Method of improving the adhesion of electroless metal deposits employing colloidal copper activator

The present invention relates to a method for promoting and improving the adhesion of an electroless metal deposit to the metal surface of a composite substrate having both a conductive metal area and an activated non-conductive surface. The process comprises treating such a substrate, subsequent to catalyzation or activation of the substrate, and prior to electroless metal deposition thereon, with an adhesion promotor compound or mixture of compounds.

BACKGROUND OF THE INVENTION 
It has been commercially desirable for many years to have plastic, glass, 
or other like non-conductive substrates provided with a metal coating or 
plating on its surface either as a continuous coat or as a patterned or 
discontinuous coating or plating. In addition, numerous related 
applications exist in regards to providing a metal coating or plating to 
composite substrates having both a conductive metal portion and a 
nonconductive portion, usually plastic. Such composite substrates are 
commonly comprised of a plastic sheet having a thin metal foil, usually 
copper, laminated or clad to the two sides of the plastic sheet leaving 
the non-conductive plastic sandwiched between two metal surfaces. Holes 
are usually drilled through the metal clad and the plastic, exposing the 
plastic where the holes are drilled. These composite substrates, after 
being electroplated, are used to produce printed circuit boards for 
electrical or electronic applications. 
So-called printed circuit boards are produced by variations of two basic 
systems, one of which is referred to as the additive system and the other 
the subtractive system. The details of both are well known but, briefly, 
in the additive system the starting composition is comprised of plastic 
with no metal foil, and the metal circuit is then built up upon the 
non-conductive substrate in the desired pattern. In the subtractive method 
a non-conductive substrate, such as epoxy bonded fiberglass, has adhered 
to two sides thereof a metal cladding or laminate, most often copper. 
Holes are drilled through the copper laminate board exposing the plastic. 
It is then deburred, chemically cleaned and rinsed. The board is then 
treated with a dilute solution of hydrochloric acid, dipped into a 
catalyst, mostly commonly a palladium-tin catalyst, to activate the 
plastic for electroless deposits, rinsed in water, treated with an 
accelerator (generally fluoroborate based) to remove the tin compound, 
again rinsed and immersed into an electroless plating bath to electrically 
connect the two metal (copper) sides by plating the inside of the holes as 
well as the exposed sides and edges of the board. A plating resist is then 
applied in the circuit pattern desired. The board is then cleaned, 
electroplated with copper followed by solder, the resist removed with a 
solvent to expose the copper that is covered and this copper is removed by 
etching, thereby providing the desired circuit. 
In all of such applications, the non-conductive portions of the substrate 
must be activated since neither electroless metal plating or electro metal 
plating can be carried out on the non-conductive portions of the substrate 
in its absence. The activation is followed by an electroless metal plating 
of a sufficient nature so that it will carry a current and permit 
subsequent electroplating. 
It is, of course, not commercially feasible to treat such composite 
substrates to activate or catalyze only the non-conductive portions and as 
a result, the entire composite substrate is immersed or dipped into an 
activating solution or colloid. In this manner of processing such 
composite substrates, the non-conductive portions are not only activated, 
but the conductive or metal portions thereof are also contacted with the 
activating solutions or colloids. Unfortunately, the activating solutions 
coming into contact with the metal conductive portions of the composite 
substrate may contaminate the metal portions and this contamination 
seriously interferes with the bond of the subsequent electroless metal 
deposit which also deposits on the metal portions. 
THE PRIOR ART 
Most present commercial activating systems rely upon one or more of the 
noble metals (Au, Ag, Pt, Pd, Ir, Rh, Ru, and Os) particularly Pd, Pt, Ag, 
Au, and most commonly palladium. For example, one of the earliest methods 
of activating such substrates involved a two-step operation involving 
first immersion of the substrate in a stannous chloride solution followed 
by a second immersion in an acid palladium chloride solution. Subsequently 
a one-step process has been employed commercially, involving a colloidal 
dispersion of palladium and tin chloride salts as disclosed in U.S. Pat. 
No. 3,011,920 to Shipley. Still another one-step process is disclosed in 
U.S. Pat. No. 3,672,923 to Zeblisky which also utilizes noble metals, 
particularly palladium. One typical example involving the metal plating of 
plastics such as acrylonitrile--butadiene--styrene copolymers (ABS) 
involves the steps of first cleaning the plastic article in a strong 
alkali bath followed by etching the article in a chemical etching bath, 
frequently a chrome etch, which serves to enhance adherence of the metal 
coatings to the surface. Following the etching step the article is rinsed 
in water and dipped in hydrochloric acid to neutralize the chrome, rinsed 
again, and then placed in the activating solution (frequently referred to 
as a catalyst, seeder or sensitizer and referred to herein as an 
activator). The most widely used activator is the colloidal dispersion of 
palladium and tin chloride in accordance with, for instance, the 
above-mentioned U.S. Pat. No. 3,011,920. After activating, the article is 
again rinsed and then placed briefly in an accelerator to remove the tin 
(which tends to interfere with adherence), rinsed again, and placed in the 
conventional electroless metal bath. The noble or non-noble metal of the 
activating solution, such as palladium, serves to catalyze or activate the 
non-conductive substrate for the subsequent electroless plating bath. 
After a few minutes in the electroless metal bath, the article will have a 
very thin coating of the selected metal of the bath thereon. It is then 
rinsed and the article may then be further plated with the same or another 
metal either by well known electroplating processes or by further 
electroless metal plating. 
The use of the colloidal palladium activation systems on composite 
substrates generally does not significantly interfere with the bonding of 
the subsequent electroless metal plating on the metal portions thereof to 
inhibit commercial production. However, on occasion, such bonding is 
inferior or poor and the process of this invention can advantageously be 
used in such systems to insure adequate and good bonding of the 
electroless metal deposits on the metal portions of the composite 
substrates. 
The use of colloidal dispersions of various metals, both noble and 
non-noble, in combination with or without additional agents to achieve 
catalyzation or activation of insulative substrates for subsequent 
electroless plating, is disclosed in many additional prior art patents, 
such as in U.S. Pat. No. 3,011,920 to Shipley, in U.S. Pat. No. 3,657,002 
to Kenney, in U.S. Pat. Nos. 3,783,005 and 3,950,570 also issued to 
Kenney, in U.S. Pat. No. 3,993,799 issued to Feldstein, and in U.S. Pat. 
No. 3,958,048 to Donovan. The problem of adhesion of an electroless metal 
deposit to the conductive metal portions of a composite substrate when a 
non-noble metal is used to activate the non-conductive portions of a 
composite substrate is more pronounced and the invention is more 
advantageously adaptable in this area and particularly where the activator 
is not a noble metal and the electroless metal bath contains copper. 
Although the non-noble metal, e.g. copper, activating colloids referred to 
in the above patents are metal oxide colloids and an oxide is deposited on 
to the substrate, it is believed that the ultimate activation of the 
non-conduction portion of the substrate is actually the metal per se as 
disclosed in the patents, since it is also believed that the oxides 
themselves will not cause activation. For example, in the copper type 
activating colloidal system disclosed, it is the resulting copper metal on 
the substrate which causes the activation permitting subsequent 
electroless metal plating thereon. 
SUMMARY OF THE INVENTION 
The present invention relates to a method for promoting and improving the 
adhesion of an electroless metal deposit to the metal portions of a metal 
clad non-conductive composite substrate which comprises treating such a 
composite substrate, subsequent to catalyzation or activation of the 
substrate and prior to electroless deposition thereon, with an adhesion 
promotor compound or mixture of compounds. The invention relates 
especially to the method for promoting and improving the adhesion of the 
electroless metal deposit to the metal portion of a composite substrate 
having non-conductive areas, such as the copper metal in the case of the 
subtractive method as applied to copper clad printed circuit boards. It 
will be understood, however, that the present invention is applicable to 
promoting and improving the adhesion of the electroless metal deposit to 
all metal clad non-conductive substrates which have been previously 
treated by a catalyst or activator to permit electroless metal deposition 
on the non-conductive portions thereof. 
DETAILED DESCRIPTION OF THE INVENTION 
The compounds which promote or improve the adhesion of the electroless 
metal deposits to the metal portion of a composite substrate having 
non-conductive areas, subsequent to catalyzation or activation of the 
substrate and prior to electroless metal deposition thereon, are hydrazine 
hydrate, ammonium persulfate, or an alkali hydroxide such as sodium 
hydroxide, or a suitable mixture of the foregoing. 
As noted above, the method of the present invention is carried out after 
the composite metal clad non-conductive substrate has been catalyzed or 
activated so as to permit a subsequent electroless metal deposit to occur 
upon the substrate. In the practice of this invention, it is desirable to 
first clean the composite non-conductive metal clade substrate in a manner 
which is well known to those skilled in the art. Thereafter, the composite 
substrate is catalyzed or activated as by treatment with a noble or a 
non-noble metal colloid or ionic solution as is described in, for 
instance, any of the above-discussed patents. After customary rinsing of 
the now activated non-conductive portions of the composite substrate, the 
same is then treated with a solution containing the above-listed compound 
or a suitable mixture of compounds. After again customary rinsing of the 
substrate which has been treated in accord with the present invention, the 
composite substrate is then immersed in an electroless bath under 
conditions which are also well known to those skilled in the art. After 
again rinsing these results a conducting substrate on which the 
electroless metal deposit is strongly, uniformly and permanently adhered 
not only to the non-conductive portions of the composite substrate but 
also to the metal portions thereof. 
The conditions under which the activated or catalyzed composite substrate 
is treated with the compounds according to the present invention are not 
critical and may be carried out by immersing the substrate in the 
solution. It has been found desirable, however, to limit the immersion 
time of the substrate in the solution containing the compounds for a 
period of time not to exceed about 5 minutes. In the case of hydrazine 
hydrate the time may preferably be from about 1-3 minutes; in the case of 
ammonium persulfate the time should ordinarily not exceed about 15 
seconds; and in the case of sodium hydroxide the time should ordinarily 
not exceed about 30 seconds. The exact times depend somewhat upon the 
concentrations of the respective compound or compounds in solution, as 
discussed below, and can be easily determined by one skilled in the art. 
If the immersion time is too short, good adhesion of the electroless metal 
deposit may not occur, and if the immersion time is too long, the 
activation of the non-conductive portions of the composite substrate may 
be unduly affected resulting in spotty electroless metal deposition on the 
activated non-conductive areas. 
As noted, the compounds may be dissolved in an aqueous solution although 
non-aqueous solution, such as alcohols can be used, so long as such 
non-aqueous solutions do not otherwise interfere or adversely affect 
either the previous catalyzation or the subsequent electroless deposition. 
The concentrations of the solutions containing the compound or compounds of 
the present invention are not especially critical. It has been found, 
however, that in the case of hydrazine hydrate it is desirable that the 
solution contain about 0.1% hydrazine hydrate by volume to saturation; in 
the case of ammumiun persulfate it is desirable that the solution contain 
about 0.5-10 grams of ammonium persulfate per liter of solution; and in 
the case of sodium hydroxide it is desirable that the solution contain 
about 0.1-5 grams of sodium hydroxide per liter of solution. It has been 
also found that if concentrations above those recited are employed, then 
the length of time during which the substrate is immersed or otherwise 
treated with the solution must be more carefully monitored and generally a 
somewhat shorter contact time than those listed above is necessary to 
prevent damage to the non-conductive areas. If solutions of lower 
concentrations are used, the immersion time should be correspondingly 
increased. 
Although the hydrazine hydrate can be used as it is or at a higher pH, it 
is advantageous to adjust the pH of the hydrazine hydrate solution to a pH 
of about 7, although, even a further reduction of the pH to about 5 will 
also be operative. Since hydrazine hydrate is weakly basic in solution, pH 
adjustment may usually be accomplished by the addition of an acid such as 
a 1% solution of phosphoric acid. Using the neutralized hydrazine permits 
greater latitute in immersion times. In the case of ammonium persulfate, 
the resulting pH is about 3-5 and with sodium hydroxide, the resulting pH 
is about 8-11. 
The temperature under which the treatment of the nonconductive substrate 
with the compound or compounds of this invention takes place is not 
critical and as a matter of convenience it is preferably carried out at 
room or ambient temperature conditions. 
As can be seen especially from the preferred times and the concentration of 
the solutions for the treatment of the composite substrate in accord with 
this invention, the controls and operating conditions required in the case 
of neutralized hydrazine hydrate are far less stringent and rigorous then 
with the other compounds. Hence, neutralized hydrazine hydrate is the 
preferred adhesion promoter. 
Electroless metal baths, particularly electroless copper baths are well 
known and generally any of these well known baths can be used for 
electroless deposition according to this invention. Although those skilled 
in the art may prefer certain electroless copper baths, applicant prefers 
electroless baths such as those disclosed in U.S. Pat. No. 3,361,580 to 
Schneble et al. 
As previously noted, the compounds of the invention are particularly 
advantageous for promoting and improving the adhesion of an electroless 
copper deposit to the copper portions of composite copper clad plastic 
boards to be used in the manufacture of printed circuit boards. These 
boards, as is well known, are generally composed of a resinous sheet such 
as epoxy-glass, phenolic glass, phenolic paper, etc. having two thin 
sheets of copper foil laminated or clad to both sides of the plastic and 
having appropriate holes drilled through both copper sheets and the 
plastic. The plastic exposed by the holes must be electro plated with 
metal to provide electrical continuity throughout the circuit board. Thus, 
the exposed plastic portions of the laminate must be activated for 
electroless metal plating, and the resulting electroless deposit must 
fully and permanently adhere not only to such exposed plastic surfaces but 
also to the metal portion of the board. 
As noted, it is a particularly novel feature of the present invention as it 
relates to catalyzed or activated printed composite boards that the 
compounds of the invention tend to promote and improve adhesion and 
uniformity of deposits of the subsequent electroless deposited metal to 
the metal portions of the composite boards. This is in clear distinction 
to the relatively poor bond that results between non-noble metal catalyzed 
or activated composite boards and the subsequent electroless metal deposit 
without the pretreatment in accord with this invention. It has been found 
that the application of the present invention not only increases adhesion 
of the electroless metal deposit to the metal portions of the substrates, 
but does not interfere, if properly applied, with the bonding and 
uniformity of electroless copper deposited to the activated non-conductive 
portions thereof. Not only does the present invention yield a strong bond, 
but the treatment in accord with this invention also brings about a 
uniform electroless metal deposit on the entire composite substrate which 
was found to be free of voids and to result in a coverage which is in all 
respects complete. 
The theory of the efficacy of the present invention is not clearly 
understood although it is theorized that compounds of the present 
invention tend to strip or remove a film from the metal clad portion of 
the substrate, which film may have been formed by the catalyzing or 
activating colloid and which may otherwise be responsible for a decrease 
in the adhesive characteristics of the subsequent electroless metal 
deposit vis-a-vis the metal portion of the composite substrate. 
The following examples are offered by way of illustration only.

EXAMPLE 1 
A 2% by volume solution of hydrazine hydrate was prepared and neutralized 
with phosphoric acid into which was immersed, for a period of about 2 
minutes, a composite substrate having both a copper clad portion and a 
non-conductive portion which had been activated by a copper type colloidal 
catalyst system resulting in the activation of the non-conductive portion 
by metallic copper. The temperature was about 70.degree.-80.degree. F. The 
substrate was then rinsed followed by electroless copper plating for a 
period of about 5-10 minutes at a temperature of about 
100.degree.-110.degree. F. After rinsing, acid dipping, rinsing and 
drying, there resulted an exceptionally uniform and strong bond to the 
metal portions of the substrate without effecting the electroless copper 
bond to the non-conductive portions. Electroless copper plating on the 
same composite substrate without the pretreatment with the hydrazine 
resulted in a good bond to the non-conductive portions thereof but a 
non-uniform and poorly adhered electroless copper deposits on the 
conductive copper portions of the substrate. 
Adhesion was determined by electroplating about 1 mil (25 microns) of 
coppeer onto the electroless copper layer, and the composite substrate was 
then mechanically destroyed in attempting to separate or peel the 
electroplate deposit. 
EXAMPLE 2 
Example 1 was repeated substituting a solution containing 7.5 gms of 
ammonium persulfate per liter of water for the hydrazine hydrate. The 
immersion time of the substrate was about 15 seconds. Substantially the 
same results were observed as set forth in Example 1. 
EXAMPLE 3 
Example 1 was repeated substituting a solution 0.5 gms of sodium hydroxide 
per liter of water for the hydrazine hydrate. The immersion time of the 
composite substrate was about 30 seconds. Again, substantially the same 
results were observed as set forth in Example 1.