CA1288802C - Permanent master with a persistent latent image for use in electrostatic transfer to a receiving substrate - Google Patents

Abstract

PERMANENT MASTER WITH A PERSISTENT
LATENT IMAGE FOR USE IN ELECTROSTATIC TRANSFER
TO A RECEIVING SUBSTRATE

Abstract of the Disclosure A method of making a permanent master and a permanent master for an electrostatic image transfer and a method of using such permanent master to electrostatically transfer a electrophotographic developed electrostatic image from the permanent master to a receiving surface are disclosed.

Description

: 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 . ' 128~3~302

-2 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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-3-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 *Trade-mark ~ ~ :

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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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12~ 2 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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~2~ 2 g 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 38~2 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 . . ~
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 . , 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 , 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 :

38~2 (.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 :~ 2~

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 ,, 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 :::
temperature of the mixture was maintained between 120F and 140F.
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~ 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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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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~29-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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-, 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 25; again.l ~

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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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Claims (24)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making a permanent master with a persistent latent image having fine line resolution for use in repeated electrostatic image transfer to receiving surfaces, comprising the steps of:
(a) coating a conductive substrate with a photosensitive material that will undergo an increase in resistivity upon exposure to a source of actinic radiation, thereby forming an electrostatically imageable surface;
(b) forming an undeveloped, non-raised persistent latent image on the photosensitive material by exposing the photosensitive material to actinic radiation through a phototool to form non-imaged areas of less electrical resistivity and a latent image area of higher electrical resistivity; and (c) charging the electrostatically imageable surface to form electrostatically contrasted non-imaged areas and a latent image area corresponding respectively to the non-imaged areas of less electrical resistivity and the latent image area of higher electrical resistivity.

2. The method according to claim 1 further comprising using a dry film photoresist as the photosensitive material.

3. The method according to claim 1 further comprising using a liquid photoresist as the photosensitive material.

4. The method according to claim 1 further comprising mounting the conductive substrate to a flexible dielectric material.

5. The method according to claim 1 further comprising mounting the conductive substrate to a rigid dielectric material.

6. A method of fabricating a toned pattern having fine line resolution on an electrically isolated receiving surface, comprising the steps of:
(a) coating a conductive substrate with a photosensitive material that will undergo an increase in resistivity upon exposure to a source of actinic radiation;
(b) forming an undeveloped, non-raised persistent latent image on the photosensitive material by exposing to an actinic radiation source to form electrostatically contrasted non-imaged areas and a latent image area, thereby forming an electrostatically imageable surface;
(c) charging the latent image area on the electrostatically imageable surface;
(d) developing the electrostatic latent image area by applying to the electrostatically imageable surface charged toner particles, the charged toner particles being directed to the latent image area of the electrostatically imageable surface to form a developed electrostatic image;
(e) establishing an electric field between the electrostatically imageable surface and the receiving surface;
(f) placing the receiving surface adjacent to the electrostatically imageable surface;
(g) transferring the developed electrostatic image from the electrostatically imageable surface at a point of transfer to the receiving surface to form non-imaged areas and a fine lined transferred tone particle image in an imaged area; and (h) fusing the transferred toner particle image to the receiving surface.

7. The method according to claim 6 further comprising the steps of (a) etching the non-imaged areas of the receiving surface to remove the receiving surface from the non-imaged areas of the receiving surface; and (b) removing the toner particles from the imaged area of the receiving surface.

8. The method according to claim 6 further comprising applying to the receiving surface a liquid comprised at least partially of a nonpolar insulating solvent.

9. The method according to claim 6 further comprising maintaining a gap between the receiving surface and the electrostatically imageable surface during transfer of the point of transfer.

10. The method according to claim 6 further comprising using a conductor laminate on which is mounted the receiving surface.

11. The method according to claim 10 further comprising using a conductive receiving surface as the receiving surface.

12. The method according to claim 8 further comprising suspending the charged toner particles applied to the electrostatically imageable surface in a liquid to form the developed electrostatic image.

13. The method according to claim 12 further comprising using a liquid comprised at least partially of a nonpolar insulating solvent as the liquid in which the charged toner particles are suspended.

14. A method of fabricating a toned pattern on an electrically isolated receiving surface, comprising the steps of:
(a) coating a conductive substrate with a photosensitive material that will undergo an increase in resistivity upon exposure to a source of actinic radiation;
(b) forming an undeveloped, non-raised persistent latent image of fine line resolution on the photosensitive material by exposing to an actinic radiation source to form electrostatically contrasted non-imaged areas and a latent image area, thereby forming an electrostatically imageable surface;
(c) charging the latent image area on the electrostatically imageable surface;
(d) developing the electrostatic latent image area by applying to the electrostatically imageable surface charged toner particles suspended in a liquid comprised at least partially of a nonpolar insulating solvent to form a first liquid layer with a first liquid surface, the charged toner particles being directed to the latent image area of the electrostatically imageable surface to form a developed electrostatic image;
(e) applying to the receiving surface a liquid comprised at least partially of a nonpolar insulating solvent to form a second liquid layer with a second liquid surface;
(f) establishing an electric field between the electrostatically imageable surface and the receiving surface;
(g) placing the receiving surface adjacent to the electrostatically imageable surface so that a gap is maintained therebetween;
(h) transferring the developed electrostatic image from the electrostatically imageable surface at a point of transfer through the liquid to the receiving surface to form non-imaged areas and a transferred toner particle image in an imaged area;
(i) maintaining the gap between the electrostatically imageable surface and the receiving surface at the point of transfer during the transfer step;
(j) fusing the transferred toner particle image to the receiving surface;
(k) etching the non-imaged areas of the receiving surface to remove the receiving surface from the non-imaged areas of the receiving surface; and (l) removing the toner particles from the imaged area of the receiving surface.

15. The method according to claim 14 further comprising using a conductive receiving surface as the receiving surface.

16. The method according to claim 15 further comprising maintaining the gap so that the first liquid surface contacts the second Liquid surface to create a liquid transfer medium across the gap.

17. The method according to claim 16 further comprising maintaining the gap between the electrostatically imageable surface and conductive surface at the point of transfer during transfer between about 3 mils and about 20 mils.

18. The method according to claim 15 further comprising using a conductor laminate on which is mounted the conductive receiving surface.

19. A permanent master for use in repeated electrostatic image transfers of fine line resolution to printed circuit board receiving surfaces, comprising in combination:
a. a conductive substrate; and b. a photosensitive material applied to the conductive substrate, the photosensitive material further having an undeveloped, non-raised electrostatic persistent latent image thereon after undergoing an increase in electrical resistivity from exposure to actinic radiation, the photosensitive material thereby having non-imaged areas of lesser electrical resistivity and a latent image area of greater electrical resistivity; and c. a charged electrostatically imageable surface formed by the photosensitive material and the conductive substrate, the charged electrostatically imageable surface further having electrostatically contrasted non-imaged areas and a latent image area corresponding respectively to the non-imaged areas of less electrical resistivity and the latent image area of higher electrical resistivity.

20. The permanent master according to claim 19 wherein the latent image area further has a charge polarity.

21. The permanent master according to claim 20 wherein the latent image area further has charged toner particles opposite in polarity to the polarity of the latent image area.

22. The permanent master according to claim 19 wherein the non-imaged areas further have a charged polarity.

23. A method of making a permanent master with a permanent latent image having fine line resolution for use in repeated electrostatic image transfer to receiving surfaces, comprising the steps of:
(a) coating a conductive substrate with a photosensitive material that will undergo an increase in resistivity upon exposure to a source of actinic radiation, thereby forming an electrostatically imageable surface;
(b) forming an undeveloped, non-raised permanent latent image on the photosensitive material by exposing the photosensitive material to actinic radiation through a phototool to form non-imaged areas of higher electrical resistivity and a latent image area of less electrical resistivity; and (c) charging the electrostatically imageable surface to form electrostatically contrasted non-imaged areas and a latent image area corresponding respectively to the non-imaged areas of higher electrical resistivity and the latent image area of less electrical resistivity.

24. A method of fabricating a toned pattern on an electrically isolated receiving surface, comprising the steps of:
(a) coating a conductive substrate with a photosensitive material that will undergo an increase in resistivity upon exposure to a source of actinic radiation;
(b) forming an undeveloped, non-raised permanent latent image of fine line resolution on the photosensitive material by exposing to an actinic radiation source to form electrostatically contrasted non-imaged areas and a latent image area, thereby forming an electrostatically imageable surface;
(c) charging the non-imaged areas on the electrostatically imageable surface;
(d) developing the latent image area by applying to the electrostatically imageable surface charged toner particles of the same charge as the non-imaged areas suspended in a liquid comprised at least partially of a nonpolar insulating solvent to form a first liquid layer with a first liquid surface, the charged toner particles being directed to the latent image area of the electrostatically imageable surface to form a developed electrostatic image;
(e) applying to the receiving surface a liquid comprised at least partially of a nonpolar insulating solvent to form a second liquid layer with a second liquid surface;
(f) establishing an electric field between the electrostatically imageable surface and the receiving surface;
(g) placing the receiving surface adjacent to the electrostatically imageable surface so that a gap is maintained therebetween;
(h) transferring the developed electrostatic image from the electrostatically imageable surface at a point of transfer through the liquid to the receiving surface to form non-imaged areas and a transferred toner particle image in an imaged area;
(i) maintaining the gap between the electrostatically imageable surface and the receiving surface at the point of transfer during the transfer step;
(j) fusing the transferred toner particle image to the receiving surface;
(k) etching the non-imaged areas of the receiving surface to remove the receiving surface from the non-imaged areas of the receiving surface; and (l) removing the toner particles from the imaged area of the receiving surface.