Conductivity and Plating

Q: What are the main takeaways for conductivity when considering the combination of plating metal deposits, passivates and topcoats/sealers?

A: Traditionally, plating metal deposits, passivates, and topcoats/sealers are used for protecting the metal parts against corrosion. For some applications, however, these deposits are required to be conductive for conducting electricity or dissipating static charges. Some key applications include battery connecting, electromagnetic interference shielding, electrostatic discharge protection, printed circuits and sensors, corrosion resistance, thermal management and so on. 

In order to make the finished parts conductive, usually all layers of the plating metal deposits, the passivates and the topcoats/sealers should be conductive because these layers cover the metal part surface one layer by another or in a series. The plating metal deposits are known to be conductive, therefore, the conductivity of the finished parts mainly depends on the passivates and the topcoats/sealers. 

However, because the passivates and the topcoats/sealers are very thin (~1 micron) and the thicknesses are comparable to the roughness of the metal deposit, the passivate and the topcoat/sealer layers have different thicknesses in different spots. As a result, the resistance of the finished parts is not only dependent on the passivates and the topcoats/sealers but also on the plating metal deposits (crystal structure, surface roughness and so on). For example, if the plating Zn deposit is from alkaline bath, the finished parts are usually conductive or have small resistance with different passivates and topcoats. For the other metal deposits, the conductivity only depends on the passivates and the topcoats/sealers.

Usually, most of the passivate layers are conductive. If coated with conductive topcoats/sealers, the finished parts are definitely conductive. If the topcoats/sealers are not conductive, the finished parts might or might not be conductive, depending on the passivates and the plating metal deposits.

Some of the passivate layers are not conductive. The finished parts have high resistance if the topcoats/sealers are not conductive but might become conductive if the topcoats/sealers are conductive.  

To find out if the finished parts are conductive or not, the direct way is to measure its resistance is by putting two leads of the multimeter to the two sides of the material. The measured resistance usually consists of the resistance of the materials and the contact resistance. For minimizing the contact resistance, some pressure is applied to the contacting surface. If the pressure is not enough, the contact resistance still makes a big contribution to the measured resistance. If the pressure is too high, the measured resistance becomes small, but that might be because the coatings are broken by the roughness of the contacting surface. Therefore, it is important to have a reliable method to measure the resistance of the coatings.