Regeneration of spent electroless copper plating solution

Disclosed is a method for removal of contaminants and replenishment of an electroless copper plating solution in order to allow reuse of the solution. Copper oxide is dissolved in the spent solution and this is followed by an electrodialysis which removes formate and carbonate ions.

BACKGROUND OF THE INVENTION 
This invention is related to an electroless copper plating process. 
Electroless copper plating is a significant step in the fabrication of 
printed circuit boards. One of the problems with the electroless plating 
process is a significant build-up of contaminants such as formate and 
carbonate ions which are detrimental to copper plating quality and bath 
stability. One approach to this problem is simply to replace the bath with 
a new one after several plating cycles (each typically 24 hours). Not only 
is such an approach expensive, it creates serious waste disposal problems. 
In present and future factory processes, it is far more desirable, if not 
mandatory to meet EPA regulations, to replenish the existing baths and 
eliminate the need to discard any material which could be harmful to the 
environment. 
In order to replenish electroless copper baths, it has been suggested to 
use electrodialysis apparatus to remove a variety of contaminants 
therefrom. (See, e.g., U.S. Pat. No. 4,805,553 issued to Krulik, and U.S. 
Pat. No. 5,091,070 issued to Bauer et al.) In practice, it has been 
extremely difficult to apply electrodialysis such that the resultant wash 
is not compromised by the addition of contaminants which cannot be easily 
disposed of, or the subsequent use of the plating solution does not 
adversely affect the final product. 
SUMMARY OF THE INVENTION 
The invention is a method for deposition of copper on an insulating 
substrate utilizing an electroless copper plating bath in a plating tank. 
The method involves removing a portion of the bath from the tank and 
adding copper oxide in order to replenish copper used up during the 
plating. Subsequently, the portion of the bath is transported to an 
electrodialysis apparatus to remove contaminants therefrom, and the 
resulting portion of the bath is removed to the plating tank.

DETAILED DESCRIPTION OF THE INVENTION 
As illustrated in FIG. 1, electroless plating of printed circuit boards, 
e.g., 10, takes place in accordance with standard techniques in a plating 
tank, 11, containing a standard plating bath, 12. For example, the bath 
could comprise a mixture of a source of copper ions from tank 22, a 
reducing agent such as formaldehyde from tank 23, a complexing agent such 
as ethylenediamine tetraacetic acid (EDTA) from tank 24, and sodium 
hydroxide from tank 25. During plating, contaminants such as formate ions 
and carbonate ions accumulate in the plating tank. In addition, copper 
ions become depleted. In order to keep the contaminant and copper levels 
within prescribed limits, a portion of the bath, 12, (hereinafter the 
"bail") is drained or pumped into a holding tank, 13. Coupled to the 
holding tank, 13, is a source of copper oxide, 14. 
The holding tank, 13, is coupled through a pump, 15, to an electrodialysis 
apparatus, 16, which will be described in more detail below with reference 
to FIG. 2. Also coupled to the apparatus, 16, through a pump, 17, is a 
tank, 18, which contains an electrode rinse solution such as a two percent 
sodium formate solution. A waste tank, 19, is also coupled to the 
apparatus, 16, through a pump, 20, in order to transport and receive a 
solution which includes the contaminants from the portion of the plating 
solution. After removal of contaminants by electrodialysis, the bail is 
pumped to recycled tank, 21. 
The basic features of an electrodialysis apparatus, 16, are illustrated 
schematically in FIG. 2. The actual machine employed in this example was 
an Electrodialyzer Model DS-O sold by Asahi Glass Company of Japan. The 
electrodialysis cell, 30, is basically a sealed chamber, 31, with a 
cathode, 32, at one end and an anode, 33, at the other end. The cathode 
was stainless steel and the anode was platinum-plated titanium. The anode 
and cathode are separated by a plurality of pairs of anion exchange 
membranes, e.g., 34, and cation exchange membranes, e.g., 35. While only 
two pairs of such membranes are illustrated, it will be appreciated that 
the cell typically includes several more pairs. In this example, ten pairs 
of membranes were utilized. The membranes were made of a styrene divinyl 
benzene polymer and were the type sold by Asahi Glass Company under the 
designation Selemion CMV and AMV membranes. 
As shown, the bail from holding tank, 13, was directed into the chamber, 
31, at alternate spaces between opposing membranes, i.e., between 
membranes 34 and 35 and membranes 36 and 37 in this example. The solution 
from waste tank, 19, which in this example comprises a 0.2 percent sodium 
formate solution, was fed into the alternate spaces between membranes not 
occupied by the plating solution, i.e., between membranes 34 and 37 in 
this example. The electrode rinse solution from tank, 18, which, in this 
example, comprises a 2 percent sodium formate solution, was fed into the 
portions of chamber 31 which include the anode, 33, and cathode, 32. 
When a DC voltage was supplied to the anode and cathode, the sodium ions, 
represented by "+" in the figure, from the bail migrated toward the 
cathode, while the formate and carbonate ions, represented by "-", from 
the plating solution migrated toward the anode. The sodium ions passed 
through the cation exchange membranes, 35 and 37, but were blocked by the 
anion exchange membrane, 34. Similarly, the formate and carbonate ions 
passed through the anion exchange membranes, 34 and 36, but were blocked 
by the cation exchange membrane, 37. Thus, the contaminants formate and 
carbonate ions were separated from the bail in the chambers between 
membranes 34 and 35 and between membranes 36 and 37, and formed sodium 
formate and sodium carbonate in the chamber between membranes 34 and 37. 
The purified bail was then pumped into tank 21 where it was introduced 
back into the plating bath, 11, when desired. The solution containing the 
sodium formate and sodium carbonate was pumped into tank 19 where it was 
disposed of in public waste treatment facilities. 
In a particular example, ten membrane pairs were employed, each with an 
effective area of approximately 0.172 square meters, a thickness of 
approximately 0.14 mm, and a distance between membranes of approximately 
0.00075 meters. The voltage applied to the anode and cathode (cell 
voltage) was 7.7 volts, resulting in a voltage across all membranes 
(membrane voltage) of approximately 6 volts and a cell current of 
approximately 2 amps. The flow rate of all solutions was approximately 120 
liters/hr. The temperature of the waste solution was held below 25 degrees 
C. by using an immersion cooling coil (not shown) in the tank 19. 
Dissolution of copper oxide in the holding tank, 13, was done while 
heating at 75 degrees C. and mechanically stirring the solution. 
Electrodialysis was performed until the formate concentration, which was 
20-25 gm/liter, fell below 1 gm/liter. 
A significant feature of the invention is the fact that copper oxide, 
rather than a copper salt, was used to replenish copper in the bail, since 
a copper salt could be environmentally hazardous. A further significant 
feature is that the copper oxide was added to the plating solution prior 
to the electrodialysis. It was discovered that when the copper oxide was 
added after electrodialysis, an appreciable amount of formate was formed 
due to the oxidation of formaldehyde and the Cannizzaro's reaction (the 
reaction of formaldehyde and sodium hydroxide). The presence of this 
formate tended to defeat the purpose of the electrodialysis. 
It was found that best results were achieved with a cell current in the 
range 1-3 amps, a membrane voltage within the range 2-8 volts, and a cell 
voltage within the range of 4-8 volts. If current is too low, ion mobility 
will be too slow, while if the current is too high, water in the solutions 
will begin to break up. Further, the flow rate should be in the range 
100-120 liters/hr. If the flow rate is too high, the solutions will not be 
within the membrane gaps for a sufficient length of time, while if the 
flow rate is too low, the process will take too long to be economical. The 
temperature of the waste solution is desirably in the range 15 to 25 
degrees C. so as not to damage the membranes or adversely affect ion 
mobility. The pH of the bail is typically in the range 10-12.