AU427451B2 - Method of stripping tin andor lead from copper/and stripping solution - Google Patents

Description

3 2.39 9

This invention relates to stripping tin and/or lead from a copper substrate, and more particularly to new

and improved compositions and method for selectively strip-

ping tini\and/or lead from copper substrates or surfaces.

Heretofore tin and tin-lead alloy deposits were

removed from copper substrates by immersion in solutions of

peroxides and an acid such as, for instance fluoboric and/

or acetic acid. Although the tin or tin-lead alloy deposits were stripped from the underlying copper surface, the copper

surface was frequently undesirably etched. Furthermore, the peroxide stripper solutions were unstable and hazardous

to work with. Ferric nitrate-and ferric chloride-contain-

ing stripper solutions have also been utilized in the past

to strip tin and tin-lead alloys from copper substrates.

Such stripper solutions also were disadvantageous due to

frequently etching the copper surface.

Furthermore, in the stripping of the tin and tin-

lead alloys from the copper utilizing the peroxide-acetic

acid containing stripper solutions, after stripping the tin

or tin-lead alloy is effected the tin tends to redeposit on

the copper. This redeposition necessitates a second strip-

ping step or operation, which is usually an immersion dip

in an alkaline or acid solution, to remove the tin re-

deposit.

A conventional alkaline stripper solution contain-

3 z39 9/ 7 ing sodium meta-nitro benzoate or benzenesulfonate has been tried for stripping tin-lead solder deposits from con- tact tabs in the preparation of printed circuit boards, but was unsatisfactory due to causing the epoxy-fiber glass board substrates to swell excessively.

In accordance with the present invention, acidic solutions especially well suited for stripping tin, lead or tin-lead alloys from copper substrates or surfaces are pro- vided. The acidic solutions comprise an acidic aqueous solution of:

at least one nitro-substituted aromatic compound having at least one NO2 group attached to the aromatic ring and a water-solubilizing substituent also attached to the aromatic ring;

an inorganic acid capable of readily. reacting with tin and lead to form water-soluble salts thereof and incapable of reacting with tin and lead to form a water- insoluble compound film on such metal surface, i.e. tin, lead or tin/lead alloy surface; and a thiourea. Such solutions constitute a considerable improvement over the prior stripper solutions for stripping tin and/or lead from a copper substrate for the reasons that: they strip the tin, lead or tin-lead alloys from the copper with- out any substantial etching or dissolution of the copper; they are free of peroxide and the hazards attendant with its presence as a constituent; there is no redeposition of tin on the copper when they are used; and therefore only a one step stripping is required as contrasted with the two step stripping necessitated by use of the prior stripper solutions; and 3 9 71 '9 excessive swelling of epoxy-fiber glass dielectric bases or boards in printed circuit board produc- tion is avoided.

The stripping method of the invention involves contacting the tin, lead or tin-lead alloy deposit on the copper substrate with the acidic aqueous solution of the nitro-substituted aromatic compound,'-tae inorganic acid capable of readily reacting with tin and lead to form water- soluble salts thereof and incapable of reacting with tin and lead to form a water-insoluble compound film on the metal surface, and the thiourea; and continuing the con- tacting of the tin, lead or tin-lead alloy on the copper pubstrate until the tin, lead or tin-lead alloy is selec- tively dissolved from the copper substrate surface. The contacting of the tin, lead or tin-lead alloy on the copper with the acidic stripper solution is usually by immersing the tin and/or lead in the stripper solution or bath. However, any other suitable way of contacting the tin and/or lead with the stripper solution can be utilized.

The nitro-substituted aromatic compound con- stituent having one or more NO 2 groups attached to a benzene-ring carbon or carbons and the water-solubilizing substituent also attached to the benzene ring is exemplified by m- and p- nitrobenzene sulfonic acids and mixtures thereof; m- and p- nitrobenzoic acids and mixtures

thereof; m- and p- nitrochlorobenzenes and mixtures thereof; m- and p- nitroanilines and mixtures thereof and m- and p- nitrophenols and mixtures thereof. Other suitable nitro-substituted aromatic compounds for use in ac- cordance with this invention are those disclosed in United

32 399/71 States Patent 2,649,361 to Spinger and Meyer and United States Patent 2,698,781 to Meyer.

Any inorganic acid capable of readily reacting with tin and lead to form water-soluble salts and which is incapable of forming a water-insoluble compound film on the tin, lead or tin-lead alloy being stripped by reaction with the tin and/or lead is utilizable in the acidic stripper solution of this invention. Exemplary of such inorganic acid constituents are a fluorine-containing inorganic acid e.g. fluoboric acid and fluosilicic acid; and sulfamic acid.

The thiourea constituent is exemplified by thio- urea per se; lower alkyl, i.e. 1-4C alkyl, thioureas, e.g. 1,3-dimethyl thiourea, 1,3-diethyl thiourea, 1,3- dipropyl thiourea, and aryl thioureas, e.g. phenyl thiourea.

The thiourea constituent is essential in the stripper solutions of this invention to prevent re-deposi- tion of stripped tin and/or lead onto the copper.

It is essential to avoid formation of a water- insoluble film on the surface of the tin, lead, or tin- lead alloy being stripped, inasmuch as the formation of such insoluble film will prevent stripping of the tin, lead, or tin-lead alloy. For this reason the inorganic acid con- stituent of the stripping bath herein, which is capable of readily reacting with tin and lead to form water-soluble

salts, should also be incapable of forming a water-insoluble

compound film on the surface of the tin, lead or tin-lead alloy by reaction with the tin and/or lead.

The constituents of the acidic stripper solutions

42 74 51 of this invention are 32.39 9/7 sua-ly- present therein in propor- tions within the following ranges:

Grams/Liter Nitro- substituted aromatic compound having one or more -NO 2 groups and the water- solubilizing substituent also attached to the aroma- tic ring--- about 0.1-180 A thiourea, e.g. thiourea per about 0.1-220 An inorganic acid capable of readily reacting with tin and lead to form water- soluble salts and incap- able of reacting with tin and lead to form a water-in- soluble film, e.g. fluo- boric about 3-500 Water-- To.1 liter The present invention can be utilized to strip tin, lead or tin-lead alloy or solder deposits from any copper substrate or surface for any desired purpose. This invention is especially eminently adapted for selectively stripping the tin-lead alloy solder deposits or layer from the copper surfaced edge contact tabs or fingers in the preparation of printed circuit boards of this type. In the embodiments of the method of this inven- tion for selectively stripping the tin-lead binary alloy solder deposits from contact tabs or fingers in the prepara- tion or processing of printed circuit boards, the method involves contacting the tin-lead binary alloy deposits of the contact tabs wherein the tabs each have a tin-lead binary alloy deposit over a copper layer with the aqueous acidic stripper solution of this invention. The contacting of the tin-lead alloy deposits of the tabs is continued until the tin-lead binary 3 39 9 P7 alloy or solder deposit is selec- tively dissolved from the copper.

The contacting of the tin-lead binary alloy de- posits of the contact tabs of the printed circuit boards is usually by immersing only the contact tabs of the printed circuit board in the acidic stripper solution. Less prefer- ably the contacting may be effected by spraying.

To facilitate the stripping of the tin-lead binary alloy or solder only from the contact tabs of the boards those portions of the printed circuit board which are immediately adjacent and contiguous with the inner- most end portions of the circuit board are blocked or masked off with a resist continuous layer which is resist- ant to and unaffected by the aqueous acidic stripper solu- tion. This masking layer should be one that is readily re- movable mnually or otherwise, and is exemplified by "ScotchXTape or an electricians dielectric tape which is capable of withstanding and is unaffected by the acidic stripper solution.

With reference to the accompanying drawing which is a diagramatic sectional view showing the immersing of the contact tabs of a printed circuit board in a bath of the acidic aqueous stripper solution of this invention for the stripping removal of a tin-lead alloy from the copper substrate of the tabs, substrate or base 5 of an epoxy resin-fiber glass laminate or of another suitable dielectric material has circuit pattern 6 thereon with "plated through" through holes 7. Circuit pattern 6 and electrical edge con- tact fingers or tabs 8 include a layer of copper foil on base 5, a chemical reduction copper layer over the foil, an

32.39 917 electrodeposited copper layer over the chemical reduction copper and a tin-lead solder layer of typical composition of 37 weight per cent lead and 63 weight per cent tin over the electrodeposited copper layer. The walls of through holes 7 have a layer of chemical reduction copper adhered directly to the walls of the epoxy resin-fiber glass

laminate of the base 5, with a layer of electrodeposited copper over the chemical reduction copper layer. The op- posite side (not shown) of the board also has a circuit pattern of a desired configuration or pattern, through hole... inlets, and electrically conductive edge contact fingers or tabs, which are comprised of the metal layers hereinbefore disclosed. The printed circuit board to this point is pre- pared in conventional manner. Strip 9 of Scotch tape, of typical width of masks or blocks off those portions of the circuit pattern 6 immediately adjacent and continu- ous with the innermost ends 10 of the contact tabs 8, so as to restrict dissolution removal of the lead-tin solder layer from only contact tabs 8. After the stripping removal of the lead-tin solder from the underlying copper of the con- tact tabs is completed, the immersed end of the circuit board is withdrawn from the bath 11 of acidic stripper solu- tion, the thus-removed board end is rinsed in water, the strip 9 is removed from the board and the exposed copper surfaces of the board contact tabs are then ready for the preferred cleaning and activation cycle prior to the plating of the gold layer, if desired, as is hereinafter described.

A thin layer of gold is usually plated onto the exposed copper surfaces of the contact tabs after the strip- ping removal of the tin-lead alloy. Any suitable method of

42745 1

3 2.39 917 plating the gold onto the copper may be utilized, prefer-

ably after cleaning and activating the copper substrate.

Immersion or ionic displacement plating of the gold onto

the copper in conventional manner is an especially suitable

method for plating the gold onto the copper of the contact

tabs and is recommended. Immersion gold plating composi-

tions are readily obtainable in commec with one such gold A,6 plating composition being that marketed as "Atomex" by

Engelhard Minerals and Chemicals Corp., 113 Astor Street,

Newark, New Jersey. The thin gold layer is typically of

thickness of 100-150 micro-inches.

Prior to plating the thin gold layer onto the

exposed copper of the contact tabs, a thin barrier layer,

of nickel is usually plated onto and over the exposed copper surfaces of the tabs. The thin gold layer is then plated onto and over the nickel layer. In the absence of this

intermediate nickel barrier layer interdiffusion of the

gold and copper may occur, and corrosion of the copper

when pores are present in the gold layer may also occur

with attendant tarnishing of the gold resulting in high con-

tact resistance. The thin nickel layer may be plated onto

the copper by any suitable method. One especially suitable

method of plating the nickel over the copper is by electro-

deposition in conventional manner utilizing an aqueous Ni

plating bath of the following exemplary composition:

9/1 Nickel 240-340

Nickel 30-60

Boric 30-40 The nickel layer has a thickness of typically 1 tenth to 3

.2 74 tenths of a mil. 3 9 /17 The tin-lead alloys.stripped by this invention areusually tin-lead binary alloys. The tin-lead binary alloys may contain, by weight, from about 1-99 tin, balance lead. The tin-lead solder alloy which can be stripped from the copper surfaces of the edge contact tabs or fingers of printed circuit boards as previously disclosed herein will typically contain, by weight, about 60% tin, balance lead.

Incidental impurities may also be present in the tin-lead

alloys stripped by this invention.

An accelerator of the formula

RCOOH wherein R is a lower alkyl, for instance methyl or propyl, or

aryl group or a hydrogen atom, is preferably utilized in the

acidic stripper solutions of this invention to accelerate

the rate of stripping removal of the tin and/or lead from

the copper substrate. Examples of such accelerators are

acetic acid, propionic acid and formic acid with propionic

acid being preferred among the two acids. Such organic

acid is utilized in the stripper solution in this preferred

embodiment in an effective amount, sufficient to accelerate

the stripping removal of the tin and/or lead from the copper

substrate. This organic acid accelerator constituent is

preferably utilized in the stripper solution in amount of

about 1.0-120 g/l, more preferably about 10-20 g/l.

When fluoboric acid is a constituent of the

stripper solutions of this invention, bofic acid is prefer-

ably also a constituent of the stripper solution or bath. Fluoboric acid undergoes a reversible reaction in the solu-

tion to form HF and H3 BO3 as set forth in the following equation: 3 399 HBF 4 +3H 2 0 =04HF+H 3BO

This reversible reaction.is not desired for the reasons

HF that is formed combines with the lead to form the

insoluble lead fluoride, i.e. Pb 2HF---PbF 2 +2H

and the HF attacks the glass constituent of the epoxy

printed circuit board thus etching the board. The addition

of the boric acid causes the reversible equilibrium reac-

tion to be shifted to the left in the direction of the de-

sired fluoboric a.cid, i.e. HBF, in accordance with the Law

of Mass Action. The boric acid constituent, as contrasted

with the reaction product boric acid of the reversible reac-

tion, is of course utilized in the stripper solution in an

amount sufficient to cause the reversible equilibrium reac-

tion to be shifted to the left, and is preferably utilized

as a constituent in an amount within the range of about

1.0-120 g/l, more preferably about 10-60 g/l.

A wetting agent is preferably also a constituent

of the acidic stripper solutions. Any suitable wetting

agent is utilizable as this constituent and the wetting agent

can be one or a plurality of such agents. Exemplary of the

wetting agents utilizable herein are nonyl phenoxy polyoxy

ethylene ethanol (Igepal CO-6o0) or tert ary octyl phenoxy

polyethoxy ethanol (Triton4X-100). A preferred wetting

agent is a cationic wetting agent or surfactant of the

quaternary ammonium imidazolinium type known as Quaternary-

0 used in the stripper solution in combination with a poly-

alkylene glycol type known as Polyglycol P-400. The

Quaternary-0 wetting agent is obtainable from Geigy Indus-

trial Chemicals Co. of Ardsley, New York and the Polyglycol

-11- 427451

32. 399/71 P-400 from the Dow Chemical Co. of Midland, Michigan.

Polyglycol P-400, which is a water-soluble polyalkylene

glycol such as a water-soluble polypropylene glycol, is

believed to function differently in the stripper solution

than does a wetting agent, and appears to form a protec-

tive film on the copper substrate after the stripping re-

moval of the tin and/or lead therefrom.

The stripper solutions or baths of the invention

preferably contain the following constituents within the

ranges specified: Grams/Liter

Nitro- substituted aromatic compound having one or more NO 2 groups and the water- solubilizing substituent also attached to the aroma- tic ring, e.g. sodium m- nitro-benzenesulfonate------- about o.1-180

Thioure a about 0.1-120

Fluoboric about 15 -360

Boric about 1 -120

Polyglycol about 1 -120 about 1 -120

Accelerator Propionic about 1 -120

To 1 liter

The following examples further illustrate the

invention: Example 1 A plurality of strips of copper-clad epoxy-fiber

glass boards each of dimensions of 1" x 12" x 0.125" were

electroplated with a tin-lead alloy electrodeposit contain-

ing, by weight, 63% tin and 37% lead. The thickness of each

-12- 427 4 5 1

3 2.3997 board was measured after electroplating and found to be

7.62 micrometers (0.0003"). One of the copper-clad boards was weighed prior to electroplating and found to weigh

22.9566 grams. The electroplated boards were then immersed in an acidic aqueous stripper solution of this invention and of the following composition:

by Weight Sodium m-nitrobenzenesulfonate---- 5.85 Thiourea 0.68 Fluoboric acid 2 7.44 Propionic 0.91 Boric 0.91 Polyglycol 1.83 Quaternary-O----- l 0.18

62.2 The tin-lead alloy electrodeposits were completely removed from the boards in 4 minutes. After the tin-lead alloy de- posits were stripped from the boards, the copper clad boards were maintained immersed in the stripper solution for an additional period of 30 minutes to 60 minutes. Upon removal of all boards from the stripper solution, the copper substrates or surfaces of the clad boards were examined and found to be bright and clean, and free of etching or pitting. The copper-clad board that had been weighed prior to electroplating was, after the afore- mentioned additional 30 minutes immersion in the stripper solution, rinsed in water, dried, and reweighed. This copper clad board weiged 22.9584 grams which amounted to an increase in weight of 0.0018 gram.

-13- 427 4 5 1

3 2.399/71 Example 2

A plurality of strips of copper clad epoxy fiber-

glass boards each of dimensions of 1" x 12" x 0.125" were

plated with tin by immersion in a proprietary immersion tin

plating bath. The thickness of the tin deposit was measured

after plating and found to be 2.54 micrometers (0.0001").

The tin plated boards were then immersed in an acidic

aqueous stripper solution of the same composition as set

forth in Example 1 herein. The tin deposits were complete-

ly removed from all boards in 1 1/4 minutes. After the

stripping removal of the tin deposits, the copper clad

boards were retained in the stripping solution for an addi-

tional period of from 30 minutes to 60 minutes.

Upon removal of the copper clad boards from the

*stripper solution, the copper substrates or surfaces of the

boards were examined and found to be clean and bright and

free of etching and pitting.

Example 3

A piece of lead foil of dimensions of 1" x 1" x

0.001" was immersed in an acidic aqueous stripper solution

of this invention and of the same composition as that of

Example 1 herein.

The lead foil completely dissolved in the stripper

solution in 12 minutes.

Example 4I

A piece of copper clad epoxy-fiberglass board was

electroplated with a tin-lead alloy deposit containing, by

weight, 63% tin and 37% lead. The board piece was weighed

just before the electroplating and was found to weigh

11.8110 grams. The thickness of the tin-lead electrodeposit

-14- 427451 on the board was measured 3 2.39 9 71 after the electroplating and found to be 13.45 micrometers (0.00053") in thickness. The thus plated board piece was then immersed in an-acidic per- oxide stripper solution of the following composition:

ML. Hydrogen peroxide

Fluoboric 128

The temperature of the stripper solution.was 250 The time required to completely strip the tin-lead deposit from the copper substrate was 15 minutes. The copper clad board portion was allowed to remain immersed in the stripper solu- tion for an additional 30 minutes after the tin-lead deposit had been stripped from the copper. The board was then rinsed in water, dried and reweighed. The board weighed 11.6846 grams and therefore 0.1264 grams were lost by the stripping. This peroxide bath was difficult to work with and was quite unstable.

Example An acidic stripping solution was prepared utiliz- ing the same constituents as employed in the stripping solu- tion of Example 1 herein except that the thiourea was omitted from this solution of Example 5. The stripping solution had the following composition:

Sby. Weight Sodium m-nitrobenzenesulfonate--- 5.40 Pluoboric acid 30.00 Propionic 1.00 Boric 1.00 Polyglycol 0.50 4274 51

32.39 971 by Weight 0.10

62.00

This solution was used full strength at room temperature as

a stripper for a tin-lead alloy. A copper clad epoxy-fiber

glass board of dimensions of 1" x 12" x 0.125", which had

been electroplated with a tin-lead alloy electrodeposit

containing, by weight, 63% tin and 37% lead and of 0.0001"

thickness over the copper, was immersed in this solution.

The time required to completely strip the tin-lead alloy

electrodeposit from the copper was 1 minute and 15 seconds.

However a gray thin coating of tin redeposited on the

copper which was unsatisfactory.

-16- 427451

Claims (6)

3 2.399/71 The claims defining the invention are as follows:

1. An acidic solution for selectively stripping tin and/or lead from a copper substrate comprising:

from 0.1 to 180 grams/liter of a nitro-substituted

aromatic compound having at least one -NO 2 group attached to the aromatic ring and a water-solubilizing substituent also attached to the aromatic ring;

from 3 to 500 grams/liter of an inorganic acid cap- able of readily reacting with tin and lead to form water- soluble salts thereof and incapable of reacting with tin and lead to form a water-insoluble compound film; and from 0.1 to 220 grams/liter of a thiourea.

2. A stripping solution according to claim 1, in which the inorganic acid capable of readily reacting with the tin and lead to form water-soluble salts thereof is a fluorine-containing inorganic acid or sulfamic acid.

3. A stripping solution according to claim 2, in which the fluorine-containing acid is fluoboric acid.

4. A stripping solution according to claim 2, in which the fluorine-containing acid is fluosilicic acid. A stripping solution according to any one of the preceding claims, which also contains an accelerator of the formula:

RCOOH wherein R is lower alkyl, aryl or a hydrogen atom.

6. A stripping solution according to claim 1, sub- stantially as hereinbefore described with reference to the Examples.

7. A method for stripping tin and/or lead deposited on a copper substrate from the copper, which comprises:

-17 427451

3 2.399 7 contacting the tin and/or lead deposit on the

copper substrate with an acidic stripping solution accord-

ing to any one of the preceding claims; and

continuing the contacting of the tin and/or lead

on the copper substrate with the acidic solution until the

tin and/or lead is selectively dissolved from the copper substrate surface.

8. A method for selectively stripping tin-lead alloy deposits from contact tabs of printed circuit boards which comprises: contacting the tin-lead alloy deposits of the con-

tact tabs each having a tin-lead alloy deposit over a copper

layer with an aqueous acidic solution according to any one

of claims 1 to 6; and

continuing the contacting of the tin-lead alloy

deposits of the tabs with the aqueous acidic solution until

the tin-lead alloy is selectively dissolved from the copper.

9. A method according to claim 8, in which the tin-

lead alloy deposit is selectively dissolved from the copper

of the tabs by immersing the tin-lead alloy deposits of

only the contact tabs in the aqueous acidic solution.

DATED this SEVENTEENTH day of AUGUST, 1972.

ENTHONE, INCORPORATED

Patent Attorneys for the Applicant SPRUSON FERGUSON

427451

3 2 2j9 9 171

LEVEL OF STRIPPER SOLUTION

C CONTACT UM ACIDIC STRIPPER TABS SOLUTION

4274 51