US3833487A - Electrolytic soft gold plating - Google Patents

Electrolytic soft gold plating Download PDF

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Publication number
US3833487A
US3833487A US00317600A US31760072A US3833487A US 3833487 A US3833487 A US 3833487A US 00317600 A US00317600 A US 00317600A US 31760072 A US31760072 A US 31760072A US 3833487 A US3833487 A US 3833487A
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United States
Prior art keywords
procedure
solution
potassium
lead
gold
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Expired - Lifetime
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US00317600A
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English (en)
Inventor
H Reinheimer
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AT&T Corp
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Bell Telephone Laboratories Inc
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Publication date
Application filed by Bell Telephone Laboratories Inc filed Critical Bell Telephone Laboratories Inc
Priority to US00317600A priority Critical patent/US3833487A/en
Priority to CA178,757A priority patent/CA1019275A/en
Priority to GB5835473A priority patent/GB1434429A/en
Priority to NL7317257.A priority patent/NL159440B/xx
Priority to BE138977A priority patent/BE808760A/xx
Priority to DE2363462A priority patent/DE2363462C3/de
Priority to FR7346014A priority patent/FR2211539B1/fr
Priority to IT70847/73A priority patent/IT1000615B/it
Priority to JP14253673A priority patent/JPS5313334B2/ja
Application granted granted Critical
Publication of US3833487A publication Critical patent/US3833487A/en
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D3/00—Electroplating: Baths therefor
    • C25D3/02—Electroplating: Baths therefor from solutions
    • C25D3/48—Electroplating: Baths therefor from solutions of gold

Definitions

  • ABSTRACT Characteristics of soft gold plating produced by electrolysis are improved by adding trace amounts of lead to the gold plating solution and by replenishing such content during use. Such characteristics include improved brightness and uniformity of deposits, better defined crystallinity of deposits, enlarged current density range, and extended solution life. 'Lead content generally less than about 1 ppm, based on the solution, does not affect purity of gold deposit as measured by ordinary analytical techniques.
  • the invention is concerned with soft gold electrolytic plating, i.e., plating of high purity gold.
  • Electrolytic soft gold plating is of appreciable significance in the fabrication of piece parts and circuit apparatus, e. g., semiconductor devices, integrated circuitry, etc. Purposes served by such deposited pure gold layers are protection of devices, conducting electric current and bonding.
  • gold plating solutions ordinarily include gold in the form of a soluble complex-generally potassium or sodium cyanoaurate (potassium or sodium gold dicyanide).
  • a soluble complex generally potassium or sodium cyanoaurate (potassium or sodium gold dicyanide).
  • Use of such a complex permits reasonable plating rates from solutions containing relatively small amounts of gold.
  • Conventional procedure calls for replenishing such bath during use by additions of the same salt. It is common toreplenish after a small fraction of the gold content has been depleted and to carry out such replenishment many times so as to have equivalent turn arounds of at least 10 or many tens.
  • Electrolytic soft gold deposits produced from commercial solutions under well controlled conditions are generally of sufficient purity. Morphological character istics are, unfortunately, variable, however, and best platings (bright yellow and of uniform fine crystallinity, I
  • Platings show no significant variation in appearance or other characteristics during long bath use and, particularly over the low range of lead addition (up to about 500 ppb), show no detectable lead content as measured by ordinary analytical techniques.
  • Gold plating baths to which the invention is applicable include those having a pH range of from 4.0 to 14.0.
  • FIGURE on coordinates of cathode potential in millivolts and time in minutes, shows the relationship between these two parameters fora plating solution maintained in accordance with the invention as well as for two unmodified solutions, one freshly prepared and the other after extensive use. Two plots, one for a density of 3 mA/cm and one for 10 mA/cm are included.
  • Plating was carried out in a one liter solution maintained at a temperature of 60 to C on a 10 cm cathode ata constant current equivalent to a density of either 3 or 10 milliampereslcm
  • Ordinate values represent cathode potential in millivolts as measured using a Luggin capillary. Abscissa values are time in minutes. Curve 1, corresponding with a fresh solution of the described composition, resulted in a bright yellow uniform coating at a current density. of 3mA/cm
  • the cathode potential increased from a nucleation value of about 900 millivolts to an ultimate value of approximately -700 millivolts, with this final stabilized value being obtained only after a period of approximately 25 minutes.
  • Curve 2 is'plotted from an old solution having been replenished with gold dicyanoaurate to about three turn arounds. Plating under the conditions noted was coarse and dark and generally unacceptable for electronic device use. It is seen that the plotted data commences at a nucleation cathode potential of a lesser value than the 900 millivolts of Curve 1. Continuing plating over the period represented resulted in no significant change in cathode potential. In fact, the particular experiment from which the data was taken wascarried on for a period of the order of 1 hour with no further change in cathode potential.
  • Curve 3 is plotted from data taken from a plating procedure as described but utilizing a solution which has been modified by the addition of 500 parts per billion of lead added as lead (II) oxide in 0.1n KOH solution. It is seen that nucleation again occurred at a cathode potential value of the order of --900 millivolts.
  • the FIGURE is believed to give an indication of the governing mechanism resulting from lead maintenance in accordance with the invention. As indicated, curves of the type shown were consistently maintained under all conditions and using all types of compositions. Bright deposits characterized by lateral growth of high crystallinity were observed where cathode potential was 900 millivolts and more positive, while dark brown coatings indicating outward random growth were obtained at values more negative than 900 millivolts. This data suggests that the trace lead content serves as a depolarizing agent during-deposition.
  • Atti Della Academia Delle Scienze di Torino, Classe di Scienze Fisiche, Matematiche e Naturali, Vol. 99, p. 1111 (1965).
  • Curve 3 Another significant characteristic difference evidenced by Curve 3 is the rapid potential stabilization at constant current level. This curve is characteristic in indicating stabilization in a period of minutes after the onset of current flow to a constant, less negative cathode potential (assuming constant pH, temperature, current density, agitation). Commercial soft gold plating baths which have not been modified in accordance with the invention may take as much as 40 minutes to reach a stabilized cathode potential under similar plating conditions.
  • Gold approximately 3 g/l to solubility limit.
  • the most common salt for plating electronic devices is potassium dicyanoaurate, KAu( CN)
  • KAu( CN) A common alternative, the corresponding sodium complex salt, is generally undesirable for electronic purposes since it may result insodium contamination.
  • the low limit of about 3 g/l permits a plating current-density of approximately 2 mA/cm. In general, lower gold content is uncommon, due to the limited current density range and frequent need for replenishment. While the'absolute limit isthe solubility limit corresponding with about 145 g/l for KAu(CN) at room temperature, a lower preferred maximum is usually specified.
  • This preferred maximum is about 120 g/l and is dictated by the desire to minimize loss of gold through dragout (significant loss of gold in solution in the wetting layer on the withdrawn cathode).
  • a gold salt content of 20 g/l is sufficient for plating to a practical maximum rate under most conditions and was used for the solutions reported in examples herein.
  • This maximum rate corresponds with a current density of about 9 mA/cm for unmodified commercial baths (beyond which platings generally have undesirable characteristics) and with about 40 mA/cm for a modified bath in accordance with the invention,
  • Additional salts 25 g/l 250 g/l are incorporated for either of two reasons; to attain (and maintain) desired pH, and/or to maintain desired ionic conductivity level. Where a buffer salt system is incorporated, it may inherently increase the conductivity to the desired level thereby eliminating need for a conductivity-increasing constituent. It will be recognized that the limits indicated are primarily practical. A minimum of 15 g/l of usual salts such as phosphate, citrate, or acetate assures a conductivity of the order of 0.015 -0.025 Mhos at a temperature of 25C. This minimum is also generally required. for most buffered systems to produce sufficient buffer action to maintain pH over reasonable life at reasonable plating rates.
  • usual salts such as phosphate, citrate, or acetate
  • the indicated maximum exceeds the quantity of buffer ordinarily required to maintain pH during expected life.
  • the solutions used for the plating procedures which resulted in the data plotted for the FIG- URE were buffered to a value of pH 10.0 by use of 100 g/l of Kid- P0, (corresponding with 70 g/l of P0 together with about 50 g/l of KOH.
  • Exemplary salts include the dibasic and tribasic phosphates (generally potassium, ammonium sodium is avoided for the same reason that it is undesirable asthe cation in the gold complex) as well as ammonium salts including citrate, sulfate, phosphate, potassium carbonate, potassium bicarbonate, potassium acetate, potassium cyanide, and corresponding acids such as phosphoric, citricand acetic acid, etc.
  • the basic member of the buffer system where ever needed, is commonly potassium, hydroxide, although other alkaline material may be utilized. Tests have also been conducted successfully in unbuffered potassium dicyanoaurate solution at higher pH (10 tol3). Salts which may be used for increasing conductivity without having a significant effeet on pH include potassium sulfate, potassium cyamate, and potassium formate.
  • the lead modification in accordance with the inven- 13, L d Additi 5 tion, has no significant effect on temperature.
  • lead level in terms f parts f metal per mon with unmodified gold solutions it is desirable to liter of solution is, from the standpoint of maintenance, operate wlthm the range of from to 9 3 desirably maintained at a minimum of 100 ppb, ale range defined as from 60 to 85 Ab0ve though 20 ppb results in improvement.
  • a preferred minimum is about 100 ppb since this Son PP P "mung the enumerated examples Set level is adequate to permit maximum plating rates and forth tabular form below- Platmg was on 3 Hull cell is sufficiently high so that the rate of depletion does not Panel Hull & Cleveland, Ohio 44102) require lead replenishing more frequently than the con- 30 50 disposed relative to the anode as to result in a ventional rate of gold replenishing.
  • rent density range of from 1 to 22 mA/cm across the Load may be added in any form which is soluble in panel. In each instance, the deposition was bright yelthe solution.
  • Formula Amount g/l Formula Amount g/l pH "C KHQPO, m0 KOH 5o 10 657() 2 KHZPO, 100 KOH 7 6570 3 KH2PO, 10o 4.3 65-70 4 (NH4)2HC,,H -,O, 50 (Nrnnso, 50 4.8-5 65-70 5 NH.H. .P0, x5 4.5 65-70 a NH,H2PO, 85 Cone.
  • rent density is permitted, e.g., up to about 40 mA/cm Atomic absorption and also emission spectroscopy with appropriate agitation. Such a density resulted in was utilized for determining lead content in gold deposhigh quality deposits where the cathode was itself pulits. While standard methods are unreliable for measursated, for example, over an amplitude of approximately ing lead levels of less than 100 ppm, averaging over a 2.5 cm with a periodicity of approximately 200 pulses large number of samples was considered to produce acper minute. Aside from this one change, which is opceptable results. As indicated above, solution lead levtional, apparatus and procedure may be conventional.
  • Carbon content in deposits also at a low level when plating in high quality solutions designed for electronic device use, is apparently lowered still further when baths are modified in accordance with the invention.
  • Procedure for the electrolytic plating of soft gold comprising biasing a first electrode to be plated cathodic relative to a second electrode, both electrodes being at least partially-immersed within an aqueous solution having a pH of from 4.0 to 14.0 and containing dicyanoaurate, characterized in that said solution consists essentially of the constituents set forth together with lead contained at least in part as a dissolved compound with the lead content expressed in terms of the element being maintained in solution by dissolving during substantially the entirety of said procedure at a level of between about 20 ppb and 2 ppm based on the said solution.
  • the buffer system includes a pH adjusting agent selected from the group consisting ofi(phosphoric acid) H PO (acetic acid) CH COOH, (potassium hydroxide) KOH, and (ammonium hydroxide) NH OH.
  • a pH adjusting agent selected from the group consisting ofi(phosphoric acid) H PO (acetic acid) CH COOH, (potassium hydroxide) KOH, and (ammonium hydroxide) NH OH.
  • Procedure of claim 5 in which the amount of gold present in the said solution is maintained between a level of from 3 grams per liter to grams per liter based on the content of dicyanoaurate in the solution.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electroplating And Plating Baths Therefor (AREA)
US00317600A 1972-12-22 1972-12-22 Electrolytic soft gold plating Expired - Lifetime US3833487A (en)

Priority Applications (9)

Application Number Priority Date Filing Date Title
US00317600A US3833487A (en) 1972-12-22 1972-12-22 Electrolytic soft gold plating
CA178,757A CA1019275A (en) 1972-12-22 1973-08-14 Electrolytic soft gold plating from bath containing lead
NL7317257.A NL159440B (nl) 1972-12-22 1973-12-17 Werkwijze voor het elektrolytisch aanbrengen van zuivere goudafzettingen.
GB5835473A GB1434429A (en) 1972-12-22 1973-12-17 Electrolytic gold plating
BE138977A BE808760A (fr) 1972-12-22 1973-12-18 Procede d'electrodeposition d'or malleable
DE2363462A DE2363462C3 (de) 1972-12-22 1973-12-20 Verfahren zum galvanischen Abscheiden von Weichgoldschichten
FR7346014A FR2211539B1 (xx) 1972-12-22 1973-12-21
IT70847/73A IT1000615B (it) 1972-12-22 1973-12-21 Procedimento di placcatura elettro litica con oro dolce
JP14253673A JPS5313334B2 (xx) 1972-12-22 1973-12-21

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US00317600A US3833487A (en) 1972-12-22 1972-12-22 Electrolytic soft gold plating

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US (1) US3833487A (xx)
JP (1) JPS5313334B2 (xx)
BE (1) BE808760A (xx)
CA (1) CA1019275A (xx)
DE (1) DE2363462C3 (xx)
FR (1) FR2211539B1 (xx)
GB (1) GB1434429A (xx)
IT (1) IT1000615B (xx)
NL (1) NL159440B (xx)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4207149A (en) * 1974-12-04 1980-06-10 Engelhard Minerals & Chemicals Corporation Gold electroplating solutions and processes

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3341233A1 (de) * 1983-11-15 1985-05-30 Robert Bosch Gmbh, 7000 Stuttgart Saures galvanisches bad zur abscheidung feiner strukturen aus weichgold sowie verfahren zur abscheidung derartiger strukturen
DE4026710A1 (de) * 1990-08-24 1992-02-27 Ant Nachrichtentech Verfahren zur goldbeschichtung von keramikschaltungen

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3380898A (en) * 1965-06-18 1968-04-30 Sel Rex Corp Electrolyte and method for electrodepositing a pink gold alloy
US3475292A (en) * 1966-02-10 1969-10-28 Technic Gold plating bath and process

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
BE484443A (xx) * 1947-08-19
US3423295A (en) * 1966-02-23 1969-01-21 Engelhard Ind Inc Gold plating
US3671408A (en) * 1971-05-25 1972-06-20 Sel Rex Corp Rhodium-platinum plating bath and process

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3380898A (en) * 1965-06-18 1968-04-30 Sel Rex Corp Electrolyte and method for electrodepositing a pink gold alloy
US3475292A (en) * 1966-02-10 1969-10-28 Technic Gold plating bath and process

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
P. F. Thompson, J. Electro Chem. Soc., Vol. 91, pp. 62 63, (1947). *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4207149A (en) * 1974-12-04 1980-06-10 Engelhard Minerals & Chemicals Corporation Gold electroplating solutions and processes

Also Published As

Publication number Publication date
FR2211539A1 (xx) 1974-07-19
BE808760A (fr) 1974-04-16
CA1019275A (en) 1977-10-18
FR2211539B1 (xx) 1976-06-25
IT1000615B (it) 1976-04-10
JPS5313334B2 (xx) 1978-05-09
DE2363462B2 (de) 1977-09-22
GB1434429A (en) 1976-05-05
NL7317257A (xx) 1974-06-25
DE2363462C3 (de) 1978-05-11
NL159440B (nl) 1979-02-15
DE2363462A1 (de) 1974-07-11
JPS4991046A (xx) 1974-08-30

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