Using the Coulter Principle to Quantify Particles in an Electrolytic Solution for Copper Acid Plating

Introduction

Acid copper plating is one electrochemical process for manufacturing printed circuit boards. The action of the ionic current in an electrolytic copper plating tank is such that, during plating, any and all suspended particulates will be forced toward the cathode (the panel that you are plating) and deposited. If the particles are large enough (5 to 20 microns), their presence can disrupt the local electrical field enough to reduce the effectiveness of the levelers present in the electrolyte. The net result of this "electrophoretic" migration is rough plating with the contaminating particles trapped in the electrolytic layer right on top of a critical circuit element. In a production setup, the plating electrolyte is usually filtered continuously using one or more 1 micron filters specially made for "polishing" electro-chemicals.

Materials and Methods

Instrument Set up and Calibration

A 100 μm aperture tube was used for the analysis. The linear dynamic range for any
aperture is 2% to 60% of its diameter. A 100 μm aperture tube will be capable of
analyzing the particle concentration and size distribution from 2 μm to 60 μm. If a different size range is needed, another aperture tube could be used.

The instrument was calibrated according to the Multisizer 3 Operator’s Manual. For
determining concentration and size distribution of particles in the sample, the results were obtained in number/mL. The Size Interpolation feature in the software was used to quantify the number of particles at different size levels. The control mode for the instrument was Volumetric Mode selecting 500 μL as the run volume.

Procedure

1. Electrolyte
As the sample is a conductive solution, it was analyzed with no further dilution into
another electrolyte. For the external electrolyte jar, 0.45μm filtered sample was used.
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

2. Sample Analysis.
a) Running the sample

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

Results

The results are expressed in particles/mL. The graph below shows the size distribution of one sample.

Particle Concentration: 3,758 particles/mL larger than 2μm. To show the repeatability of the method, the same sample was analyzed five consecutives times. Table 1 shows the 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 
5  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% 

Table 1. Repeatability of results

 

In addition to 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 

Conclusions

As the Coulter Principle is the highest resolution technology available for sizing and counting particles, it is an excellent tool for monitoring cleanness of the electrolytic solution in the copper plating process processes.

The procedure described here may also be used to evaluate filtration efficiency during the cooper plating.

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