Method and solution for producing gold coating

Details Method and solution for producing gold coating Method and solution for producing gold coating

2000-07-07

2002-01-08

Klemanski, Helene (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Metal-depositing composition or substrate-sensitizing...

C106S001260, C427S443100, C427S437000, C205S085000, C205S181000, C205S187000, C205S265000, C205S267000, C205S271000

Reexamination Certificate

active

06336962

ABSTRACT:

BACKGROUND OF THE INVENTION
Gold layers are used to produce solderable, gluable and bondable surfaces on circuitboards and other electronic components and to produce electric contacts. Such layers were originally produced by electrolytic methods. However, there are some major disadvantages to methods where electrolytic gold deposition can be used. In many cases, gold cannot be deposited electrolytically at all, because surfaces that electrically insulated from one another must be gold plated.
Therefore, electroless methods have been developed, whereby gold can be deposited from a suitable solution without external electric contacting. Baths used in such methods are used in particular in the production of high-grade circuitboards or circuit substrates for integrated circuits. The final layer required for the soldering process is usually formed from nickel or a nickel alloy deposited in an electroless operation and a thin layer of gold deposited on top of that. The gold layer is only 0.05 &mgr;m to 0.2 &mgr;m thick. Thinner layers cannot adequately protect the underlying nickel. On the other hand, thicker layers cannot be deposited without permanently damaging the nickel layers.
Solutions for electroless deposition processes contain reducing agents such as ascorbic acid, hydrazine, hydroxylamine, dimethylaminoborane, trimethylaminoborane or formaldehyde. Thiourea and its derivatives have also been proposed as reducing agents. These solutions contain complex gold compounds such as halogeno complexes, cyano complexes, thiosulfato complexes or sulfito complexes as the gold salts.
U.S. Pat. No. 5,202,151 describes a method of electroless deposition of gold, where gold(I) ion complexes of thiosulfate or sulfite are used together with thiourea, derivatives thereof or hydroquinone as the reducing agent. In addition, sulfite may also be added to the solution to stabilize it. The pH of the solution is in the range of 7 to 11. According to the specifications in this U.S. Patent, gold is deposited on nickel, cobalt and gold layers at a maximum rate of 1 &mgr;m/hour. It is mentioned that the deposition rate is too low at a pH lower than 7.
International Application WO-A 92/00398 describes a combination of aqueous baths for electroless deposition of gold. The precoating bath contains disulfitoaurate(I) complexes, an alkali or ammonium sulfite as stabilizer, a reducing agent and another complexing agent. The reducing agents are aldehydes, such as formaldehyde, or derivatives thereof, and ethylenediamine, for example, may be added as an additional complexing agent. A second bath contains, in addition to dicyanoaurate(I) complexes, cobalt(II) salts and thiourea. The gold layers are again deposited on nickel or nickel alloy surfaces. The precoating bath is adjusted to a maximum pH of 8.
U.S. Pat. No. 5,318,621 describes an electroless metal deposition solution for silver and gold, containing a non-cyanide metal complex with thiosulfate, sulfite and ethylenediaminetetraacetic acid as the complexing agent and at least one amino acid to increase the deposition rate. Examples are given there for producing silver layers on copper surfaces coated with nickel layers. The solutions have a pH in the range of 7 to 9, preferably 7.5 to 8.5.
International Application WO-A 92/02663 describes a stable, electroless acid gold bath containing tetracyanogold(III) complexes, at least one complexing agent having carboxyl groups and phosphonic acid groups and an acid, where the pH of the solution is set to less than 1. The gold layers are deposited, for example, on layers of iron
ickel/cobalt alloys, nickel or tungsten surfaces at the rate of 0.2 to 0.5 &mgr;m in 30 minutes.
U.S. Pat. No. A 5,470,381 discloses an electroless gold deposition solution containing tetrachlorogold(III) complexes or gold(I) complexes with thiosulfate or sulfite as the complexing agent, plus ascorbic acid, a pH buffer and organic sulfur compounds to stabilize the solution to prevent spontaneous decomposition. The solution is adjusted to a pH of 5 to 9, preferably 6 to 8. The gold layers are applied to a substrate to which has been applied first a 3 &mgr;m thick layer of nickel and then a 3 &mgr;m thick layer of gold. The deposition rate is 0.6 to 1 &mgr;m/hour.
U.S. Pat. No. 5,364,460 describes another electroless gold deposition bath which contains a gold-sulfito complex and a reducing agent from the group of hydrazine, ascorbic acid, trimethylaminoborane and dimethylaminoborane. The solution also contains, for example, amino acids or aminobenzoic acid to increase the deposition rate. The pH of the solution is in the range of 6 to 11, preferably 7 to 9. The deposition rate is given as approximately 0.1 to about 5 &mgr;m/hour. It is stated that the deposition rate is too low when the pH is below 6. The layers are deposited on copper, nickel/boron or nickel/phosphorus layers on circuitboards or ceramic chip carriers.
One of the known deposition solutions contains, in addition to the gold salts, considerable quantities of reducing agent(s), such as ascorbic acid, boranes, hydrazines or formaldehyde. However, this causes the solutions to be susceptible to precipitation of metallic gold. There have been various attempts to improve the stability of the solutions against this spontaneous decomposition: for example, organic sulfur compounds have been added to the solution (U.S. Pat. No. 5,470,381) or nonionic surfactants or polymers have been used as stabilizers (U.S. Pat. No. 5,364,460).
However, it has been found that it is very difficult to stabilize the reductive gold baths. Even if no instability can be found by a test in which a precipitate of gold from the solution is tested, gold may nevertheless under some circumstances be deposited on surface areas that should not have a catalytic effect for gold deposition. In the production of finely structured electronic components in particular, this wild growth poses problems, so that even the slightest stability problems must be carefully prevented.
Therefore, baths containing no reducing agent have been developed. Such baths contain thiourea, for example, or a complexing agent such as ethylenediaminetetraacetic acid or salts thereof to accelerate the charge exchange of the gold ions with the less noble metal substrate. Most proposals have been for deposition of gold layers on nickel or nickel alloy surfaces.
Such a bath is described in British Patent Application No. 1,022,061. This bath contains a gold salt and ethylenediaminetetraacetic acid as a complexing agent and also, for example, nickel, cobalt or copper ions bound to salicylic acid, for example in a complex to permit deposition of thick gold layers. The pH of the bath is at least 6.5 and is therefore unsuitable for coating a substrate which is already coated with a photoresist or a screen printing resist. The gold layers are deposited on a nickel substrate.
Nickel is dissolved in deposition of gold. It is therefore very important to interrupt the gold deposition process after a certain amount of time to prevent excessive loss of nickel due to corrosion. In this case, the deposited gold layer could lose contact with the underlying gold layer, so that the gold layer would become delaminated. If delamination of gold occurs, the gold layer becomes worthless for any further processing, e.g., soldering operations.
For these reasons, the final layers of nickel or a nickel alloy and a layer of gold above it have been successful only for soldering applications and as a contact layer for ultrasonic bonding of integrated semiconductor circuits by the ultrasonic bonding technique. However, nickel/gold layers are not suitable for thermosonic bonding techniques, because the layer thickness that can be achieved is not sufficient. For this case, the above-mentioned electroless gold baths could be used, because thicker gold layers could also be produced with them. However, such baths have the disadvantages mentioned above. In some cases, these baths are also alkaline and therefore cannot be used for processing circuits with photoresists and screen print

Affiliated with

Also associated with

Rating

Method and solution for producing gold coating does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Method and solution for producing gold coating, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and solution for producing gold coating will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2862691