High resolution liquid photopolymer coating patterns over irregular printed wiring board surface conductors

High resolution photoimaging with liquid photopolymers of pastelike consistency overlying irregular surface topography such as provided by conductors extending from the surface of a printed wiring board substrate is obtained without air entrapment or starvation of the superimposed liquid polymer. Thus, for example, solder mask coatings of high resolution may be prepared. This is achieved in two photosteps, the first of which is a partial polymerization obtained by radiating with the polymer surface exposed to air, thereby polymerizing the layer portion unexposed to air adjacent the substrate surface and leaving the air exposed surface in the liquid state. This step uses an image with a greater dimension of opaque pattern area than that used in the subsequent step, which obtains high resolution by surface contact of the image with the polymer. The radiation source in each case is non-collimated. The resulting product because of the partial polymerization covering the surface produces no starvation over the rough surface because of pressing the second image into surface contact. The liquid polymer layer covers rough surfaces without entrapment of air. By radiating when the surface is exposed to air, noise causing poor resolution is eliminated, such as from surface interface reflections and pinholes in opaque surfaces, etc. A layer of liquid photopolymer is disposed upon its permanent substrate cured through only part of its thickness adjacent the substrate to leave a liquid tacky outer surface.

TECHNICAL FIELD 
This invention relates to liquid photopolymer coatings on printed wiring 
boards, and more particularly it relates to method for assuring sufficient 
thickness of the polymer coatings over irregular surfaces with high 
resolution photo pattern reproduction on liquid polymers of pastelike 
consistency, and the products so obtained. 
BACKGROUND ART 
Liquid photopolymers of paste like consistency are in use for 
photopatterning coatings on printed wiring boards as evidenced by my U.S. 
Pat. No. 4,424,089, Jan. 3, 1984 for Photoprinting Process and Apparatus 
for Exposing Pasteconsistency Photopolymers. In that art the photopattern 
image bearing surface is pressed into contact with the polymer to produce 
high resolution coating patterns with non-collimated radiation of low 
intensity permissible because the polymer does not have its outer surface 
exposed to air. It is important to keep heat down and photospeed up in 
this manner. 
Liquid polymers are imaged in U.S. Pat. No. 4,436,806, F. J. Rendulic, et 
al., Mar. 13, 1984 with the outer surface exposed to air thus requiring 
greater exposure energy and generating considerable heat. Furthermore to 
obtain high resolution coating patterns, collimated radiation is required 
to eliminate the noise from scattering at the interfaces and particularly 
from the air gap interface. 
When the liquid polymer layers are superimposed over irregular surfaces, 
such as those encountered on printed wiring boards having conductor traces 
extending about 0.001 inch above the substrate surface, they have the 
significant advantage of covering the rough surface without entrapment of 
air, which would lower resolution and adversely affect curing and 
adhesion. 
However when the phototool with image, generally a flexible film layer, is 
pressed into place into intimate air free contact with the liquid polymer 
layer superimposed over the wiring traces, then the liquid polymer tends 
to be displaced from the wiring traces and thinned or starved. Thus, the 
following photopatterning step will leave a deficient thickness of 
covering over the wiring traces. When solder masking is required, for 
example, a layer of substantial thickness is required over the conductors 
for proper adhesion, insulation and resistance to the heat and stresses of 
soldering. This problem is addressed by this invention. 
Another problem introduced by the use of non-collimated radiation is the 
inconsistency with high resolution patterns. Thus surface interfaces, such 
as between a liquid polymer layer and a plastic film carrying the desired 
image pattern, can cause reflections of the radiant energy which 
introduces noise tending to polymerize the polymer in unwanted places. 
Thus in printed wiring boards undercutting may occur to cause reduced 
adhesion, short circuits, open circuits or the like. This has been 
resolved by using collimated radiation for photoexposure, which is 
expensive. 
It is therefore a general object of this invention to resolve the problems 
of the prior art as set forth hereinbefore ato obtain high resolution 
photopatterning of liquid polymers using uncollimated radiation. 
It is a further object of the invention to provide a method of producing 
superior products with high resolution photoimaged patterns produced with 
liquid photopolymers, with low photo energy and resulting heat while 
resolving problems of flow of the liquid polymer off of roughly contoured 
surface peaks, such as produced by conductor traces superimposed on the 
surface of a printed wiring board substrate. 
Another general object of the invention is to produce a cured polymer layer 
in its permanent substrate position which has a tacky surface for further 
processing. 
DISCLOSURE OF THE INVENTION 
This invention resolves the inconsistent properties and photoresponse 
characteristics presented when it is desired to use inexpensive liquid 
photopolymers and inexpensive noncollimated radiation of low energy level 
to produce high resolution photopatterns with assurance of adequate 
thickness of the liquid photopolymer superimposed on conductor traces or 
other surfaces protruding from the substrate surface of a printed wiring 
board, even when a phototool image is pressed into air excluding intimate 
contact with the liquid polymer. 
The property of the liquid photopolymers that polymerization is inhibited 
by air is employed in this invention to reduce stray noise effects from 
uncollimated radiation, so that high resolution images may be produced. 
Thus, considerably greater radiant energy is required to polymerize the 
liquid polymer when the surface is exposed to air. However this is 
inconsistent with the high resolution desired, where surface contact of 
the phototool carrying the image to be reproduced is required. To resolve 
this inconsistency the photopatterning process takes place partially with 
the phototool in surface contact and partly with the phototool separated 
by an air gap from the surface of the liquid polymer. 
In a first embodiment two sequential photopolymerization steps respectively 
are made through separate photo images, with an air exposed polymer 
surface in the first step and the image in contact with the polymer 
surface in the second step. The first and second images are related by 
having the opaque portions of different areas. The first photo step with 
the air gap is then taken with the larger area opaque pattern image with 
enough radiant energy to polymerize only the portion of the polymer layer 
thickness adjacent the substrate carrying the layer and therefore not 
exposed to air. This substrate, typically a printed wiring board with 
superimposed wiring traces extending from the substrate surface, then has 
the polymer layer polymerized to a depth covering the wiring traces with 
sufficient thickness, typically 0.001 inch, to assure a good solder mask 
coat for example. Then a liquid unpolymerized outer surface layer permits 
the intimate air free lamination of the second image into surface contact, 
so that about the same amount of radiation energy will polymerize the 
polymer layer throughout its depth. Therefore with the final imaging step 
through the desired pattern with less opaque image area the high 
resolution final image is prepared. The liquid polymer in all cases has 
pastelike consistency when laid down and merged with the image carrying 
phototool by squeegee pressure as taught in my U.S. Pat. No. 4,424,089. 
The advantage of this embodiment is the assurance that the squeegee 
pressure does not displace or starve the liquid polymer layer over the 
edges of the wiring traces, but rather assures a minimum acceptable 
thickness of polymerized polymer over such surface irregularities. 
In a second embodiment the partial surface contact is achieved with a 
contoured phototool surface with the transparent portions of the image in 
surface contact with the polymer and the opaque portions separated by an 
air gap from the polymer surface. Thus any stray radiation resulting from 
the non-collimated source at those areas where polymerization is not 
wanted is inhibited by the air-inhibiting characteristic of the polymer. 
This has the advantage of a single radiating step and a single phototool 
image. 
More generally the invention is directed to a partial curing of a liquid 
photopolymer through its thickness with a desired pattern in its permanent 
substrate position to leave the outer surface tacky for further 
processing. The further processing may be typically a higher resolution 
photocure through the full layer thickness. Thus a liquid photopolymer 
layer in permanent place on its substrate is photocured through a selected 
opaque phototool pattern shape in two successive radiation steps, with 
differing areas imaged, the first radiation step producing an intermediate 
product condition and the second radiation step producing a permanent 
product condition. The second step, typically is one of high resolution 
obtained by contact printing of an image in surface contact with a tacky 
liquid state surface portion of the polymer layer. 
Further objects features and advantages of the invention will be found 
throughout the following description, drawing and claims.

THE PREFERRED EMBODIMENTS 
As may be seen from FIG. 1 non-collimated radiation, indicated by arrows 
15, is directed through a phototool transparency 16 having opaque image 
forming patterns 17 thereon for selectively blocking out the radiation. 
For purposes of this invention non-collimated radiation is such radiation 
that significant energy is contained in rays that are non-parallel, 
thereby tending to cause reflections at interface surfaces and 
particularly air-to-plastic surfaces, which have enough misdirected energy 
to cause undesirable curing of the polymers. 
The image pattern typically is used for photoimaging solder mask coatings 
on the printed wiring board 18 to cover wiring traces 19 and leave through 
hole connection pads 20 free to receive solder. A photopolymer layer 21 is 
superimposed over the surface of the wiring board 18, covering both the 
insulation panel substrate 22 surface and the wiring 19 and other surface 
irregularities superimposed thereupon. The lightly dotted stippling 
indicates the polymer in liquid paste consistency form and the heavy 
stippling the polymer in polymerized form as converted when subjected to 
radiation through the transparency 16. 
Polymers as used in this invention may be either (1) those with free 
radical photo-initiators which react substantially instantly to radiation 
for polymerization, and thus are preferred for low energy fast response, 
or (2) cationic curing substances which gradually convert some ingredients 
to acids during radiation for acid curing of the remaining ingredients. 
An air gap 23 is disposed between the phototool 16 and the surface of the 
polymer 21 by appropriate mounting of the substrate and the phototool. The 
properties of the liquid photopolymer selected cause its polymerization in 
response to radiation to be inhibited in the presence of air. Thus the 
radiation 15 passing through the transparent regions of the phototool 16 
will polymerize the darkly stippled underneath portions 25 of the polymer 
layer disposed on the substrate surface and thus not exposed to air, while 
the surface exposed to air remains in liquid form. The radiation energy 
can be controlled to control the thickness of the liquid surface and the 
polymerized lower portion. The conductor trace 19 is thus covered with a 
hardened polymerized layer of the photopolymer. Typically, the conductor 
19 extends about 0.001 inch from the substrate surface, the liquid polymer 
layer is about 0.003 inch thick and is polymerized about 0.002 inch to 
leave a liquid polymer depth of about 0.001 inch. This assures a minimum 
acceptable solder mask covering of at least 0.001 inch thickness. 
Otherwise had the phototool 16 been pressed, such as by squeegee action, 
into contact with the liquid polymer surface the thickness over the 
conductor would tend to be starved, particularly at the corners of the 
conductor trace 19, by being drawn into the gaps between the traces, etc. 
Other surface irregularities can cause the same effect. 
The foregoing step gives very poor resolution for several reasons. The 
non-collimated radiation 15 with the air gap 23 will cause a spreading of 
the pattern from the desired area exactly under the transparent regions of 
the phototool image. Also reflections from the surface interfaces will 
cause noise that tends to disturb high resolution. reproduction, although 
it is significantly inhibited by the air surface exposure of the liquid 
polymer. In addition the phototool has oversized opaque regions 17, which 
do not correspond with the eventual desired high resolution pattern. 
Note that this process is also particularly advantageous for tenting over 
holes 28 in the printed wiring board substrate. In the printed wiring 
board arts, particularly where solder mask coatings are desired, the 
hardened polymer surfaces inhibit about 85% of the surface area, leaving 
about 15% with the polymer removed, from those areas generally defined in 
negative type polymers by about 15% opaque constituency of a 
phototransparency image used in the radiating process step. 
Accordingly the second radiation step as illustrated by FIG. 2 is provided 
by this invention starts with a significant portion of the polymer area 
cured. The printed wiring board 18 following the preceding exposure step 
of FIG. 1 is now overlaminated with a clear plastic thin film layer 30 by 
means of a squeegee or the like. This may tend to squeeze a little of the 
liquid form polymer into the through holes as at 31. However the broad 
base polymerized regions 25 of FIG. 1 support the squeegeeing step with 
little rearrangement of the liquid polymer state outer surface layer and 
certainly provides a further thickness of polymer over the conductor 
traces to be shielded and insulated in the solder bath process of solder 
mask coating embodiments. 
The thin plastic film transparent layer 30 can serve as a phototool by 
having the final image pattern opaque regions carried thereby. However if 
this gives registration problems, a superimposed phototool 33 may be used 
wherein the opaque regions 34 are in close proximity with the liquid 
polymer surface in order to assure high resolution final reproduction. The 
surface may have some irregularities, exaggerated in the drawing, but 
since the air is excluded from the polymer surface it is more sensitive to 
radiation and the final polymerization step can take place with about the 
same radiated energy as before used to simplify equipment cost. Thus, this 
is a `contact` printing step that gives high resolution faithfully 
reproducing the image borne by phototool 33. 
In this step the finally polymerized regions 35 are defined by the opaque 
image portions 36 of lesser area than those of FIG. 1. Thereby any poor 
resolution reproduction from the off contact printing step of FIG. 1 is 
simply covered to increase the extent of fully polymerized surface area 
and the unpolymerized liquid state polymer 37 remaining is simply washed 
out to form the solder pad regions 40 in the final product shown in FIG. 
3. Therein the covering thickness of hardened polymer 41 over the 
conductor trace 19 is that obtained in FIG. 1 with the additional 
thickness, typically 0.001 inch, of the liquid layer, now polymerized. 
In the embodiment of FIG. 4, only a single radiation exposure step is 
required. Here again the resolution is increased by using the photopolymer 
property that polymerization is inhibited in the presence of air. There 
can be noise introduced for example by pinholes in the phototool image 
opaque regions, in addition to the reflections, etc. hereinbefore 
described when inexpensive non-collimated radiation is employed. In this 
embodiment to correct for this, a relief surface on the contact face of 
the phototool 50 is provided with the transparent regions 51 in contact 
with the liquid polymer 21 and an air gap S2 disposed between the opaque 
regions 54 and the surface of the polymer 21. 
In this way the polymerized regions 55 result from the contact printing 
step due to the radiation 15, while the polymer remains in liquid state 
beneath the opaque regions 54. Because of the air gap 52 much greater 
radiation power would be needed to make the stray radiation from the 
noncollimated radiation source effective for polymerization causing noise. 
Thus high resolution photopatterning is effected with a single radiation 
step while improving that resolution obtainable with full contact of an 
entire phototool surface. 
In particular this invention provides for the partial curing through the 
thickness of a liquid polymer layer in residence in permanent position on 
a substrate to leave a tacky liquid outer surface for further processing 
and a cured hard patterned underlying layer that may cover wiring patterns 
or irregular surfaces with a known thickness of cured and hardened 
polymer. This permits a first radiation step of controlled radiation 
energy to cure only from the desired depth down to the substrate. This is 
less critical when an air sensitive polymer is used with an air surface 
during exposure. 
Excellent resolution may be obtained by contact printing on the tacky 
surface with a final photopattern after a first non-contact air interposed 
first printing step in an approximate or other preliminary pattern.