Quantification of Particles in an Electrolytic Solution for Copper Acid Plating Using the Coulter Principle

PCB manufacturing involves acid copper plating whereby suspended particles are deposited during plating by forcing them toward the cathode using the ionic current in an electrolytic copper plating tank. The presence of larger particles of 5-20 µm can affect the local electrical field, thus reducing the efficacy of the levelers in the electrolyte.

This "electrophoretic" migration leads to rough plating with impurities confined in the electrolytic layer covering a key circuit component. Hence, the plating electrolyte is subjected to continuous filtration with one or more 1µm filters, which are specially designed for "polishing" electro-chemicals.

Experimental Setup and Procedure

For this experiment, a 100 µm aperture tube was used, which can analyze the particle concentration and size distribution range of 2-60 µm. Another aperture tube may be needed to meet the requirement of different size ranges. The Multisizer 3 was configured and calibrated as described in the user manual. The particle concentration and size distribution results were reported as number of particles per mL.

The number of particles was quantified at different size levels using the ‘Size Interpolation’ feature in the Multisizer 3 Software. Volumetric Mode was set as the control mode for the Multisizer 3, selecting a run volume of 500 µL. The analysis of the sample was performed without dilution, as it is a conductive solution. This analysis involved the use of 0.45µm filtered sample for the external electrolyte jar.

The sample information was entered in the Multisizer 3 software. A 100µm aperture was used to analyze the sample contained in a round bottom beaker. The aperture tube and electrode were cleaned between runs.

Experimental Results

The experimental results are reported in particles/mL. The size distribution of a sample is presented in Figure 1. Table 1 presents the results to show the reproducibility of the method by analyzing the same sample consecutively for five times. The number of particles with a size of more than 2 µm was 3,758/mL.

Besides determining the total concentration of particles, the concentration of the particles beyond pre-set size levels can also be determined with the ‘Interpolation Points’ feature in the Multisizer 3 software.

Experimental results.

Figure 1. Experimental results. Image credit: Beckman Coulter

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%


Since the Coulter Principle technology provides highly accurate particle size and count results, it is ideally suited to monitor the purity of the electrolytic solution used in the copper plating process. The procedure discussed in this article can also be applied in the evaluation of the efficiency of filtration employed in the copper plating process.

Beckman Coulter Life Sciences - Auto-Cellular and Proteomics

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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