EP0107109B1

EP0107109B1 - Electrolytic copper plating solutions and a method for their application

Info

Publication number: EP0107109B1
Application number: EP83109814A
Authority: EP
Inventors: John Houman
Original assignee: LeaRonal Inc
Current assignee: Shipley Co Inc
Priority date: 1982-09-30
Filing date: 1983-09-30
Publication date: 1988-02-24
Also published as: ATE32611T1; WO1984001393A1; EP0107109A2; JPS59501829A; EP0107109A3; JPS6250559B2; DE107109T1; DE3375732D1

Abstract

Acid copper electroplating solutions containing the reaction product of (1) a compound of the formula <CHEM> wherein R1 and R2 are lower alkyl radicals of with 1 to 6 carbon atoms, a hydrogen atom or mixtures thereof and R4 is an alkali metal, hydrogen, magnesium, or the groups SX or SSX, wherein X is an alkali metal, hydrogen or magnesium, or a compound of the formula <CHEM> wherein R3 is an aromatic, heterocyclic or alicyclic radical containing 3 to 12 carbon atoms and R4 is an alkali metal, hydrogen, magnesium, or the groups SX or SSX where X is an alkali metal, hydrogen or magnesium, (2) a compound of the formula XR1-(S)n-R2-SO3H wherein R1 and R2 are the same or different and are alkylene radicals containing 1 to 6 carbon atoms, X is hydrogen or -SO3H and n equals 2 to 5, and (3) acrylamide in a sufficient amount to increase the brightness of the deposit and/or to prevent the formation of cracks during thermal shock.

Description

The invention relates to electrolytic copper plating solutions and a method for their application.
The invention relates especially to the electrodeposition of copper for decorative use and more particularly to the electrodeposition of copper on substrates having sharp corners such as formed by the holes drilled into copper clads for the production of printed circuit boards.
Circuit boards are generally prepared by laminating copper clad to both sides of a plastic sheet such as an epoxy-glass. Holes are then drilled through the copper clad and the plastic, exposing the plastic. The exposed plastic must then be plated to effect conductivity from one side of the board to the other. This is generally accomplished by treating the plastic with an activator by well known processes, subjecting the entire circuit board to electroless deposition of copper to render the areas receptive to electrolytic copper depositions, and then plating the board and the internal surfaces of the holes by electrodeposition of copper. The sharp corners formed by the perimeter of the holes adjacent the top and bottom of the board must also be plated. This can be accomplished by many different copper electroplating solutions presently on the market, but the copper plate at the corners has a tendency to develop cracks when the boards are subjected to thermal shock necessitated by further processing of the boards.
It is the object of the present invention to provide an improved copper electroplating solution and a method of its application for substrates having sharp corners as well as for substrates subjected to thermal- shock, and additionally to improve the brightness thereof.
This invention relates to acid copper electroplating solutions comprising a soluble copper salt, free acid and a solution obtainable by reacting

(1) a compound of the formula
wherein R, and R₂ are lower alkyl radicals with 1 to 6 carbon atoms, a hydrogen atom or mixtures thereof and R₄ is an alkali metal, hydrogen, 1/2 magnesium, or the groups SX or SSX, wherein X is an alkali metal, hydrogen or 1/2 magnesium, or a compound of the formula
wherein R₃is an aromatic heterocyclic or alicyclic radical containing 3 to 12 carbon atoms and R₄ represents an alkali metal, hydrogen, 1/2 magnesium or the groups SX or SSX wherein X is an alkali metal, hydrogen or 1/2 magnesium;
(2) a compound of the formula
wherein R, and R₂ are the same or different and are alkylene radicals containing 1 to 6 carbon atoms, X is hydrogen or -S0₃H and n equals 2 to 5; and
(3) acrylamide in methanol, optionally together with sodium hydroxide, and adding concentrated sulfuric acid thereto under reflux until gassing has ceased or no precipitate or turbidity is present.

The invention also comprises a method of electroplating substrates having sharp corners to prevent the formation of cracks at the corners due to thermal shock, using the electroplating copper solution according to the invention.
The compounds that can be used to react with the compounds according to above item (2) and the acrylamide are represented by the following formulae:
wherein R, and R₂ are lower alkyl radicals with 1 to 6 carbon atoms, a hydrogen atom or mixtures thereof, and R₄ is an alkali metal, hydrogen, 1/2 magnesium, or the groups SX or SSX, wherein X is an alkali metal, hydrogen or 1/2 magnesium, or
wherein R₃ is an aromatic, heterocyclic or alicyclic radical containing 3 to 12 carbon atoms, and R₄ represents an alkali metal, hydrogen, 1/2 magnesium, or the groups SX or SSX where X is an alkali metal, hydrogen or 1/2 magnesium.
The compounds found to be the most advantageous to date are the soduim salts obtained from the following compounds: tetraalkylthiuram disulfide,
wherein R, and R₂ are methyl or ethyl or mixtures thereof, 2,2'-dithio-bisbenzothiazole,
and 2-mercaptobenzothiazole
When reacting compounds such as (3) and (4) with sodium hydroxide, the compounds are split, predominantly but not exclusively between the -S-S- bond forming the sodium salts. Thus with formula (3), wherein R is ethyl, after reaction with sodium hydroxide would form predominantly two moles of
plus minor amounts of
and
and formulas (4) and (5) would form
with minor amounts of
The sodium salts of the compounds (3), (4) and (5) can readily be prepared by known means by heating the compounds dissolved in a solvent such as methanol (preferably under reflux) with sodium hydroxide. The compound of formulas (3), (4) and (5) are available commercially and marketed under the marks TUADS^O, ALTAX° and CAPTAX^O, respectively, by R. T. Vanderbilt Company, Inc.
The second reactant is an alkylene sulfide compound having terminal acid group(s). This compound corresponds to the general formula
wherein R_i, R₂ and X have the meaning as defined above.
Examples of a number of specific compounds coming within the scope of the above formula are set forth in column 2 of U.S. Patent 3,328,273 issued to Creutz et al on June 27,1967. It is preferable to use the alkali metal salts of the above compounds.
The most advantageous alkylene sulfide known to date is di(sodium 3-sulfonate-1-propyl) sulfide:
The third reactant is acrylamide.
The exact chemical nature of the reaction product is not known. The product resulting from these reactions is hereinafter referred to as "reaction solution".
The invention includes the use of oxyalkylene polymers as brightening and leveling agents in combination with the "reaction solution". The oxyalkylene polymers have been found to materially increase the brightness and leveling of the deposits. The polyalkylene glycols, such as polyethylene glycols, methoxy polyethylene glycols and the polypropylene glycols, have been found to be particularly advantageous.
The oxyethylene or oxypropylene polymers can be surfactants, anionic, nonionic or cationic. Anionic and nonionic are preferred. These types of surfactants are well known and lists of specific polymers can be obtained by consulting any standard text on the subject such as the various volumes of Kirk-Othmer Encyclopedia of Chemical Technology or the industrial literature. It is the presence of the ethylene oxide or propylene oxide groups that is most important. The compounds should have at least about 8 mols of ethylene and/or propylene oxide and be soluble in the bath solution. Combinations of polyethylene and polypropylene glycols and/or surfactants can also be used.
The amounts of the oxyalkylene polymers can be about the same as is usually employed in acid copper baths. A sufficient amount should, of course, be used to obtain the brightness and leveling desired which will in turn depend on the ultimate use intended. Generally about 0.1 to 0.5 g/I or ml/I can be employed.
Additional brighteners, grain refiners or leveling agents known in the art can also be added to the plating solutions of this invention in addition to or in place of the oxyalkylene polymers as will be apparent to those skilled in the art.
As noted above, the copper deposited according to this invention is useful as decorative use, in the electronic industry generally, and for the conduction of electricity on substrates that do not have sharp corners or on articles where thermal shock is not a problem. The amounts of the reaction products employed in the acid copper plating solutions may therefore differ depending on the result desired, but in any event the amounts should be sufficient to improve the brightness and smoothness of the metallic deposits over that obtainable from the basic plating solutions. When a substrate is plated having sharp corners, such as circuit boards which are subjected to thermal shock, the amounts should be sufficient to prevent cracks in the deposit at the corners when the plated substrate is subjected to thermal shock. As far as it is known today, the amounts to accomplish both of these results will be substantially the same. Small amounts, as little as about 0.1 mill, have been found sufficient to accomplish this purpose. Larger amounts, such as 1 ml/I, can of course also be employed so long as it does not adversely affect the plating operations or the advantages of this invention. No upper limit has been determined. It is, of course, advantageous to use as little of the "reaction solution" as practicable to obtain the results desired.
The acid copper plating solutions to which the "reaction solution" can be added are conventional and well known. The two essential constituents are a copper salt, such as copper sulfate, and an acid, such as sulfuric acid. The salt furnishes the metal ions and the acid serves to reduce the resistivity or promote conductivity. These baths typically contain between about 70-250 g/I of copper sulfate and 30 to 250 g/I of sulfuric acid.
The "reaction solution" can be formed by dissolving compounds of formulas (1) and/or (2), such as tetralakylthiuram disulfide sodium salt in a suitable solvent, adding a bis(3-sulfoalkyl) disulfide salt to the reaction mixture together with acrylamide under reflux. Concentrated sulfuric acid is then added (dropwise in the laboratory) during the reflux and continued until gassing has ceased or no precipitate or turbidity is present. The reactants can be the mixtures as described above.

Example 1

2.6 g oftetraethylthiuram disulfide is dissolved in a sufficient amount of methanol and 0.78 g of sodium hydroxide. The reaction mixture is refluxed for 30 minutes to commplete the reaction and the volume of the resulting solution is increased by 50% to 100% with water to clear it from turbidity. 3.52 g of bis(3-sulfopropyl) disulfide disodium salt and 8.0 g of acrylamide are then added while continuing the reflux for about 30 minutes to an hour. Concentrated sulfuric acid is added dropwise during the reflux and continued until no more gassing or precipitate or turbidity is present. The color of the solution, during the sulfuric acid addition, changes from a dark greenish-yellow to pale yellow-colorless. The reaction medium is then diluted with water to a volume of 1 liter, to give the "reaction solution".
The exact proportions of the reactants are not very critical but best results to date are obtained by using stoichiometric amounts. The reaction can include additional reactants so long as they do not affect the function and advantageous properties of the resulting reaction solution. For example, 0.6 g of formaldehyde can be added to the methanol solution and reacted with the sodium hydroxide before the addition of the disulfide compound and the resulting reaction solution has substantially the same advantageous properties.

Example 2

A 7.5 I (2 gallon) tank and an Hull cell was used on a acid copper plating solution of the following composition:
The plating bath was operated at 23.9°C (75°F) in a Hull cell with air agitation at a current of 2 amps for 10 minutes. The plating bath in the 7.5 I (2 gallon) tank was operated at identical parameters, but at a current density of 1.6 A/dm² (15 ASF) for an hour.
Printed circuit boards with the holes drilled therein after being activated and electrolessly plated with copper were plated in this tank. The copper deposit on the circuit board was smooth and semi-lustrous over current density range of 0.2-2.1 A/dm² (2 to 20 ASF) and showed no signs of corner cracks after thermal shock.

Example 3

The procedure of Example 2 was followed except that the following material was also incorporated into the plating bath:
The copper deposit on the plated material was very bright and leveled over a current density range of from 0.1-10.7 A/dm² (1 to 100 ASF) and showed no signs of corner cracks after thermal shock.

Example 4

The procedure of Example 2 was followed except that the following materials were also incorporated into the plating bath:
The deposit on the plated material was very bright and leveled in the current density range of from 0.1-10.7 A/dm² (1 to 100 ASF). The deposit on the printed circuit board plated in the 7.5 1 (2 gallon) tank was very bright and leveled, and showed no signs of corner cracks after thermal shock.
The thermal shock test to which the plated boards are subjected in the above examples is conventional. After the boards are baked for about an hour at 150°C, they are cooled to room temperature and allowed to float on one side in molten solder at 288°C for 10 seconds, then turned over and allowed to float on the solder on the other side for 10 seconds. The boards are then removed and inspected for cracks.

Claims

1. An acid copper electroplating solution comprising a soluble copper salt, free acid and a solution obtainable by reacting

(1) a compound of the formula

wherein R, and R₂ are lower alkyl radicals with 1 to 6 carbon atoms, a hydrogen atom or mixtures thereof and R₄ is an alkali metal, hydrogen, 1/2 magnesium, or the groups SX or SSX, wherein X is an alkali metal, hydrogen or 1/2 magnesium, or a compound of the formula

wherein R₃ is an aromatic heterocyclic or alicyclic radical containing 3 to 12 carbon atoms and R₄ represents an alkali metal, hydrogen, 1/2 magnesium or the groups SX or SSX wherein X is an alkali metal, hydrogen or 1/2 magnesium;

(2) a compound of the formula

wherein R, and R₂ are the same or different and are alkylene radicals containing 1 to 6 carbon atoms, X is hydrogen or -S0₃H and n equals 2 to 5; and

(3) acrylamide in methanol, optionally together with sodium hydroxide, and adding concentrated sulfuric acid thereto under reflux until gassing has ceased or no precipitate or turbidity is present.

2. The electroplating solution of claim 1 in which the copper sulfate and the free acid is sulfuric acid.

3. The electroplating solution of claims 1 or 2 in which (1) is an alkali metal salt obtained from tetraalkylthiuram disulfide, 2,2'-dithio-bis-arylthiazole, or 2-mercaptoarylthiazole and (2) is di(alkali-3-sulfonate-1-alkyf) sulfide.

4. The electroplating solution of claims 1 or 2 in which (1) is the sodium salt obtained from tetramethylthiuram disulfide, tetraethylthiuram disulfide or mixtures thereof, 2,2'-dithio-bisbenzothiazole or 2-mercaptobenzothiazole and (2) is di(sodium-3-sulfonate-1-propyl) sulfide.

5. The electroplating solution of claims 1, 2, 3 or 4 containing a brightening and/or leveling agent.

6. The electroplating solution of claim 5 in which the agent is an oxyethylene and/or an oxypropylene polymer containing at least about 8 ethylene or propylene groups.

7. A method of electroplating copper from acid copper solutions on substrates having sharp corners to prevent the formation of cracks at the corners due to thermal shock which comprises electroplating copper on the substrate from an electroplating solution as claimed in any one of claims 1 to 6.