the range of from about 5 kV to about 
20 kV. 
Thus, a static electrical gradient field is established between the 
positively charged electrode wire 34 and the negatively charged electrode 
provided by the copper foil 20. The wire is positioned in laterally 
spaced, generally parallel relationship relative to the major plane of the 
foil, and the foil with the film thereon are moved, at least at the top of 
roller 38, in a direction that is generally parallel relative to the plane 
of the foil and which is generally transverse relative to the longitudinal 
axis of wire 34. The electrical potential between wire 34 and foil 20 is 
uniformly distributed along the length of wire 34 under the conditions 
outlined above and the film 24 and foil 20 are moved at a constant rate so 
that the influence of the gradient field is uniformly applied to the film 
24 as it passes through the electrical gradient field. Since film 24 is 
formed from a dielectric material, the described action creates a 
generally uniform static electrical charge that is applied evenly to film 
24. 
With reference to FIG. 4, the static electrical charge in the dielectric 
film 4 is believed to be distributed gradiently so that a positive charge 
accumulates on the surface of the film adjacent foil 20 as the film and 
the foil approach the merger point 28. The positive charge on the lower 
surface of film 24 and the negative charge on the foil 20 are strongly 
attractive and evenly distributed so that the negatively charged foil 20 
and the positively charged lower surface of the film 24 are attracted and 
caused to converge, and the same are drawn or forced together with an 
evenly distributed force pattern such that air and other gaseous materials 
are squeezed or driven out from between the film and the foil to thus 
produce an essential absence of gaseous material (or vacuum) between the 
foil and the film. 
The process thus described leaves a substantial static charge on the film. 
Such charge may interfere with further processing of the foil. 
Accordingly, in the preferred form of the invention, a grounding mechanism 
in the form of a static ribbon 48 is provided to ground and remove static 
charges from film 24. Ribbon 48 is disposed in laterally spaced 
relationship relative to wire 34 and is positioned far enough away from 
wire 34 such that it does not interfere with the attractive forces 
operating to expel air from between the foil and the film. This distance 
may vary from a few inches to several feet. 
Ribbon 48 is illustrated schematically in FIG. 5 and comprises an elongated 
conductive wire or rod 58 which extends across the width of film 24 and a 
plurality of strips 60 of conductive foil. The strips 60 are attached to 
wire 58 and preferably are like tinsel. The strips 60 are preferably 
flexible so that the free ends thereof simply drag on film 24 as the same 
moves past in the direction of the arrow. Strips 60 are distributed along 
the entire length of wire 58 so that the entire lateral extent of film 24 
is contacted simultaneously by strips 60 as the film moves away from 
electrode 34. Element 58 is connected to ground 56. Thus, static charges 
are effectively substantially removed from film 24. 
The net result of the process is that an essential absence of air or other 
gaseous material is created between film 24 and foil 20. This absence of 
air constitutes a vacuum which, in conjunction with any residual remaining 
static charges, causes the film to conform intimately to the contours and 
shape of the foil and to cling tightly thereto in a covering, protecting 
relationship. Accordingly, the foil may be moved and subjected to further 
processing while the film remains thereon in its covering and protecting 
position. The film 24 may readily be removed from the foil 20 when the 
time comes to do so simply by lifting a corner of the film and peeling the 
film back away from the corner by hand. 
The linear speed of foil web 20 and plastic film 24 as the same pass 
electrode 34 in the direction of the arrows in FIGS. 3 and 4 may be varied 
to suit other process parameters. 
Ideally such speed may range from about 80 to about 200 feet per minute. 
However, the removable bonding strength between web 20 and film 24 does 
not seem to be affected to any great degree by the linear speed of web 20 
and film 24 as the same pass under electrode 34. 
Removable bonding strength may be measured using an Instron Model 1130 
universal testing machine. When this machine is used to separate 2 inch 
wide strips of material using a 500 gram load cell it appears that the 
removable bonding strength does vary to some degree with voltage. The 
relationship appears to be essentially a straight line function in the 
voltage ranges mentioned above. Thus, at an applied voltage of 8 kV the 
measured bond strength is approximately 0.68 g/in, at 12 kV the bond 
strength is approximately 1.36 g/in and at 16 kV the bond strength is 
approximately 2.04 g/in. These relationships may be expressed by the 
following equation: 
EQU Bond strength; g/in=(0.17.multidot.kV)-0.68 
To utilize the materials wound up on roll 30, the material is unwound from 
the roll and the web material is cut across the web to present individual 
sheets of material. In the case of the present invention, when the 
material is unwound from roll 30 and cut, the resultant product will be a 
protected conductive foil assemblage 40 as illustrated in FIG. 6. 
Assemblage 40 is made up of a piece of conductive metallic foil 120 which 
has a side 120b and a side 120a. As illustrated in FIG. 6, the side 120a 
is covered by plastic film layer 124 and thus cannot be seen, except for 
the edge thereof. Side 120b of foil 120 is adapted for bonding to a 
dielectric support by a lamination process involving pressing between 
platens of a laminating press. Thus, side 120b has been pre-treated for 
enhanced bond strength, improved peel strength resistance to thermal and 
chemical degradation and stabilization against oxidation. Plastic film 
layer 124 overlies side 120a in covering, protecting relationship thereto. 
As explained previously, plastic film layer 124 is removably joined to the 
foil sufficiently to permit movement and further processing of the foil 
with the film layer remaining in said covering, protecting relationship to 
the side 120a of the foil by virtue of the fact that there is an essential 
absence of gaseous material between the film and the foil whereby the film 
conforms intimately to the contours and shape of the surface of side 120a 
of foil 120 and clings tightly thereto. 
After assemblage 40 has been cut from the material drawn from roll 30, the 
assemblage 40 is laid-up with a coextensively sized dielectric support 
layer 42. Layer 42 preferably may be a prepreg as described above and 
which contains a curable laminating resin. During the lay-up procedure, 
the prepreg 42 is simply brought into contact with the side 120b of foil 
120. The laid-up materials are thus ready for the lamination process to 
bond the prepreg 42 to the side 120b of foil 120 while plastic film 124 
remains in its covering, protecting relationship relative to side 120a of 
the foil. 
FIG. 6 represents a protected conductive foil assemblage which comprises a 
prepreg dielectric support layer 42 and a single piece of protected foil. 
For other applications, however, it may be more desirable to lay-up the 
prepreg 42 with a protected layer of foil on each side thereof. This is 
illustrated in FIG. 7, where the prepreg 42 is illustrated as having a 
foil layer 120 on each side thereof. And in each instance, the foil layer 
120 is protected by a corresponding plastic film layer 124. After lay-up 
and lamination, the assemblage of FIG. 7 takes the form of the laminate 44 
shown in FIG. 8. Laminate 44 comprises the prepreg 42, the pieces of 
metallic foil 120 which have one of their sides bonded to the prepreg 42, 
and the plastic film layers 124 disposed in covering, protecting 
relationship relative to the other (outer) sides of the foil layers 120. 
In the case of the preferred embodiments described above, the plastic film 
layer is removably joined with the foil by use of evenly distributed 
static electrical forces which force the film layer and foil together with 
an evenly distributed force pattern to drive gaseous material out from 
between the film and the surface of the foil to produce an essential 
absence of gaseous material (or vacuum) between the film and the foil. The 
vacuum thus created causes the film to conform intimately to the contours 
and shape of the foil surface so that the film clings tightly to the foil 
to permit movement and further processing of the foil while the protective 
film remains in place thereon in covering, protecting relationship 
thereto. This is an improvement over the procedures utilized in said 
co-pending application Ser. No. 07/347,841, where an adhesive material is 
placed in the marginal area of the foil. In that case the foil and film 
layer may be pulled apart manually or the marginal area may simply be cut 
off of the assemblage whereby the plastic film is readily separated or 
removed from the foil. This is a valuable procedure; however the added 
steps needed to apply the adhesive, and the waste resulting from removal 
of the marginal area result in a procedure which is less efficient and 
effective than the preferred procedures of the present invention. Likewise 
the procedures wherein the film is heated to cause it to conform to the 
topography of the foil and/or adhere to the marginal areas are less 
efficient than the present procedure where the film and foil are forced 
together only by static electrical forces and held together thereafter by 
the absence of gaseous material (or vacuum) created when the static forces 
squeeze the air out from between the foil and the protective film.