Note: Descriptions are shown in the official language in which they were submitted.
302
: C-9441
PERMANENT MASTER WII'H A PERSISTENT
LATENT IMAGE FOR USE IN ELECTROSTATIC TRANSFER
TO A RECEIVING SUBSTRATE
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Background of the Invention ~ -
: 5 This invention relates generally to a method of
: high resolution electrostatic trans~fer of a high density
image to a nonporous, nonabsorbent conductive receiving
: surface. :More specifically, it pertains to the
: permanent master:used to make such a transfer, the
~: 10 methodlof creating a persiste~nt latent image on a
permanent:master employing a photosensitive material,
such:as a:liquid or dry film photoresist, and the method
of transferring from that master. The permanent master
that~may be repeatedly used to produce high resolution
5~ and high density im~ges on~receiving surfaces, such as
printed circuit boards. . :
~ The production of conductive wiring patterns on
;.; : : an insulating substrate employing a dry film resist by
:~: use of photoimaging and other techniques to produce a
:20 printed circuit board typically employs a five
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step process. Regardless of whether a tenting method or
a hole-plugging method is employed, the five distinct
steps have included laminating or coating a
photosensitive dry film resist on at least one
conductive surface of an insulating substrate, forming a
wiring pattern on the dry film resist by use of artwork
or a phototool and exposing the dry film resist to
actinic radiation through the transparent areas of the
phototool, developing the circuit board by removing the
unexposed portions of the negative working dry film
resist, etching the conductive substrate from the
circuit board in all non-imaged areas not beneath the
desired conductive wiring pattern which is still covered
with the dry film resist, and finally stripping or
i5 removing the dry film resist covering the desired wiring
pattern from the non-etched portions of the conductive
substrate. This five step process must be repeated for
each circuit board produced. ~
' During the exposure s~tep-in the standard dry ~ '
~20~ f~ilm process, sufficient radiation exposure~levels and
exposure times~are desired to produce~straight sidewalls
in~the dry film resist;that ar~e the r'esult of a pattern
of the cross-linking of polymers in the dry film. These
straight sidewalls~should be normal~to the conductor ~ '
~25~ ~ surface. Practicallyj~ however, for~example in the ~ ' '-
standa~rd~negative~work~ing dry fllm~photoresist print~ànd
e~tch~proces~s,~ eLther underexposure occurs, producing a
sidéwall~e~dge ~tha~ undercuts the desired resist pattern,
or~overexposure'occurs~ producing a~sidewall edge in~the'30;~ ~dr~y film photoresist that increases the width of the dry
fi~lm~photores~st~a~t~ the base of th~e resist and the
surface of the conductor causing a~foot~ Both of~these '`
conditions vary the width~of the ultimate conductive
pattern r~om th~at~;which~is~desired, beyond the planned
~35~ and engineered tDlerance or overage of the line widths
in the conductor surface.
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The development step during this process
ideally should dissolve away the unexposed and,
therefore, uncross-linked areas of the negative working
dry film resist to produce an edge in the dry film
resist on the conductor surface that is equal in width
to the pattern on the phototool and normal to the
conductor surface. Practically, however, either
underdevelopment or overdevelopment of the dry film
photoresist occurs. Underdevelopment produces a buildup ~-
of resist residue in the sidewall zone or developed
channels that is sloped toward the adjacent sidewall~
resulting in smaller spaces between the adjacent lines
than is desired. When overdevelopment occurs the
unexposed film resist edge is undercut, producing larger
1~5 than desired spacing~ between adjacent lines.~
Additionally, there is the potential for some rounding
at the top of the resist surface sidewall edges.
his inability to accurately reproduce the
; phototool throughout the thickness of the dry film
2;0 ~ resist~affects the fine line resolutlon and reproduction~
char~acteristics of the reproduced c;ircuit pattern.~ ~As~
circuit boards have become more~complex and stacking of
multiple boards has become prevalent, the need for
hiyher~ density,~ finer resolution circuit patterns has
~2~5~ evolved.~ Resolution has~;been viewed as the abllity~to
reliably~produce the sma~llest line and space between~
adjacent~lines~th~a~t~can be reliably carried through the
a~orementioned five step processing. The thinness~or
smallness of the lines that can survive development and
. 3a~ ; the~narrowness~oE~the gap~or space~between the adjacent
lines in the circuit pattern have led to fine l~ine
reso1utian~and r~eproduction~standards~in ~the printed
circui~t board~indus~try calling for about~3.1 mil line
and~space dimensions~or the d~evelopment of~about 6.3
line pair~s per millimeter. These standards are used to
define the desired density of the circuit board.
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The attempt to apply the principles of
. ~ xerography to transfer developed electrostatic latent
images from a photoconductor's electrostatically imaged
surface to a receiver sur~ace with high resolution and
high density images has encountered difficulty. The
major source of this difficulty stems from the fact that
circuit boards consist of a nonporous or nonabsorbent
substrate, such as metal, like copper, or a plastic,
` like the polyester film sold under the tradename of
MYLAR. This nonporous and nonabsorbent receiving
~" surface causes the image being transferred, especially
when attempted with a liquid toner, to become distorted
or "squished".
Xerogr~phic techn iques solved the problem of
trans~erring an image to absorbent receiving surfaces,
such as paper, by transferring the images formed by
toner particles across a gap. The gap has either been
an air or a combination air-liquid gap. Attempts to
translate this gap transfer technology to nonporous
substrates, however, resulted in image "squish" and the
realization that the gap space and the voltage must be
; ~ carefully controlled to produce an acceptable
transferre-d toner image with the proper resolution an~
density. If the voltage and the gap space or distance
between the photoconductor or the electrostatically
imageablle surface and the conductive receiving surface ~
are not carefully controlled, electrical arcing across
the~gap will occur. This can cause pin-holes in the
transferred toner image by 2ermanently damaging the
electrostatically imageable surface. This is~esp~ecially
; significant in print and etch applications used to
manufacture orinted circult boards.
lso, it has been found with nonporous
receiving substrates that both the photoconductor or
;~ ~ electrostatically imageable surface and the receiving
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conductive surface must be stationary at the point of
transfer of the toner image to achieve a transferred
image of high resolution.
An additional problem is presented in
transferring the developed latent image
electrostatically to a nonabsorbent substrate, such as
copper. The metal or copper sur~ace forming the
conductive receiving surface, as well as the
electrostatically imageable surface, is uneven so that
the spacing between the electrostatically imageable
surface and the conductive receiving surface must be
sufficient to avoid contact between the uneven surfaces
of the photoconductor and the conductive receiving
surface.
These problems are~solved in the process of the
present invention by providing a method of making~a
persistent latent image and a permanent master using a
photosensitive material, such as a` liquid or a~dry film
resist. ~The persistent latent image on the
20~ ~ photosensitive material is used to transfer an
electrophotographically developed electrostatic latent
image from the~electrosta~tically imaged surface of the
; permanent master ~o a receiving~surface which is
preferably, but~not exclusively, a~conduct1ve,
25~ nonabsorbent,~and nonporous receiving surface of the ~ -
type used to produce multiple printed~circuit boards
wi~th a~desired~conductlve pattern. The permanent master
does not have thè uncross-linked areas of the
photosensitive~material dissolved~away to obtain the
30~ dielectric~con~trast nec;essa~y to effect the image
transfer.
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Summary of the Invention
It is an object of the present invention to
provide a method for achieving non-contact high
resolution electrostatic transfer of a developed high
density electrostatic latent image from a permanent
master directly to a nonporous and nonabsorbent
substrate.
It is another object of the present invention
to obtain a high density electrostatic latent image on
an electrostatically imageable surface through the use
of a dry film resist that serves as a permanent and
reuseable master.
It is another object of the present invention
that the method of electrostatic transfer can be
;15 utilized with a permanent master as the
electrostatically imageable surface.
It is yet another object of the present
~; ; invention to provide a permanent master and a method of
making a permanent master that can~be used to produce
20~ ` multiple copies of the desired~resist pattern on a
conduc~tive substrate in preparation for~etching to
produce the conductive circuit~wiring pattern;on printed
circuit~boards.
It is;~a feature of the present~ in~ention that
~25~ the~photosensitlve~material of~the~permanent master is
charged after~ the latent image is;formed and~that the~
reusable permanent master is capable of resolving about
.l mil~line and~space, or the equivalent of~18~line~
pair~per~millimeter.~
30~ It is another feature;of the present invention
that the conduct~ive backing material or~the substrate
supporting~the permanent master~ is elec~trically grounded
and the~conductive reaeiving surface~is~electrically
isolated from ground~during the electrostatic transfer.
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It is still another feature of the present
invention that the difference in conductivity of the dry
film photoresist between the imaged and non-imaged areas
on the master is employed to form a persistent
electrostatic latent image.
It is another feature of the present invention
that the persistent latent image is electrostatically
charged, developed and transferred to a conductive
receiving surface across a liquid-filled gap.
It is another feature of the present invention
that the distance or spacing of the gap between the
electrostatically imageable surface and the conductive
receiving surface is between about 3 mils and about 20
mils and the distance is maintained by the use between
the two surfaces of spacer means which are electrically
isolated from ground.
It is still another feature of the present
invention with a permanent master used as the
electrostatically imageable surface that the
electrostatic latent image on the electrostatically
imageable surface is persistent and reusable as a master
to produce large quantities of printed circuit boards
without the need to expose the dry film or liquid
photoresist for each circuit board copy.
It is a further feature of the present
invent~on that only the top surface oE the dry film
photoresist image on the master is used to form the
developed image prior to transfer to the conductive
receiving surface.
It is yet another feature of the present
invention that the transfer oE the developed
; electrostatic image to the conductive receiving surface
is accomplished by directly applying D~C. voltage to the
conductive receiving surface, rather than corona
charging.
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It is an advantage of the method of the present
invention that high resolution transfer of the toner
particles forming the developed latent image is obtained
on the conductive receiving surface without image
distortion.
It is another advantage of the present
invention that there is no damage or abrasion to the
permanent master that is the electrostatically imaged
surface during the transfer process so that the surface
may be continually reused.
It is still another advantage of the present
invention that high resolution transfer is achieved
because there is no contact between the developed toner
particles on the permanent master that is the
electrostatically imaged surface and the conductive
receiving surface.
It is yet another advantage of the present
invention that the power requirements can be reduced to
accomplish the electrostatic transfer because of the use
of direct applied voltage~ rather than corona charging
which causes air ionization.
It i5 still another advantage of the present
invention that a faster and lower cost method of making
printed circuit boards is achieved because of the
~75 elimination of the repeated exposure and development
steps r!equired o dry film or liquid photoresists for
each circuit board.
These and other objects, features and
~; ~ advantages are obtained~by the use of a permanent master
and the method of fabricating a toned pattern on an
electrically isolated nonabsorbent conductive surface by
~first establishing an electrostatic latent image on a
photosensitive material on a permanent master that is an
electrostatically imageable surface. The
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electrostatically imageable surface is charged. chargedtoner particles are then developed to a latent image
area on the photosensitive material of the master and
transferred to the conductive receiving surface.
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~rief Description of the Drawings
The objects, features and advantages of the
invention will become apparent upon consideration of the
following detailed disclosure of the invention,
especially when it is taken in conjunction with the
accompanying drawings wherein;
FIGURE 1 is a diagrammatic illustration of the
prior art print and etch printed circuit ooard
fabrication steps; and
FIGURE 2 is a diagrammatic illustration of the
process of the present invention employing a permanen~
master that is reusable to produce multiple copies of a
desired conductive wiring pattern on an insulating
dielectric layer by the migration of charged toner
particles Erom the master across a liquid-filled gap to
a conductive receiving surface.
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Detailed Description of the Preferred Embodiment
FIGURE 1 shows the standard five step process
that has been previously employed in the production of
printed circuit boards. Each one of the circuit boards
produced has routinely required the application of a dry
film to a conductive substrate, such as copper, that is
laminated to a nonconductive substrate, such as
fiberglass epoxy, with pressure and heat. A mask is
then applied over the dry film to permit selective
exposure from a light source or other source of actinic
radiation to produce the desired pattern. Development
takes place by removing the uncross-linked dry film,
leaving only cross-linked dry film with the desired
pattern. Etching with an acid etchant removes the
conductive copper substrate from between the areas of
cross-linked dry film. Finally, stripping the dry film
~ from the remaining conductive copper substrate exposes
; the desired circuit pattern. This is commonly known as
the print and etch process.
In the process of the present invention,
however, a permanent master is produced with the use of
a photosensitive material or coating, such as a dry film
or liquid photoresist over a conductive substrate.
Thereafter, dry film or liquid photoresist is not
~ employeld to produce the desired conductive wiring
patterns from the permanent master on the product
circuit boards.
The permanent master is used as an electro-
statically imageable surface as shown in FIGURE 2. A
conductive backing has a photosensitive material, such
as a dry film or liquid photoresist, applied to it on at
least one side. This photosensitive material undergoes
a change in resistivity UpGn exposure to actinic
radiation because of the cross-linking of-the polymers
in the material. A persistent image is formed on the
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photosensitive material by actinicly radiating through a
mask or by "writing" the desired pattern with a digital
laser pen. Either method produces electrostatic
contrasts or differences in the resistivity between
imaged and non-imaged areas on the photosensitive
material. The electrostatically imageable sur~ace is
isolated from ground and charged with a corona charging
device to produce the charged latent image.
The electrostatically imageable surface is then
developed by the application, through surface
adsorption, of a liquid comprised at least partially of
a nonpolar insulating solvent that serves as a liquid
carrier for toner particles that are charged oppositely
to the charge of the electrostatically imageable
surface. This application can be accomplished by
flooding, dipping or spraying the electrostatically
imageable surface. The charged toner particles are
directed to the latent image area of the
electrostatically imageable surface to form or develop
the latent image.
Thus developed, the image is formed on the
electrostatically imageable surface according to the
persistent latent imageis pattern on the permanent
~ master. The developed image thus is ready for transfer
i ~ 25 ~ to an electrically isolated conductive receiving surface
to produce a circuit board with the desired conductive
wiring pattern.
~; The conductive receiving surface is firs~
coated with a liquid that comprises at least partially a
~30 nonpolar insulatLng solvent. The solvent is the same
as, or an equivalent to that which is applied to the
electrostatically imageable surface and may be applied
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by sponge, squegee, rubber roller or other means capable
; of applying a thin continuous film. The solvent should
~35 ~ pr-ferably hav- a high resistivity and a low viscosity
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-~ ?er~it the charged toner particles to mi~rate or flow
~ ouyn the solvent from the charged electrostatic
l~tent i~age area on the electrostatically imageable
surface to the conductive receiving surface. The
, solvents are generally mixtures of ~9-Cll or
C9-C12 branched aliphatic hydrocarbons sold under
tneTrade-mark Isopar G and Isopar H, respectively,
manufactured by the Exxon Corporation, or equivalents
thereof. The electrical resistivity is preferably on
the order of at least about 109 ohm-centimeters and
the dielectric constant preferably is less than about
3 1/2~ The use of nonpolar insulating solvents with
; these characteristics helps to ensure that the pattern
of charged toner particles is not dissipated.
After a D.C. voltage optimally between about
; 200 to about 1200 volts is applied to the conductive
receiving surface to establish an electric field between
the electrostatically imageable surface and the
conductive receiving surface, the surfaces are moved
close enouyh together to create a completely liquid
transfer medium by the contact of the two layers of
nonpolar insulating solvent. The first liquid surface
; of the first layer of nonpolar insulating solvent on the
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electrostatically imageable surface and the second
~25 liquid surface o~ the second layer of nonpolar
insulat~ing solvent on the conductive receiver surface
join together to fill the gap between the two s~rfaces.
he voltage necessary to establish the electric field
between the electrostatically i~ageable surface and the
~conductive receiving surface operably can be between
about 200 to about 3500 vol~s, but is preferably between
aoout 200 and about 1500 volts and optimally is as
stated above. The ability to transfer a high resol~ltion
age is a function of the combined factors of the
3~ toner, the liquid carr1er, the gap s~acing and the
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voltage applied. Generally, a greater gap spacing
requires a higher voltage to effect a high quality, high
resolution image transfer~
A uniform spacing across this gap is maintained
by the use of spacer strips or gap spacers, seen in
FIGURE 2, which are electrically isolated ~rom ground.
The developed image from the electrosta-tically imageable
surface is transferred across the gap through the liquid
medium to the conductive surface to form an imaged area
in a pattern similar to that of the phototool where the
transferred toner particles are present and non-imaged
areas where the particles are absent.
The transfer of the developed image across the
liquid-filled gap takes place at the point of transfer
by maintaining a first plane taken through the
electrostatically imageable surface parallel to a second
plane taken through the conductive receiving surface.
The electrostatically imageable surface and the
conductive receiving surface at the point of transfer
~0 should have no relative motion occuring between them,
although the point of transfer could be a stationary or
rolling point of transfer~ A drum or web, or a
stationary flat surface could be employed for the
electrostatically imageable surface, transferring the
developed image across the gap to a flat and stationary,
or a moving conductive receiving surface. The moving
conductive receiving surface could be a rolling drum or
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a web or o~her appropriate means. The electrostatically
imageable and conductive receiving surfaces must be held
30 ~ iD place at the point of transfer~ such as by a vacuum,
or alternately could be accomplished by magnetically or
electrostatically holding the surfaces in place across
the gap.
This gap between the electrostatically
imageable surface and the conductive receiv1ng surface
lS preferably maintained between at least about 3 mils
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(.003 inch) and about 10 mils (.010 inch) by the use of
spacer strips of the desired ~hickness. By maintaining
the gap greater than about 3 mils, the inconsistencies
or irregularities in the two surfaces are separated
sufficiently to prevent any contact from occurring
between the two surfaces and any possible abrasion or
scratching from occurring to the surEace of the master
or electrostatically imageable surface.
The spacer strips or gap spacers are selected
from either conductive materials, such as metal, or
nonconductive materials, such as polyester film sold
under the tradename MYLAR, or cellophane. The strips
must be electrically isolated from ground and be of
uniform thickness. The uniform thickness insures that a
uniform gap spacing is obtained between the
electrostatically imageable surface and the conductive
receiving surface. The spacer strips preferably should
be placed outside of the image area.
By applyiny the first and second layers of
nonpolar insulating solvent in sufficient thickness to
the electrostatically imageable surface and the
conductive receiving surface to fill the gap
therebetween, the first liquid surface and the second
liquid surface of the first and second layers of the
nonpolar insulating solvent join together to form a
continuous liquid transfer medium at the point of
transfer of the charged toner particles between the
electrostatically imageable surface and the conductive
~` ~ receiving surface. By traveling through a continuous
~30 ~ ~ sea of liquid transfer medium, there are no surface
tension forces which the charged toner~particles must
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overcome that could hinder their migration from the
electrostatically imageable surface to the conductive
receiving surface. The charged toner particles are
directed through the liquid transfer medium formed by
the joining of the two layers of nonpolar insulating
solvent at this point of transfer by the electric field
that is applied at the point of transfer.
As is diagrammatically illustrated in FIGURE 2,
the charged toner particles with their predetermined
charge, migrate from the oppositely charged cross-linked
imaged area with the photosensitive material on the
electrostatically imageable surface to the conductive
receiving surface as individual or grouped particles.
The conductive receiving surface is laminated onto an
insulating dielectric layer, such as a fiberglass
epoxy. The applied electric transfer field causes the
toner particles to migrate through the liquid transfer
medium of the nonpolar insulating solvent and attach to
the conductive receiving surface to create imaged areas
where the toner particles are present and non imaged
areas where they are absent.
Since the photosensitive material, such as a
dry film or liquid photoresist~ on the electrostatically
imageable surface acts as a master electrostatic image
plate,!and the resistivity difference between the imaged
and non-imaged areas on the electrostatically imageable
~ ~ sur~ace remains relatively constant in most instances
-~ ~ for sustained periods of time dependent upon the
photoresist used, multiple copies can be made by the
electrostatic transfer method. To repeat the procedure,
excess nonpolar insulating solvent and excess toner
particles on the electrostatically imageable surface
should be removed, such as by rinsing, followed by a
physical wiping or squeeging. Any residual electric
35 ~ charge on the electrostatically imageable area should be
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discharged, such as by charging the photosensitive
material's surface with an alternating current corona.
The desired electrostatic latent image pattern
remains in the photosensitive material by using the
material's ability to retain differences in resistivity
for relatively long periods of time after having been
exposed to actinic radiation to form cross-linked imaged
areas of increased resistivity and non-imaged areas
unexposed to the actinic radiation which remain the less
resistive or background areas. The photosensitive
material, such as a dry film resist, typically is ~ormed
of polymers which become cross-linked to form the imaged
areas of greater electrical resistivity that should be
an order of magnitude, or greater, more dielectric than
the background or unexposed areas, dependent upon the
process speed of the image transfer. Faster process
speeds will require greater resistivity differentials.
For fine line image transfer the resistivity of
the photosensitive material can range between 10l7 to
10ll ohm-centimeters, depending upon the material.
The background or non-imaged areas should typically have
an electrical resistivity of 10l ohm-centimeters.
For comparative purposes, the resistivity of various
materials, including some traditionally considered as
insulating materials, are known. Electronic grade Mylar
polyestlr film has a resistivity of about 1017
ohm-centimeters, aluminum oxide ceramic about 10l6
ohm-cen~imeters, various grades of mica from 1013 to
1017 ohm-centimeters, unexposed Dynachem AX dry film
about 1013 ohm-centimeters, and various grades of
Union Carbide's*Bakelite copolymer pla tics from about
107 to 10 6 ohm-centimeters. DuPont's Riston 3615
dry film has a resistivity of aDout lolo
ohm-centimeters.
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These imaged areas, formed by the exposure of
the photosensitive material to actinic radiation, are
the only areas of increased resistivity that hold a high
voltage charge when charged by a D.C. charge corona, if
the conductive backing is electrically grounded. The
non-imaged or background areas with the lesser
electrical resistivity very rapidly release or leak the
charge through the grounded conductive backing. The
charged toner particles suspended in the nonpolar
insulating solvent are oppositely charged to these
latent imaged areas so that the charged toner particles
are attracted to them. This then permits the transfer
of these charged toner particles from the
electrostatically imageable surface across the liquid
gap to the conductive receiving surface as previously
described.
Once the toner image is formed by the toner
particles in the imaged area on a conductive receiver
surface, the particles are fused to the conductive
receiving surface by heating, as illustrated
diagrammatically in FIGURE 2. The heat can be provided
either by the use of an oven or directed warm air
hrough an air slot so that the heat is supplied for a
finite period of time sufficient to reach the
temperature at which the binder or polymer forming the
toner particles will solvate in the liquid which is
entrained within the transferred image. The fusing, for
example, can occur for about 15 to about 20 seconds at a
- ~ ~ temperature greater than ahout 100C and up to about
180C.
Therea~ter the non-imaged areas are etched to
produce the desired conductive wiring pattern in the
unetched conductive receiver surface which is overcoated
with the toner particles. The etching step utilizes a
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solution that cannot remove the conductor material from
the areas of the conductive recelving sur-Eace protected
by the toner particles, but does attack and remove the
conductor material from the areas unprotected by the
toner particles. The particular type of etchant
employed depend~, in part, on the conductor material
being etched and the type of resist being used, so that
both acid and very mild alkaline etching solutions are
possible Eor use. For example, when the conductive
receiving surface is copper, an etchant comprising
acidic cupric chloride is preferably used.
The final step in the electrostatic transfer
process to form the copy is the stripping step. During
this step the toner particles are appropriately removed
or stripped from the imaged areas, such as by rinsing
with methylene chloride, acetone, an alkaline aqueous
solution or other suitable solution.
In order to exemplify the results achieved, the
following examples are provided without any intent to
limit the scop~ o~ the instant invention to the
discussion therein. The examples are intended to
illustrate the manner in which a permanent master with a
;~ persistent conductive latent image on the
electrostatically imageable sur~ace can be obtained and
how the gap spacing and voltage levels can be varied to
achie~e successful electrostatic image transfer. The
examples also illustrate, whether a photoconductor or a
permanent master is used as the electrostatically
imageable surface, how successful electrostatic ima~e
transfer can be achieved without the need or the
application of a dry film or liquid resist to each
conductive receiving surface prior to the transfer of
the developed latent image from the electrostatically
imageable surface.
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~ 2xample l
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A liquid toner was prepared for use by
preparing the following raw materials in the amounts
shown in a high speed disperser:
Raw ~aterial ~ ~
*ISOPAR H 1248.6 so~vent-carrier
UNIREZ 7059 439.2 alcohol insoluble
(Union Camp) maleic modified
rosin ester
Allied AC 307.8 linear polyethylene
Polyethylene 6A
; BAKELITE 1584.0 ethylene-
DPD 6169 ethylacrylate
(Union Carbide) copolymer 20~ shock-
cooled suspension in
~ ISOPAR H
: phthalocyanine229~2 coloring agent
~ green pigment
: Alkali Blue G158.4 coloring agent -
~ 20 pigment
: These components were mixed at a speed of 8000
-~ rpm for 10 minutes while maintaining the temperature of
~:~ the ~ixture between 160 and 220Fo
~;~ f 606 Grams of an amphipathic graft copolymer
~: ` 2 5 system was prepared by mixing 104.3 grams of lauryl
ethacrylata and 44.7 grams of methyl methacrylate, both
available from Rohm and Haas, and 3.0 grams of azobis - .
: isobutonitrile, available from DuPont as ~azo 64.
Next 108.2 grams of an amphipathic copolymer
` ~ stabili~er was prepared according ~o the proce~ure
described hereafter. In a l liter reaction flask
e~uipped with a stirrer, a thermometer and a reflux
-.; condensor is placed 400 grams of petroleum ether ~b.p.
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90-120C) and the same is the heated at atmospheric
pressure to a moderate rate of reflux. A solution is
made of 194 grams lauryl methacrylate, 6.0 grams of
glycidyl methacrylate and 3.0 grams of benzoyl peroxide
paste (60 percent by wt. in dioctyl phthalate) and
placed in a 250 ml. dropping funnel attached to the
reflux condensor. The monomer mixture is allowed to
drip into the refluxing solvent at such a rate that it
requires 3 hours for the total amount to be added.
After refluxing 40 minutes at atmospheric pressure
beyond the final addition of monomer, 0.5 grams of
lauryl dimethyl amine is added and the refluxing is
continued at atmospheric pressure ~or another hour.
Then 0.1 gram hydroquinone and 3.0 grams methacrylic
acid are added and refluxing continued under a nitrogen
blanket until about 52 percent esterification of the
glycidyl groups is effected (about 16 hours). The
resulting product is slightly viscous straw-colored
liquid.
345.8 Grams of ISOPAR H from Exxon Corporation
was added to the 108.2 grams of the amphipathic
copolymer stabilizer and the aforementioned quantities
; ~ of lauryl methacrylate, methyl methacrylate and azobis
isobutnitrile to form the 606 grams of amphipathic graft
copolymer system. Polymerization was effected by
heating this solution to about 158F under a nitrogen
atmosphere for about 4 to about 20 hours.
606 Grams of additional ISOPAR H was added to
the above solution and mixing was continued for 10
minutes at 8000 RPM while the temperature was maintained
between about 160F and 180F.
;~ Finally, 3578 grams of ISOPAR H was added, the
mixer speed reduced to 1000-2000 RPM, and mixing
~- continued for 30 minutes. During this last step, the
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temperature of the mixture was maintained between
120F and 140F.
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-22- ~
~ ext ~ liquid toner concentra~e was prepared by
combining the following in a static attritor-ty2e
~aterialAmount (~rams) Description
Predispersion ~ix 1022.7 liquid toner
predispersion
Carnauba wax 58.3 wax
polymer dispersion 83.3 amphipa~hic polymer
dispersion as
prepared in Ex. XI
of Kosel U.S. Patent
No. 3,900,412
*Neocryl S-100462.4 amphipathic polymer
dispersion
available from
Polyvinyl Chemical
Industries, Div~ of
Beatrice
730 Main St.
Wilmington, MA 01887
ISOPAR H 694O5 solvent carrier
These components were milled for three hours at
300 RPM and a temperature of about 75F to create a
toner concentrate. The toner concentrate.was futher
diluted to about 1 to about 2 percent solid to create
the working solution for use in electrostatic imaging.
A cadmium sulfide photoconductor overcoated
. with a MYLAR polyester film layer (typical of the NP
process type) was corona charged and then light exposed
to a circuit trace pattern from about 0.75 to about 2.70
~icrojoules/square centimeter in a*Canon Model L824
~:~ copier to create a charged latent im~ge. The charqed
latent image was developed by applying the liquid toner
to the overcoated cadmium sulfide photoconductor that is
~; the electrostatically imageable surface. The
electrostatically imageable surface of thi~
photoconductor is mounted over an inner aluminum
substrate drum., The drum was removed
*Trade-mark
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from the copier. A high voltage power source had its
ground lead connected to the interior of the drum and
its positive lead connected to the copper surface of the
conductive receiving surface. Cellophane spacer strips
or gap spacers were used between the drum and the
conductive surface. The conductive surface was coated
with a liquid that included the nonpolar insulating
- solvent. The electrostatically imageable surface of the
drum was coated during the development step. 1000 Volts
D.C. current was applied and the gap was set at 10 mils.
The cadmium sulfide drum was manually rolled
across the spacer strips to create points of transfer of
the latent image from the electrostatically imageable
surace to the conductive receiving surface of copper.
The image transfer was successful with the image
- possessing excellent resolution and good density.
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Example 2
The cadmium sulfide photoconductor drum of
Example 1 was cleaned and dried. The same liquid was
utilized with the same photoconductor latent image
obtained in the same apparatus as in Example 1. The
drum was wetted with the liquid transfer medium and the
steps Example 1 were repeated. The liquid transfer
medium was applied to the conductive receiving surface.
500 Volts of D.C. voltage was applied and the same gap
space was set as in Example 1. The imaye transfer was
successful, but the amount of the transferred toner
particles forming the transferred image was less than
the amount in Example 1 and was very light over the
entire image area. There appeared to be insufficient
voltage applied to transfer the majority of the toner
particles over a 10 mil gap.
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~25-
Example 3
The same steps and liquid transfer medium as
employed in Example 2 were repeated. The conductive
receiving surface was wetted with the liquid transfer
medium by applying to the conductive receiving sur~ace
with a squeegee. The spacer str ip5 were set at 3 mils
to achieve a uniform 3 mil separation between the two
surfaces and a voltage of 1000 volts was employed.
A clear high resolution image with good density
; 10 was obtained but some void areas appeared in the image.
The image was uniform and distinct.
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Example 4
The same steps and liquid transer medium were
employed as in Example 3, but the gap spacers were 3 mil
thickness to establish the 3 mil gap between the
electrostatically imagea~le surface and the conductive
receiving surface. 200 Volts D.C. current was applied
to establish the electric field. ~ very clear high
resolution image was transferred from the
elec~rostatically imageable surface to the conductive
receiving surface, which exhibited good reflectance
image density. The density of the pad areas and line
traces, however, was somewhat less than that achieved in
Example 3 because all of the toner particles apparently
were not transferred.
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Example 5
The same steps and liquid transfer medium were
employed as used in Example 2, but the liquid was
squeegeed on. The gap spacers were 1 mil thick to
establish a 1 mil gap between the electrostatically
imageable surface and the conductive receiving surface.
1000 Volts D.C. was applied to create the electric
field. The transferred image had good image density,
but there were many hollow spots due to arcing. Some
image distortion was present, apparently due to the
closeness of the two surfaces and the resultant
"squishing" of the toner particles. The use of a
system, such as a vacuum hold-down system, which permits
the receiving substrate to be held rigidly flat would
have reduced the image distortion. Most of the
-~ transferred image was not distorted.
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128B802
-28-
Exam~le _
A 4" x 5" electrically conductive substrate of
aluminum foil, CDA alloy #1145-H18, full hard temper
with a scratch brush finish, was selected as the
conductive substrate for use in making the
electrostatically imageable surface of the permanent
master. The aluminum substrate was checked for need of
cleaning or degreasing. If necessary, the substrate can
be cleaned with methyl chloride, methylene chloride or
trichloroethylene to promote good adhesion of the
photoresist to the cleaned surface during the subsequent
lamination step. In this particular instance cleaning
was not necessary. DuPont Riston 215 dry film
photoresist was laminated to the substrate as the
photosensitive material. The lamination was
accomplished with the use of a Western Magnum Model
XRL 360 laminator made by Dynachem of Tustin, CA. The
~; lamination was carried out at a roll temperature o~
about 220F and a speed of about six feet per minute.
A protective top layer of approximately .001 inch thick
; polyethylene terephthalate, hereafter PETI film was
retained over the dry film photoresist of the aluminum
foiljRiston 215~laminate.
he laminate was exposed to actinic radiation
through a~negative phototool us~ing the Optic Beam 5050
exposure unit manufac~ured by Optical Radiation
Corporation. The exposure was accomplished after the
laminate cooled to room;temperature following the
laminating process. The exposure level was
approximately 250 millijoules. The phototool was a
Microcopy~Test Target T-10 resolution test chart, with
groups of bars vaxying from 1.0 cycles or line pairs pe~
millimeter to 18 cycles or line pairs per millimeter,
and is sold by Applied Image, Inc.~of Rochester, New
York.~
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The master thus formed was allowed to stabilize
for about 30 minutes to allow photogenerated
cross-linking to be completed and then the protective
layer of PET film was peeled away from the dry film
photoresist. ~he aluminum foil under the
electrostatically imageable surface was then grounded,
and the surface was corona charged so that the ima~ed
area received a positive charge. After a suitable time
delay up to several seconds duration to allow the
background areas to discharge, the charged latent image
was then electrophotographically developed by applying
~ the liquid toner of Example 1 to the electrostatically
; imageable surface. The electrostatically imageable
surEace was then rinsed with Isopar H solvent carrier to
remove excess toner particles. The permanent master was
then ready for transfer of the developed image from the
master to a conductive receiving surface.
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-30-
A 4" x 5" electrically conductive substrate of
copper was mounted on a glass epoxy support substrate
known as FR 4. The conductive surface of the laminate
was checked to ensure that it was clean and not i~ need
of degreasing. If necessary, the substrate can be
cleaned with methyl chloride, methylene chloride or
trichloroethylene to promote good adhesion of the
photoresist to the cleaned surface during the subsequent
lamination step. In this particular instance cleaning
was not necessary. DuPont RiSton 3615 dry film
photoresist was laminated to the surface, using~the
Western Magnum Model XRL-360 run at a speed of six feet
per minute and a roll temperature of about 220F. A
protective top layer of approximately .001 inch thick
PET film was retained over the dry film photoresist of
the copper/Riston 3615 laminate.
After allowing the laminate to cool to room
temperature for about 10 to 15 minutes the dry ~ilm was~ --
~ exposed in two steps to actinic radiation to a negative
phototool. A pre-fogging for about five seconds by an
ADDALUX Model 1421-40 unit, produced by Berkeley
Technical Corporation of WoodSide, NY was accomplished
~at an qnergy level of about 25 millijoules. ~he second
25~ step of the exposure process i~nvolved;~exposing ~the -~
negative~phototool and the electrostatically imageable
surface to an energy level of approximately 475
mlllijoules or an exposure time of about 55 seconds.
The negative phototool was a Mi¢rocopy Test Target T-10
30 ~ resolution test c~art sold by Applied Image, Inc. of
Rochester, New~York. The bar groups on the ph~to~tool
varied~the line pairs from 1.0 cycles or line pairs per
millimeter to 18 line pairs per millimeter.
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The exposed electrostatically imageable surface
was then allowed to cool to room temperature for about
30 minutes, thereby permitting cross-linking in the dry
~ilm to complete. The protective layer of PET film was
peeled awayO The copper substrate was grounded and the
electrostatically imageable surface was corona charged
so tha~ the imaged area received a positive charge.
After a short delay of about a second or more to allow
background areas to discharge, the charged persistent
image was then electrophotographically developed with
liquid toner of Example 1. Excess toner particles were
rinsed from the developed permanent master with Isopar H
solvent carrier without allowing the toner to dry. The
; developed persistent image on the electrostatic master
was then ready for transfer to a conductive receiving
surface.
The electrostatic master thus formed was laid
flat on a generally flat working surface. Two three
(03) mil thick MYLAR poIyester spacer strips were
placed along a pair of parallel and opposing edges of
;~ the master outside of the developed image area.
A flexible conductive receiving surface of 1/2
ounce copper foil laminated to a 1 mil thick Kapton
polyimide insulating layer was wrapped around and
securedjby lap taping the edges to a 1 1/2 inch diameter
; drumO The receiving surface was wet with a layer of
~sopar H solvent carrier by immersing the cylinder.
Alternativelyr the receiving surface could be coated by
pouring the liquid thereover.
30 ~ ~ An electrical potential of about 800 volts was
established to create an electric field across an
approximately 3 mil gap. The conductive receiving
surface of copper foil was charged with positive
polarity with respect to the electrically conductive
copper substra~e of the master for use with the
negatively charged toner particles.
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The 1 1/2 inch diameter drum with the
conductive receiving surface secured thereto was rolled
over the spacer strips on the edges of the master. AS
the roller passed over the master at each discrete point
of transfer the toner particles were transferred from
the master to the conductive receiving surface.
The conductive receiving surface was then
exposed to a fan for up to about 30 seconds to dry the
non-imaged areas that comprise the background areas.
The non-imaged areas should be dried while the imaged
areas remain wet so the polymers in the toner particles
can solvate in the solvent carrier and not run outside
of the imaged areas. An air knife can also be used to
effect the drying of the non-imaged areas.
The transferred image on the conductive
receiving surface was then fused by placing in an oven
for about 30 seconds. The temperature of the oven prior
to opening was about 180C. The fusing is
accompIished through a temperature ramping that
~ 20 effectively occurs when the oven door is opened to place
;~ the conductive receiving surface inside because of the
resultant temperature drop within the oven. The oven
temperature gradually increases to the approximate
~180C temperature level after the oven door is closed
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While the preferred method in which the
principles of the present invention have been
incorporated is shown and described above, it is to be
understood that the present invention is not to be
limited to ~he particular details or methods thus
presented, but, in fact, widely different means and
methods may be employed in the practice of the broader
aspects of this invention.
For example, to effect transfer the electric
field established between the electrostatically
imageable surface and the conductive receiving surface
can be charged with either positive or negative
polarity, depending upon the charge of the toner
particles, to direct the charged toner particles across
the liquid medium. Charged toner particles o~ negative
polarity will be attracted to a positively charged
conductive receiving surface or will be repelled by a
negative back charging of the electrostatically
imageable surface. If charged to~ner particles of
positi~e polarity are used, they will be attracted to a
negatively charged conductive receiving surface or
repelled by a positive back charging of the
electrostatically imagea~le surface.
Similarly, in the development of the
electrojstatically imageable master surface, alternate
methods can be used. Negatively charged toner particles
will be attracted to a positively charged latent image
or vice versa. In the instance of reversal development
where the background areas are exposed, the desired
image areas on the electrostatically imageable surface
will be uncharged and the surrounding non-image areas
will be charged the same as the toner particles to cause
the charged~toner particles to be repelled from the
non-image areas onto the desired image area. Also, the
nonpolar insulating solvent can equally well be mineral
spirits, as long as it possesses high resistivity and
low viscosity.
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The gap spacing can equally well employ a
~eb-to-~eb arrangement that will hold the
electrostati~ally imageable surface and the conductive
receiving surface at the desired distance.
The electric field can be established in
several ways. F~r example, with a conductive receiving
surface, such as the copper laminate, or in the case of
a dielectric material, such as MYLAR polyester film,
backed by a conductive surface the electric field is
created by direct charging. Where a dielectric
receiving surface, such as MYLAR polyester film, is used
front or back charging via conventional corona charging
or roller charging can be employed.
The electrostatically imageable surface can be
a photoconductor, such as a cadmium sulfide surface with
a MYLAR polyester film or a polystyrene or a
polyethylene overcoating, a selenium photoconductor
surface, or suitable organic photoconductors such as
carbazole and carbazole derivatives, polyvinyl carbazole
and anthracene. where the electrostatically imageable
surface uses a persistent latent image as a permanent
~; master, the surface can be zinc oxide, or organic
photoconductors developed with toner which is fused onto
the master, or a dry film or liquid photoresist.
;~ 25 I The type o~ photosensitive material applied to
the conducti~e backing to make the permanent master may
vary as long as it is permanently imageable and
possesses the correct resistivity characteristics. For
example, where dry film resists are used, the films may
~e aqueous, semi-aqueous or solvent based.
Photoconductive insulating films of zinc oxide dispersed
in a resin binder may also be used.
~; ~ The process disclosed herein has been discussed
~in the context of producing printed circuit boards. It
should be noted, however, that the electrostatic image
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transfer process from a permanent master is equally well
acceptable for use in the production of labels, high
speed production of documents and photochemical
machining or milling. The permanent master can be
employed with liquid or dry toners. Dry toners can be
applied to the photosensitive material that is used to
make the permanent master by magnetic brush, particle
cascade or particle bed systems.
The scope of the appended claims is intended to
encompass all obvious changes in the details, materials
and arrangements of parts which will occur to one of
skill in the art upon a reading of the disclosure.
Having thus described the invention, what is
claimed is:
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