CA1182953A

CA1182953A - Method and bath for the electroless plating of gold

Info

Publication number: CA1182953A
Application number: CA000418444A
Authority: CA
Inventors: John L. Little
Original assignee: Mine Safety Appliances Co
Current assignee: MSA Safety Inc
Priority date: 1982-01-25
Filing date: 1982-12-23
Publication date: 1985-02-26
Also published as: JPS602387B2; GB2114159B; FR2520385B1; IT1160454B; IT8319210A0; JPS58133367A; DE3247144C2; FR2520385A1; DE3247144A1; GB2114159A

Abstract

TITLE
METHOD AND BATH FOR THE
ELECTROLESS PLATING OF GOLD
ABSTRACT
The present invention relates to a method and bath for the electroless plating of gold using an aqueous solution of trivalent gold, a ligand and a tertiary or secondary amine borane reducing agent.
Preferably, a stabilizer such as 6-ethoxy-2-mercaptobenzothiozole is used in the bath.

Description

FIEL~ OF THE INVENTION
The invention relates to the electroless plating of gold without the use of a cyanide complex.
BACKGROUND QF T~E INVENTION
Electroless gold plating, that is where gold is deposited on a catalytic surface without electricity, is becoming a known and recognlzed method. Many ofthe methods of electroless gold plating utilize cyanide to form a complex with the gold, such as U.S. Patent Nos. 3,700,469 and 4,162,337, or utlli~e a cyanideto stabllize the bath, for example, U.S. Patent No. 3,589,916. Where cyanide isused 9 a strong reducing agent such as an alkali-metal borohydride or alkylamineborane must also be used. The use of cyanides, however, creates disposal and health problems which are preferably avoided.
One method for the electroless deposition of gold without the use of cyanlde has been described in U.S. Patent No. 4,142,902. Disclosed therein is the use of a soluble ether substituted tertiary amine borane as a reducin~ agent.
The plating baths of that invention meet the environmental ob~ectives, but the plating rate has been slower than commercially desirable, and bath stability at elevated temperature is poor.
Accordingly, it is an object of the present invention to provide a method and bath for the electroless plating of gold whlch a~ords co~mercially desirable plating rates at elevated t~mperatures, but does not involve ~he use of cyanides as either a complexing agent or as a bath stabilizer~
SU~MARY OF THE IN~RNTION
Generally, the present invention involves the use of a stable. aqueous bath comprising a soluble salt of tetrachloroaurate ion and at least one inorganic ligand which complexes with the gold. A water 01/28/82 342468 1 101 65.00CK
01/2B/82 342468 1 102 20.00CK
soluble tertlary amine borane or the secondary amine borane, diisopropylam~ne borane, is used as the reducing agent. Preferably, catalytic poison stabilizersare added when bath temperatures above 35C are used. These stabilizers inhibit ,.`~` - 1 - ''`$' the spontaneous decomposition of the bath. The pH of the bath ls ~aintained between 8.5 and 14.
In the method of the present invention, it is necessary to heat, pre-ferably to boiling, the aqueous solution containing the gold anion and the ligand.
The reducing agent and stabilizer are then added to the solution after it has cooled. In the event the gold anion and ligand are mixed together without heating, the bath will rapidly decompose upon the additlon of the reducing agent.
It is preferably to use soluble salts such as KAuC14 or NaAuC14 to prepare the gold solution. The preferred range of gold concentration is from about 0.003 to 0~006 moles/liter, but suitable baths can be prepared with gold concentrations from 0.001 to 0.05 moleslliter. The most desirable inorganic ligands to buffer the gold and comple~ with it include inorganic borates/ o- andm-silicates, phosphates, pyrophosphates or carbonates. Specifically, sodium metaborate in an amount from 0.06 to 6.0 moles/liter, sodium phosphate in an amount from 0.02 to 1.0 moles/liter and sodium carbonate in an amount from 0.02 to 1.5 moles/liter have been found desirable.
The reducing agents of the present invention are soluble tertiary amine boranes in an amount of from 0.004 to 0.020 moles/liter, or diisopropylamine borane in the range of from 0.002 0.08 moleslliter. The tertiary amines consist of the class described in U.S. Patent No. 4,255,194, namely trialkyamine borane ~laYing th~ formula R1R2R3NBH3, where RlR2, and R3 are methyl or ethyl groups and N alkyl substituted Morpholine borane having the general formula:

/~ ~
o ~ > N:BH3 \

where R is an alkyl group preferably having from 1 to 3 carbon atoms, but havinga solubility of at least 0,004 moles/iite~.

The preferred reducing agents comprlse trimethyl am-lne borane (TMAB) in an amount from 0.004 to 0.15 moles/liter; methyl morpholine borane in an amount from 0.065 to 0.12 moles/liter and diisopropylamine borane ln an amount from 0.0002-0.03 moles/liter.
For baths of the present invention malntained above 35C i~ is deslrable to utilize a catalytic poison stabilizer. Suitable stabilizers include mercap-tans, such as 6-ethoxymercaptoben7Othiazole (EMBT), the preferred stabilizer;
cyanohydroquinones, such as 2,3-dicyanohydroquinone; organic iodine compounds 9 such as iodopyridinol; alkali metal iodates; organics containing sulfur doubly bonded to carbon, such as 3-e~hyl-2-thioxo-4-oxazolidinone or ethylaxanthic acid.
Other types of compounds, especially sulfur-containing organics also stabilize the baths of the present invention. Most of the stabillzers are effectlve in anamount of 1 to 5 ppm. The cyanide ion is not effective as a stabilizer, and amounts less than one ppm will stop plating altogether.
Regeneration of the bath is accomplished by addition of a solution of gold complex and solid reducer to the depleted bath at room temperature. The gold solution is made by dissolvlng the soluble salt of tetrachloroaurate ion in a small quantity of water, adding proportionally about half a~ much ligand as was used in the original make up of the bath, and then heating to a boil to complex the gold. More than one complete turnover of gold in the bath has been accom-plished without a reduction in plating rate or a loss of plate quality.
PREPARATION OF BATHS OF PRESENT INVENTION
In the preferred method of preparatlon, a soluble salt of tetrachloro-aurate ion ls dissolved in water and a solid salt containing the inorganic ligand is added. When the resulting solution is heated to a gentle boil, the color of the solutlon will change from yellow to either very light yellow or become color-less. Failure to heat ~he solution as described, however, will result in catastrophic bath decomposition when the reducer is added.
After cooling to room temperature, a stabilizer is preferably added to the solution of gold complex. Thereafter an aqueous solution of amine borane reducing agent is added and the combined solutions are stirred. The resulting Z~3S3 bath is brought up to the desired volume, if necessary, with distilled water andthereafter filtered through a medium porosity paper to remove any undlssolved material.
PREPARATION OF PLATING SUBSTRATES
. ~ . .
Copper, nickel, mild steel and stainless steel can be plated in accor-dance with the present invention without pre-treatment9 exeept preferably de~
greasing or oxide removal, such as, for example, using a solution of concentrated hydrochloric acid. Electrolessly plated nickel and palladium, as well as gold (plated from a cyanide-free bath) can be directly plated in accordance with the present inventio~ without pre-treatment. ABS plastic, on the other hand, should be first etched and treated with tin chlorides and palladium chloride or gold chloride solutions as should ceramics such as aluminum oxide. Glass is first abraded and then treated in the same manner as ceramics.
Other substrates requlre sensitization procedures which are commonly known ln the art~

A bath waR made up using preparation procedures set forth above to provide the followlng concentration: KAuC14-3H20; 2.0 g/l; Na3P04 12H20p 20 g/l,2-mercaptobenzothiazole (MBT), 1.2 mg/ml; and trimethylamine borane (T~AB),

2.0 g/l. The pH was about 11.9. The bath, which was maintained at 50C, pla~ed a palladium chloride sensitized nickel workpiece at the rate of 25 ~ in/hr, witha plating load of 36 cm2t1. The resulting plate was very smooth and shiny, and th~ plate hardness was found to be 94 DPN using a 25 gram load.

A bath was made up as in example 1 with the following concentration of ingredients: K~uC14-3H~0, 2.7 g/l, Na2C03, 15 g/1; MBT 10 mg/1; TMAB, 4.0 g/1.
The bath pH was about 10.8. Tin chloride~palladium chloride treated mlldly etched glass was plated at 45C. Chemical analysis of the plate showed that it contained about 0.15% boron.

5~

A bath was made up as ln example 1 with the following conl~entration of ingredients: KAuC14.3H20, 2.0 g/l; NaB02, lO g/l, methylmorpholine borane, 5 g/l; MBT, 1.6 mg/l. The pH was adjusted to 12.6 with sodium hydroxide solution.
At 60~ the observed plating ra~e was 15,u in/hr on palladlu~ chloride sensitized nickel sheet.

A bath was made up as in example l with the following concentration of ingredients: KAuCl4.2H20, 4.0 g/l; Na3P04.12H20, 40 g/l; TMAB 4.0 g/l, and 6-ethoxy-2-mercaptobenzothiazole (EMBT), 2.0 mg/l. Fifty ~icroinches of electroless nickel-phosphorous was first plated on a copper sheet, and then the workpiece was rinsed with distilled water and placed in the electroless gold bath. P]ating rates of 20-25 ~ in/hr were obæer~ed at 55C.
E2~MPLE 5 __ A bath was made up as in example 1 with the following concentration of ingredients: KAuC14.2H20, 4.0 g/l; Na3P04.12H20, 20 g/l; T~AB, 1.0 g/l; 3-ethyl-2~thio~o-4-oxazolidinone, 5.5 mg/l. At 55C the bath plated at 31~u in/hr at a plating load at 24 cm2/1. The plate was very smooth and shiny and adhered well to the nickel substrate.

A bath was ~ade up as in example l with the following concentration of ingredients: KAuC14.2H20, 5.2 g/l; ~T, 1.2 mg/l; NaB02 10 g/l; and diiSRrPYl-amlne borane, 8 g/l. At a ba~h temperature of 60C, gold was plated on catalized nickel sheet at 15~u in/hr.

. . . _ A bath was made up as in example 1 with the following concentration ingredients: KAuC14.2H20, 2.0 g/l; NAB02, 10 g/l; MBT, 1.5 mg/l; and TMAB, 2.0 g/l. An ABS plastic sample was etched and catalyze and plated at 55C at a rate of 25 ~ in/hr.

Microhardness measurements were made with a 25 g load on electroless gold plate at least 1.5 mil in thickness. Microhardness was found to be 90-100 DPN, which is slightly harder than pure gold.
Poration as determined by a standard nitric acid fume test was one por/cm2 or less at 50 ~ in plate thickness over smooth, clean substrates. Rougher or improperly cleaned substrates showed greater poration.
While the presently preferred embodiments of ~he invention have been described, the invention may be otherwise embodied within the scope of the appended claims,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An aqueous bath for the electroless plating of gold comprising of an aqueous solution of gold complexed with an anion selected from the group consist-ing of borates, carbonates, phosphates, pyrophosphates and o- or m- silicates, atertiary amine borane or diisopropylamine borane present in an amount of at least 0.002 moles/liter and having a pH of from 8.5 to 14.

2. An aqueous bath for the electroless plating of gold comprising:
(a) trivalent gold having a concentration of about 0.001 to 0.05 moles/
liter;
(b) an inorganic ligand selected from the group consisting of soluble borates, o- or m- silicates, phosphates, pyrophosphates and carbonates present in an amount of from 0.02 to 6.0 moles/liter; and (c) a tertiary amine borane having a solubility of at least 0.004 moles/
liter or at least 0.002 mole/liter of diisopropylamine borane; and (d) a pH of 8.5 to 14.

3. An aqueous bath for electroless plating as set forth in Claim 2 wherein said tertiary amine borane is selected from the group consisting of tri-alkylamine boranes, and N alkyl substituted morpholine boranes.

4. A method of plating gold to a prepared substrate comprising:
(a) preparing a solution comprising an aqueous solution of trivalent gold and adding thereto an inorganic ligand comprising a borate, phosphate, carbonate, pyrophosphate or o- or m-silicate anion;
(b) heating said solution to complex said gold and ligand anion;
(c) adding to said solution after it cools a tertiary amine borane in an amount of from 0.004 to 0.020 moles/liter or diisopropylamine borane in an amount of from 0.002 to 0.08 mole/liter to form the plating bath; and (d) placing said substrate in said bath until the desired plate thickness is obtained.

5. A method set forth in claim 4 wherein said tertiary amine is selected from the group consisting of trialkylamine boranes and N alkyl sub-stituted morpholine boranes.