Solution for stripping a layer of tin or tin-lead alloy from a substrate by means of a spraying operation

The invention relates to a chemical solution capable of selectively stripping a layer of tin or tin-lead alloy from a substrate, generally of copper, by means of a spraying operation, without requiring the use of any specially designed machine, inasmuch as the solution does not exhibit any particular aggressiveness in respect of the metal materials of which the spraying machines are usually made, and is nevertheless able to selectively strip and maintain in solution the tin or the tin-lead alloy, without etching the copper substrate. The solution comprises mainly an acid of organic nature appertaining to the group including the class of the alkylsulfonic acids having a number of carbon atoms from 1 to 4, such as in particular the methanesulfonic acid. The solution also contains an aromatic nitro-substituted compound including a solubilizing group, which is adapted to oxidize the tin-lead alloy and to facilitate the etching action of the organic acid. A further component of the solution is formed by thiourea or alkyl-substituted or aryl-substituted thioureas, whose function consists in preventing the tin from being deposited again onto the copper substrate.

BACKGROUND OF THE INVENTION 
The present invention relates to a solution for selectively stripping a 
layer of tin or tin-lead alloy from a substrate, which solution is 
particularly suitable for a spraying operation. 
The necessity of providing a solution capable of selectively stripping a 
layer of tin or tin-lead alloy from a substrate, generally of copper, by 
means of a spraying operation, originates from an evolution of the printed 
circuits manufacturing technique. In the currently used technique the 
stripping of the alloy from the copper substrate is carried out only in 
the proximity of the connectors, whilst with the new technique of leveling 
in hot air it is necessary to carry out a complete stripping of the 
tin-lead alloy, whose only function is to provide during the etching stage 
a protective coating, from the entire copper surface. With such technique 
it would be highly desirable to carry out the stripping of the alloy by 
way of a spraying operation, inasmuch as the productivity would be a 
greatly increased and the necessity of manual operations would be reduced. 
However, with the baths presently available in commerce, it is not possible 
to carry out this operation with the normal spraying machines, because 
these baths are highly corrosive in respect to the metal materials of 
which the machines are usually made, and therefore to use these baths it 
is necessary to convert the machines or to design new machines specially 
conceived for this specific purpose. In fact, the metallic components 
(recirculation pump, nozzles etc.) of the spraying machines available at 
present are generally made of titanium, a metal which resists very well to 
the ammoniacal ambient of the alkaline etchers normally used with such 
machines, but is quickly and strongly attached by the inorganic acids, 
such as chromic acid, nitric acid, and particularly fluoboric acid and its 
derivatives, as well as by other components, such as the peroxide 
solutions, which are contained in the formulations of the baths presently 
available in commerce for the stripping of layers of tin and tin-lead 
alloys. 
BRIEF SUMMARY OF THE INVENTION 
The object of this invention is to provide a solution capable of 
selectively stripping the layer of tin or tin-lead alloy from a substrate 
of copper or of any other metallic or non-metallic material, by means of a 
spraying operation, by using the normal etching machines whose metallic 
part is generally made of titanium. 
This object is attained according to the invention by using acids of 
organic nature which appertain to a group having the peculiar 
characteristic of not being aggressive in respect of titanium, i.e. 
alkylsulfonic acids having a low number of carbon atoms, and the present 
invention generally contemplates the use of acids having a number of 
carbon atoms from 1 to 4. A most suitable acid for practical application 
has proved to be methanesulfonic acid. These acids have the property of 
being able to form very soluble salts of tin and lead, and, consequently, 
to solubilize a large quantity of tin-lead, thus ensuring a good yield. 
To selectively strip the tin or the tin-lead alloy from the copper 
substrate it is necessary to combine the action of these organic acids 
with that of a mild oxidizing agent which is inert in respect of the base 
laminate of copper and at the same time is not aggressive in respect of 
the titanium. This result is obtained by the use of aromatic nitro- and 
polynitro-derivatives made soluble by the introduction of hydrophilic 
groups of the type --SO.sub.3 H, --COOH and their salts. 
A third component of the bath is represented by thiourea or its aliphatic 
or aromatic derivatives having a low molecular weight, whose specific 
function is to inhibit any redeposition of the tin onto the copper 
substrate, which redeposition otherwise could take place just during the 
operation of etching the alloy. 
The active components of the solution according to the invention may be 
present therein in very variable concentrations, depending on the desired 
rate of attack, on the quantity of tin-lead which it is desirable to 
maintain in solution, etc. In particular, the organic acid may be present 
with concentrations ranging from 1 g/l to saturation, but the optimal 
range is from 30 g/l to 120 g/l. The concentration of the aromatic 
nitro-derivative containing a solubilizing group may vary from 0.5 g/l to 
saturation, but the best results are obtained when this concentration is 
comprised in the range from 20 to 70 g/l. The thiourea derivative may be 
present in an amount ranging from 0.1 to 50 g/l, preferably from 1 to 10 
g/l. 
A solution having the composition described hereinabove has a very high 
attack rate when used by spraying, but a more moderate rate of attack when 
used by immersion. In its preferred field of application, i.e. by a 
spraying method, the said solution is capable of uniformly attacking a 10 
microns thick tin-lead layer in 60-90 seconds, without the necessity of 
using any particular rate promoters. The treatment by immersion is less 
preferable because of its lower rate of attack, but it is applicable as 
well. 
A solution having the formulation according to the present invention is 
operative within a wide range of temperatures, but the level which is 
preferable for reasons of operation simplicity is the ambient temperature. 
Obviously, the rate of attacking the tin or the tin-lead alloy increases 
with the treatment temperature. 
The subject-matter solution is capable of indifferently stripping layers of 
tin or of binary tin-lead alloys, in which the two metals may be present 
in whatever ratio.