Coating processes – Immersion or partial immersion – Metal base
Patent
1996-09-13
1998-04-14
Utech, Benjamin
Coating processes
Immersion or partial immersion
Metal base
4274431, 106 113, 106 123, 106 126, B05D 118
Patent
active
057389144
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the use of a compound. In particular, the present invention relates to the use of ethylene diaminedisuccinic acid "EDDS ".
BACKGROUND OF THE INVENTION
Electroless plating solutions are used to deposit metals on to substrates. The electroless plating process is very, sensitive and dependent on many factors such as the stability of the complexant and the resultant complexes used to deposit metals on substrates. In addition, the crystallinity, the homogeneity and the adherence of the deposited metal films are dependent on factors such as the stability constant of the complexant used.
A commonly used electroless plating solution is an electroless copper plating solution. In a typical process, a substrate, such as a circuit board, is placed in a solution of copper ions. At the metal/solution interface copper ions are reduced and copper is then "deposited" on to the substrate.
However, there are problems with electroless copper plating solutions. For example, copper hydroxide precipitates can be formed at high pH. Also, copper metal will homogeneously nucleate when present in solutions containing high levels of copper ions. Moreover, if the rate of deposition in the coating process is too fast one can get an uneven and/or an inadherant coating which, for some applications, is undesirable. Also, if excessive copper ions are deposited uncontrollably, the life of the plating solution is reduced and the process will become inoperative if there are no free copper ions.
In order to try to overcome these problems a chelating agent is added to the electroless copper plating solution to restrict the levels of free copper ions. A typical chelating agent is EDTA (ethylene diamine tetra-acetic acid).
Whilst EDTA may reduce to some extent the onset of copper hydroxide precipitation and control the level of copper deposition, it does suffer from the drawback that it is non-biodegradable. The manufacturer of, for example, circuit boards is then faced with the problem of how to remove the EDTA prior to disposal of the rinse water.
The present invention seeks to overcome the problems associated with the use of EDTA in electroless metal plating processes.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided an electroless metal plating solution comprising metal ions to be reduced for plating on to a substrate and a chelate for the metal ions to prevent or reduce metal hydroxide precipitates forming and to buffer the amount of metal ions available for reduction, wherein the chelate is at least EDDS.
According to a second aspect of the present invention there is provided a process of plating a substrate with a metal comprising exposing the substrate to the electroless metal plating solution as defined above.
According to a third aspect of the present invention, there is provided the use of EDDS in an electroless metal plating solution as a biodegradable chelate.
The term "EDDS" includes racemic EDDS or optically active isomers thereof, such as (s.s)EDDS. and active salts and active derivatives thereof. Preferably the term means (s.s)EDDS or salts thereof. Preferably the EDDS is (s,s)EDDS. More preferably the EDDS is (s,s)EDDS as prepared by the process of PCT/GB94/02397 filed 2 Nov. 1994.
Suitable metals to be reduced include nickel and copper.
Preferably the metal to be reduced is copper.
The term "active" means the ability to prevent metal hydroxide precipitates forming and to buffer the amount of metal ions available for reduction, but also have the property of being biodegradable.
The term "at least EDDS" means that other chelates may be present. Preferably the chelate is EDDS alone.
The term "substrate" includes integrated circuits, circuit boards, conductor circuits on printed circuit boards and ceramic substrates, electromagnetic wave shielding for telecommunication applications and engineering substrates.
In more detail, the present invention is based on the surprising discovery that the use of EDDS, which is biodegradable, prevents metal hydr
REFERENCES:
patent: 3158635 (1964-11-01), Kezerian et al.
patent: 4704233 (1987-11-01), Hartman et al.
patent: 5316898 (1994-05-01), Ueda et al.
patent: 5569443 (1996-10-01), Wilson et al.
The Associated Octel Company Limited
Utech Benjamin
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