Method for making printed circuits

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. 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. 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 from 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 direction, (S,B) each through 90.degree..

BACKGROUND OF THE INVENTION 
This invention is related to printed circuits for use in electrical and 
electronic equipment. In particular, it is an improved method of making 
printed-circuit boards and of producing printed circuits on surfaces other 
than conventional printed-circuit boards. This invention is also concerned 
with making printed circuits and is especially concerned with a method of 
making a printed circuit in which a layer is applied in a desired pattern 
to a substrate. 
An element that is common to almost all electronic equipment is the printed 
circuit, typically in the form of a printed-circuit board (PCB). A PCB is 
generally made by laminating copper foil to a board. A desired pattern 
that includes conductors in the plane of at least one surface of the board 
is placed upon the board, and holes are drilled or punched for the 
mounting of components. This pattern is typically realized by placing a 
photosensitive resist on the board, exposing a photograph of the desired 
pattern on the resist, and developing the resist to produce a protective 
coating over the pattern. The remainder of the resist is treated to remove 
undeveloped resist from portions where it is desired to remove the copper. 
The board is then treated with a process that removes the exposed copper. 
When the remaining resist is then removed, the desired pattern remains in 
the copper. Mounting holes for components are drilled or punched at 
desired locations somewhere in the course of this process, either before 
or after the removal step. 
The materials most commonly used for PCBs are either polymerized epoxy 
materials containing glass fibers or paper bound by impregnated synthetic 
resins such as phenolic materials. The latter group is often referred to 
generically as synthetic-resin-bonded paper (SRBP). Boards made of either 
of these types of materials are clad with copper on one side or both 
sides, with the heat and pressure of the cladding or laminating process 
helping to cure the resin. The material cost of an SRBP PCB is typically 
about half as much as that of an epoxy fiberglass board of the same 
surface area, so there is a potential cost advantage when the SRBP board 
can be used. There are various bases for the selection between epoxy 
fiberglass and SRBP material. For example, the epoxy fiberglass boards are 
generally higher in strength and are preferred for use in equipment that 
may be subject to vibration. 
A particular problem of circuit design that leads to complication in PCBs 
is the fact that not all electronic circuits can be made with their 
connections in a single plane. It is sometimes necessary to make bridging 
connections between different portions of a circuit. A considerable amount 
of ingenuity goes into the design and layout of PCBs to minimize such 
bridging connections. However, sometimes it is impossible to avoid them. 
In such a case, it is possible to solder jumper wires between the portions 
of the circuit that are to be connected. This is especially undesirable 
for long runs, and it is better avoided even for short runs. A better 
solution is to use PCBs that have more than one conducting layer. The 
simplest of these is a two-sided PCB. This is a board that has copper 
laminated to both sides. Separate patterns are etched on the two sides to 
effect the desired circuit layout and cross-connections. However, in order 
to make such cross-connections, and to complete the connection between the 
two sides of the board, it is normally necessary to use epoxy fiberglass 
because of the necessity of plating holes through the board to connect the 
top layer to the bottom layer. Holes that are drilled in an SRBP board are 
adequate to support the leads of components that are placed in the board 
for soldering, but they are not normally clean enough when drilled or 
punched to permit satisfactory electroplating of connections between 
layers of the board. There is thus a long-felt need in the PCB industry 
for a way of making PCBs with crossover connections on single-sided SRBP 
boards without using soldered jumper wires. 
The same considerations apply equally as much to boards having more than 
two layers. These boards, referred to as multilayer boards, are often used 
in more complicated circuits where one set of bridging connections is not 
enough. As with the two-sided board, it is normally necessary to use epoxy 
fiberglass for such PCBs. It can be seen that the result of a need for 
crossed connections in the typical etched copper PCB leads to an increased 
cost because of the need for epoxy fiberglass in the PCB instead of the 
less-expensive SRBP boards. 
The problems just described become extreme in the case of PCBs for 
keyboards. A keyboard for a typewriter, computer or the like typically 
generates an electrical signal when a key is depressed to make an 
electrical contact or an inductive or capacitive coupling. Such a coupling 
is made directly or indirectly between two separated conductors on the 
board or on a flexible plastic membrane that is spaced apart from the 
board and makes a conductive or field contact when pressed toward the 
board. When the keyboard is the typical typewriter or computer keyboard, 
it is impossible or nearly impossible to avoid crossed connections. The 
physical size of the hands of an operator also sets a limit to the minimum 
size of a PCB for a keyboard, since it is necessary to place a keyboard 
switch on the PCB at a location directly beneath the key to be depressed. 
As a result, the typical PCB for a keyboard is of the order of ten to 
eighteen inches by four to seven inches (25-45 cm. by 10-18 cm.) This size 
requirement has caused the PCB to become a significant part of the cost of 
a typewriter or computer keyboard. It is not normally possible to use an 
SRBP PCB because of the need for crossovers and the attendant plated 
connecting holes. The result is a relatively expensive epoxy fiberglass 
PCB, laminated and etched on both sides. This cost could be greatly 
reduced if it were possible to use an SRBP board that contained a circuit 
on only one side. 
A second problem in the manufacture of PCBs for keyboards is the fact that 
keyboards either have pairs of exposed electrical conductors that are 
bridged by another conductor or coupled capacitively or inductively to 
make an electrical connection when a key is depressed, or else have 
flexible membranes that couple to the board when pressed. It is necessary 
to apply some form of protection to the exposed electrical conductor so as 
to minimize the buildup of corrosion that would interfere with the making 
of the electrical connection. This is most commonly done by etching a 
copper pattern of interlaced combs, parallel conductors or the like and 
plating gold to the combs to provide a contact surface that is conductive 
electrically and that is not readily corroded by exposure to the 
atmosphere. Gold may be plated to the copper either by electroplating or 
by electroless plating. Either of these represents an additional element 
that contributes to the cost of preparing a PCB for a keyboard. 
The usual intended use of a PCB is to serve as a mount for components such 
as resistors, capacitors, diodes and transistors. Any of these components 
is typically inserted by placing its leads into holes in the PCB which is 
then passed through a wave-soldering process to attach the components 
physically and electrically to the PCB. During the process of 
manufacturing the PCB, the board is typically coated in part with an 
organic polymer solder resist to prevent solder from adhering to the 
covered regions. If the PCB is one designed for a keyboard, the resist is 
typically deposited so as to cover conductors on the keyboard surface but 
is masked to leave the comb or other keyboard switches exposed for 
operation. 
A use that is analogous to that o f PCBs is the manufacture of electrical 
cables or the like by depositing conducting material on flexible plastics 
such as mylar. This is often done by some form of printing process such as 
screen printing . Most, if not all, of the materials, typically plastics, 
that are used for flexible cables or flexible flat conductors are not 
adapted for wave-soldering, and it is therefore necessary to make 
compression connections or the like at the ends of the cable or flat 
conductor. As a result, there is no way to attach resistors or capacitors 
to flexible material, and a rigid PCB is therefore used with the flexible 
material to serve as a component mount. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an alternative method 
of making electrical interconnections. 
It is a further object of the present invention to provide a better way of 
making printed circuits. 
It is a further object of the present invention to provide a better way of 
making printed-circuit boards. 
It is a further object of the present invention to provide a method of 
making crossed electrical connections on a single side of printed-circuit 
board. 
It is a further object of the present invention to provide a method of 
producing resistors by a printing process on the printed-circuit board. 
It is a further object of the present invention to provide a method of 
producing capacitors by a printing process on a printed-circuit board. 
It is a further object of the present invention to provide a method of 
producing a printed circuit on a flexible substrate. 
It is a further object of the present invention to produce a printed 
circuit including resistors and capacitors on a substantially planar solid 
surface. 
It is a further object of the present invention to produce a printed 
circuit including resistors and capacitors on a curved surface. 
It is a further object of the present invention to enable the use of 
synthetic-resin-bonded paper printed-circuit boards by effecting 
multilayer printing on one side of the board. Other objects will become 
apparent in the course of a detailed description of the invention. 
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 plane to provide electrical isolation of portions of a circuit 
from each other. 
A method in accordance with the invention permits production of a printed 
circuit on a flexible substrate. Resistors, capacitors, conductors and 
insulators may be produced on a substantially planar solid surface or on a 
curved surface.

DETAILED DESCRIPTION OF THE INVENTION 
The examples that follow represent particular applications of the 
invention. In the case of those applications to keyboards for typewriters 
or computers or the like, the scale is fixed by the size of the hands of 
an operator, and the resulting necessity to separate the keys physically. 
These dimensions fix the size of such a circuit board. Some of the 
examples relate to conventional printed-circuit boards in which components 
will be inserted and wave-soldered. The possibility of limiting the 
printing to one side of a board also makes it possible to print a circuit 
on any surface on which it can be printed and on any substance that is 
compatible with the printing process. Thus, the plastic of a car 
dashboard, the case of a radio, a flexible plastic or paper transfer 
medium, or a heat sink could be used as a substrate on which to print a 
circuit according to the present invention. The examples shown here were 
also produced by a screen printing process, which represents a preferred 
method, but it is clear that any method of printing that will handle the 
conducting, resistive, carbon and solder-resist inks will be adaptable for 
the present process and for products produced by the process. The other 
possibilities for doing this include transfer printing, lithography, air 
brush, hand brush, and the like. The examples that included the printing 
of. resistors and capacitors produced such components having values that 
were repeatable within tolerance ranges of five to ten per cent. These 
values are thus comparable to those achieved with ordinary discrete 
components. 
EXAMPLE 1 
An SRBP board approximately 14 inches by 4 inches by 1/16 inch contained a 
laminated layer of copper that had been etched to produce conducting paths 
and appropriately placed comb connectors to be bridged by conducting pills 
when a key was depressed. The combs were plated with gold. In order to 
function properly, this board needed connections that would cross certain 
conductors without making electrical contact with them. The crossing 
connections were achieved by screen printing with an ink containing a 
polymer resist at the crossings. The resist was cured using ultraviolet 
radiation. A conducting path connecting the desired points was then 
printed by a screen process over the cured resist, and the printed 
conductor was then cured at an elevated temperature. This produced a 
functioning printed-circuit board for a keyboard that was printed on only 
one side of the PCB and that had no bridging wire connections. 
EXAMPLE 2 
An SRBP board approximately 14 inches by 7 inches by 1/16 inch had been 
laminated with copper and the copper had been etched to leave an 
appropriate pattern of electrical connections for a keyboard. This pattern 
was placed on a single side of the phenolic board. Each of the contact 
pads for the keys was formed by a screen printing of a conductive ink. 
This ink was then cured by heating. The contact pads and also exposed 
conductors for compression connectors at an edge of the board were then 
covered by screen printing with an ink containing carbon, and the ink was 
cured by heating it. A screen printing was then made to apply an 
insulating polymer resist to all areas of the board except the exposed 
conductors for the key pads and for compression connections at an edge of 
the board and at those areas where components were to be inserted and 
wave-soldered. The cured carbon ink functioned as a solder resist that 
protected the contact pads from solder during the wave-soldering process. 
If the board had been subjected to hand soldering, the carbon ink would 
have protected the contact pads from damage by heat. 
EXAMPLE 3 
A keyboard of FR-4 epoxy fiberglass having dimensions of approximately 14 
inches by 7 inches by 1/16 inch had been laminated and etched on one side 
to leave copper conductors in a pattern appropriate for a keyboard. Layers 
of insulating resist were screened in desired crossover patterns on the 
side of the board that carried the copper and were cured by ultraviolet 
radiation. A screen printing was then made with a conducting ink to apply 
conducting strips on the crossovers and also print the contact areas for 
keys. This was cured by heating. The areas of screened conductors were 
then subjected to a further screening process to cover the screened 
conductors with ink containing carbon. This was then cured by heat. The 
board was then equipped with resistors by screen printing with resistive 
ink in desired locations. After the printed resistors were cured by heat, 
a protective layer of solder resist was applied and was then cured by 
ultraviolet radiation. The solder resist left open areas for the 
application of solder. Because of the carbon covering, this board could be 
subjected to flow soldering without damaging the screened connectors and 
components. 
EXAMPLE 4 
An SRBP board approximately 1 inch by 2 inches by 1/16 inch had been 
laminated with copper foil on one side, and the foil was etched to leave a 
d.RTM.sired circuit pattern. A portion of the board was covered with a 
screened resist which was then cured by ultraviolet light. A conducting 
ink was printed by a screen process over the resist to make a conducting 
crossover. The conducting ink was also placed over an area approximately 
1/4 inch square to form one plate of a capacitor. The conducting ink was 
cured by heat. Resistors were deposited in desired locations by screening 
resistive ink which was then cured by heat. A layer of resist was 
deposited over the conducting area to provide a dielectric material for 
the capacitor. This was then cured by exposure to ultraviolet light. A 
second conducting layer was printed by screening to cover the resist and 
form the second plate of the capacitor and also to connect the capacitor 
at a desired point on the copper lamination, and the conducting ink was 
cured by heating it. The result was a circuit board with connections and 
components. A protective layer of solder resist was applied with holes for 
solder points, and the resist was cured by ultraviolet radiation. 
EXAMPLE 5 
An SRBP board of dimensions approximately 2 inches by 1 inch by 1/16 inch 
received a screened pattern of conducting ink which was then cured by 
heat. A pattern of screen resistive inks was placed by a screen printing 
process in desired locations, and the resistive ink was cured by heat. 
Selected locations of the screened conductor were then plated with copper 
in an electroless plating tank. A protective resist was then screened over 
all but those portions of the board that were to be exposed for contact. 
This included the areas that were given the copper plate, which readied 
that region of the board for soldered contact, either manual or automatic 
soldering. As an alternative, nickel could equally as well have been 
plated to provide an appropriate soldering surface. The conductors formed 
by conducting ink will not generally withstand the heat associated with 
hand soldering, although they may stand up under wave-soldering. 
EXAMPLE 6 
An SRBP board 14 inches by 7 inches by 1/16 inch was screen-printed with a 
conducting ink in a pattern appropriate to a keyboard. The ink was cured 
by heat, and crossing connectors were placed by the process described 
above. Resistive ink in controlled patterns was printed and cured by heat 
to produce desired resistors as described above. A layer of solder resist 
was then screened on the board in a pattern that left substantially square 
openings at the locations of the typewriter keys. The resist was cured by 
exposure to ultraviolet light. The cured resist served as a mechanical 
spacer for a membrane with appropriately placed conductors that was laid 
over the resist. This provided a method of making a membrane-switched 
keyboard without the necessity of tooling to cut openings in a piece of 
plastic to space the membrane from its mating electrical connection. The 
connections made here were capacitive, but they could equally as well have 
been conductive or inductive. The process could also have been applied to 
make switches using two parallel membranes that were spaced apart by 
screened and cured resist rather than using a rigid board as one connector 
support. 
All of the boards described in the examples above have in common the fact 
that all of their printing is done on one side of the board. Some boards 
have been made in the past with jumper wires that are wave-soldered as a 
part of the process of making the board, but this is seldom a good 
solution to the problem of bridging connectors and it is often unfeasible 
if the run of conductors to be bridged is of any length. Another problem 
that can be overcome much more simply than the present practice is that of 
providing shielding against radio-frequency interference (RFI shielding). 
One reason for the use of multilayer boards is to place a shielding layer 
protecting portions of the circuit. This is done much more simply on a 
single layer board on one side by depositing and curing a conducting layer 
over a layer of resist. 
The process of the present invention is adaptable to print as many as 
thirty to fifty layers, one on top of the other. The practical minimum 
spacing between adjacent conductors is of the order of 0.01 inch. A 
screened layer after curing can typically be controlled in thickness to 
plus or minus 5 microns. Capacitors of various values ranging up to 1000 
picofarads may be made as described in the examples, with tolerances to 5 
per cent. 
In some circumstances, pinholes in one of the applied layers may cause 
problems, especially in an insulation layer where pinholes may cause 
unintentional and catastrophic connection between conducting layers. In 
order to reduce the risk of this problem arising, the or each insulating 
layer may be applied in two stages, so that two coatings are applied with 
variation of coating direction or slightly positional variation between 
the coatings. For example, when applying an insulating layer using screen 
printing the first stage comprises applying a screened coating with the 
direction of application along the X-axis of the screen and work-piece. 
The second stage comprises applying a further screened coating with a 
variation from the original coating, for example either by rotating both 
the screen and the work-piece through 90.degree., relative to the coating 
direction, and/or with the screen offset relative to the work-piece, e.g. 
in the X and/or Y direction, by a minimal amount e.g. a small part of a 
millimeter. 
The Table is a listing of the inks that were used in a screen printing 
process to produce the examples above. 
TABLE 
Screen Printing Process Inks 
1. Resist. This is a cross-linking polymer sold under the trademark 
"Photocoat." The examples used type 2G which is not flexible, although a 
style 3G is available that is flexible. The examples used a resist that 
was curable by ultraviolet radiation. A heat-curable resist could equally 
as well have been used. 
2. Resistive Ink. All of the examples used a polymer thick-film ink sold 
under the trade name "Matthey-Lec" R-4000 series. This is available in a 
range of resistivities and is cured by heat. 
3. Carbon Ink. This is a polymer thick-film ink containing carbon black. It 
is cured by exposure to heat. 
4. Conductive Ink. This is a polymer thick-film ink containing silver 
flakes. It is cured by exposure to heat. 
In the manufacture of printed circuits it is known to apply a number of 
layers in desired patterns to the surface of a substrate, commonly an 
insulating board. The layers may be applied in a variety of ways, for 
example by first applying a sheet of copper material to the board and then 
etching away unrequired regions to leave a desired pattern or by applying 
coatings of suitable material to the surface of the board in a desired 
pattern to achieve the required effect. It is with this latter operation 
that the invention is concerned. The layers which are applied may be in 
the form of a conductive material or may be a resist which may be an 
electrically insulating material and/or a material which resists damage, 
e.g. by etching fluid or other operations, for example soldering. Solder 
is non-adherent to most resists and such resists are commonly unaffected 
by temperatures encountered during soldering. 
In some circumstances pinholes in one or more of the applied layers may 
cause problems, especially in an insulation area where pinholes may cause 
unintentional and, potentially, catastrophic connection between conducting 
layers. 
It has been found that the risks of pinholes arising may be reduced by 
applying the, or each, insulating layer in two stages with the substrate 
disposed in a different relationship to coating apparatus in each stage, 
so that two coatings are applied with, for example, variation of coating 
direction or slight positional variation between coatings. This 
substantially reduces the risk that pinholes will cause a problem. 
In one aspect the invention may be considered to provide a method of 
applying a layers 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, 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. 
Preferably, in methods in accordance with the invention the coating 
apparatus used is a screen printing apparatus. In one methods the 
relationship of the board to the apparatus is changed by moving the 
position of the screen of the apparatus relative to the substrate through 
a very small distance e.g. a small part of the millimeter e.g. 0.05 mm. 
However, although this technique is satisfactory for some applications, 
the various conductors and other applied layers of many modern printed 
circuits are so close to one another that even this very small positional 
adjustment is unacceptable. 
In another method in accordance with the invention which is, in some 
circumstances, an improvement over the method outlined in the last 
preceding paragraph, two separate but identical screens each having the 
same pattern are used. The first coating is applied using one of the 
screens and then this screen is removed and the other screen positioned in 
registration with the substrate and the second coating applied. The cost 
of production of suitable screens is very high and this method requires 
two screens. From an accuracy point of view the use of more than one 
screen can lead to added problems, for example distortion of the two 
screens may differ e.g. if the screens are of slightly different tensions. 
Thus, while this method using two screens may be preferable to that 
described in the last preceding paragraph, it is still desirable to 
achieve further improvements in accuracy for some uses, as well as to 
avoid the expense of two screens, if possible. 
In another preferred method in accordance with the invention where the 
coating apparatus is a screen printing apparatus, the relationship of the 
substrate to the I apparatus is changed by rotating both the substrate and 
the screen through an angle, suitably 90.degree., relative to the coating 
direction of the apparatus. 
In carrying out the preferred method in accordance with the invention, 
preferably a number of identical circuit patterns are printed on a 
substrate; the circuit patterns are suitably printed to provide a 
corresponding number of individual circuit boards which are subsequently 
separated from the remainder of the substrate. Suitably the substrate is 
provided by an insulating sheet material on which the circuits are printed 
and from which the individual circuit boards are severed after printing. 
In this preferred method the regions to which the desired pattern is to be 
applied are positioned such that the same pattern may be 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 a centre of the substrate. The first coating is applied 
with 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.degree.), and then the second 
coating is applied. Suitably the fixed angle is 180.degree. and four 
regions to which a pattern is to be applied are present. By making this 
change in position of the pattern applying portion relative to the 
substrate, the second coating is applied to each region using a different 
pattern applying portion from that used to apply the first coating to the 
same region; the chance of a damaging pinhole appearing in precisely the 
same position of two different applying portions is very small. 
In the preferred method in accordance with the invention this risk is 
reduced even further by not only ensuring that the second coating is 
applied to each region by a different pattern applying portion from the 
first coating but also that the second coating is applied in a different 
coating direction. In this preferred method, which uses a screen printing 
apparatus, the screen and substrate are rotated in opposite directions, 
each being rotated through an angle of 90.degree.. 
There now follows a detailed description to be read with reference to the 
accompanying drawings of a method of making a printed circuit board 
embodying the invention. It will be realized this method has been selected 
for description to illustrate the invention by way of example. 
In the accompanying drawings: 
FIG. 1 is a diagrammatic plan view of a screen with a corner broken away 
revealing a substrate above which the screen is positioned, in a position 
for applying a first coating; and 
FIG. 2 is a similar plan view showing the screen and substrate in position 
for applying a second coating. 
A screen 10 of a screen printing apparatus is positioned above a substrate 
provided by a rigid insulating sheet material 12 in contact with a surface 
of the sheet material 12, forming a base on which printed circuit boards 
are to be formed. The screen 10 and sheet material 12 are positioned for 
the first coating in register with one another as shown in FIG. 1, that is 
with datums D1 and L1 is alignment and with datums D2 (not shown) and L2 
in alignment. In register for the second coating (FIG. 2) the datum D1 
overlies the datum L2 whilst the datum D2 overlies datum L1. Desired 
patterns P1-P4 to be printed are shown in the drawings. The patterns are 
produced by multiple image photographic techniques and are, therefore, 
basically identical, although arranged in appropriate positions and 
orientations to enable the carrying out of the illustrative method. 
In carrying out the illustrative method, with the screen 10 positioned 
relative to the sheet material 12 as shown in FIG. 1, coating means of the 
screen printing apparatus is moved relative to the screen in a coating 
direction C to apply a first coating of material direction C to apply a 
first coating of material, for example a resist, through the screen 10 to 
deposit the desired pattern P1-P4 onto the base sheet material 12; 
material M1 deposited on the sheet material 12 is indicated in FIG. 1. 
After the first coating has been applied, the screen 10 is separated from 
the sheet material 12 and the screen 10 and sheet material 12 are rotated 
about an axis A at the centre of the base and perpendicular to the base 
and to the screen 10. The screen 10 is rotated in the direction indicated 
by the arrow S in FIG. 1 and the base sheet material 12 is rotated through 
an angle of 90.degree. as indicated by the arrows A, B about the axis A 
until they occupy the positions in which they are shown in FIG. 2. The 
screen 10 and sheet material 12 are put into register by ensuring that the 
datums are appropriately aligned. In this instance, however, the datum D1 
is aligned with the datum L2 whilst the datum D2 (not shown) is aligned 
with the datum L1 (see FIG. 2). After the screen 10 and sheet material 12 
have been moved into register, they are again moved into contact with one 
another and a second coating is applied by moving the coating means (e.g. 
a squeegee) in the coating direction indicated by the arrow C in FIG. 2. 
The second coating is applied over the first coating but the second 
coating is applied by means of different ones of the patterns P1-P4 formed 
in the screen. For example, the second coating applied over the material 
M1 shown in FIG. 1 is applied by the pattern P1 in the screen 10 whilst 
pattern P3 in the screen 10 applies a second coating over material M2, the 
first coating of which was applied by the pattern Pl Thus, not only is the 
second coating applied by coating means travelling in a coating direction 
C across the screen at an orientation of 90.degree. to that in which the 
first coating is applied but also the actual patterns through which the 
second coating is applied are different. By this means it is ensured that 
the risk that any pinholes will occur in the applied material is 
practically zero. 
After all the necessary layers have been applied to the sheet material 12 
individual circuit boards are severed from the sheet material 12 by a 
suitable means such as, for example, routing. Each of the boards carryies 
material applied by one of the patterns P1-P4, there being four separate 
boards manufactured in this method.