Printed circuit board and method using thermal spray techniques

A method of making printed circuit boards, particularly 3-Dimensional circuit boards, wherein a surface layer of micron sized hollow spheres, beads or spacers are ruptured or fractured in a pattern defining the electrical circuit. The circuit pattern is thermally sprayed with molten copper particles. The molten copper particles shape themselves to the nooks, crannies and undercuts of the fractured spheres or beads to mechanically lock the electrical circuitry to the board. The current carrying capacity can be adjusted by the thickness of the sprayed metal. The overspray does not adhere to smooth non porous surfaces adjacent the fractured spheres or beads.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
The present invention relates to printed boards and methods and processes 
of manufacturing same particularly three dimensional printed circuit 
boards. Currently, the art of printed circuit board manufacturing involves 
both subtractive and additive techniques. The subtractive processes are 
currently the most popular PC manufacturing technique and it involves a PC 
board which is entirely coated with copper. The holes are drilled and 
sensitized and electroless copper flashes used. The PC board artwork is 
used then to construct an acid-resistant shadow mask on the copper so that 
when submerged in acid the copper is eaten away leaving a pattern of 
conductive wires in the copper. This conductive pattern is the boards 
printed circuit. 
In the additive process, the acid etching aspect is eliminated and this 
allows the manufacturers to improve their circuit board density and 
product yield. See Chapter 16 of the text "The Design and Drafting of 
Printed Circuits" by Darryl Lindsey, 1979, 1984, Published by Bishop 
Graphics entitled "Manufacturing Process of PC Boards". 
In either system, a number of chemicals and chemical byproducts are used 
and/or generated, which creates problems when they cannot be reused and 
must be stored or disposed of safely. 
There are problems with producing three dimensional printed circuit boards 
using these techniques. New techniques are required for this type of 
printed and plating is not trouble free with these new surfaces and 
substrate materials. The object of the present invention is to adapt the 
thermal spray bonding or adherence techniques disclosed in our U.S. Pat. 
Nos. 4,751,113; 4,521,475; 4,618,504; 4,714,623 (incorporated herein by 
reference) to forming printed circuit boards and particularly three 
dimensional printed circuit boards. 
According to a preferred embodiment of the present invention, a mixture of 
resin and hollow micron-sized spheres or beads is applied to the surface 
of an electronic circuit board by injection molding or by lamination or 
other composite technology processes. A circuit pattern is etched in the 
surface by either laser etching, engraving, or other mechanical means such 
as high speed machining or abrasion. These processes serve to fracture the 
hollow micron-sized spheres or beads directly under the surface. The 
fractured surface is cleaned ultrasonically or with high velocity liquid 
spray to remove surface debris. 
The top surface of the circuit board must be without microporosity or 
surface defects which would serve as anchor sites for sprayed metal. Very 
fine metal particles may find slight porosity on the surface and will tend 
to stick to the surface at these points. Normally, they can easily be 
scraped off the surface and hence do not affect the integrity of the 
circuit board. 
In the preferred embodiment, the circuitry is formed slightly below or 
flush with the surface. 
As an alternative method of preparing a printed circuit board utilizing the 
thermal spray processes according to the invention, the surface on which 
the circuit pattern is to be formed is screened with a circuit pattern 
onto the surface using a resin such as an epoxy. Secondly, the surface is 
"dusted" with hollow micron-sized spheres or beads in a manner disclosed 
our U.S. Pat. No. 4,714,623 so that the hollow micron-sized spheres or 
beads stick to the wet epoxy circuit pattern and are engulfed into the wet 
circuit pattern until the pattern is saturated with hollow micron-sized 
spheres or beads. Thirdly, the pattern is allowed to cure then it is 
lightly abraded to produce the voids, nooks, crannies and undercuts which 
serve as anchor sites for the sprayed metal. The surface is cleaned to 
remove any surface debris and, fourthly, the surface is then thermally 
sprayed with copper or other conductive metal. 
In a third process, which somewhat resembles the second process or method 
and partakes of the process claimed in our U.S. Pat. No. 4,751,113. After 
silk screening the resin in the desired circuit pattern, and while the 
resin is still "wet" or uncured, it is thermally sprayed with copper and 
then the resin is cured. This third method is more dependent on the curing 
rate of the resin and various parameters such as potlife which determine 
the appropriate time to spray. Also, the gun distance must be such that 
the pattern is not distorted by the impingement of the molten particles. 
Additionally, the type resin used is a factor i.e. whether it's a uv 
ultraviolet (uv curing resin, a resin that cures only with heat) or a 
resin that cures with a combination of ultraviolet and heat. 
All the above methods of thermally spraying an electronic circuit board 
depend on a non-porous smooth resin or substrate surface that prevents the 
sprayed particles from adhering accept where the pattern is established. 
The overspray is easily collectible and can be reprocessed for respraying 
whereas acids and other solutions used in the fabrication of other circuit 
boards are practically totally lost and present problems in storage and 
disposal. 
Considering the large number of steps in fabricating conventional printed 
circuit boards (as is disclosed in the article referenced above), thermal 
spraying according to the present invention is a very economical process 
especially for surface mount boards. Thermal spraying may now be 
competitive with very fine lines or double sided boards which require 
plating through holes. In cases where plating through holes is required, 
inserts or tubular eyelets can be placed into the holes and crimped, and 
the conductive metal sprayed circuit pattern makes the connection between 
circuits on opposite sides of the board. In a preferred embodiment, the 
sprayed deposits should remain or be 2 to 3 mils below the top surface of 
the board. If the spray deposit fills the groove and starts to become 
flush with the surface, then the sprayed particles may spread from the 
edge of the pattern and thereby create rough edges. This may be 
objectionable for closely spaced combination lines as well for aesthetic 
or cosmetic reasons.

DETAILED DESCRIPTION OF THE INVENTION 
In the preferred embodiment of the invention illustrated in FIG. 1a 
substrate 10, which may be a conventional fiber reinforced phenolic board, 
is coated with a mixture of resin (preferably an epoxy resin) and 
non-conductive inorganic hollow micron-sized spheres or beads 12 by 
injection molding, by lamination or any other composite technology 
process. For example, a layer of resin, filled with the hollow 
micron-sized spheres or beads can be screened, colandered or otherwise 
applied in a smooth layer to the surface 10S of board 10. 
Alternatively, a layer of resin can be applied to board 10. Prior to curing 
of the layer of resin, the hollow micron-sized spheres or beads may be 
sprayed upon the surface of uncured resin (in the manner disclosed in our 
U.S. Pat. No. 4,714,623) to thereby form one or more layers of hollow 
micron-sized spheres or beads or a mixture of hollow micron-sized spheres 
or beads and resin may be applied to the substrate surface 10s and then 
cured. 
As an alternative, a molded or cast or syntactic foam 10mf in which the 
voids have migrated to the surface 10mfs forms the surface to which the 
conductor material is ultimately adhered. Substrate 10 with the adhering 
hollow micron-sized sphere or bead layered 12 in the next phase has a 
dielectric film 13 applied to the surface 12 either by spraying from a 
sprayer 14 or applied by an adhesive 15 so as to be conformal to all 
points of the surface 12. 
In the next phase illustrated, the circuit pattern is etched through the 
dielectric film or conformal coating 13 and into the hollow micron-sized 
sphere or bead layer 12. In the preferred embodiment the circuit pattern 
16 is etched into the surface by a laser 17 which is controlled by a 
microproccessor 18 using multi-axis programmable tooling or controller 
18t. It will be appreciated that the laser can be stationary and the 
circuit board manipulated both in x-y axis as well as z axis. In other 
words, for maximum efficiency of the laser etching, the laser beam should 
be maintained perpendicular to the surface being etched. Thus, in 
connection with the three dimensional circuit boards shown in FIG. 1b, the 
circuit board and/or the laser can be manipulated so as to maintain the 
surfaces perpendicular to the surface being etched. 
As shown in FIG. 1a the balloon enlargement of an etched pattern, the laser 
beam burns a groove through the dielectric film and ruptures the bond coat 
hollow micron-sized sphere or bead filled resin layer 12 to form groove 
20. It will be appreciated that circuit connection pads 21 can be formed 
in an area larger than the width of the groove for connection to external 
circuitry or for receiving the pins of board mounted circuit components 
such as integrated circuit chips, resisters, capacitors, coils, 
transformers and the like. Instead of the preferred laser etching, the 
circuit pattern can be formed by engraving or other mechanical means such 
as high speed machining or abrasion. In any case, the etching, abrading, 
engraving, etc. process serves the function of fracturing the hollow 
micron-sized spheres or beads directly under the surface in the circuit 
pattern desired to thereby form undercuts, nooks, and crannies which are 
illustrated in cross section in the balloon enlargement of phase three of 
FIG. 1a. The fractured surfaces are cleaned ultrasonically or with high 
velocity liquid spray to remove surface debris and than it is dried. 
In phase 4, the engraved surface is thermally sprayed with copper (or other 
conductive metal or material which can include super-conductor substances 
which thus become an easy way to form these materials as wire conductors 
in any desired circuit pattern, on 3-D surfaces, etc.) to establish a 
conductive path for electronic circuitry. Typically, the paths are 
elongated pathways which are essentially grooves formed in the surface of 
the fractured hollow micron-sized spheres or beads. The top surface of the 
circuit board e.g. the dielectric film or layer 13 in this embodiment or 
the resin forming the hardened resin encapsulating the hollow micron-sized 
spheres or beads. Normally, the fine metal particles can be easily scraped 
off the surface because of the non porous nature of the resin surface and 
hence do not effect the integrity of the circuitry, the circuit preferably 
being slightly below or flush with the surface. For very fine conductive 
lines, thermal spray apparatus using finely powdered copper is preferred 
since this forms smaller molten particles or droplets which fill the 
groove and hollow micron-sized spheres or beads. The thickness of the 
conductor elements can be adjusted to accommodate for the current carrying 
capacity of the individual lines. In cases where the circuitry is known to 
be high current carrying circuitry a few more passes with the thermal 
spray makes the conductors slightly thicker e.g. a larger conductor area 
thereby carrying a larger current. 
As noted above, the thermal spraying of the electronic circuit board 
depends on a smooth surface that prevents the sprayed particles from 
adhering except where the pattern of fractured or ruptured hollow 
micro-sized spheres or beads is established. The overspray is easily 
collectible and can be reprocessed for respraying which, in contrast to 
conventional PC board manufacturer where acids and other solutions are 
used in the fabrication of printed circuit boards, such materials are 
practically total losses and present problems of disposal and compliance 
with EPA restrictions. 
Considering the large number of steps in conventional fabrication of 
printed circuit boards, as is reflected in the article referenced above 
from the Lindsey text, the present invention utilizing thermal spraying 
offers a very economical process especially for surface mount boards. 
Where a double sided board is required, and holes are bored either using 
the laser or other hole boring techniques, instead of plating the holes, a 
tubular conductive insert such as copper rivets can be placed the hole and 
crimped into place so that when the conductor patters are sprayed in the 
laser etched grooves or conductor pattern, the spray contacts the tubular 
inserts to form the through connection. 
In the preferred embodiment, the metal sprayed deposit should be two to 
three mils below the top surface of the board. If the spray deposit fills 
the groove and starts to become flush with the surface then the spray 
particles spread from the edge of the pattern and thereby can create rough 
edges which may be objectional for closely spaced lines as well as for 
cosmetic or aesthetic reasons. For these reason, it is preferred that the 
groove be deep enough so that the conductor filling the groove is 
sufficient to perform the function desired and yet be two to three mils 
below the top surface of the board as shown on the balloon enlargement 
coupled to phase 4 of the diagrammatic block diagram of FIG. 1a. 
A three dimensional circuit board is shown in FIG. 1b and, the processing 
steps are essentially the same. In the phase three and phase four process 
steps, the circuit pattern is etched by either manipulating the circuit 
board so that the surface facing the laser is perpendicular thereto or the 
laser is manipulated for this purpose. Both the laser axis and the work 
piece can be simultaneously manipulated, if desired. Similarly, the 
thermal spray apparatus is preferably aligned perpendicular to the groove 
and the surface because when the circuit board the groove therein is held 
at any angle to the thermal spray, there is differential angular buildup 
of the metal on one side or the other side of the groove. Hence, the 
thermal spray should be perpendicular to the surface of the circuit 
pattern. However, if preferential build up on one side of the laser etched 
grove is desired, that can be accomplished by adjusting the relationship 
of the axis of the spray apparatus to the axis of the groove. Referring to 
the molded or cast structural foam in which the voids which have migrated 
to the surface illustrated in 1A as 10mf, this can be syntactic foam resin 
coating described in our U.S. Pat. Nos. 4,521,475 and 4,618,504. The 
hollow micron-sized spheres or beads are likewise described in detailed in 
these reference patents. The copper is preferably high conductivity copper 
and depending upon the application including the width and depth of the 
conductor lines to be formed, the copper can be in the form of wires or 
powders which are supplied to the thermal spray gun apparatus. 
Referring now to the embodiment shown in FIG. 2, the PC board substrate 30 
has the upper smooth surface thereof 30ss cleaned and a silkscreen 31sc 
having the circuit pattern 32 formed therein as a conventional silkscreen 
forming process applied to the surface 30ss and then the screen pattern of 
33 is screened or squeegeed through the silkscreen 31scm. While the resin 
circuit pattern 33 is stilled uncured, the hollow micron-sized spheres or 
beads 35 are dusted from a supply 36 onto the uncured circuit pattern 33 
and then the dusted pattern is cured. This is somewhat similar to the 
process disclosed in our U.S. Pat. No. 4,714,623. After the hollow 
micron-sized sphere or beads filled resin circuit pattern 33 is cured, the 
hollow micron-sized spheres or beads are fractured and/or ruptured by 
fracturing or abrading wheel 37 and the remains are removed. 
Thereafter, a thermal spray gun 38 sprays molten copper on the surface. The 
copper particles preferentially adhered to the fractured hollow 
micron-sized sphere or bead pattern by bonding onto the voids, undercuts, 
and nooks, and crannies forming anchor sites for the copper. This 
overspray is easily collectible for reprocessing or respraying purposes as 
described above in connection with the embodiment described in connection 
with FIG. 1a. In a preferred embodiment the resin in an epoxy and the 
dusting of the surface of the hollow micron-sized spheres or beads is such 
that the hollow micron-sized spheres or beads stick to the wet circuit 
pattern and are engulfed into the wet pattern until the wet pattern is 
saturated (by `wet` we mean `uncured`). FIG. 3 illustrates a third 
technique which is similar to the method disclosed in our U.S. Pat. No. 
4,751,113. In this case, the circuit pattern is screened onto the circuit 
board 45 and the uncured uv resin 46 is in the pattern of the circuit to 
be formed. Instead of using hollow micron-sized spheres or beads, the 
surface is exposed to ultraviolet followed by a thermal spray of copper 
into the partially cured resin. The ultraviolet lamps 47 can be used in 
conjuction with thermal spray gun 38 or separately. As indicated in phase 
three, additional copper is sprayed to build to a predetermined thickness 
if a higher current capacity is required. This last method is more 
dependent on the curing rate of the resin and various parameters such as 
potlife, which determine the appropriate time to spray. In addition, the 
gun--to work--distance must be considered since if it is too close, the 
circuit pattern may be distorted. Additionally, the type of resin used is 
a factor, i.e. a UV(ultraviolet) curing resin or a resin that cures with 
heat only or a resin that cures with a combination of heat and 
ultraviolet. The Henkel Corporation, ITEC, Dymax have developed two part 
systems. In the Henkel system, for example, called Duocure (.TM.) IPM 
(Inter Penetrating Polymer Network) a two part system of alphatic 
polyisocynate and a second component of an alphatic poloyl dissolved in a 
UV curable multifunctional acrylate. After adding a photo-initiator, the 
acrylate portion can be cured first using radiation then the cure of the 
polyurethane is completed using thermal curing. Both radiation and heat 
can be applied simultaneously. The ratios of acrylate and polyurethane can 
be tailored to a specific application. 
While the invention have been shown and described with reference to a 
preferred practice of the invention, it would be understood that this 
disclosure is for purposes of illustration and various omissions and 
changes may be made thereto without departing from the spirit and the 
scope of the invention as set forth in the claims appended hereto.