Coating processes – Immersion or partial immersion – Metal base
Reexamination Certificate
2002-02-19
2004-10-19
Wong, Edna (Department: 1753)
Coating processes
Immersion or partial immersion
Metal base
C106S001260, C205S125000, C205S187000
Reexamination Certificate
active
06805915
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an electroless copper plating solution mainly used for a formation of circuits in electronic parts, an electroless copper plating process and process for producing a circuit board each using said electroless copper plating solution; and particularly to a plating solution and plating technique each using glyoxylic acid but not using formaldehyde having a high volatility as a reducing agent for copper ion.
BACKGROUND OF THE INVENTION
In JP-A-7-268638, there is proposed a plating process which comprises carrying out a plating while filtering a plating solution with the aim of preventing a body to be plated from a deposition of sodium oxalate precipitate formed in an electroless copper plating solution.
In JP-A-61-183474, there is mentioned a technique of using glyoxylic acid as a reducing agent in electroless copper plating solution. In this document, it is mentioned that NaOH or KOH is used for alkalifying the pH of electroless copper plating solution, and especially that KOH is more desirable than NaOH because potassium oxalate is higher than sodium oxalate in solubility of an oxalate salt which is an oxidized product of glyoxylic acid.
SUMMARY OF THE INVENTION
An electroless copper plating solution usually comprises copper ion, a complexing agent for copper ion, a reducing agent for copper ion and a pH adjusting agent.
As the reducing agent for copper ion, formaldehyde, glyoxylic acid or a salt of glyoxylic acid has been used generally. In plating solutions, an ion of oxidized product of reducing agent accumulates. In the case of using formaldehyde as a reducing agent for copper ion, the accumulating substance is formate ion. In the case of using glyoxylic acid as the reducing agent, the accumulating substance is oxalate ion.
On the other hand, as the pH adjusting agent in electroless copper plating solutions, NaOH has been generally used.
When NaOH was used as a pH adjusting agent and glyoxylic acid was used as a reducing gent, there has arised a difficulty that sodium oxalate is so small in solubility that precipitate of sodium oxalate was formed in the plating solution in the way of plating process. If such a solid precipitate is deposited on a body to be plated, no plating film can be formed in the area to which the precipitate is attached, and there is formed the so-called “void”.
In the case of using glyoxylic acid, the accumulation of oxalic acid in a plating solution is caused not only by the plating reaction but also by the Cannizzaro reaction. In the case of using glyoxylic acid, the Cannizzaro reaction is expressed by the following scheme:
2CHOCOOH+2OH
−
→C
2
O
4
2−
+HOCH
2
COOH+H
2
O
Since reaction rate of this reaction increases with elevation of temperature of plating solution, the progress of Cannizzaro reaction can be suppressed by keeping the plating solution at a low temperature. JP-A-2000-144438 discloses a plating apparatus equipped with a plating-practicing chamber and a circulation tank for circulating the plating solution. In this apparatus, the Cannizzaro reaction can be suppressed by always keeping low the temperature of plating solution present in the circulation tank for storing the plating solution.
Further, in this document, it is also mentioned that methanol can be added to the plating solution in some cases for the purpose of preventing the deterioration of plating solution caused by the Cannizzaro reaction, even though it is not expressly mentioned there whether the Cannizzaro reaction is that of formaldehyde or that of glyoxylic acid.
However, it is mentioned there that the method of adding methanol does not suppress the Cannizzaro reaction itself but the effect of this method has a certain limitation. That is, in the practical use of electroless copper plating solution, no successful case of suppressing the Cannizzaro reaction by the use of methanol has ever been known.
In “Hyomen Gijutsu (Surface Technology)”, Vol.42, No.9, Pages 913-917 (1991) and The 6th national convention Record of Purinto Kairo Jissou Gakkai (Japan Institute of Printed Circuit), Pages 101-102, there is mentioned that, in the electroless copper plating solutions using glyoxylic acid as a reducing agent, the Cannizzaro reaction can be suppressed by using KOH as a pH adjusting agent more effectively than by using NaOH as a pH adjusting agent. Further, these reports refer to the higher solubility of potassium oxalate as compared with that of sodium oxalate, too.
In the case of these reports, nevertheless, NaOH was still used for realizing an alkaline pH value (pH=12.5) at the time of preparing the plating bath. As its result, precipitation of sodium oxalate corresponding to the quantity of sodium ion introduced into the plating solution in the initial stage was unavoidable in the case of these reports.
In the case of using glyoxylic acid as a reducing agent for the electroless copper plating solution, there have so far been problems that the reacting quantity of Cannizzaro reaction is larger, the plating solution is less stable, and the cost is higher than in the case of using formaldehyde as the reducing agent.
As to the stability of plating solution, it can be considered that a high reacting quantity of Cannizzaro reaction results in an elevation of salt concentration in the plating solution, which brings about a reduction of dissolved oxygen concentration in the plating solution and thereby an instability of plating solution.
Further, in the case of using glyoxylic acid, the progress of Cannizzaro reaction or the plating reaction brings about accumulation of oxalic acid, namely the oxidized product of glyoxylic acid, in the plating solution. In the prior electroless copper plating process, it has been conventional to carry out the plating process while adding NaOH to keep the plating solution alkaline. Thus, there has been a problem that, since sodium oxalate is low in solubility, the crystal of sodium oxalate precipitates in the plating solution and deposited on some area of base board, where no plating layer can be deposited and a void is formed.
With the aim of avoiding the rise in the salt concentration in the plating solution and the formation of precipitate of sodium oxalate, a method of adding KOH as a pH adjusting agent for keeping pH value alkaline during the plating treatment has been studied.
The description of “Hyomen Gijutsu (Surface Technology)”, Vol.42, No.9, Pages 913-917 (1991) shows, however, that such a method of using KOH gave so low a Cannizzaro reaction-suppressing effect as 15-40%, as compared with the case of using NaOH. Although the suppressing effect was 40% as compared with NaOH when one hour had passed after start of the plating, the suppressing effect dropped to 15% when 5 hours had passed after start of plating (“Hyomen Gijutsu (Surface Technology)”, Vol.42, No.9, Page 915, line 16 of the main text). Since electroless copper plating solution is usually used for a long period of time, the tendency of decreasing the suppressing effect with time is a fatal disadvantage.
Further, as is mentioned in “Hyomen Gijutsu (Surface technology)”, Vol.42, No.9, Page 913-917 (1991) and JP-A-61-183474, potassium oxalate is higher than sodium oxalate in solubility and hence the use of KOH as a pH adjusting agent is considered advantageous from the viewpoint of preventing the precipitation of oxalate.
However, since a plating solution contains other salts such as complexing agent, additives, counter anion of copper ion, etc. in large amounts, the precipitate of potassium oxalate can be formed at a concentration lower than its saturation solubility in pure water.
Further, in “Hyomen Gijutsu (Surface Technology)”, Vol.42, No.9, Page 913-917 (1991), paragraph of experimental method and in Table 1 of “The 6th national convention Record of Purinto Kairo Jissou Gakkai (Japan Institute of Printed Circuit)”, Pages 101-102, there is mentioned that NaOH was used for the purpose of making the pH value of plating solution alkaline (pH=12.5) at the time of preparing the plating b
Akahoshi Haruo
Iida Tadashi
Itabashi Takeyuki
Kanemoto Hiroshi
Nishimura Naoki
Dickstein , Shapiro, Morin & Oshinsky, LLP
Hitachi , Ltd.
Wong Edna
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