Method of making printed circuit boards

A process of making printed circuit boards in which the individual substrates involved are hot set formed from a suitable thermosetting resin and filler composition, with the cure being limited to the resin B stage. The side surfacing of the individual substrates that is to bear the metallic electrically conductive circuiting is catalyzed and then electrolessly plated with copper or the like electrically conductive metallic material to a thickness in the range of from about 50 to about 100 millionths of an inch, and thereafter the thusly plated substrates are hot set to fully cure same and perfect the bond between the substrate and its plating. Each substrate then has a masking and non-conductive resist applied to its plated side (that is to bear the circuit), in a pattern that defines the printed circuit configuration and exposes the underlining electroless plating in the configuration of the circuit for electroplating, after which the substrate is electroplated with copper or the like in the circuit defining voids of the resist thereafter, the resist is stripped from the substrate, and the electroless plating thereby exposed is removed by etching.

This invention relates to a method of making printed circuit boards, and 
more particularly, to a method of forming, curing, and electrolessly 
plating dielectric non-metallic circuit board substrates, and separately 
electroplating of the circuitry on the substrates so as to provide for 
maximized adhesion between the conductive metal forming the finalized 
circuit, and the dielectric non-metallic substrate, and minimizing plating 
removal requirements to complete the circuit board. 
Rhodenizer et. al. U.S. Pat. No. 3,666,549 brings out that the manufacture 
of printed circuit boards involving a non-metallic dielectric substrate 
and a metallic circuit printed thereon basically follow two distinct 
method approaches, namely the subtractive method and the additive method. 
The present invention is directed to employing basically the additive 
method of making printed circuit boards, with a major objective being to 
so process the boards that adequate adhesion between the non-metallic 
dielectric substrate and the conductive metal forming the circuit is 
assured, thus overcoming the major obstacle to successful additive circuit 
board processing. 
The principal object of the present invention is to provide an essentially 
additive method of making printed circuit boards, pursuant to which the 
dielectric thermosetting material substrates that are to become the base 
piece of the individual boards are initially cured only to their "B 
stage", with the full cure being effected after the surfaces of the 
substrates that are to bear the circuitry are thin metal plated. 
Another principal object of the invention is to provide an additive method 
of making printed circuit boards, in accordance with which "B stage" cured 
substrates, after being thin metal plated, as by employing electroless 
plating techniques, on the areas of same that are to bear the printed 
circuiting, are full cured under hot set conditions of temperature and 
pressure to maximize the bond between the plating and the substrate, as 
well as effect full cure of the substrate. 
Still another important object of the invention is to provide an additive 
method of making printed circuit boards in which the plating and metal 
removal requirements are minimized while providing for maximized adhesion 
between the metallic material forming the circuitry and the dielectric 
non-metallic theremosetting material forming the substrate. 
Other important objects of the invention are to provide an additive method 
of making printed circuit boards that is readily practiced using existing 
equipment and techniques for hot setting the board substrates, electroless 
plating and electroplating of same, and etching of the plating as needed 
to finalize the shaping of the electrically conductive metallic material 
forming the circuitry of the board. 
In accordance with the invention, the substrates employed to form the base 
piece or body of the circuit board are formed utilizing suitable hot 
setting procedures and equipment whereby a suitable thermosetting resin 
and filler mixture, such as epoxy or phenolic, fiberglas filled 
compositions, to shape through pressure and heat the base structure 
involved into the familiar planar, usually quadrilateral configuration, 
but with the cure of the resulting substrate being limited to the resin "B 
stage". The substrate surfacing that is to bear the printed circuiting is 
then suitably catalyzed and then electrolessly plated with a film of 
copper or the like to a thickness in the range of from about 50 to about 
100 millionths of an inch. 
The resulting intermediate product is then hot set processed, utilizing 
appropriate conditions of pressure and heat to fully cure the substrate 
and perfect the bonding of the film thereto. As part of this procedure, 
the surfacing of the substrate bearing the film is overlaid with a sheet 
formed from a suitable elastomer, such as silicone rubber, that is 
coextensive with the film to insure even distribution of the pressure over 
the film. 
Where the circuit boards are to have through openings in same for circuit 
connectors and mounting of the board, as the case may be, these openings 
are formed in the substrate prior to the final cure step, to take 
advantage of the relatively soft nature of the substrate in its B stage. 
Where such through openings are in the portion of the electrolessly plated 
surfacing of the substrate, such through openings are through plated by 
the electrolessly plating procedure. 
The resulting intermediate substrate product may then be stored until 
needed or immediately processed further in accordance with the invention, 
whereby a masking resist is applied to the metallic films that are to bear 
printed circuiting, in a configuration that leaves exposed on the metallic 
plating the underlying plating in the configuration of the circuit, after 
which the exposed metallic film portions are electroplated with copper or 
the like electrically conductive metallic material to build up the 
thickness of the circuit leads and terminals. Thereafter, the masking 
resist is stripped from the substrate and the thus exposed electrolessly 
plated film portions are etched away from the substrates to leave the 
desired printed circuiting on the substrate that will now have maximized 
adhesion to the substrate while having only that section thickness that is 
needed to adequately serve printed circuit functions without leaving 
excess metallic material on the board, and with freedom from etching under 
cutting.

However, it is to be distinctly understood that the specific drawing 
illustrations and disclosure provided are supplied primarily to comply 
with the requirements of the Patent Laws, and that the invention 
contemplates modifications and variations that will be obvious to those 
skilled in the art, and which are intended to be covered by the appended 
claims. 
BACKGROUND TECHNOLOGY OF INVENTION 
The additive method of making printed circuit boards disclosed herein is 
concerned with board substrates formed from dielectric non-metallic 
thermosetting resinous materials that are generally known as resinoids. 
As disclosed in "Plastics", by J. Harry DuBois and Fredrick W. John 
(published 1967 by Van Nostrand Reinhold Company), at page 18, 
theremosetting compounds may be compared with concrete. Concrete is a 
mixture of cement and sand which has been hardened by a chemical action in 
the presence of water. The cement may be said to be a binder for the 
particle of sand and the sand serves as a filler, decreasing the cost, and 
providing body and substance to resist dimensional changes. 
Chemical changes takes place when the cement "sets", thus producing a solid 
body of resulting solid substance that retains the form in which it was 
cast and cured. 
In a typical thermosetting material, such as a phenol-formaldehyde 
compound, the analogy with concrete provides an excellent means for 
understanding the thermosetting reaction. In this case, the binder or the 
"cement" is a chemically produced resin, resulting from the partial union 
of phenol and formaldehyde. A phenol-formaldehyde resin having suitable 
fillers incorporated in same, such as fiberglas, and the resulting mixture 
is then pressed to shape in heated dies. The combination of heat and 
pressure on the compound or mixture in the mold causes it to become 
plastic and flow to the desired contour as defined by the mold. Continued 
heat and pressure complete the chemical union of the phenol and 
formaldehyde with the result that the binder welds the particles of the 
filler into a single mass that cannot be softened again by heat. The 
resulting product now resist chemicals which would have dissolved the 
resin before it passed through the thermosetting reaction in the mold. 
Materials of this type are said to be "hot set" or "hot molded" when they 
are hardened by the addition of heat and pressure, as is the case with 
thermosetting compounds (see "Plastics" by J. H. DuBois, published 1943 by 
American Technical Society, at page 21). 
By way of contrast, thermoplastic materials may be considered similar to 
sealing wax, which is a dense hard substance at room temperatures, but 
when heated becomes soft and pliable and may be molded. When the shaped or 
molded wax again cools to normal temperatures, it again becomes dense and 
hard and retains its new shape; unlike thermosetting resins, thermoplastic 
materials may undergo softening by heating and hardening by cooling again 
and again. As there is no chemical change in thermoplastic material during 
heating and cooling cycles of molding, the same chemicals which would 
attack it before molding will still attack it after molding (see 
"Plastics" by J. H. DuBois, supra, at pages 4 and 5). 
Thermosetting materials or resinoids are considered to be "cured" in being 
hot set from a soluable or fusable condition to a substantially 
insoluable-infusable condition by chemical action. This processing is 
analogous to the vulcanization of rubber (see Plastics Dictionary by 
Thomas A. Dickinson, published 1948 by Pitman Publishing Corporation, 
pages 77 and 241). 
As the above cited Plastics Dictionary by Dickinson sets forth, at pages 
23, 38, and 76, thermosetting resin materials are considered to have three 
stages of cure, as follows: 
The "A stage" is the early period of the reaction during which the 
thermosetting resin is both soluble and fusible. 
The "B stage" is the intermediate period of reaction during which the 
thermosetting resin materials soften when heated and swell in contact with 
liquids, but do not completely fuse or dissolve. This is the preferred 
stage for thermosetting resin materials for molding compositions of same, 
and it is sometimes called the resitol stage. 
The "C stage" is the final period of reaction of the thermosetting resin 
materials, during which the thermosetting resin becomes infusable and 
insoluable; this is approximately the state of the resin in the finished 
article formed by same, and this stage is sometimes called the resite 
stage. 
Circuit board substrates are conventionally formed from suitable 
thermosetting resin compositions, of which glass-epoxy or phenolic based 
types are generally preferred for making the substrate, because of their 
dielectric properties as well as resistance to structural deformation or 
warpping due to temperature and humidity variations. An example of 
material in this category is fiberglas filled phenolformaldehyde 
thermosetting composition, although, as is well known in the art, quite a 
number of thermosetting compositions of this general category are employed 
for making substrates, with specific compositions being designed for 
specific applications as needed. 
In FIG. 1, reference numeral 10 generally illustrates a printed circuit 
board of the one sided or single sided type comprising substrate 12 that 
is formed from one of the usual thermosetting resin compositions that is 
hot set to define a planar body 14 of quadrilateral configuration 
(rectangular in the illustrated embodiment) of suitable thickness in 
defining an upper side surfacing 16 to which printed circuiting 18 is 
applied, an underside 20 that is free of printed circuiting, and three 
through openings 22 that in the form shown are drilled or punched in the 
substrate 14 for purposes of mounting the board 10 in its position of 
operation. The printed circuit 18 itself comprises plurality of pads or 
terminals 24 and traces or leads 26 that are formed from a suitable 
electrically conductive metallic material such as copper or nickel, and 
that are adhered to the top surfacing 16 of the substrate 12. In the 
specific printed circuiting 18 illustrated, a number of the pads or 
terminals 24 do not have through openings for connectors, while others do, 
such as the through openings 28 about which the pads or terminals 24 at 
the right hand end of the board 10 are formed. 
The board 10 is electrically arranged in the manner shown in FIG. 10 of my 
U.S. Pat. No. 4,214,153, granted July 22, 1980, in which the circuit board 
there illustrated is arranged for use in a tape reader for programmable 
controllers. 
FIGS. 2 and 3 illustrate the general arrangement of a typical two sided or 
double sided printed circuit board, in which the printed circuiting is 
applied to both sides of the board. The board 30 comprises suitable 
substrate 32 shaped to have the elongated quadrilateral configuration 
illustrated and having a top side surfacing 34 that bears printed 
circuiting 36 and an underside surfacing 38 which bears printed circuiting 
40. As has already been indicated, the board 30 in FIG. 2 is viewed from 
its top surfacing 34 while in the showing of FIG. 3 the printed circuiting 
36 is omitted and the printed circuiting 40 is illustrated as viewed from 
the top surfacing side 34 of the board 30, and thus through the board 30, 
so as the relation of the two circuitings 36 and 40 will be more readily 
apparent. The circuiting 36 defines larger pads or terminals 42, smaller 
pads or terminals 44, and leads or traces 46, with the circuiting 36 
illustrated having its pads or terminals 42 disposed about through 
openings 48 of the board 40. The board substrate is solid or imperforate 
underlying the smaller pads or terminals 44. 
The circuiting 40 of the side surfacing 38 comprises only the larger pads 
or terminals 42A and traces or leads 46A. The terminals 42A of the 
specific circuit board 30 illustrated are applied about two of the through 
apertures 48 at either end of the board, with the third through opening 48 
at the right hand end of the board not having a terminal 42A applied 
thereabout, and with the remainder of such openings 48 of the board side 
surfacing 38 (that are not associated with a terminal 42A) are omitted, 
but they would have the positioning indicated in FIG. 2 (laying the 
showing of FIG. 2 on top of the showing of FIG. 3). 
The showings of FIGS. 1-3 are provided primarily to specifically illustrate 
the general nature of circuit boards that are made in accordance with the 
practice of this invention. The invention of this application is not 
concerned with the specific configuration of the circuitings 18, 36 and 40 
that are illustrated, but rather the manner of application of the 
terminals and leads there illustrated, or any other printed circuit 
arrangement of leads and traces that may be desired in connection with or 
for printed circuit boards. However, the methods hereinafter described 
take account of the fact that single or one sided circuit boards of the 
type indicated in FIG. 1 may be handled somewhat differently than double 
sided circuit boards that are represented by the showings of FIGS. 2 and 
3. 
APPLICANT'S CIRCUIT BOARD MAKING METHODS 
In accordance with the invention, circuit board substrates, as represented 
by the substrates 12 and 32, are made using a suitable thermosetting 
composition therefor and hot set procedures and equipment in connection 
therewith, with the specific substrates being produced being shaped as 
desired for specific applications. As is conventional, the substrate 
forming materials, such as the phenol-formaldehyde resin and fiberglas 
filler hereinbefore referred to, or any other thermosetting composition, 
is applied to, for instance, a suitably arranged compression molding 
process molding machine or other appropriate hot set equipment, several 
forms of which are diagrammatically illustrated in the references cited 
above. The hot setting of the materials involved to form the individual 
substrates proceed in accordance with standard practices, except, in 
accordance with the present invention, the hot setting processing is 
terminated, and the substrates are removed from the mold, when in their 
resitol or B stage of cure, with these terms having the meaning defined by 
the cited Thomas A. Dickinson Plastics Dictionary (at page 38). 
The resitol stage or B stage cured substrates form the base structure of 
the printed circuit board, in accordance with this invention. Of course, 
for mass production purposes, quite a number of the substrates in question 
may be made up and suitably stored until needed for further processing in 
accordance with the practice of the invention. It is important, however, 
that the B stage cure status of the substrates be maintained, and it may 
thus be appropriate under some circumstances to store the thus formed 
substrates in cool storage areas or perhaps even under refrigerated 
ambient conditions. 
Thereafter, one way of processing the individual substrates in accordance 
with the invention is as follows: 
The substrate to be made into a finished circuit board in accordance with 
the practice of the invention, assuming its B stage cure status still 
obtains, is first catalyzed, for instance, in the manner suggested by said 
Rhodenizer et. al. patent at column 5; suggested is a two step activation 
procedure using stannous chloride in hydrochloric acid followed by a dip 
in palladium chloride in hydrochloric acid. Alternately, as suggested by 
the reference indicated, the catalyzation may be effected by a one step 
procedure employing a tin-palladium hydrosol such as that disclosed in 
U.S. Pat. No. 3,532,518. It may be also be desired to subject the 
catalized substrate to an accelerating solution, as, for example, a dilute 
solution of suitable acid or alkali, as suggested by Rhodenizer. 
Thereafter, the side surface of the substrate that is to bear the printed 
circuiting has applied thereto a thin film of a suitable electrically 
conductive material, such as copper, by way of conventional electroless 
metal plating procedures. The metal deposit contemplated is to be over the 
entire side surfacing of the substrate side involved, but it is a feature 
of the invention that this film be minimal to conserve the valuable metal 
involved and minimize the amount of the metal that needs to be removed by 
etching at the final stages of the Applicant's method for avoiding 
undercutting of the pads and traces. 
Specifically, it is preferred that the electrolessly plated film have a 
thickness in the range of from approximately 50 millionths of an inch to 
approximately 100 millionths of an inch. Typical compositions of 
electroless copper baths suitable for practice of the invention are 
disclosed in U.S. Pat. Nos. 2,874,072, 3,075,855, and 3,095,309. The 
purpose of the electrolessly plated film is to provide an initial 
conductive surface on the side surfacing of the substrate that is to bear 
the printed circuiting in order to facilitate electro disposition of metal 
plating in the appropriate place in accordance with the later described 
electro plating steps of the invention. The thickness of the electroless 
metal coating should be as thin as practical; preferably it is no more 
than a 100 millionth of an inch and exceeds a 50 millionth of an inch only 
to the extent needed for effectively performing the electroplating step or 
steps that are to follow. 
Where the substrate involved is to have both side surfaces of the substrate 
bear printed circuiting, as illustrated by the board 30, the substrate 
involved is dipped into an appropriate electroless copper bath so that the 
substrate involved is totally electrolessly plated within the thickness 
range indicated. The electroless plating will thus also be through the 
through holes 48 (if such holes have been drilled) and along the side 
edges and end edges of the substrate; in other words, the entire surfacing 
of the substrate at this stage will have an electroless copper coating. 
Where the board is of the single sided type, in connection with which the 
printed circuiting is applied only to one side of the board, it is only 
necessary to plate the one side of the board involved. One way of 
effecting this is to cement together two of the boards by applying a 
narrow band of adhesive along their margins and clamping or holding them 
together until the adhesive hardens. The pairs of bonded together 
substrates are then electrolessly coated in the same manner as substrates 
that are to be made into double sided boards, after which the bonded 
together boards are separated for further processing in accordance with 
the invention. 
Thereafter, the electroless plated substrate is subject to suitable hot 
press procedures to complete its cure through the resite or C stage. In 
this processing of the board, it is a feature of the invention that the 
board, electrolessly plated as indicated, be subjected to both pressure 
and temperature conditions that affect the full C stage cure, with the 
board under the indicated pressure conditions having the pressure evenly 
applied across the surfacing of the electroless plating involved. 
One way of doing this is diagrammatically illustrated in FIG. 5 for the 
substrates that are to be double sided boards. In the showing of FIG. 5, a 
compression molding apparatus 60 is illustrated comprising a fixed base 
member 62 having a planar compression face 64, and a movable plunger 
member 66 that is suitably hydraulically actuated to move vertically in 
either direction, as indicated by the arrow 68, and having a planar 
compression face 72 to be applied against substrates disposed on 
compression face 64 for this purpose. 
The mold apparatus 60 includes suitable movement guide means, and heating, 
actuating, and other conventionally needed or desired equipment of a 
conventional nature that is not illustrated since anything of this type of 
a conventional nature will serve the purpose. Members 62 and 66 may be 
formed from steel or the like. 
In using the mold apparatus 60, double sided substrates of the same size 
and shape, for instance, substrates 32, are applied to the compression 
surface 64, which for production purposes should be in substantial 
parallelism with corresponding surface 72. In loading the apparatus 60, 
plunger member 66 is suitably spaced from base member 62, and a thin sheet 
of a suitable elastomeric material, such as silicone rubber, is applied on 
the surface 64 of the die in centered relation thereto, where indicated at 
80, and one of the substrates 32 that has been electrolessly copper film 
plated as described above is laid thereon and covered by a succeeding 
sheet 80 of the same elastomeric sheeting. The substrates 32 that have 
been electrolessly copper film plated as described above and the 
elastomeric sheets 80 are stacked in alternating relation, as indicated in 
the showing of FIG. 5 so that each electrolessly plated side surfacing of 
the substrate 40 will have bearing thereagainst one of the elastomeric 
sheets 80. The stack 82 to be processed by the operation of the apparatus 
60 is completed by a final elastomeric sheet 80, and then the apparatus 60 
is run through its compression stroke, to place the stack 82 under 
suitable conditions of pressure, coordinated with appropriate conditions 
of temperature by heating the apparatus 60 in any conventional manner so 
that the cure of the substrates 32 being processed advances through the 
indicated C stage. During this period, the electroless film plating is 
also compressed against the substrate 32 on which it is plated, and at the 
side surfacing of same that is to bear the printed circuiting, thereby 
effecting, in accordance with the practice of the invention, significantly 
improved adhesion between the individual substrates and the conductive 
metal films electrolessly applied thereto. While the reasons for achieving 
the improved adhesion that results are not fully understood, it is 
believed that the result comes by way of a combination of factors involved 
in this step of the procedure whereby not only are the substrate and the 
film that has been electrolessly applied thereto pressed thereagainst 
subject to both heat and pressure, but the substrate is advanced from its 
B stage cure through its C stage cure, whereby apparently the chemical 
changes taking place as the cure advances through C stage effect an 
appreciable degree of "cementing" of the electroless metal film to the 
substrate, in addition to the substrate mass itself being irreversibly 
hardened through full cure. 
When full cure has been achieved, as is determined by conventional 
instrumentation for indicating pressures, temperatures, timing, and the 
like, the apparatus 60 is opened for removal of the stack 82 and 
application thereto of a fresh stack 82 to be similarly processed. 
Where the substrates being processed are to be single sided, in forming a 
stack 82 for same the lowermost elestomeric sheet 80 may be omitted 
assuming that the first substrate, for instance, a substrate 12, is 
applied to the mold surface 64 with its unplated side engaging surface 64. 
The upwardly facing and plated side surfacing of the substrate 12 has 
applied thereto a elastomeric sheet 80, which is followed by the next 
substrate 12 having its electrolessly plated side engaging the first 
elastomeric sheet 80. The next substrate 12 is applied to the stack with 
its unplated side engaging the unplated side of the substrate immediately 
preceeding it, and has an elastomeric sheet 80 applied to its plated side 
on which in turn the next substrate 12 is plated with its plated side 
engaging the sheet 80. The stack is completed by alternating the 
substrates 12 in pairs and the elastomeric sheets 80 between the passed 
substrates, with the final substrate, in having its unplated side facing 
upwardly, not requiring a sheet 80 overlying same. Full cure is then 
achieved as already described as described. 
The substrates 32 or 12 being processed are preferably of the same shape in 
perimeter, and the elastomeric sheets 80 are preferably of similar 
shaping, although sheets 80 may desirably exceed the size of the 
substrates 32 and 12 in marginal dimensioning for ease of application. 
Sheets 80 may be formed from silicone rubber sheeting having a thickness 
in the range of from about 0.003 inch to about 1/16th inch; other suitable 
elastomeric material will serve the purpose, the main thing being that 
sheets 80 adequately pass heat and pressure therethrough. The sheets 80 
insure uniform application of pressure to the plated surfaces of 
substrates 12 and 32, as the substrates 12 and 32 themselves generally 
will not be perfectly flat, by serving as mediums for applying heat and 
pressure thereto that also readily conform under pressure to the substrate 
surfacing shapings for full and uniform engagement or contact therewith. 
The fully cured substrates 12 and 32 may now be either directly processed 
in accordance with the invention to provide completed printed 
circuitboards, or stored until needed, or shipped to processers who 
manufacture printed circuit boards. In any event, the substrates that have 
been electrolessly copper film plated and fully cured as described above 
are processed to the final printed circuit board position as follows: 
The board is first cleaned as needed by a suitable rinsing step or the 
like, to insure that the electrolessly applied copper film is clean, and 
then the substrate has applied to its film covered side surfacing a 
suitable resist coating, the function of which is to expose only those 
portions of the film that are to define the contour of the printed 
circuiting to be provided by the finished board, and to cover the 
remaining portions of the film to preclude electroplating of same in the 
electroplating step that is to follow. 
In the practice of the invention, it is preferred that a suitable chemical 
resist be applied to the substrate film utilizing appropriate silk screen 
equipment and techniques that are designed to produce coverage of the 
non-circuit areas of the substrate film in question with the resist while 
leaving the circuit forming areas of same free of the resist. Thus, the 
silk screening is arranged to apply the resist to the substrate film in a 
negative pattern that leaves voids in the resist which define contouring 
or outlining identical to the contouring of the printed circuiting that is 
to be ultimately provided by the board being processed. Equipment of this 
type involves the resist being squeegeed through the screen by appropriate 
squeegeeing techniques. 
Suitable chemical resists that may be employed for the purpose are the 
Dynacure products SR-28 or SR-30ST, offered by Dynachem Corp. of 
Minneapolis, Minnesota, but any conventional chemical resist that is 
dielectric or electrically insulating in nature will be satisfactory. 
The substrate having its electroless metallic film resist covered as 
indicated, is then electroplated to plate in the voids defined by the 
resist the electrically conductive material that is to have the contour of 
the printed circuiting involved and that is to be built up to the 
thickness appropriate for printed circuiting electrically conducting 
requirements. However, it is a feature of this invention that the 
electroplating thickness provided, combined with the thickness of the 
electroless plating, be no more than is appropriate for good electric 
conducting purposes to avoid the common problem of undercutting at the 
etching stage of the Applicant's method. 
The substrate to be electroplated is applied to conventional electroplating 
equipment, which is facilitated because of the application of the 
electroless metal film on the side surfacing of the substrate that is to 
receive the printed circuiting. Thus, in using such equipment, a single 
connection may be made to the substrate, and specifically its electroless 
coated metallic film at any point on the conductive surface of the 
substrate, whereby the electroless coated film is made the cathode of the 
conventional electroplating equipment in its bath, with the result that 
the material to be electroplated on the substrate in the voids defined by 
the resist will be applied thereto in accordance with conventional 
electroplating techniques. Preferably this material is copper or nickel. 
The application of the resist to the substrates and the electroplating of 
same will be essentially the same for double and single sided substrates, 
except, of course, for the double sided substrates, which are to be double 
sided printed circuit boards, the application of the resist and the 
electroplating is effected to both sides of the substrate. 
Where the printed circuit configuration as defined in negative pattern by 
the resist includes one or more through holes or openings 48, the 
electroplating is applied through such holes, this being illustrated by 
the showing of FIG. 4 wherein the terminals 42 and 42A at a substrate 
opening 48 are illustrated in section, with the electroless film being 
illustrated at 90 and the electroplating layer being indicated at 92. It 
is to be noted that the film 90 and the layer 92 cover the substrate 
surfacing 94 that defines the through opening 48 as well as the substrate 
top surfacing 34 and the substrate undersurfacing 38 (to the extent that 
the terminals 42 and 42A are laid out or formed about the through openings 
48) are illustrated in section, with the electroless film being 
illustrated at 90 and the electroplating layer being indicated at 92. It 
is to be noted that the film 90 and the layer 92 cover the substrate 
surface in 94 that defines the through opening 48 as well as the substrate 
top surfacing 34 and the substrate undersurfacing 38 (to the extent that 
the terminals 42 and 42A are laid out or formed about the individual 
through openings 48). 
Where the through openings 48 do not lie within the contouring of the 
printed circuiting that is to be provided, they will be covered by the 
resist and thus will not be through plated. Examples are the through 
openings 22 shown in the single sided board 10 of FIG. 1, which are board 
mounted openings and are not provided for electrical connectors. The 
through openings 28 of the board 10 are through plated, but, of course, 
the terminals 24 thereof are formed only on one side of the board. At the 
undersurfacing 38 of board 30, where the through holes 48 do not lie 
within a terminal 42, the electroless film 90 will cover or coat the 
portion of the hole 48 adjacent side surface 34, and the resist will coat 
the portion of such hole 48 adjacent surfacing 38, so that such holes 48 
are not through plated. 
In accordance with the practice of the invention it is preferred that the 
electroplating step apply the electroplated metal to the electrolessly 
deposited metal in an amount that lies in the range of from approximately 
one-half thousandths to approximately 3 thousandths inch, depending on the 
application to which the board is to be put. 
After the electroplating is completed, the deposited electroplate can be 
further electroplated with protective metal such as gold or silver, or 
with solder and serve both as a resist in subsequent etching procedures, 
as well as a protective coating and also serving to facilitate subsequent 
attachment of accessory electronic components to the completed board. This 
may be done utilizing suitable metal plating techniques. 
After the electroplating has been completed, the resulting board is then 
stripped of the chemical resist by dipping the board in a suitable solvent 
as needed to fully strip the chemical resist from the electrolessly plated 
film. A suitable resist remover is a twenty-five percent aqueous solution 
of sodium hydroxide (Na OH) at a temperature of approximately 110 degrees 
F. 
Thereafter the board is emersed in a suitable etchant, such as dilute 
nitric acid, to strip the noncircuit areas of the board of all conductive 
metal. 
In the practice of the invention, it is ordinarily not necessary to 
protectively plate the built up portions of the printed circuitry, which 
have been built up in the present instance by electroplating of copper 
thereon. While during the etching procedures, some copper will be removed 
from the electroplated metal exterior, while the electrolessly applied 
copper film is being removed exteriorily of the electroplated metal, the 
combined electrolytically applied metal and the electrolessly applied 
metal film remaining underlying same together will have a section or 
thickness appropriate for printed circuit electro conductivity. The 
thinness of the electrolessly applied film that is removed is such that 
during the etching step, the etching time requirements are so short that 
there is no time for the etchant to make any significant undercutting of 
the pads and traces. This minimal etching time is a significant and 
critical aspect of the invention for that purpose, and should be closely 
controlled to effect removal of all the exposed electrolessly plated metal 
which avoids undercutting of the board pads and traces. 
With the etching step completed, the boards are suitably rinsed and dried. 
The processing of the circuit boards in accordance with the indicated 
version of the invention then is complete, and they are ready for 
packaging and shipping for distribution purposes, or direct application to 
electronic apparatus, depending on the circumstances. 
In practicing the invention, the electroless film may be in the form of 
nickel, as suggested by said Rhodenizer et. al. patent with reference to 
U.S. Pat. Nos. 2,532,283, 2,990,296, and 3,062,666, as illustrating nickel 
electroless baths. 
As to the resist employed, instead of using a suitable chemical resist, a 
suitable photoresist composition may be employed, with the photoresist 
being processed in accordance with existing technology relating to same, 
which will depend on whether the photoresist is of the positive or 
negative type. Suggestions with regard to photoresist use are contained in 
the cited Rhodenizer et. al. patent. 
The electroplating of the electrolessly applied film may be effected using 
nickel instead of copper, or other suitable electrically conductive metals 
as desired, with the electroplating equipment being varied according, 
following conventional procedures. 
It will therefore be seen that the invention provides a method of making 
printed circuit boards that is adapted for practice with essentially 
conventional apparatus, while simplifying the procedures involved and 
utilizing only that amount of the electrically conductive metals needed to 
result in printed circuiting depth or section of adequate conductivity 
without unnecessary depth. The minimizing of the depth of thickness of the 
electroplated metal is an important factor in avoiding undercutting of the 
electrically conductive material outlining the circuiting since 
undercutting when etching tends to occur in a dimension that corresponds 
to the thickness of the plated metal removed by etching. As undercutting 
necessarily weakens the adherence of the plated metallic materials to the 
non-metallic substrate, it is important that excessive undercutting be 
avoided. 
In practice of the invention, the electroplating build up of copper or the 
like on the electrolessly plated metal should only be enough to meet the 
electrically conductive needs of the circuit board circiting. As 
indicated, a simplified approach preferred for this invention involves not 
masking the electroplated metal to protect same with etching, so the 
amount of the electroplated deposit under such circumstances must take 
into consideration that portion of the electroplated deposit that will be 
removed by the etching procedures. 
It is believed that the application of the electrolessly plated film to B 
stage cured substrates and the cure completion effected by the practice of 
the invention are important factors in the improved adherence of the 
printed circuit forming metal to the substrate, that is obtained by the 
practice of the invention. 
Variations in the herein described method are contemplated depending on the 
needs and facilities of the processors who do the actual circuit forming 
steps. 
For instance, the processor may start with blank substrates, unplated, free 
of apertures, and B stage cured. After sizing of the board, the processor 
may drill or punch the board through holes, with increased tool life and 
speed due to the softer nature of the B stage material. Deburring needs 
are also eliminated, as drilling of previously plated substrates forms 
metallic burrs that must be deburred. After formation of the substrate 
through holes, the processor proceeds with the electroless plating, C 
stage cure, and the electroplating and related steps herein disclosed. 
Alternatively, the substrates may be made up for the processor in blank 
(through hole free), B stage cured, and electrolessly coated on one or 
both sides. The processor would then drill the needed through holes and 
provide the C stage cure, and resist coat, and electroplate, etc., as 
hereindisclosed to complete the board. 
The substrates may also be made up for the processor in blank (through hole 
free), B stage cured, electrolessly plated on both sides for a full 
electroless film coating, C stage cured, the holes drilled, then fully 
electrolessly coated again to so coat the through openings, then resist 
coated, electroplated, etc., as herein disclosed to complete the board. 
The foregoing description and the drawings are given merely to explain and 
illustrate the invention and the invention is not to be limited thereto, 
except insofar as the appended claims are so limited, since those skilled 
in the art who have the disclosure before them will be able to make 
modifications and variations therein without departing from the scope of 
the invention.