CA2006585A1

CA2006585A1 - Improved method for making printed circuits

Info

Publication number: CA2006585A1
Application number: CA002006585A
Authority: CA
Inventors: John Michael Lowe; Joseph Devonport
Original assignee: Individual
Current assignee: Technology Applications Co Ltd
Priority date: 1988-12-24
Filing date: 1989-12-22
Publication date: 1990-06-24
Also published as: GB2227887A; GB8928563D0; GB8830251D0; GB2228232A; KR910700599A

Abstract

1523d ABSTRACT OF THE DISCLOSURE
Printed circuits are produced by screen printing or the like to deposit conductors, resistors, capacitors and insulators. Crossover connections are made by covering the conducting portions to be crossed with an insulating material that serves as a base for a printed crossing conductor. Exposed conductors, as for keyboards and compression connections, are protected from the development of high-resistance corrosion products by printing or overprinting them with an ink that deposits a carbon layer. The method permits the use of rigid printed-circuit boards having crossed connections without requiring the use of a two-sided board. The equivalent of multilayer boards can be achieved by printing repeated layers of conductors.
components and insulators on the same side of the board, or by printing on two sides of a board.
Interconnection between adjacent layers can be made as a part of the printing process. Resistors can be printed by screening or otherwise depositing a controlled amount of a resistive ink. Capacitors can be produced by printing conductive layers separated by insulating layers. The process may produce printed-circuit boards containing resistors and capacitors without the necessity for inserting and wave-soldering components. If such a board is to contain components in addition to those printed by the process of the present invention, a printed carbon layer can be used as a solder resist to protect printed conductors during the wave-soldering process, and to protect exposed contacts, as for keyboards, during wave-soldering and in use. The process also permits the printing of a conducting layer as a shield against radio-frequency interference or as a ground place to provide electrical isolation of portions of a circuit from each other. A method of making a printed circuit board comprises applying a first and second coating in a desired pattern to a substrate, the substrate (12) being disposed in a different relationship to coating apparatus used in applying a second coating fro the relationship in which it is disposed in applying the first coating. Preferably a number of identical boards are printed on a substrate (12) from which they are subsequently separated. The individual boards are positioned such that the same pattern may be applied when the substrate (12) is presented to coating means in any of a number of orientations each separated from the next by a fixed angle of rotation about an axis (A) at right angles to the substrate (12), the first coating being applied in one such orientation and the second after the screen and substrate respectively have been rotated in opposite directions, (C, B) each through 90°.

Description

15~2d Inven~ors: John M. I~w~ ZO~ 5 -l 523d Joseph Da~3"pc~r~r I~SPROV~D Ml~T~O~) ~FOR ~Il~G PRIN~EI) CIRCIJITB

BA~ o~l~!~

This inv~n~ion is rela~ed to printed circuits for use in electrical and el~ tronic equipment. In parti~:ular, it is an improved method o:~ making prin-ted-circ:~lit ~oards and of p.roduc:ing printed circults vn ~urfac~s o~her than conven~ional printed-circllit boards. This inven~ion is also concerned with malcin~ printed circ:uits ~nd is especially con~erned With a me~.hod of making a printed Ci~llit ir~ which ~ l~Lyer iS applied in a desired pattern to a s~strate~
P~n element th~ ls co~anon to alJnost all electronic equipment i5 t~le printed çir¢uit, typic~ally in the ~orm OI a printPd-circ~uit ~oard ~PCl~)~ A PCE~ is gener~lly made by laminating copper foil to a }~oa~d. A d~sired pat~ern that in(;:lu~le~ c:onduc:~ors in the plane o~ ~t - least ~ne sur~aC~e of the board is plac:ed upon the ~oard, and h~les are driIled or punched for the mo~nting o$ components. ~his pa~tern is t~pically realizçd ~r plac~ing a photos~nsitive res~st on the board, expo~ing a photos~raph of the de~ired pattern on ~he resi~t, and developi~g the re~is ~o produ~.e a protecti~e c:oa~ing ove~ th~ pa~tern. The remainder of 2(3~ 8~
,~

the resist i~ treated to remove undeveloped resist from p~rtion3 where it i5 desired to remove th~ copper. The board i~ then treated with ~ process ~hat removes the expossd copper. When ~he rem~ining resist is then re~oved, the desired pattern remains in the copper.
Mounting holes for components are drilled or punched at desired lo~a~ions som~whers in the course of this proce~s, either before or after the removal step.
The ~aterials most comm~nly used for PCBs are 0 either polymeri~ed epoxy ma~erials containing glass fibers or paper b~und by imp~egna~ed syn~hetic resins such ~ phenolia mate~als. The la~te~ group is of~en rcfer~ed to generically a~ syn~hetic-resin-borlded paper (SRBP). Boa~ds made o ei~her of ~hese types of materials are clad with copper on one side or both sidesl w~th ~he heat and pr~ssure of the cladding or laminating proce~s helping to cure the resin. The ma~erial cost of an S~BP PCB i~ typically about hal~ as much as that of an epoxy fib~glass ~ard of the same surfa~e area, ~o th~re is a po~enti~l co~t advan~ge when the SRBP b~rd can be used. There are various bases for ~he selection b~tween epoxy fiberglass and SRBP m~terial~ For example~ the ep~xy fiberglass ~o~rds are ~ene~ally higher in strength ~nd are preferred for use in equip~ent ~h~t may be subje~t ~o vibra~ion.
A particular probl~m ~f cirouit design tha~ leads to complica~ion in P~Bs is ~he ~at ~ha~ not all electroni~ circui~ can be m~de wi~h their conne~ions 3~ in a single plane. I~ is sometimes necessary to make ~ridging ~onnections between different portions of a circuit, A considerable amoun~ of ingenui~y goes into ~he design and layou~ o~ PC~s to ~inimize suc~ bridging connection~ How~ver, ~ome~imes i~ is impossible to 3~ a~oid ~hem~ In suGh ~ ca e, it i~ possible ~o solder ,, , s~

-jumpe~ wires bet~een the portions of ~he circuit th~t are to be connect~d. mis 1~ ~specially undesir~ble for long runs, and it is b~tter avoided ev~n for short r~n~. A better s~lution i~ to use PCBs that have more than one condu~ting layer. Tha ~implest of these is a two-sided PCB. ~his i~ a hoard that has copper .la~inated to both sid~s. Separat~ patterns ~re etched on the two ~ides to ePf~t t~ desired circui-t layou~
and cxoss-conne~tion~. How~er, in order to make such cros~-connections, and to ~o~plete the ~onnection be~ween the two si~e~ o~ ~he ~oard, it is normally necessary to use ep~xy fiberglas~ because of ~he ne~essity of pl~ting ~oles ~rough the board to connect the top layer ~o th~ bott~m~layer. Holes that are drilled in an SRBP board are adequate to support the l~ads of components ~h~t a~e placed in the ~oard for sol~ering, but the~ are no~ normally clean enough when drilled or ~unched to p~rmi~ ~atis~actor~
eleotroplating o~ ¢onnections between layers of the board, There is thu a l~ng-felt need in the PCB
industry for a way o~ ma~ing PC~s with cr~sso~er connections on single-sided SRBP boards without using : solde~ed jump~r wir~s.
The ~a~e con~iderations apply equally as m~ch to boards ha~ing ~ore than two layers. These boa~ds, referred ~o ~9 multil~yer boards, are often used in more complic~ted ~ir~uits where ~ne set of br~dging connections is not ehough~ ~ with ~he two-sided ~o~rd, it is normally necessaty to use epoxy fiber~lass for such P~B~ It ~an be s~n th~t the result of a need fo~ ~rossed csnn~ ns in the typical etched copper PCB l~ds to an incxea~e~ ~05~ beca~se of the need ~or epoxy ~iberglass ~n the P~ inste~d of the less-e~p~nsive SRBP boards~
~he problem~ ~s~ de~cribed beco~e extr~me in the case of PCBs for X~yboards. A Xeyboard for a typewriter, compu~ or the like typically generates an ele~txical ~ign~l when a key is depressed to make an electrical ~ont~ct or ~n induative or capacitive coupling. Su~h a coupling ~s made dir~ctly or indirectly between two sep~rated conductors on the boar~ or on a flex.lble pl~sti~ me~br~ne that is spaced ap~rt from the board and make~ a condu~tive or field contdct when pre~se~ ~oward the ~oa~d. Wh~n the 1~ k~ey~oard is ~he typical typewri~er or computer ke~boar~, it i~ impo.~sible or nearly impossible to ~oid crossed connections. The physical size of the hands of an operator also set~ a limit to the minimum size of a P~B for a keyboard, ~inc~ it is necess~ry to place a keyboard swit~h on th~ P~B at a location dire~ly beneath the key t~ ~e ~epressed. As a result, ~he typical PC~ for a keyboard i~ of -the order of ten to eighteen inches by four to seven inches (25-45 cm.
~y 10-18 cm.~ This s~ze requirement h~s caused the PCB to beco~e a significant pa~t of the cost of a ~ypew~i~er or comp~er k~yboard. It is not normally possible to us~ an SRBP P~ because o~ the need ~or cros~overs.and the at~endan~ plated connecting holes.
~he re~ult is a rela~ively expe~ive epoxy fiberglass ~5 PCB, lamina~ed and etched on hoth sides. ~his cost could be gre~tly r~du~d i~ it were possible to use an SRBP ~oard ~ha~ contained a ~cuit on only on~ side.
A s~cond pr~blem in th~ manufactu~e of PCBs for keyboards is the f~ct ~hat Xeyboards either have pairs of exposed elea~ric~ ondua~ors th~ ~re ~ridged hy ~nother conductor or coupled cap~ci~ively or inducti~ely to make an elec~rical ~onne~ion when a key is depresse~, o~ el~e ha~ flexi~le membranes that ~o~pl~ ~o the board wh~n pr~sed. It is necessary to apply ~ome f~-rm o~ pro~ection to the exposed electrical . ~

condu~tor ~o a~ to ~inimize the buildup of corrosion that would interfere with t~e malcing of the electri~al conn~ction~ This is mo~ com~only done by etchinq a copper p~ttern o~ interlac~d com~s, parallel conductors or the like ~nd plating ~ld to the oombs to provide a cont~c~ surfac~ that i~ conductive electrically and that is not readily aorroded ~y exposure to the atmosphere. Gold m~y b~ plated ~o the copper either by electroplating or by ele~roless plating. Either of these represents an add!itio~al element that contributes to thR cost of prep~ring a PCB for a ~eyboard.
The usual intende~ use Qf a PCB is to se~ve as a mount for compon~nts ~uch a~ resistors, capacitors, diodes and transis~or~. Any ~ these components is lS ~ypic~lly inser~ed by pla~ing i~s ~eads into holes in the PCB whic~ is then p~ssed through a wave-soldering process to at~ch the components physically and electri~ally to ~he P~B. Durin~ the process of manufacturing ~he PCB, the b~ar~ is typically coated in part with ~n organic polymer solder resist to prevent solder f~om adheri.~ to:tAhe ~ove~ed regio~s. If the PC~ is one de~igned for a keyboard, the resist is typically depo~ited so ~ to cover conductors on the ~eyboard surfa~e but i~ ~a~ked ~ leave the com~ or other keyboard s~it~hes exposed fo~ opera~io~, A USQ tha~ is analog~us to that of P~Bs is the manuf~cture o~ electrical c~hlQs or the like by deposi~ing aonduc~ing mate~i~1 on flexi~le plastiçs su~h as mylar. ~rhis is o~en ~one by some fo~m of printing p~cess such as screen print}ng . Mos~, if not all, of the ~akerials, typically plasti~s, th~ are used for flexi~le cables or flexible ~la~ ~onduc-~ors are no~ ad~pted ~or wa~-sol~erin~, and i~ is the~efore necessary to make co~pression conne~tions or ~he like at the ends of th~ c~bl~ or flat conductor. As a 3~ 5 - 6~

r sul~, ther~ is no w~y to a~ach resistors or cap~citox~ to flexible material, and a rigid Pc~ is thereore used with the flexible material to serve as a componenk ~ount.
STJM ~ ~O,F ~HE I ~ I QN
It is an object of ~e present invention to pro~ide an al~ernative method of making electrical irlterconn~ctions.
It i~ a furth~r object o~ the pres2nt inven~ion to provide a be~ter wa~ of making printed circuits.
It is a fur~her ob~ect of the presen~ invention to provide a be~ter way of ma~ing printe~-cir~uit boards It i~ a fur~her o~ec~-o~ the presen~ invention to ~S pro~ide a ~thod of ~aking 4ros~ed elec~rical connections on a ~ingle side of printed-circuit board.
Xt is a further o~ect o~ the pr~en~ invention to provide a ~thod of producing resistors by a printing process on the prin~ed-cira~i~ board.
It is a further ob~e~ ofi the present invention to provide a method o producing capacitors by a printin~
process on a printed-~ircu~t ~o~rd.
It is a f~rther object o~ the pres~nt inven~ion to provide a method of pr~ducing a printed ~ircuit on : 25 ~exible ~ubstrate.
I~ is a fur~her object of the p~esent invention to produce ~ p~inted circuit inclu~iny resis~ors ahd capaci~or6 on a subs~ntialIy planar solid sur~ace, It i~ ~ further ob~t o~ ~he present inYention ~0 produce a printed cir~uit in~l~ding resistors ~n~
capacitors on a c~rved surfa¢e.
It i~ ~ further objec~ of ~h~ pre~ent invention to enable ~he use of synthe~ic-resin-bonded p~per : printed-cir~uit boards by e~ectiny multilayer printing on one side o~ the ho~rd.

~ , ~

~ 8'3 Other objects will become app~rent in th~ course of a detailed de~ription of ~he lnvention, Print~d circuit~ are produce~ by screen printing or the like to deposit conductors, resistors, cap~citors and insul~tors. crossover connections are made by covering the ~onducting portions to be crossed with an in~ulat~ng ~te~lal t~ serves as ~ base for a print~d crossing aonductor. Exposed conductors, as for keyboards ~nd compresslon connectiorls, are pro~ected lo fro~ the development o~ high-resistance co~rosion product~ by printing or ove~printing them with an ink that deposit~ ~ car~on layer. The method pe~mits the use of rigid prin~ed-circuit bo~rds ha~ihg crossed connections without requiring the use o~ a ~wo-sided ho~rd. The equiv~lent of ~ultilayer boa~ds can be achieved by printing repe~t~d layers of conductors, compon~nts and ins~lators on the same side of the board, or ~y printing on two~sides of a board.
Interconnection b~tween adjacent layers can ~e made as a part of the printing pxo~ess~ Resistors c~n be printed by screening or otherwise depositing a controlled amoun~ of a resistive ink. Cap~citors can be produced by print~ng cond~ctive layers separated by insulating lay~rs. ~he p~OC2~ ~ay produce printed~circu~t board~ con~ining resistors and cap~citors wi~hout th~ neG~si~y for inserting and wave-soldering co~ponents. If such ~ board is to ~ontain components i~ addition ~o those printed by the process of ~he preqen~ invention, a prin~ed carbon layer can be u~ed ~ old~r re~is~ to proteot printed condu~ors ~uring ~he w~ve-soldering proces$, ~n~ ~o p~o~ect exp~sed conta~s, ~s for key~oards, duriny wave-solder~n~ and in use~. ~he p~ocess alao permits the printing o~ a conduoting Iayer ~s a shield against 3~ radio-frequ~ncy in~erference or a~ a ground plane to x~çj~s provide electrical isolation of por~jon~ of a circuit from e~ch other.
A mQthod in accor~ance wi~h the invention permits production of a printed ci~cuit on a flexible substrate. Resistor6, capa~i~ors, ~onductors and insulator~ may be produ~d on a substan~ially planar solid surface or on a ~urved ~urface~

BRIEF I~ESC~I~:[O~oF ~: D~AWINGS
Fig. 1 is a diagramma~ic plan view of a ~creen wi~h a corner broken away revealing a substr~te above which ~he screen is positioned, in a position for ~pplying a first coating; and Fig. 2 i~ a ~lmilar pIan ~iew showing the sc~een and subs~r~e in po~ition for applying a second coating.
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D~TaL~ED DE$~IP~ION ~F ~H~ ~VENTION
The examples ~h~t ~ollow ~epresent particular applications of the in~ntion. In the case of those applications t~ ~eybo~rds for -~ypewriters or computers or the like, the s~alR is ~ix~d ~y the size of the hand~ of an operator, ~nd the re~ ing necessity ~o sep~rate ~he keys physica~ly. ~hese dimens~ons fix the ~5 size of ~uch a ~ircuit ~o~rd~ So~e of ~he ex~mples r~la~e to ~onven~ional printed-~ircuit boards in which ~ co~ponents will ~e ~nserted and wave-sold~red. The : po~ibility of limiting $he printin~ to one side of a board also makes it possible to print a oircuit on any ~face on which it ~an ke.printed ~nd on any ~ubst~nce ~hat is compatible wit~ the printin~ process. Thus, the pl~5tiC of a ~ar dashbo~d, tXe aase of a radio, a flexible pl~stic or pap~r transfer medium, or a he~t si~k could be ~s~d a~ a substrate on which to print cir~uit according So th~ pre~ n~ inven~ion. Th~

, .~ , :' .

20~3f;~ ~5 _ g_ exa~ples shown her~ were ~lso pro~uced by a screen printing process, whic~ repre~en~s a preferred method, but it i~ clear that any ~ethod of printing th~t will h~ndle the conduc~ing, re~istive, carbon and solder-resist inks wil~ be adaptable for the present p~ocess ~nd for pr~ducts produced by the process. The other possib~lities for doing this include transfer p~inting, li~hography, air bru~h, hand brush, and the like. ThP exampleS that ~ncluded ~he printing of resisto~s and c~pacitors produced such components having valu~s th~t were repeatable within tolerance ran~es of ~i~e to ten per ce.nt. These values are thus compar~ble to t~ose achie~ed with ~rdinary discre~e component~
~xam~
An SRBP board a~roxim~tely 14 in~.he.~ by 4 inchQs by 11l6 inch co~tained ~ lamina~ed layer o~ copper that had been etc~ed to pr~duçe condu~ting pat~s and 2~ appr~priately placed co~b connectors to be bridg~d by conducting pills when a key was depressed. The combs were pl~ed with gold. In order to function properly, this b~arA needed connectivns ~hat would cross ~ertain conducto~s without m~king electrical contact with them, The cros~lng connec~ions were achieved by screen printing with an ink containing a pol~er resis~ at the crossings. ~he resi~ was cured using ul~raviolet ~diation. A ~onduct~g path connecting ~he desired po nts w~s then prin~ed ~y a sc~een process over the cu~ed resist,: ~nd ~he prin~ed condu~tor was then cured at an ~leYat~d ~emperztur~ This produced a functioning printed ~ircuit bo~d ~or a keyboard that was printed. ~n only one sid~ of the PC~ and that had no ~idgin~ wixe conn~ctions.
3~ .

xAamPl~, ~
An S~P board approximately 14 inches ~y 7 in~hes by lJ16 inch had bflen lamln~ed with copper and the copp~r had been etched to leave an ~pprop~iate pattern of electri~l connect~on~ ~or a keyboard. Thi5 pattern was placed on a single ~ide of the phenolic board.
Each of the contact pad~ ~or ~he keys was formed by a ~creen p~lnting o~ a c~nd~c~ive in~. This ink was then cu~ed by heati~g. The ~ontact pads ahd also ~xposed condu~tors for c~mpr~sion conne~tors ~t an e~ge of the ~o~rd were then covered by screen printing with an ink containin~ car~on, and the ink was cured by h~ting it. A screen printing was ~hen made to apply an insulating polymer r~ist ~o all areas of ~he board except the exposed conductor~ f or the key pads and f or compresslon conneatiOnS at ~n edge of ~he board and at ~hose areas where ~omponent~ we~e ~o be inser~ed and wave-soldered. ~he cured: ~arbon ink functioned as a s~lder resi~ that pro~ected the contac~ pads ~rom solder du~ing the wave-~oldering process. If the board had been subjected to hand so~dering, the carbon ink would have proteoted the contact pads from damage by hea~.

E~B~
A keyboard o~ FR-4 epoxy ~be~glass having di~ensions o~ approx~ately 14 inch s by 7 inches by 1/~6 inch h~d ~een lamlnat~d and ~tahed on one side to le~ve copper conducto~ in a pa~ern appropriate for a keyboard. Layers of insulating resist were screened in desired ¢rossover pa~erns on the sid~ of the b~ard that carr~ed ~he aopp~r ~nd we~e cured by ultraviolet r~dia~ion. A ssreen print~ng was then maae ~ith a conducting ink ~o apply aonducting strips on the aro~30vers and al~o print ~h~ contac~ ~eas for keys.

3~
. ., -- 11--q'his was cured by heatlngO The area~ of screened cond~ctors were th~n subjected to a further screening proce~s to cover the screened conductors with ink containing car~on. Thi~ was then ~ured by heat. ~he bo~rd Was then equipped ~ith resistors by screen printing with re~iYtiv~ ink in desired locations.
After the printed resis~o~s were cured by heat, ~
protective layer of solder resist was applied and was then cured by ultr~violet r~diation. 'r~e solder resist l~ft open areas for the applic~ion of solder. Because of the carbon ~verin~, ~his board could be subjected to flow solderin~ without damaglng ~he screened connec~rs and co~ponen~3.
' Ex~mple ~
An SRBP board approxima~ely 1 inch by 2 inc~es by 1/16 in¢h had ~een lamina~ed wi~h copper foil on one side, and ~he foil w~s etched t~ leave a desired ci~cuit pat~ern. A porti~n of the board was COvered zo with a ~c~eened ~sis~ which was then ~ured by ultraviolet light. A conduc~ing ink wa~ pxinted by a s~reen proce~s over the r~ist to make a conducting crossove~. The Gondu~ing ink ~as als~ placed over ~n ~rea app~ximateiy 1~4 ~n~h square to fo~m one pla~e of ~ capacitor. ~he condu~ting ink was ~ured hy heate R~sistor~ were dep~si~ed in de~ired locations by screening resistivè ink which was then cured by heat.
A layer o~ re~ist w~ ~epo~ d over the ~ond~c~ing area ~o pr~vide a dielectric material for ~he c~pa~itor. Thi~ wa ~h~n cured by expo~ure to u~tr~iolet light. A se¢ond con~u~ting layer was printed by screening to cover the re~is~ and form the second plate of ~he capa~it~r ~nd al~o to connect ~he capacitor ~t a d~ired poi~t:on ~he aopper lami~ation, and t~e ~nduc~in~ ink waæ aured by heati~g i~. ~he - 12- X~3~

~e~ult wa~ a circuit board wi~h connections and component~, A prot~¢~ive layer of solder ~e5ist waC
appli~d with hole~ for ~older points, and the re5ist was ~ured by ultraviolet radia~ion.

~x~elç 5 An S~BP board ~f dimen~ion~ approximately 2 inch0s ~y 1 inch by l/l~ inch x~ceivRd a s~reened pattern o~
condu~ting ink which ~as th~n cured by ~eat. A pattern of æcreen reæist~ve inks w~ p~a~ed by a sare~n printin~ pro~ess ~n desired l~cations, and -~he re~istive ink wa~ cure~ by heat. 5ele ted locations of the sc~eened conductor weXe then plated with ~oppe~ in an elec~roleæs platlng tanX, A pro~ective resist was th~n screened over all ~Ut:thO~e portions of the board that were to he exposed ~or ~ontact~ This includqd the area~ that wPre ~i~sn the copp~r plate, which readied that r~gion o~ the b~rd for sold~red cont~ct, ei~her manual or automati~ soldering. As an alternative, nickel coul~ equally aæ well h~ve been plated ~o ; provide an appropri~e sol~ering su~face. The conducto~s formed by ~onducting ink w~ll not generally wiShstand the heat ~sso~la~ed with hand ~oldering, althouyh ~hey may stand up under wave-soldering.

~ : .
A~ SRBP board 1~ inch~ by 7 in~hes by l/16 ~nch wa~ scraen-prin~ed wi~ a condu~ing ink in ~ patte~n appropria~ k~yboard. Th~ ink was ~ured by heat, and crossing aonn~cto~s were p~c~d by the procçss ~escribed akove. Registive ink in ~on~rolled pa~terns was prlnted and ~ured by heat ~o produce desired resi~tor~ as de~ribed abov~. A layer of solder resist w~ then sc~eened on the board in a pat~ern tha~ left : 35 suhs~antially sq~are openings at the loc~tions of the ~' ' . ' , typewrite~ keys. Th~ re~ist w~s cu~ed b~ exposure ~o ult~aviolet light~ Th~ cured resist served as a mechanic~l ~pacer .~o~ a ~embr~ne wi~h appropriately pla~ed aonductors tha~ was laid over the resist. ~his provided a m~thod Qf ~aking a membrane-~witChed keyboard without the neces~i~y of tooling to cut open.ing~ in a pieae of pla~tic to space the membrane fro~ its ma~ing ~leatric~l conneation. The connections made he~e w~r~ capacitive, but they could e~ually as well have ~een ~onduati~e oX inductive. The process could also have been applied to make ~witches using two parallel membranes that were spaced apart ~y screened and cured resist rather thah using a rigad board as one connector support.
All of the board~ des~ribed in the examples above have in co~m~n the fa~ that all of ~heir printin~ is done on one side o ~he board. Some boards have been made in ~he pas~ wit~ j~mper wires that ~re wave 301der~d as a part of~the process o~ makiny the board, bUt this i3 seldom a go~d solution to the problem of bri~gi~g connectorY and i~ is often unfeas~le if the run of condu~t~rs to be hrid~ed is of any length~ Another problem that can be overcome much mo~e simply than ths presen~ prac~ice is that of providing sh~elding against ~ad;o-~r~quency interference ~RFI shieldi~g). One reason for the use o~ multilayer boards is to pl~co a shieldin~ layer pro~ecting po~tionS of ~he circui~. ~his is done much ~ore simply ~n a single la~er boaxd on one side by depo~iting and curing ~ ~ondu~ing layer oYer a layer D$ re i~t.
~h~ pro~ess o the pre~en~ inven~ion is adaptable to prin~ as many ~ thirt~ ~o ~ifty layers, one on top of the o~her. The prac~ical minimum spacing b~tween 3~ adjacent co~du~or~ f the order of O.ol inch. A

- 14- ~n~i~8~

screened layer ~fter curing c~n typically be controlled in thickne~s ~o plu~ ox min~s 5 microns. ~pacitors of variou6 value~ ranging 4p to 1000 pico~arads may be made as de~cribed in th~ examples, wi~h tolerances to S per cent.
In som~ ci~cums~ance~, plnholes in one of the applied layers may aause probl~ms, espeCia~ly in an insulation ~ayer wh~re p~Jihole~ may cause unintentional : ~nd c~ta~trophia connection betwe~n conducting layers.
In order to reduce the risks of this problem ~rising, the or each in~ulat~ng layer may be applied in two : ~tages, so tha~ two coating~ a~e ~pplied ~ith ~riation of coating direction or slightly p~sitional vari~tion between ~he coatin~s~ For example, when applying an 15 insulating layer usin~ s~reen printing t~e first stage comprises applying a ~reened co~ing with the dire~ion of appllcation a~ong the X-axis of the screen and work-piece. Th~ sec~nd ~tage a~mprises applying a further screened coatinq w~t~ riation from the 20 ori~in~l co~ing, for example ~ er by rotating both the screen and the work-piece t~rough ~0, rel~tive to the coating direG~ion, and/o~ with the s~reen offset relative to tne work-piece, e.g. in the X ~ndJor Y
direction, by a m~ni~al a~un~ e.g~ ~ small part of a 25 millimetre.
T~e Table i~ a lis~ln~ of the inks th~t wPre used in a screen printing process ~o produce ~h~ examples above.
~0 ., :

, ' . .

:
. ' ' , .

- 15~

__________~_______._~________~;_ __ _________________ Tl!~BLE

SCREE~ PRINTIN~: P~OC~ESS I~KS

. ~ç~ ni~ ~ a cro~-linking polymer sold under ~he ~radem~r~ "Phot.o~o~t.
The exa~ple~ u~ed type ~G which i~ n~
flexible, al~h~ugh a style 3G is availablQ t~t i~ ~lexible. The examples u~ed a re~ that was curable by ultraYiol8t r~d iation~ . A
heat~curable re~ could equally as wel~ h~e ~e~n u~ed.

2. ~esi~tiv~ ~nk. All~o~ the examples used a polymer thick-film ink ~old under the trade n~me 'IM~tth~y L~" R-4000 series.
~his is a~aila~le in a r~nge of resis~ivl~ies ~nd ls cured by hea~.

3 Ç~n_ln~ T~ i5 a~p~lymer thick-fil~ lnk con~aining c~rbon black.
It i~ cured by ~xp~ure to heat.
sgn~Y~ n~ This is a polymer thic~-~ilm ink con~aining silver fla~es. ~ cured by ~xpos~re to he~.

~5 _ ___ _ ________________________ ' - 16~ 8 In th~ manu~actur~ of p~inted cir~uits it i~
known to apply a number o~ layers in desired patterns ~o t~e surface o~ a 3ub6trate, c~mmonly ~n insulating board. ~he layers ~ay be applied in a variety of way~, ~or example ~y firs~ applying a sheet of copper ~aterial t~ the bo~rd and ~hen etching away unrequired r~gions to lea~e a de~ired pattern or by applying ¢oatin~s of suitable ~aterial to ~he surface of the board in a desired patt~rn to a~hiev~ the required effect. It is with this lat~er operation ~hat the invention i~ concerned~ The layers which are applied ma~ be in the form of a conducti~e materi~l or may be a resist which may b~ an elect~ically in~ulating materi~l andlor a material which resists damage, e.g.
by etching fluid o~ other oper~tions, ~or example soldering, Solder is non-adhe~ent to most resists and such resi~t~ are com~only una~ected by temp~ratures encounte~ed during sol~ering.
In some circu~stances pinholes in one or more of the ~pplied layers ~ay cau e problems, especially in an insulation area where pin~les may c~use unintentional and, potentially, catastrophic connec~io~ betWeen conduc~ing lay~rs.
It has been found that the risks o~ pinholes ar~sing may be reduced by app~ying the, or each, insul~ting l~yer in two sta~es wi~h the substrate dispo~ed in a di~ferent relationship to ~oating ~ppara~Us in eaah ~tagej so ~hat t~o coatings are - applied with~ for exampl~, ~aria~ion o~ Coatirlg dire~tion oX llgh~ position~l v~iation between coatin~s~ Ihis su~stan~ially r~du~e~ the risk ~hat pinhol~s will ~ause a proble~.

In one ~speot th~ inYen~i~n may b~ Gonsidered to pr~vide ~ method o~ applying a layers in a desired ~ 17~ iS ~ ~

pattern to a substra~e ~or a prin~ed circuit compri~ing applying a fi~t coa~ing in the desired pattern to the subs~rate and applying a second eoat~ng i~ the desire~ pat~ern to the subs~rate, the subs~rate being disposed in a diff~ren~ relationship to Coatihg - apparatu~ ~5~ in app~yin~ the seaond coating from the relationship in wh~ah it ig aisposed to coating appa~atu~ u~ed in ~pplying the fir~t coating.
Prefer~bly, ~n met~ods in accordance wit~ the inv~ntion the coating appar~tus ~sed is a screen printin~ app~atuS. In one methods the r~lationship of t~e ~oa~d to the apparatus is chan~ed by moving the position of ~he soreen of the appar~tus relative tu : the ~ubstrate through a very ~ll dis~ance e.g. a small part of the: ~illi~et~e e.g. 0~05 mm. However, ~lthough ~his ~e~hnique is ~tisf~cto~y for some appli~tions~ t~e v~riou~ condu~o~s and ot~er applied layers of many modern printe~ circ~its are so close to one another tha~ even thi~ very sm~ll positional adju~tme~t i.s una~ep~able.
In anoSher method in acco~dance wi~h the invention which is, in ~ome ci~um~tances, an improvement o~er th~ method outlined in th~ last precedin~ paraqr~ph/ two separate but iden~ical screen3 ea¢h ha~ing ~he s~me~p~tern are used. The ~irst ~oat~ng iS applied usin~ one of t~e screens and then thi8 screen i~ remoYed ~nd the other screen positioned in regis~ration with ~h substr~te and the ~econd ~t~ng applied. Th~ cost o~ production of ~uitable ~reens is very high ~nd this method requireS
tw~ screen3. F~om an ac~Uracy poin~ sf view the use of more th~n one screen:c~n le~d ~ added problems,.
: for ex~mple dlstortivn ~ ~h~ ~wo screens may differ e,g. if the ~creen~ are of sli~htly diffe~ent 3S t~nsion~, ~hus,.whil~ thl~ ~e~od using t~o screens , -- lg~

~ay be prefe~able to that dss~ribed in the last preceding paragraph, it i~ still desir~ble to achieve further improvem~nt~ in accuracy for so~ uses, a~
well as to ~void the exp~nse ~ two screens, if possi~le.
In another preferr~d method in accordance with the invention whs~e th~ coating appara~us is a screen printing apparatu~, thR relation~hip of the subs~rate to the 1 appa~at~ chang~d by rotatin~ bo~h the lo subs~rate an~ the screen through an angle, suitably 90, relative to th~ coa~ing direc~ioh of the ~ppa~atus In ca~rying ou~ ~he preferred metho~ in a~cordance with ~he invention~ pre~erably a number of identi~al ~iraUit p~tern~ are printed on a su~strate;
the ~ircUit pa~tern~ are suitably printed to provide a correspondin~ nu~ber o~ in~ivi~ual circui~ bo~rds which ~re subsequently sep~rated fro~ the remainder of the substrate~ Suitably t~ su~strate is provided by an insu~ting sheet ~a~erial on which the circuits are printed ~nd f~om whi~h ~h~ individual ~irc~it boards are s~vered a~ter printing. In ~his preferred method the re~ions to whlch ~he desir~d pa~tern is to be applied are positlo~ed such ~t the sa~e p~ttern may 2S ~e applied to the substrate when the substrate is presen~ed to a pat~ern applying porti~n of the coating app~ratus in any of ~ num~er o~. ~ri~ntations, each separated from t~ nex~ by ~ fixed angle of rotation about an axi~ at ri~ht angles to the substrate a centre of the substra~e. ~he ~irst coating is applied With the ~ubstrate in o~e ~uch orientati~n, t~e substrate and patt~rn applying por~ion are ~her~aft~r rela~ively ~otated th~ough s~d fixed angle (or a multiple ther~of other ~han 360), and ~h~n ~he ~cond co~ing is ~ppliedO Suita~ly the f ixe~ ~ngle is 180 .

;.J~

and four region~ to whlch a pattern i~ to be applied ar~ prQ~en~. ~y ~aking thi~ ~harlge in position of the pattern appLy~n~ po~tion r~lative to t~e substrate, the s~cond co~ting is ~ppli~d ~o each region usihg a diff~rent pattern applying p~rtion from that used to apply the fir~ coating to the same region; -the chance of a dam~gin~ plnhol~ ~ppearing in preoisely the ~ame position of two different applying portions is very small~
In tha preEerred me~hod in ~ccor~ance with the inve~tion this ri~X i~ ~educed even further by not only ensuring ~hat ~.he second co~ting is applied to each region by a diff~ren~ pat~ern applying portion from the firs~ coating bUt al~o that ~he second coating is applied in a dlfferent co~ting direction.
In this preferred me~hod, which u~es a screen printing app~r~tus, the -~re~n and substra~e ar~ rotated in opposite directi~n , ea~h ~eing rot~ted through an angl~ of ~0~. .
There now follow5 a qe~ailed description to be read ~ith reference ~o t~ ac~ompanying drawings of a method of making ~ printed ~ircui~ board embodying -~he invention~ It will be reaIized this method has been ~elec~ed ~or des~ription to illustr~e the invention by ~ay of ~*ample.
In the acçompanying drawings:
Fiyure l is a diaqramm~ic plan ~iew of a screen with ~ ~orn~r broken a~ay rev~ling a substrate above whicb ~he screen is po~itionedf in a position for 3~ ~pplying a ~irs~ coa~ing; ~nd Fi~ure:~ is ~ si~ilar plan view showiny the scr~en ~nd suhstra~ in posi~ion for applying a second coa~lng.
A scre~n 10 ~ a screen printin~ app~ra~us is positioned ~oY~ a ~ubstr~t~ pr~vide~ by a rigid ins~llating sheet material 12 in contact with a surface of the she~t m~te~ial 12, forming a base on which prin~ed cir~ui~ ~oards are to he formed. The screen lo and ~hee~ material 12 are po~itioned for the first ~oating in r~gister wi~h ons ~nother as shown in ~igure 1, ~h~t is with da~m~ D1 and Ll is alignment anq with datum~ D2 (not ~hown) and LZ in align~ent.
In register or ~he ~e~ond coat.ing (Figure 2) the datum D~ overlies the datum L~ ~hilst the datum D2 overlles d~tum L1. De~ired patte~ns ~1 - P4 to be printed a~e shown in ~he drawing~. ~he patterns are produced by multiple i~age ph~ographic techniques and ~re, there~ore, b~s~ally identi¢al, although arr~nged in appropriate po~iti~ns ~nd orien~ations to enable the carrying ou~ o$ the illustrative meth~d.
In c~rying out khe illustrativ~ me~hod, with the soreen lo positioned relative to the sheet material 1 as shown in Fi~Ure 1, coating means of the screen printlng appa~atus i~ ~oved relative to t~e screen in . a coating ~rection C to apply a first ~oating o~
material ~irection C to apply a fi~k ooa~ing of material~ ~or example ~ r~is~, through the scre~n 10 t~ deposit ~he de~i~ed patt~rn P1 - P4 ont~ the base shee~ material 12; ~ateri~l ~1 deposited on the sheet material 12 is indi~ated in Figure 1. After the first coatin~ has bsen applie~ the s~reen 10 is sepa~ated ~ro~ the sheet m~terial 12 and the scr~en lo and sheet materi~ are ro~ted abou~ an axis A ~t the cent~e of the base and perpendi~ular ~o ~he ~as~ and to the screen lo. The screen lO i~ ro~ted in the direction i~dicated b~ ~he arrow S in ~igure 1 and the base sheet mat~rial 12 is ro~ted throug~ an angle of ~0 as indi~ated ~y the a~roW~ A, B about the axi~ A until :: they ocaupy.the positions in which ~hey are shown in Figure 2. ~he scre~n lO and -~heet material 12 are put into ~egister by ensuring tha~ the datums are ~ppropriately a~igned. In th~ stan~e, however, the datu~ aliyned ~it21 th~ da~um L2 whilst the datum ~2 (no~ shown~ ~g aligned wi~ the datum ~l (see Figure 2). ~f~er the .~creen lO ~nd sheet mate~ial 12 h~ve be~n moved into reyi~te~, they are again moved i~to con~act Witl1 on~ another and a second coating is applied by moving the coating means (e.g. a squeegee~
in the co~tlng direq~ion indicated ~y the arro~ C in l~igure 2. The second ~02ti~CJ i5 app~ied over the first coating but the second ~oating is applied by means of di~fe~ent ones of the patterns Pl - P4 formed in the ~creen. For exampl~, the second coating applied o~er the ~aterial Ml shown in Fi~ure l is pp~ y ~be pa~ern Pl in the s~reen lO whil~t pattern P3 in the ~reen lO ~pplies a second co~ting over ~aterial M2, the fir~t Goating of which was ~pplied by the pattern Pl. Th~s, not only is the second coating app~ied by c~ating means tr~elling in ~o a co~ting di~ection ~ across ~le screen ~t an orientation of ~o to t~at in which the first coating is applied but also the actual patternS through which the second co~ting is applied are different. By this means i~ is e~sured that ~he risk that any pinholes will occur in ~he applied material .is practically zero.
~fter all ~he ne~essary layers have been applied to the sheet material 12 individu21 ~ircuit bo~rds ar~
severed fr~m the sh~et mat~rial 12 by a suitable means such as, for exa~ple; rou~ing. Each of the bo~rds ~arryies ~aterial applied by one of the patterns Pl -P4, there being four ~ep~ra~e boar~s manuf~ctured in this method.

Claims

1. A method of effecting an electrical connection between a first electrical conductor that is affixed to a surface and a second conductor that is affixed to the surface, the connection crossing a third electrical conductor that is affixed to the surface without making electrical contact with the third conductor, the method comprising the steps of:
(a) placing an insulating polymer on a desired path that bridges the third conductor;
(b) curing the polymer;
(c) placing a conducting layer on the desired path between the first and second conductors and on the resist; and (d) curing the deposited conducting material.

2. The method of Claim 1 wherein the surface is a printed-circuit board and wherein the first, second and third electrical conductors are made of copper formed by laminating copperfoil to the board and etching to remove copper from portions of the board.

3. The method of Claim 1 wherein the surface is a printed-circuit board and wherein the first, second and third electrical conductors are made of a conducting ink that is printed in a desired pattern on the board and cured.

4. The method of Claim 1 wherein the step of placing the insulating polymer comprises the steps of printing an insulating ink in the desired path by a screen printing process and curing the ink.

5. The method of Claim 1 wherein the step of placing the insulating polymer comprises the steps of printing an insulating ink in the desired path by a transfer printing process and curing the ink.

6. The method of Claim 1 wherein the step of placing the insulating polymer comprises the steps of printing an insulating ink in the desired path by a lithographic printing process and curing the ink.

7. The method of Claim 1 wherein the step of placing the conductive layer comprises the steps of printing a conductive ink in the desired path by a screen printing process and curing the ink.

8. The method of Claim 1 wherein the step of placing the conductive layer comprises the steps of printing a conductive ink in the desired path by a transfer printing process and curing the ink.

9. The method of Claim 1 wherein the step of placing the conductive layer comprises the steps of printing a conductive ink in the desired path by a lithographic printing process and curing the ink.

10. The method of claim 1 comprising in addition the steps of:
(a) depositing resistive ink in a desired pattern on the surface to serve as a circuit resistor; and (b) curing the resistive ink.

11. The method of Claim 10 wherein the step of depositing comprises the steps of screen printing the resistive ink in the desired pattern and curing the ink.

12. The method of Claim 10 wherein the step of depositing comprises the steps of transfer printing the resistive ink in the desired pattern the curing the ink.

13. The method of claim 10 wherein the step of depositing comprises the steps of lithographing comprises the steps of desired pattern and curing the ink.

14. A printed-circuit board made by the process of Claim 4.

15. A printed-circuit board for a keyboard made by the process of Claim 4.

16. A printed-circuit board made by the process of Claim 11.

17. A printed-circuit board for a keyboard made by the process of Claim 11.

18. A method of producing a contact pad for a printed-circuit board for a keyboard comprising the steps of:
(a) printing a pattern of the contact pad in a conductive ink in a desired location on the printed-circuit board;
(b) curing the conductive ink;
(c) printing the pattern in a carbon ink over the cured conductive ink; and (d) curing the carbon ink.

19. The method of Claim 18 wherein the steps of printing comprise screen printing the conductive ink and screen printing the carbon ink.

20. A method of producing a capacitor by screen printing on a surface comprising the steps of:
(a) printing a first layer of conducting ink on the surface in a predetermined pattern;
(b) curing the conducting ink;
(c) printing an insulating ink in a pattern that substantially covers the first layer;
(d) curing the insulating ink;
(e) printing a second layer of conducting ink on the cured insulating ink in a pattern that is spaced so as to be close to the first layer while avoiding electrical contact with the first layer; and (f) curing the second layer.

21. A method according to any one of the claims 1 to 13 and 18 to 20 wherein at least one of the layer of insulating polymer is applied in two stages.

22. A method according to claim 21 wherein the first stage comprises applying a screened coating layer in a coating direction, rotating both screen and the surface on which the layer is to be applied through 90°
relative to the coating direction and applying a further screened coating layer in the same coating direction.

23. A method according to claim 21 wherein the first stage comprises applying a screened coating layer in a coating direction, offsetting the screen relative to the surface on which the layer is to be applied in the coating direction and/or at 90° to the coating direction by a minimal amount and applying a further screened coating layer in the coating direction.

24. A printed circuit produced on a single side of a substrate comprising a first set of electrical conductors carried on a surface of the substrate, a layer of an insulating polymer applied on a portion of said surface of the substrate and bridging a first of said electrical conductors on the surface, and bridging electrical conductor carried on the surface of said layer of insulating polymer opposite said surface of the substrate so that it is insulated from said first electrical conductor and in electrical contact with at least a second and a third of said electrical conductors on said surface at opposite sides of said first electrical conductor.

25. A printed circuit according to claim 24 comprising a second layer of insulating polymer applied on portions of said surface but which leaves exposed parts of said first set of electrical conductors, and a second set of electrical conductors, carried partly on said second layer of insulating polymer but making electrical contact with some of said first set of electrical conductors at said exposed portions.

26. A printed circuit according to claim 25 comprising at least one further layer of insulating polymer applied on said surface and/or on parts at least of previously aplied insulating layer and conductors, which leaves exposed parts of at least one set of previously applied electrical conductors.

27. A printed circuit comprising at least one set of electrical conductors carried by a substrate including a first conducting area connected to a first electrical conductor to provide one plate of a capacitor, a layer of dielectric material applied over said area and a second conducting area carried by said layer to form a second plate of the capacitor, and connected to a second electrical conductor.

28. A printed circuit according to claim 27 wherein said second electrical conductor belongs to the same set as said first electrical conductor.

29. A printed circuit according to claim 27 comprising a first set of electrical conductors including said first electrical conductor a first layer of insulating polymer including said layer of dielectric material applied to said substrate and to parts at least of said first set of electrical conductors and a second set of electrical conductor carried by said substrate and said first layer.

30. A method of applying a layer in a desired pattern to a substrate for a printed circuit comprising applying a first coating in the desired pattern to the substrate and applying a second coating in the desired pattern to the substrate being disposed in a different relationship to coating apparatus used in applying the second coating from the relationship in which it is disposed to coating apparatus used in applying the first coating.

31. A method according to Claim 30 wherein the coating apparatus is a screen printing apparatus and the relationship of the substrate to the apparatus is changed by moving the position of the screen of the apparatus relative to the substrate through a very small distance between application of the first and second coatings.

32. A method according to Claim 30 wherein the coating apparatus is screen printing apparatus and the relationship of the substrate to the apparatus is changed by rotating both the substrate and the screen through an angle of 90° relative to a coating direction of the apparatus.

33. A method according to Claim 30 wherein a number of identical or substantially identical circuit boards are printed on a substrate from which they are subsequently separated the boards being positioned such that the same pattern maybe applied to the substrate when the substrate is presented to a pattern applying portion of the coating apparatus in any of a number of orientations each separated from the next by a fixed angle of rotation about an axis at right angles to the substrate at a centre of the substrate in one such orientation, the substrate and pattern applying portion are thereafter relatively rotated through said fixed angle (or a multiple thereof other than 360°), and then the second coating is applied.

34. A method according to Claim 33 wherein the fixed angle is 180°.

35. A method according to Claim 34 wherein the apparatus is a screen printing apparatus and the screen and substrate are rotated in opposite directions each through 90°.

36. A method according to Claim 4 wherein the substrate is a sheet material on which the circuits are printed and from which the individual circuit boards are severed after printing.

37. A method according to Claim 30 wherein the apparatus is a screen printing apparatus having two substantially identical screens one of which is used to apply the first coating and the other to apply the second coating.
1523d