Aqueous electroless nickel plating bath and method of using the same

Abstract

An electroless nickel plating solution and a method of using the same to produce a nickel deposit having a phosphorus content that remains at about 12% throughout the lifetime of the electroless nickel plating solution is disclosed. The electroless nickel plating solution comprises (a) a source of nickel ions; (b) a reducing agent comprising a hypophosphite; and (c) a chelation system comprising: (i) one or more dicarboxylic acids; and (ii) one or more alpha hydroxy carboxylic acids. The electroless nickel plating solution may also comprise stabilizers and brighteners.

Claims

1. A method of producing an electroless nickel phosphorus deposit on a substrate, wherein the electroless nickel phosphorus deposit has a phosphorus content of about 12%, the method comprising the steps of: contacting the substrate with an electroless nickel phosphorus plating solution comprising: a) a source of nickel ions; b) a reducing agent comprising a hypophosphite; c) a chelation system comprising: i) one or more dicarboxylic acids; and ii) one or more alpha hydroxy carboxylic acids; and d) a brightener comprising bismuth; for a period of time to provide a nickel phosphorus deposit on the substrate having a phosphorus content of about 12%; wherein the electroless nickel phosphorous plating solution has a lifetime and produces a nickel phosphorous deposit having a phosphorus content that remains at about 12% throughout the lifetime of the electroless nickel phosphorous plating solution wherein the electroless nickel phosphorous plating solution deposits the nickel phosphorous deposit on the substrate at rate of at least 0.6 mil/hour.

2. The method according to claim 1, wherein the brightener comprises about 2 to about 4 mg/L bismuth.

3. The method according to claim 1, wherein the lifetime of the electroless nickel phosphorous plating solution comprises at least 3 metal turnovers.

4. The method according to claim 3, wherein the lifetime of the electroless nickel phosphorous plating solution comprises at least 5 metal turnovers.

5. The method according to claim 1, wherein the electroless nickel phosphorous plating solution comprises: a) about 30 to about 40 g/L of hypophosphite; b) about 30 to about 40 g/L of lactic acid; c) about 3 to about 6 g/L of succinic acid; and d) about 25 to about 35 g/L of malonic acid.

6. The method according to claim 5, wherein the electroless nickel phosphorous plating solution comprises: a) about 33 to about 36 g/L of hyphophosphite; b) about 33 to about 36 g/L of lactic acid; c) about 4 to about 5 g/L of succinic acid; and d) about 28 to about 31 g/L of malonic acid.

7. The method according to claim 1, wherein the electroless nickel phosphorous plating solution has a pH of about 5.2 to about 6.2.

8. The method according to claim 1, wherein the electroless nickel phosphorous plating solution comprises a stabilizer, wherein the stabilizer comprises one or more iodine compounds selected from the group consisting of potassium iodate, sodium iodate, and ammonium iodate.

9. The method according to claim 8, wherein the stabilizer does not comprise the metals cadmium, lead, hexavalent chromium or mercury.

10. The method according to claim 1, wherein the electroless nickel phosphorous plating solution further comprises a sulfur compound.

11. The method according to claim 10, wherein the sulfur compound is saccharin.

12. The method according to claim 1, wherein the electroless nickel phosphorous plating solution deposits the nickel phosphorous deposit on the substrate at a rate of at least 0.9 mil/hour.

13. The method according to claim 1, wherein the electroless nickel phosphorous deposit is capable of passing a standard nitric acid test, wherein the standard nitric acid test comprises immersing the electroless nickel phosphorous deposit on the substrate in a concentrated nitric acid solution for 30 seconds, wherein the electroless nickel phosphorous deposit on the substrate passes the nitric acid test if no discoloration of the electroless nickel phosphorous deposit is observed.

14. The method according to claim 7, wherein the electroless nickel phosphorous plating solution has a pH of about 5.6 to about 5.7.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) The present invention relates generally to an electroless nickel plating solution comprising: a) a source of nickel ions; b) a reducing agent comprising a hypophosphite; and c) a chelation system comprising: i) one or more dicarboxylic acids; and ii) one or more alpha hydroxy carboxylic acids; wherein the electroless nickel plating solution produces a nickel deposit having a phosphorus content that remains at about 12% throughout the lifetime of the electroless nickel plating solution.

(2) The use of the chelation system described herein in the electroless nickel plating solution produces a nickel deposit having a phosphorus content that remains in the 12% range throughout the life of the bath. This is unique in nickel phosphorus systems, because normally the phosphorus content starts at about 10% to 11% and then climbs to 12%.

(3) The nickel ions are introduced into the bath employing various bath soluble and compatible nickel salts such as nickel sulfate hexahydrate, nickel chloride, nickel acetate, and the like to provide an operating nickel ion concentration ranging from about 1 up to about 15 g/L, more preferably about 3 to about 9 g/L, and most preferably about 5 to about 8 g/L.

(4) The hypophosphite reducing ions are introduced by hypophosphorous acid, sodium or potassium hypophosphite, as well as other bath soluble and compatible salts thereof to provide a hypophosphite ion concentration of about 2 up to about 40 g/L, more preferably about 12 to 25 g/L, and most preferably about 15 to about 20 g/l.

(5) The specific concentration of the nickel ions and hypophosphite ions employed will vary depending upon the relative concentration of these two constituents in the bath, the particular operating conditions of the bath and the types and concentrations of other bath components present.

(6) The temperature employed for the plating bath is in part a function of the desired rate of plating as well as the composition of the bath. The plating bath is preferably maintained at a temperature of between about room temperature and about 100° C., more preferably between about 30° and about 90° C., most preferably between about 40° to about 80° C.

(7) The complexing of the nickel ions present in the bath retards the formation of nickel orthophosphite which is of relatively low solubility and tends to form insoluble suspensoids which not only act as catalytic nuclei promoting bath decomposition but also result in the formation of coarse or rough undesirable nickel deposits. The inventors have also found that the addition of the chelators described herein does not affect the phosphorus content of the deposit or hurt the nitric acid test. That is, unlike any of the currently known high phosphorus electroless nickel deposits, the electroless nickel phosphorus deposit of the present invention maintains phosphorus content throughout the life of the bath and does not fail nitric acid testing. In fact, the inventors of the present invention have not been able to change the phosphorus content of the deposit from 12% with any of the tests that were carried out.

(8) The one or more dicarboxylic acids are selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid and pimelic acid and the one or more alpha hydroxy carboxylic acids are selected from the group consisting of glycolic acid, lactic acid, malic acid, citric acid and tartaric acid. Malonic acid is most preferred.

(9) In one preferred embodiment, the plating solution comprises: a) about 30 to about 40 g/L, more preferably about 33 to about 36 g/L, of hypophosphite; b) about 30 to about 40 g/L, more preferably about 33 to about 36 g/L, of lactic acid; c) about 3 to about 6 g/L, more preferably about 4 to about 5 g/L, of succinic acid; and d) about 25 to about 35 g/L, more preferably about 28 to about 31 g/L of malonic acid.

(10) The use of the chelation system described herein in the electroless nickel plating solution produces a nickel deposit having a phosphorus content that remains in the 12% range throughout the life of the bath. This is unique in nickel phosphorus systems, because normally the phosphorus content starts at about 10% to 11% and then climbs to 12%.

(11) The electroless nickel plating solution preferably has a pH of between about 5.2 to about 6.2, more preferably about 5.6 to about 5.7. When the pH of a conventional high phosphorus bath is raised above about 4.9 to 5.0, the phosphorus content of the bath drops and the plating speed increases. This has not allowed a high phosphorus bath to plate above a plating speed of about 0.5 mil/hour and achieve an acceptable phosphorus content of greater than 10%. However, using the unique chelation system described herein, the inventors of the present invention have been able to obtain a deposit having a phosphorus content of 12% from a plating bath having a pH of 5.7 and at a plating rate of at least about 0.9 mil/hour.

(12) The electroless nickel plating using the chelation system described herein is also capable of handling a sulfur compound such as a compound bearing one or more sulfur-containing groups such as —SH (mercapto group), —S— (thioether group), C═S (thioaldehyde group, thioketone group), —COSH (thiocarboxyl group), —CSSH (dithiocarboxyl group), —CSNH.sub.2 (thioamide group) and —SCN (thiocyanate group, isothiocyanate group). The sulfur-containing compound may be either an organic sulfur compound or an inorganic sulfur compound. Specific compounds include compounds selected from the group consisting of thioglycolic acid, thiodiglycolic acid, cysteine, saccharin, thiamine nitrate, sodium N,N-diethyl-dithiocarbamate, 1,3-diethyl-2-thiourea, dipyridine, N-thiazole-2-sulfamylamide, 1,2,3-benzotriazole 2-thiazoline-2-thiol, thiazole, thiourea, thiozole, sodium thioindoxylate, o-sulfonamide benzoic acid, sulfanilic acid, Orange-2, Methyl Orange, naphthionic acid, naphthalene-.alpha.-sulfonic acid, 2-mercaptobenzothiazole, 1-naphthol-4-sulfonic acid, Scheffer acid, sulfadiazine, ammonium rhodanide, potassium rhodanide, sodium rhodanide, rhodanine, ammonium sulfide, sodium sulfide, ammonium sulfate etc. thiourea, mercaptans, sulfonates, thiocyanates, and combinations of one or more of the foregoing. The inventors of the present invention have found that an electroless nickel plating solution using the chelation system described by herein is capable of handling one of the above described sulfur compounds as a stabilizer without failing nitric acid testing. It was previously believed that a high phosphorus plating compositions containing a sulfur compound would fail nitric acid testing. Typically, stabilizer systems for high phosphorus electroless nickel include iodine compounds with small amounts of lead or antimony or tin. Small amounts of bismuth will also fail nitric acid testing and thus the use of bismuth has never been an acceptable alternative for use in high phosphorus systems.

(13) In one embodiment, the present invention describes an ELV-compatible system that contains iodine as the stabilizer for the electroless nickel plating bath without the inclusion of any heavy metals such as lead or antimony. In one preferred embodiment, the electroless nickel plating solution of the invention contains about 100 to about 140 mg/L of an iodine compound, more preferably about 110 to about 130 mg/L, and most preferably about 115 to about 125 mg/L of the iodine compound. Suitable iodine compounds include potassium iodate, sodium iodate and ammonium iodate. In a preferred embodiment, the iodine compound is potassium iodate.

(14) In addition to the iodine compound, the stabilizer component may also preferably contain a sulfur compound. One suitable sulfur compound is saccharin which is used in an amount of between about 150 to 250 mg/L, more preferably about 175 to 225 mg/L, and most preferably about 190 to about 210 mg/L. Other sulfur compounds described herein would also be usable in combination with the iodine compound to stabilizer the electroless nickel plating bath.

(15) The electroless nickel plating bath may also comprise a brightener system. In one embodiment, the brightener system of the invention comprises a bismuth/taurine brightener system comprising about 2 to about 4 mg/L, more preferably about 2.5 to about 3.5 mg/L of bismuth and about 0.5 to about 3 mg/L, more preferably about 1.0 to about 1.5 mg/L taurine. In addition, the pH of the plating bath was increased to 6.1 because the stabilizer would be expected to slow down the plating rate. In this instance, a plating deposit was produced having a phosphorus content of 12%, a gloss of 120 and a plating rate of about 0.75 mil/hour.

(16) In another embodiment, the present invention relates generally to a method of producing an electroless nickel phosphorus deposit on substrate, wherein the electroless nickel phosphorus deposit has phosphorus content of about 12%, the method comprising the steps of: contacting the substrate with an electroless nickel phosphorus plating solution comprising: a) a source of nickel ions; b) a reducing agent comprising a hypophosphite; and c) a chelation system comprising: i) one or more dicarboxylic acids; and ii) one or more alpha hydroxy carboxylic acids; for a period of time to provide a nickel phosphorus deposit on the substrate having a phosphorus content of about 12%; wherein the electroless nickel plating solution produces a nickel deposit having a phosphorus content that remains at about 12% throughout the lifetime of the electroless nickel plating solution.

(17) The lifetime of the electroless nickel plating solution is defined in terms of metal turnovers (MTO). In one embodiment, the lifetime of the electroless nickel plating solution comprises at least 3 metal turnovers, more preferably, the lifetime of the electroless nickel plating solution comprises at least 5 metal turnovers.

(18) The plating rate of the electroless nickel solution on the substrate is preferably at least 0.5 mil/hour, more preferably at least 0.9 mil/hour.

(19) Additionally, depending on the high phosphorus system, the stress of the deposit is normally in the range of between about 20,000 and 30,000 which is too high for many applications. The inventors of the present invention have also discovered that thiourea may be continuously added to the replenisher solution to maintain a stress of less than 15,000 PSI tensile at 5 MTO's, more preferably, less than about 2500 PSI tensile at 5 MTO's.

(20) A range of about 0.2 to about 2.0 mg/l/MTO of thiourea, more preferably about 0.5 to about 1.5 mg/l/MTO of thiourea in the replenisher solution was found to reduce the stress of the deposit to about 2100 PSI and 5 MTO's.

(21) The duration of contact of the electroless nickel solution with the substrate being plated is a function which is dependent on the desired thickness of the nickel-phosphorus alloy. The contact time can typically range from as little as about one minute to several hours. A plating deposit of about 0.2 to about 1.5 mils is a typical thickness for many commercial applications, while thicker deposits (i.e., up to about 5 mils) can be applied when wear resistance is desired.

(22) During the deposition of the nickel alloy, mild agitation may be employed, including, for example, mild air agitation, mechanical agitation, bath circulation by pumping, rotation of a barrel for barrel plating, etc. The plating solution also may be subjected to a periodic or continuous filtration treatment to reduce the level of contaminants therein. Replenishment of the constituents of the bath may also be performed, in some embodiments, on a periodic or continuous basis to maintain the concentration of constituents, and in particular, the concentration of nickel ions and hypophosphite ions, as well as the pH level within the desired limits.

(23) The invention will now be illustrated according to the following non-limiting example:

Example 1

(24) A chelation system was prepared comprising:

(25) TABLE-US-00001  34 g/L lactic acid 4.1 g/L succinic acid  30 g/L malonic acid

(26) This chelation system was added to an electroless nickel plating solution comprising:

(27) TABLE-US-00002  6 g/L nickel sulfate 20 g/L sodium hypophosphite
Temperature:
pH:

(28) It was observed that the phosphorus content remained in the 12% range throughout the lifetime of the bath.

(29) Additions of medium phosphorus chelators and sulfur compounds to the bath did not affect the phosphorus content of the bath or hurt the nitric acid test.

(30) The nitric acid test is a quality control test for electronic components. The standard nitric acid test is a test of passivity and consists of immersing a coated coupon or part into concentrated nitric acid (approximately 70 wt. %) for 30 seconds. If the coating turns black or grey during the immersion, it fails the test.

(31) In this instance, coatings prepared in accordance with Example 1 passed the nitric acid test.

(32) In addition, the neutral salt spray (NSS) test is a measure of the degree of corrosion, blistering, or under-creep of the test samples after exposure to very harsh weathering conditions in a controlled environment. It is conducted according to AS 2331.3.1 (Methods of test for metallic and related coatings). This accelerated test consists of a solution of salt and water sprayed at test samples for a continuous period of 1,000 hours. The test simulates the performance of the coated mesh in a coastal and corrosive environment.

(33) Coatings prepared in accordance with Example 1 also passed the NSS test.

(34) The nitric acid test is actually a test of passivity and was originally developed by the RCA Labs in New Jersey in the 1960's as a quality control test for incoming electronic components. The standard nitric acid test is an immersion of a coated coupon or part into concentrated nitric acid (70 percent by weight concentration) for 30 seconds. If the costing turns black or grey during the immersion, it fails the test.