Non-flammable and strippable plating resist and method of using same

This invention is directed to a non-flammable plating resist with improved adhesion and edge covering characteristics which permits its selective removal by an ordinary laser ablation process or an energy efficient laser assisted process, and to the preferred method in using same to selectively plate a metal substrate, such as a nickel plated, electrically conductive metal with a precious metal. The preferred method comprises the steps of applying a thin uniform layer of such resist onto the substrate and drying the same in situ to a thickness greater than 100 microinches and less than 500 microinches, and drying same in situ, where such resist (a) is resistant to deterioration by chemical plating solutions of such precious metal, (b) is readily strippable in alkaline solutions, and (c) has a flashpoint in excess of 100.degree. F. A preferred formulation for such resist comprises, by weight, the following: ______________________________________ Styrene acrylic co-polymer 61.5% suspension Butyl cellosolve 3.1% Butyl carbitol 7.4% Water, essentially the balance. ______________________________________ Thereafter, selected areas of said resist coated substrate are subjected to a controlled excimer laser pulse to remove such resist within said areas by an ablation process or a "patch blow off" energy efficient process, followed by plating the exposed areas of said substrate with a precious metal, and stripping said resist from the remaining portions of said metal substrate.

RELATED APPLICATIONS 
This invention represents an improvement on co-pending application, Ser. 
No. 232,357 directed primarily to a waterborne plating resist. More 
particularly, this invention relates to a plating resist which is more 
effective by virtue of its improved handling property of a flashpoint in 
excess of 100.degree. F., when practicing the invention in co-pending 
application, Ser. No. 180,417, where selective plating is achieved by the 
technique of laser ablation of the resist in certain areas, followed by 
plating the exposed areas of the substrate. 
FIELD OF THE INVENTION 
This invention is directed to an improved plating resist, and to the method 
which includes the selective removal thereof for subsequent plating of the 
underlying substrate by a precious metal. 
BACKGROUND OF THE INVENTION 
The present invention relates to a method which allows for the selective 
plating of a metal substrate, such as an electrical contact. The selective 
plating thereof, such as by plating with a precious metal, is achieved 
herein by first applying to all surfaces of such contact a thin coating of 
a sprayable styrene-acrylic co-polymer resist, drying the resist, 
preferably followed by the selective removal thereof by a technique called 
laser ablation, precious metal plating of such selective areas, and 
removal of the remaining resist from such contact. However, the plating 
resist of this invention is also applicable to other plating methods such 
as by the use of a mask or other means for selective plating as known in 
the art. 
A preferred embodiment of this invention is the selective plating of 
electrical terminals. Typically, such terminals are stamped and formed 
from metal strip and are attached to a carrier strip which is useful for 
strip feeding the terminals through successive manufacturing operations. 
One necessary manufacturing operation involves plating; i.e., 
electroplating, the electrical contact surfaces of the strip fed terminals 
with precious metal or semi-precious metal, such as gold or alloys 
thereof. Such metals are characterized by good electrical conductivity and 
little or no formation of oxides that reduce said conductivity. Therefore 
these metals, when applied as plating, will improve conductivity of the 
terminals. However, the high cost of these metals has necessitated 
precision deposition onto the contact surfaces of the terminals, and not 
on surfaces of the terminals on which plating is not necessary. By being 
able to initially control the selective resist removal procedure, one can 
ultimately control and limit the application of the precious metals. 
Ablation is defined as the process of removal of a part such as by melting 
or vaporization. The laser is the mechanism by which one may achieve the 
selective melting or vaporization. By the use of different lasers, 
particularly ones utilizing a broadly differing wavelength, the general 
process of laser ablation is affected. For example, by the use of an 
excimer laser, operating in the U.V. range, coupled with a resist 
essentially transparent to the wavelength of such laser, and a metal 
substrate from which the resist is to be removed and which absorbs such 
wavelength, as taught in co-pending application, Ser. No. 180,417, a 
different laser assisted material removal process is observed. The latter 
process, which is more energy efficient, may be termed "patch blowoff." 
That is, rather than melting or vaporizing the resist, an interesting 
phenomena occurs at the resist/substrate interface resulting in the 
overlying resist being blown off, essentially as solid particles. Thus, 
while laser ablation has been broadly used to define any process where a 
laser is used to assist in a material removal process, it will be 
appreciated that the certain parameters applied will render the various 
approaches quite distinctive. 
Returning now to the broad concept, it can be acknowledged that selective 
removal of a resist may be accomplished by the technique known as laser 
ablation. Reports have appeared in the literature regarding attempts at 
laser ablation of polymer coatings on metals, and regarding methods of 
multi-shot removal of polymer coatings on non-metals. R. Srinivasan et al, 
in the JAP 59, 3862 (2986) and JVST, B1, 923 (1983) describe, for example, 
the use of excimer laser wavelengths which are strongly absorbed directly 
in the polymer itself to achieve removal of polymer by chemical 
bond-breaking or heating to vaporization, or a combination of both. 
However, the authors found that polymer ablation occurs when the laser 
light is absorbed within about the first 0.2 micron or less of the polymer 
surface. Then only that polymer material within the characteristic 
absorption depth was removed In order to remove a thicker polymer film, 
such as is necessary for most electroplating requirements, multiple laser 
shots would be required. The use of multiple shots is much less desirable 
than single shot removal One problem associated with the method of 
Srinivasan et al, wherein the laser light is directly absorbed in the 
polymer, is that choosing a laser wavelength too strongly absorbed in the 
polymer necessarily implies a small absorption depth and small thickness 
removed. On the other hand, choosing a wavelength too weakly absorbed in 
the polymer precludes depositing sufficient energy per unit volume of 
polymer to achieve ablation. The compromise value between these extremes 
dictates that no more than about 0.3 micron per pulse can be removed in 
the best case. Cole et al, in Mat. Res. Soc. Symp. Proc. 72, 241 (1986), 
concur with Srinivasan et al in this finding The above process represents 
the current state of the art on excimer laser ablation of polymers. 
In U.S. Pat. No. 4,671,848 to Miller et al, a method for the removal of a 
dielectric coating from a conductor, by means of a focused, high energy 
radiation source, is taught. More particularly, in said method a laser 
source is focused to a point above the coating which results in a plasma 
ionized region being formed. As a consequence, the coating is removed in a 
preselected region on the underlying conductor. In other words, the laser 
ablation depends on absorption of laser light by ionized air or other 
plasma and transmission to the dielectric. A difficulty of this method is 
the ability to control and adjust the air breakdown so as to ensure there 
is no damage to the conductor, i.e. underlying substrate, and to achieve 
removal of the residual layer. Another difficulty is that only a small 
area corresponding to the tight focus region can be removed on each shot. 
Miller et al state that extended areas are to be ablated by multiple shots 
while moving the workpiece, or the laser focus. 
Notwithstanding such prior art teachings, co-pending application, Ser. No. 
180,417 discovered, among other features, a more energy efficient laser 
process alternative to complete ablation of the material being removed. A 
single excimer laser shot which is not appreciably absorbed by the coating 
material penetrates through the entire thickness of the coating and causes 
heating of the metal substrate surface. The heated metal substrate causes 
vaporization of a thin layer of the coating material next to the metal, 
thereby destroying the bond of the coating to the metal and providing 
expanding gasses which cause the blowoff of a patch of coating material 
covering the total area of the exposed region. The entire thickness of the 
coating in the patterned exposure area is thereby removed with only a 
single laser shot with the utilization of only a fraction of the energy 
required to volatilize the entire volume of the material so removed. 
Thus, an important recognition to come from the work of such co-pending 
application was the criticality in correlating the operating parameters 
and characteristics of the laser with the properties and characteristics 
of the plating resist and the properties and characteristics of the 
underlying metal substrate. It is known from such prior work that the 
resist must be sufficiently transparent to allow most of the laser 
radiation to pass through without appreciably decomposing the coating so 
as to provide heating of the metal surface in the intended ablation patch 
area. 
However, it has now been discovered that additional attributes are 
necessary. Through continued investigation, the present invention has 
found that in order for a resist coating to be removed by the above 
mentioned energy efficient process, it must have additional attributes 
such that the resist coating will: 
a) soften enough when heated by the laser heated metal substrate surface to 
allow stretch tearing at the patch peripheral boundary between heated and 
non-heated material to insure patch removal; 
b) not soften so much when heated by the laser heated metal substrate 
surface so that "balloon burst" failure occurs, i.e. rupture tear failure 
of the center of the ballooning heated patch first, thus relieving blister 
pressure and allowing the patch edges to fall back into the intended 
ablation patch area; 
c) adhere strongly to the metal substrate so that the resulting resist 
sidewall bordering the removed areas maintains a sharply defined profile 
with no undercutting or breaking thereof; 
d) provide excellent edge coverage; and 
e) possess a high flash point, i.e. outside flammable category. 
as well as preserve the previously established requirements that the resist 
coating will: 
a) be sufficiently transparent to allow most of the laser radiation to pass 
through the coating so as to provide heating of the metal surface in the 
intended ablation patch area; 
b) gasify enough when in contact with the laser heating metal substrate 
surface to provide an expulsion means for the patch being removed; 
c) adhere strongly enough to protect the covered surface during plating 
operations; 
d) not deteriorate in the plating bath chemical environment; 
e) be strippable after the plating operation is completed; and 
f) be able to be applied by spraying or printing. 
As a result of the improved formulation, the plating resist hereof provides 
for a superior performance, the manner of which will become apparent from 
a reading of the specification which follows. 
SUMMARY OF THE INVENTION 
This invention is directed to a plating resist which permits its selective 
removal by a laser ablation technique or an energy efficient laser 
assisted "patch blowoff" process that exhibits improved properties with 
regard to: 
a) handling characteristics by being classified as non-flammable, 
b) edge coverage of object being resist coated, 
c) adhesion to the substrate, and 
d) maintenance of resist edgewall profile integrity along removed patch 
boundaries; 
and to the preferred method in using same to selectively plate a metal 
substrate, such as a nickel plated, electrically conductive metal. The 
method comprises the steps of spraying or printing a thin uniform layer of 
said resist onto said substrate and drying same in situ to a thickness of 
greater than 100 microinches and less than 500 microinches, where said 
resist is resistant to chemical plating solutions, is readily strippable 
in alkaline solutions, and has a flashpoint in excess of 100.degree. F. A 
preferred formulation for such resist comprises, by weight, the following: 
______________________________________ 
Styrene acrylic co-polymer 
61.5% 
suspension 
Butyl cellosolve 3.1% 
Butyl carbitol 7.4% 
Water, essentially the balance. 
______________________________________ 
Thereafter, selected areas of said resist coated substrate are subjected to 
a controlled excimer laser pulse to remove the resist within said areas, 
following by plating the exposed areas of said substrate with a precious 
metal, and stripping said resist from the remaining portions of said metal 
substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
This invention, in its preferred embodiment, is directed to an improved 
formulation for a plating resist, and to a method of using same in the 
selective plating of a metal substrate, such as an electrical contact or 
continuous strip of stamped and formed contacts 
A significant characteristic of such improved formulation is that it may be 
defined as non-flammable; this is by virtue of its "flashpoint" being 
above 100.degree. F. This is a significant development as OSHA regulations 
establish handling or use requirements that are particularly stringent for 
"flammable" materials, i.e. those materials having a "flashpoint" below 
100.degree. F. It will be appreciated that for a commercial operation 
using "flammable" materials, considerable attention, time and effort must 
be given to various safety precautions and equipment. However, by being 
able to shift from "flammable" to "combustible", the next higher 
classification, significant commercial advantages can be gained by the 
removal of certain operating restrictions. 
It will be understood that in the selective plating of electrical contacts, 
for example, the major cost, and hence the area for potential savings, 
lies in the precious metal deposition step. The entire operation is 
motivated by the need to improve techniques so as to be able to minimize 
gold use, by way of example, and to limit the deposition thereof to only 
the contact areas. 
It has been discovered that the improved formulation for the resist hereof 
possesses a higher molecular weight than other known polymer resists, 
which may account in part for its better adhesion to the metal substrate, 
and to its superior edge coverage. In any case, this means that gold 
deposition on substrate edges is essentially eliminated. Further, by 
virtue of its improved adhesion to the metal substrate, the resist profile 
through the laser processed areas reveals a sharply defined sidewall with 
no undercutting or breaking thereof. That is, until time for the stripping 
takes place, the integrity of the resist is maintained. These improved 
characteristics are achieved with a preferred resist formulation 
comprising, by weight, the following: 
______________________________________ 
Styrene acrylic co-polymer 
61.5% 
suspension* 
Butyl cellosolve 3.1% 
Butyl carbitol 7.5% 
Water, essentially the balance. 
______________________________________ 
*a reaction mixture of about 90% of Jonacryl56 and about 10% of 
Jonacryl585, produced by S. C. Johnson Company, Racine, Wisconsin 
It is contemplated that variations may be made to this formulation; 
however, an increase or decrease of as little as 4% in the amount of 
Jonacryl-56 in the reaction mixture may be damaging. Nonetheless, 
variations are acceptable to the extent that the "flashpoint" thereof is 
not significantly lowered to below about 100.degree. F. 
The unique features of the plating resist of this invention can best be 
appreciated by reviewing the preferred process of using same. 
Specifically, such resist must be capable (I) of being selectively removed 
by focusing a laser beam thereon, (2) resistant to the chemical plating 
environment wherein plating of the ablated areas takes place, and (3) 
readily removed after the chemical plating of the ablated areas is 
complete by an alkaline solution as known in the art. The resist of this 
invention possesses these qualities as well as the further qualities noted 
above. 
In any case, with the plating resist in place and suitably dried on the 
metal substrate, selected areas thereof may be subjected preferably to an 
excimer laser pulse or pulses. Excimer lasers for example, form a group of 
pulsed high-pressure gas lasers which emit various ultraviolet 
wavelengths, depending on the lasing medium, such as 193 nm, 248 nm, 308 
nm and 351 nm. 
It is important to the success of the method hereof that the proper laser 
and wavelength range be coordinated with both the metal substrate and the 
plating resist. Since it is necessary that the laser energy be absorbed 
mostly in the resist for the traditional ablation process method, and 
mostly in the substrate for the energy efficient "patch blowoff" process, 
as preferred herein. The absorbing tendencies of the resist vary with 
wavelength, so a suitable wavelength must be selected for the process 
intended. In the energy efficient "patch blow-off" method, the plating 
resist of this invention must possess a low optical absorption 
coefficient, e.g. with absorption coefficient preferably no greater than 
about 1000cm-1 for a 150 microinch coating thickness, so as to transmit 
the laser energy therethrough. 
The theory supporting the proposition that the laser energy must be 
absorbed at the metal substrate is set forth below. It is theorized that 
most of the laser energy goes through the resist to the substrate, 
preferably a nickel plated substrate. There, about 55% is absorbed in 
roughly 30 nm of metal. For an energy density below about 1.8 J/cm.sup.2, 
the nickel evidently remains below its melting point (1455.degree. C.), 
but nevertheless transfers enough energy to a thin adjoining layer of 
resist to vaporize it. This simultaneously destroys the bonding layer 
holding the resist to the nickel, and supplies the expanding vapor needed 
to drive the remaining solid resist away from the surface. For higher 
energy densities, the bonding layer is likewise destroyed. However, a 
superficial layer of nickel melts resulting in a reflowing of the nickel 
over the substrate. Thus, depending on what is to be achieved with the 
underlying nickel substrate, there is latitude with regard to the laser 
energy densities. For single shot resist removal, energy densities of at 
least about 0.90 J/cm.sup.2 are required Between about 0.90 and 2.5 
J/cm.sup.2, little change to the surface of the nickel substrate is 
expected. At a higher energy density (3.0 J/cm.sup.2), or multiple shots, 
polishing of the nickel substrate may be expected And, with such polishing 
the quality of the plating, by reducing substrate porosity, will be 
observed. To review, an energy density level of at least about 0.90 
J/cm.sup.2 is needed to achieve single shot removal of the plating resist 
of 150 microinches thickness. However, to affect the surface 
characteristics of the plated substrate, higher energy densities, on the 
order of about 2.5 J/cm.sup.2, or higher, are required. 
With the selected areas suitably removed by one of the methods described 
above, the resist coated metal substrate may be subjected to metal 
plating. Since, as noted above, the plating step is a chemical process 
which can attack the resist coating, resistance to such attack is 
important. The plating resist of this invention is particularly resistant 
to such attack by either of the conventional plating techniques, i.e. 
immersion plating and electrolytic plating. Since immersion plating is 
very sensitive to surface contaminants, such as oxides or organic 
contaminants, a desirable treatment is to first immerse the resist coated 
and laser processed substrate in an activation solution to remove surface 
oxides and contaminants prior to plating, such as with gold. Thereafter, 
such substrate can be immersion plated in a Technic Oromerse "N", a gold 
plating solution, for 90 seconds at about 80.degree. C., and removed. 
Electrolytic plating, another plating method, may be used to obtain a thick 
hardened gold plating. To clean and activate the surface, an activation 
bath, such as described above is recommended. In such cleaned conditions, 
the substrates can then be electroplated in a bath having the following 
parameters: 
______________________________________ 
* gold concentration 
2 tr. oz./gal 
* nickel concentration 
2100 ppm 
* pH 4.6-4.8 
* temperature 120.degree. F. 
* efficiency @ 40 ASF 
40 50% 
and 120.degree. F. 
______________________________________ 
With such plating parameters, a gold deposit thickness of 30 microinches 
can be achieved in a dwell time of from 45 to 60 seconds. By either 
practice, suitable gold plating may be achieved on the laser processed 
areas. 
The final step is the removal or stripping of the remaining plating resist 
from the metal substrate by a cathodic removal process. In a preferred 
practice, this is accomplished by passing said resist coated metal 
substrate through an alkaline solution, such as sodium hydroxide, 
maintained at a pH of about 9.5, a temperature of about 140.degree. F., a 
voltage of about 2 volts, for about one minute, and drying the resist-free 
metal substrate.