Monitoring the Cleanliness of an Electrolytic Solution Used for Copper Plating

Acid copper plating is an electrochemical process used to manufacture printed circuit boards. The action of the ionic current in an electrolytic copper plating tank is such that while plating, any and all suspended particulates are forced toward the cathode which is the panel that is being plated deposited. If the particles are large enough (size 5 to 20 µm), their presence can disrupt the local electrical field enough to reduce the effectiveness of the levelers present in the electrolyte.

This electrophoretic migration results in rough plating with the contaminating particles trapped in the electrolytic layer formed on top of a critical circuit element. In a production setup, the plating electrolyte is normally continuously filtered using one or more 1 µm filters specially made for "polishing" electro-chemicals.

Materials and Methods

Instrument Set up and Calibration

A 100 μm aperture tube was utilized for the analysis of this case study. The linear dynamic range for any aperture is 2% to 60% of its diameter. A 100 μm aperture tube can analyze the particle concentration and size distribution from 2 to 60 μm. In case a different size range is needed, another aperture tube can be used. The instrument was calibrated as per the Multisizer 3 Operator’s Manual. In order to determine concentration and size distribution of particles in the sample, the results were obtained in number/mL. The size Interpolation feature in the software was utilized to quantify the number of particles at different size levels. The control mode for the instrument was Volumetric Mode choosing 500 μL as the run volume.

Procedure

Electrolyte

Since the sample is a conductive solution, it was analyzed with no further dilution into another electrolyte. For the external electrolyte jar, a 0.45-μm filtered sample was utilized. In the sample information dialog of the Multisizer 3 software the information below was entered:

         Sample Volume: 150 mL
          Electrolyte Volume: 0 mL
          Analytical Volume: 500 μL

Sample Analysis

  • Running the sample

A round bottom beaker having the sample preparation was analyzed using a 100 μm aperture. The aperture tube was flushed prior to each analysis. After each run, the electrode and aperture were rinsed before proceeding to the next sample.

Results

The results are expressed in particles/mL. The graph in figure 1 below shows the size distribution of a single sample.

Size distribution of a reference sample.

Figure 1. Size distribution of a reference sample.

Particle Concentration: 3,758 particles/mL larger than 2 μm.

In order to establish the repeatability of the method, the same sample was analyzed five consecutive times. Table 1 shows the results.

Table 1. Repeatability of results.

  Particles Mean d10 d50 d90
Run per mL μm μm μm μm
1 3,758 2.73 2.01 2.15 3.80
2 3,294 2.73 2.02 2.16 3.71
3 3,654 2.69 2.02 2.14 3.50
4 3,744 2.77 2.01 2.13 3.70
4 3,714 2.72 2.02 2.14 3.57
Aver. 3,633 2.73 2.02 2.14 3.66
C.V. 5.3% 1.0% 0.0% 0.6% 3.2%

Along with the total concentration of particles, by using the Interpolation Points feature in the software, it is possible to determine the concentration of particles above pre-set size levels.

Particles per mL larger than Diameter (μm)
3,758 2
668 3
323 4
187 5
58. 10
1 25
0 50

Conclusion

As the Coulter Principle is the highest resolution technology present for sizing and counting particles, it is an excellent tool for monitoring cleanness of the electrolytic solution in the copper plating process. The procedure described here may also be used to evaluate filtration efficiency during the cooper plating.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.

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