A negative-acting photoimageable composition comprises PA1 A) between about 30 and about 90 wt % of a binder polymer which is soluble in an appropriate developer, PA1 B) between about 5 and about 30 wt % of an epoxy resin, PA1 C) between about 0 and about 30 wt % of a polyol cross-linking agent for the epoxy resin B), and PA1 D) between about 5 and about 15 wt % of a photosensitive composition of the formula: ##STR1## where the Zs are the samne or different and are selected from CX.sub.3, X, and H; the X's are the same or different halogens, and R is any chemical moiety consistant with the photoimageable composition as a whole.

The present invention is directed to a negative-acting photoimageable 
dielectric composition which may be used, for example, to form the 
permanent innerlayers of a multilayer printed circuit board, and to a 
method for forming such a board. It is also directed to a novel method for 
manufacturing multilayer printed circuit boards by the selective plating 
of the dielectric layers, eliminating the need for the standard copper 
foil inner layers, and the photodefinition of vias, thereby eliminating 
the need for drilled holes in most instances. The invention is also 
directed to the use of the photoimageable dielectric composition as a 
solder mask. 
BACKGROUND OF THE INVENTION 
Multilayer printed circuit boards have traditionally comprised a stack of 
individual printed circuit boards or innerlayers separated by dielectrical 
material. The circuitry of the several innerlayers is electrically 
connected by bored and plated through-holes. Multilayer printed circuit 
boards provide circuitry in a three-dimensional array and are therefor 
advantageously space-saving, relative to individual printed circuit 
boards, which provide at most two layers of circuitry on a two-sided 
board. 
These printed circuit boards are commonly provided with internal ground and 
power planes. These internal planes are frequently solid sheets of copper 
interrupted only by clearance holes (the perforations required for 
electrically isolating the through-hole pattern of the printed circuit 
board). Ground and power planes provide power voltage and current and 
ground connections for the components of the multilayer printed circuit. A 
second function of the ground and power planes is to provide 
electromagnetic shielding for the multilayer printed circuit board and 
reduce the electromagnetic and radio frequency interference. Multiple 
ground and power planes and additional ground planes on the surface layers 
with the conductive pattern are common. 
Multilayer circuits enable formation of multiple circuits in minimal 
volume. They typically comprise a stack of layers with layers of signal 
lines (conductors) separated from each other by dielectric layers having 
plated through-holes known as vias providing electrical interconnections 
between the layers. 
Current processes for fabricating multilayer boards are extensions of 
methods used for fabricating double-sided boards. The method comprises 
fabricating of separate innerlayers having circuit patterns disposed over 
their surface. A photosensitive material is coated over the copper 
surfaces of a copper clad innerlayer material, imaged, developed, and 
etched to form a conductor pattern in the copper cladding protected by the 
photosensitive coating. After etching, the photosensitive coating is 
stripped from the copper, leaving the circuit pattern on the surface of 
the base material. Following formation of the innerlayers, a multilayer 
stack is formed by preparing a lay up of innerlayers, ground plane layers, 
power plane layers, etc., typically separated from each other by a 
dielectric prepeg (a layer consisting of glass cloth impregnated with 
partially cured material, typically a B-stage epoxy resin). The outer 
layers of the stack comprise copper-clad, glass-filled epoxy board 
material with the copper cladding comprising exterior surfaces of the 
stack. The stack is laminated to form a monolithic structure using heat 
and pressure to fully cure the B-stage resin. 
Interconnections or through-holes, buried vias, and blind hole 
interconnections are used to connect circuit layers within a multilayer 
board. The buried vias are plated through-holes connecting two sides of an 
innerlayer. Blind vias typically pass through one surface of the stack and 
pass into and stop within the stack. Regardless of the form of 
interconnections, holes are generally drilled at appropriate locations 
through the stack, catalyzed by contact with a plating catalyst and 
metallized, typically with electroless copper that is overplated with 
electrolytic copper, to provide electrical contact between circuit 
innerlayers. 
The uses, advantages and fabricating techniques for the manufacture of 
multilayer boards are described by Coombs, Printed Circuits Handbook, 
McGraw Hill Book Company, New York, 2nd edition, pp. 20-3 to 23-19, 1979, 
incorporated herein by reference. 
Multilayer boards have become increasingly complex. For example, boards for 
main frame computers may have as many as 36 layers of circuitry, with the 
complete stack having a thickness of about 1/4 inch. These boards are 
typically designed with 4 mil wide signal lines and 12 mil diameter vias 
for interconnections between signal line layers. For increased 
densification, it is desired to reduce signal lines to a width of 2 mils 
or less and vias to a diameter of 2 to 5 mils or less. 
The photoimageable dielectric coatings for printed circuit boards on the 
leading edge of the technology must be capable of being processed in a 
minimum number of steps. Their dielectric and photolithographic 
properties, flexibility, and intercoat adhesion also must be excellent. A 
low photospeed, high moisture resistance, and good adhesion to a plated 
metal are also important properties of such coatings. 
This invention is directed to negative-acting photosensitive dielectric 
compositions. The processing of negative-acting photoresists generally 
follows the sequence of applying a solution of the resist to a copper foil 
laminated to an epoxy resin base, drying and baking the resist to expel 
the solvent, exposing the resist to actinic radiation through a patterned 
photomask to define an image, dissolving the non-exposed portions of the 
resist in a developer, such as an alkaline aqueous developer to delineate 
the irnage, rinsing, and in some instances post-baking the imaged resist. 
The areas void of resist are then either etched, plated with metal, or 
filled with a conducting polymer. As discussed above, the use of copper 
foil is eliminated by the method of this invention. 
This invention's currently preferred embodiments are closely akin to the 
invention described in U.S. Ser. No. 08/801,682 filed 18 Feb. 1997. The 
invention described therein is directed to a positive-acting 
photoimageable composition which has a similar function to the 
photoimageable composition of the present invention. The positive-imaging 
composition in that application contains a hydroxy-functional novalac 
resin, a cross-linkable resin, such as an epoxy resin, a naphthoquinone 
diazide, and at least one thermally labile halogen-containing cure 
catalyst. The novalac resin is normally soluble in alkaline aqueous 
solution, such as a sodium hydroxide solution. However, the naphthoquinone 
diazide acts to inhibit the novalac resin from being dissolved until the 
naphthoquinone diazide is exposed to actinic radiation; whereupon, the 
naphthoquinone diazide undergoes a rearrangement such that it facilitates 
the dissolving of the novalac resin in alkaline aqueous solution. The 
cross-linkable resin, particularly as catalyzed by the cure catalyst, 
undergoes a post development cure that renders the layer hard and 
permanent. 
While the above-identified application details advantage of positive-acting 
photoresists, there are also advantages to negative-acting resists. 
Negative-acting resists tend to have faster photospeeds. Also, in 
formulations of the type discussed in the above-identified application and 
in the instant application, there tends to be out-gassing in 
positive-acting resists, whereas negative-acting resists of the type 
described herein do not exhibit outgassing. 
It is an object of this invention to provide a novel method for 
manufacturing multilayer printed circuit boards by the selective plating 
of the dielectric layers, eliminating the need for the standard copper 
foil inner layers. 
It is another related object of this invention to provide a 
negative-acting, photoimageable dielectric composition whose post-develop 
image is highly stable, both chemically and thermally. 
It is still another object of this invention to provide a multilayer 
printed circuit board having permanent innerlayers made of a 
negative-acting photoimaged dielectric composition. 
It is yet another object of this invention to provide a method for making a 
multilayered printed circuit board by which the vias are photodefined. 
These and other objects of the invention which will become apparent from 
the following description of the invention. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided a negative-acting 
photoimageable composition comprising 
A) between about 30 and about 79 wt % of a binder polymer which is soluble 
in an appropriate developer, 
B) between about 5 and about 30 wt % of an epoxy resin, 
C) between 0 and about 30 wt %, preferably at least about 5 wt %, of a 
cross-linking agent for the epoxy resin B), and 
D) between about 1 and about 15 wt %, preferably at least 5 wt %, of a 
photosensitive composition of the formula: 
##STR2## 
where the Zs are the same or different and are selected from CX.sub.3, X, 
and H; the X's are the same or different halogens, and R is any chemical 
moiety consistant with the photoimageable composition as a whole; 
the weight percentages being based on total weight of A) plus B) plus C) 
plus D). Preferably both Zs are CX.sub.3. Preferably all Xs are Cl. 
Preferably, the binder polymer is developable in alkaline aqueous solution, 
and most preferably the resin is a novalac resin having sufficient 
hydroxyl functionality to be developed in alkaline aqueous solution. 
DETAILED DESCRIPTION OF THE INVENTION 
To better understand the invention, the following definitions have been 
adopted: 
Photoimageable dielectric coating means an organic dielectric coating 
composition capable of imaging by exposure to activating radiation and 
development to yield a relief image and become an integral part of a 
multilayer board. It may be applied as a liquid coating composition and 
dried to a tack-free coating or as a dry film. Preferably, the dielectric 
constant of the coating does not exceed 4.5. 
Imaged opening means a relief image of (1) recesses or channels defining a 
pattern of conductors or (2) openings for interconnections, within a 
dielectric coating. Imaged openings are subsequently selectively 
metallized, whereby metal is contained within the recesses of the relief 
image. 
Substantially means largely but not wholly that which is being specified so 
that the difference is inconsequential. 
Unless otherwise noted, the total of compositions A) plus B) plus C) plus 
D) are considered to comprise 100 parts by weight. Weight percentages of 
each at A), B), C), and D) are also expressed relative to this total, and 
other minor components are expressed as weight percentages relative to 
this total. 
In accordance with the preferred aspect of the invention, all, or the major 
portion, of the binder A) for the photoimageable dielectric composition of 
this invention is a novalac resin. The novalac resin imparts alkaline 
aqueous solubility to the composition due to the hydroxyl functionality of 
the resin. The hydroxyl functionality of the novalac resin may also react 
with the epoxy resin(s) B) during the photochemical reaction which occurs 
during exposure to actinic radiation and during any thermal crosslinking 
composition which might be effected subsequent to exposure and 
development. 
Novalac resins are commonly known products of the acid catalyzed 
condensation of a phenol and an aldehyde such as formaldehyde, 
acetaldehyde, and furfural. The term phenol is used herein to mean phenols 
as a class and includes alkylphenols such as cresol, the xylenols, and 
butylated phenolic novalacs. Suitably, the amount of the novalac resin 
binder may be from about 30 to about 90% by weight of the total 
composition, but it is preferred to use from about 40 to 70%, of the 
novalac in the photoimageable dielectric coating composition of this 
invention. The resins are widely available from many chemical suppliers. 
Herein, the epoxy resin B) and the cross-linking agent C), undergo, in the 
presence of the cure catalyst D), a photo-initiated cross-linking reaction 
which renders exposed portions of the photoimageable composition insoluble 
to alkaline aqueous solution. If not fully reacted during exposure, 
further cross-linking is effected by heat, such that the composition may 
undergo a post-development hardening. If the binder is the preferred 
binder, i.e., a novalac resin, or any other hydroxy-functional binder 
polymer, the hydroxyl functionality of the binder A) may also take part in 
both the photo-initiated cross-linking reaction and in any subsequent 
thermal cure. 
The amount of epoxy resin used in the composition of the invention may be 
from about 5 to about 30%, but it is preferably from about 10 to about % 
by weight. Epoxy resins in general are suitable for purposes of this 
invention. Preferably, the resin has an epoxy equivalent weight of between 
about 70 and about 4000, more preferably between about 100 and about 500. 
A particularly suitable epoxy resin for use in the present invention is 
trimethyol propane triglycidyl ether such as that sold as Heloxy.RTM. 48. 
Epoxy novalacs are another example of suitable epoxy resins. Still another 
suitable epoxy resin is epoxidized polybutadiene sold by Elf Atochem under 
the trademark POLY BD 605 and having a viscosity of 2500 cps @ 25.degree. 
C. and an epoxy equivalent weight of 260. 
The preferred cross-linker C) for the epoxy resin B) is a compound having 
multi-hydroxyl functionality, i.e., a polyol. Substantially any polyol may 
be used including multi-functional monomers, hydroxyl-functional 
polyesters, hydroxyl functional polyethers, etc. Preferred polyols for 
cross-linking the epoxy resin B) have --OH equivalent weights of between 
about 70 and about 4000, more preferably between about 100 and about 1000. 
In a preferred photoimageable dielectric coating composition of this 
invention, a butylated phenolic novalac is used as the cross-linking 
polyol. The butylated phenolic novalac imparts flexibility to the cured 
photoimageable dielectric coating of this invention. An example of a 
butylated novalac cross-linker is sold as Santolinko.RTM. EP-560. 
OH-functional binders and OH-functional cross-linking agents are generally 
stable in the presence of epoxy resins; however, upon activation, by 
actinic radiation, of the photo-sensitive compound, the --OH groups of the 
cross-linker C) (and any --OH groups on the binder A)) react with the 
epoxy resin B), producing a cross-linked structure that is insoluble in 
alkaline aqueous solution, such as NaOH solutions. Photosensitive 
compositions D) of the type used herein, have been used previously as 
catalysts for effecting thermal-cure of epoxies. In fact, above-noted U.S. 
application Ser. No. 08/801,682 uses catalysts within the scope of 
photosensitive compositions to effect post-development hardening of the 
positive-acting photoimageable compositions described therein. Herein, it 
is found, that these thermal catalysts are also photosensitive, and when 
used in sufficient amounts, i.e., about 5 wt % and upward, will promote 
photo-initiated cross-linking of epoxy resins. It is believed that these 
compounds, when exposed to actinic radiation, give off halogen ions which 
promote the cross-linking of --OH and epoxy groups. The R group in the 
generalized formula above may be any group, even H, which is otherwise 
compatible with the composition as a whole, but preferably is an 
unsaturated group, such as an aromatic or substituted aromatic group or a 
styryl group. Different R groups may be used to according to the 
absorbance maximum, whereby the absorbance maximum may be tailored to the 
source of actinic radiation. Some specific examples of photosensitive 
compounds useful in the present invention include, but are not limited to: 
2,4-trichloromethyl (4-methoxyphenyl) 6-triazine, CAS no. 3584-23-4; 
2,4-trichloromethyl (4-methoxynaphthyl) 6-triazine, CAS no. 69432-40-2; 
2,4-trichloromethyl (piperonyl) 6-triazine, CAS no. 71255-78-2; and 
2,4-trichloromethyl (4-methoxystyryl) 6-triazine, CAS no. 42573-57-9. 
Mixtures of these compounds may also be used. Of course, these compounds 
also act in their usual function as thermal cure catalysts. Generally, 
only a portion of the photosensitive compound is depleted during the 
photoinitiated reaction, and residual photosensitive compound is generally 
available to promote a post-development thermal hardening. Of course, 
other thermal cure catalysts may be used in conjunction with the 
photosensitive compound D), but these are not necessary, and, therefore, 
not preferred. 
Cross-linkers C) as alternatives to polyols include glycoluril, 
benzoquanamine, melamine and urea. 
Fillers may be used in amounts of from about 2 to about 10% by weight in 
order to control the flow of resins therein while the composition is being 
cured at elevated temperatures. Fumed silica such as that sold as 
CAB-O-SIL.RTM. and SYLOIDO.RTM. is an example of a useful filler. The 
CAB-O-SILO.RTM. M-5 silica is particularly useful. 
A leveling agent such as those sold as MODAFLOW.RTM. in the amount of from 
about 0.2 to about 3% by weight is also useful. A flexibilizing agent such 
as a poly(vinylmethyl ether) sold under as LUTANOL.RTM. M is particularly 
useful in the preparation of a dry film of this invention. An effective 
amount of the flexibilizing agent is from about 10 to about 20 percent by 
weight. 
The dielectric constant of the photoimageable dielectric coating 
composition of this invention preferably is not greater than 4.5 and more 
preferably is not greater than 3.5. The resolution is preferably 
sufficient to provide line widths of 10 mils or less, more preferably 5 
mils or less, and still more preferably about 2 mils or less. 
The photoimageable dielectric coating composition of this invention may be 
applied to a substrate as a liquid, then dried, imaged by exposure to UV 
light and development, and cured. Or it may be cast as a dry film for 
storage and subsequent lamination onto a substrate for imaging and curing. 
The liquid coating composition may be coated onto the surface of a 
substrate in a variety of ways, including screening, roller coating, 
curtain coating, and spray coating. A water-miscible solvent such as 
propylene glycol methyl ether acetate may be used in amounts necessary to 
adjust the viscosity of the photoimageable dielectric coating compositions 
of this invention to suit the coating method and coating thickness 
desired. The dielectric substrate for the printed circuit board use may 
be, for example, a glass-epoxy construction or a polyimide. The coating is 
tack-dried at about 90.degree. C. for about 30 minutes and then irradiated 
through a mask in an image pattern by ultraviolet light having a wave 
length in the range of from 350 to 450 nm. The total dosage of UV light is 
from 100 to 800 mj/cm.sub.2. The exposed coating is then developed in a 
0.17 to 0.3 normal aqueous solution of sodium hydroxide at from 80.degree. 
to 100.degree. F. (27.degree.-38.degree. C.), rinsed, and cured at a 
temperature of from 140.degree. to 175.degree. C. for about one hour. 
Dry film may be made by drawing down the liquid coating composition with a 
Baker bar at a setting of from about 4 to about 20 and drying it in a 
convection oven or tunnel dryer for about 2 to about 60 minutes at from 
about 35.degree. to about 105.degree. C. to obtain films ranging from 
about 0.5 mil to about 3 mils thick. The dry film may then be laminated 
onto a dielectric substrate, such as a polyimide fllm or an epoxy resin 
impregnated glass fiber board either at room temperature or at an elevated 
temperature, e.g., 180.degree. F. (82.degree. C.). A Hot Roll 
(DYNACHEM.RTM. Model 300 or 360) laminator may be used at a speed of 1 to 
5 feet per minute, a roll pressure of 40 to 60 psi (0.28-0.41 MPa), and a 
roll temperature of 225.degree. to 300.degree. F. (113.degree.-150.degree. 
l C.). Vacuum laminators, such as models 724 and 730 sold by Morton 
International, Inc. may be used, also. In conventional vacuum lamination, 
in addition to heat and vacuum, mechanical pressure is brought to bear 
against the dry film. In what is known as a "slap down" process, a rubber 
blanket is used to press the dry film against the substrate. During vacuum 
lamination, the photoimageable dielectric dry film is heated to a board 
surface temperature of 55.degree.-90.degree. C. with a cycle time of 30-90 
seconds and a slap down cycle of 4-12 seconds. A post lamination bake may 
last for about 30 minutes at about 90.degree. C. (194.degree. F.) but it 
may be eliminated under certain conditions. The laminate is then 
irradiated through a mask in an image pattern by ultraviolet light and 
developed as described above. 
High resolution relief images including openings that are approximately 
equal to the thickness of the coating may thus be achieved. By use of such 
coatings, imaged openings for interconnections and conductors can be of a 
size equivalent to the resolution capability of the dielectric coating and 
the method of imaging and may be in any desired shape. 
Selective metal deposition in the imaged openings may be performed in a 
conventional manner in the process of this invention. It is characterized 
by the selective metallization of the relief image of the dielectric 
coating without an increase in the surface resistivity of an underlying 
substrate between conductor lines. Plating can occur only on those areas 
where the resist has been removed (additive production of the circuits), 
or on the entire surface including areas where the resist was removed by 
development as well as upon the resist itself (subtractive production of 
the circuits) or some degree of plating falling between these extremes 
(semi-additive production of the circuits). For a discussion of the 
specifics of circuitry and interconnect creation see U.S. Pat. No. 
4,847,114 (Brach et al), the teachings of which are incorporated by 
reference. 
If the additive process is chosen, then plating will occur in those areas 
where the photoimageable dielectric coating has been removed by 
development and upon the surface of the dielectric in a defined manner 
such that defined circuitry and interconnects are created. Thus, the 
plating itself will define the circuitry and other features desired. In 
the additive process, the photoimaging of the permanent dielectric will 
create and define the circuitry and other surface features desired as well 
as the holes and vias which will interconnect the various layers of the 
circuitry package. 
If the substrate to be coated is a circuit, the process may comprise 
formation of a dielectric coating over a circuit with imaged openings 
defining interconnections. The walls of the imaged openings in the 
dielectric coatings contain metal as it deposits during plating and 
assures a desired cross sectional shape of the deposit. The process is 
repeated sequentially to form sequential layers of circuits and 
interconnections. 
In the subtractive procedure, the entire surface will be plated. The 
circuitry and other features will be defined by subsequent etching of the 
plated metal. In the subtractive manner, the photodefinition of the 
permanent resist will typically be used for the creation of holes and vias 
which connect the various layers of the circuitry package, thus 
eliminating the need for drilling holes. 
The plating solution is typically an electroless copper plating solution 
well known in the art and typically comprises a source of cupric ions, a 
complexing agent to hold the ions in solution, a reducing agent to reduce 
the cupric ions to metallic copper in the presence of the catalyst-e.g., 
formaldehyde and a pH adjustor. Typical copper plating solutions are 
disclosed in U.S. Pat. Nos. 4,834,796; 4,814,009; 4,684,440; and 
4,548,644, the teachings of each of which are incorporated herein by 
reference. 
The invention is described above primarily in terms of a composition having 
a hydroxyl-functional binder polymer, such as a novalac resin. However, 
other binder systems could be used, including a binder which is not 
soluble in alkaline aqueous solution but which is soluble in suitable 
organic developing solvents. A carboxylic acid functional binder polymer 
may be used, in which case, the composition may be developed in weaker 
bases, such as sodium bicarbonate solutions. However, whereas 
OH-functional binders have long-term stability in the presence of the 
epoxy resins, carboxylic acids react more readily with epoxy groups and 
therefore would have to be provided as a two-part composition with the 
parts admixed at the point of application. 
The coating composition and process of this invention are further described 
in the following examples, which are not limiting in any way. All parts 
are by weight unless otherwise indicated and all components are 100% 
solids unless otherwise indicated.