Coulter Counter Application in Copper Plating

A basic laboratory set-up to demonstrate the copper plating electrochemical process

A basic laboratory set-up to demonstrate the copper plating electrochemical process

The manufacturing of printed circuits often makes use of an electrochemical process known as acid copper plating. One of the potential pitfalls in this process is contamination of the electrolyte with particles, so manufacturers must keep a close eye on the numbers and sizes of any particulates present. Which is where a Coulter Counter comes in…

The panel being plated acts a cathode, attracting any particles suspended in the liquid and depositing them on its surface. Large particles, in the region of 5 to 20 µm, can have a disruptive effect on the local electric field. This may be sufficient to prevent the ‘levellers’ (additives used to smooth out the deposition) from doing their job effectively.

The result of this interference is that the plating is rough and the contaminant particles are trapped right on top of a crucial element of the circuit, within the electrolytic layer. To prevent this, manufacturers usually employ a 1 µm filter, or more than one filter, to remove particles constantly.

Assessment of how ‘clean’ the electrolyte is can be carried out using the Coulter Principle and a Coulter Counter. An account of how to do this is given on the Beckman Coulter website, www.beckmancoulter.com, where you will find an absolute treasure trove of useful literature on application of techniques. Just look for the type of instrument in which you are interested – in this case Coulter Counter – and then click on ‘Literature’ in the right-hand column.

I won’t go into the full details here, but essentially the method described there gives clear results, expressed in terms of particles per ml, and plots the size distribution in a graph. In the particular procedure described, a particle concentration figure of 3,758 per ml is obtained for particles above 2 µm in size. Repeated analysis shows excellent consistency in the results.

Using the Interpolation Points facility in the Coulter Counter software, the particle concentration for particles above any pre-specified size can be determined.

It can be concluded that because the Coulter Principle is the technology that offers the greatest resolution of all in particle sizing and counting, it is the ideal tool for manufacturers wishing to assess how clean their copper plating electrolyte is. It can also be used to test the efficiency of the filters used.