Trelic’s picture of the week 04/2018

Could this occur in your product?

 

This week’s picture is something beautiful, but lethal for your product: corrosion.

Last week’s picture mentioned corrosion as a phenomenon likely occurring if you drop your phone into salty water. This week we continue with the same topic by giving an example of how salt corrosion affects the surface of a printed circuit board (PCB).

The turquoise corrosion product is a result of a rather long salt spray test. Under the corrosion product there is a copper trace with electroless nickel immersion gold (ENIG) surface finish and a plated through hole (PTH) via structure. Both the copper pad and via structure have corroded during testing. Before testing the structure looked like this:

Salt spray corrosion testing is one of the harshest reliability tests available. However, it is very effective way to test corrosion resistance of products used in corrosive environments. A salt spray test is therefore very suitable for automotive products, especially in countries which use road salt during winter time. Another application field for salt test are medical and wearable electronics applications, because salt spray is a can be used to mimic the effect of body sweat on the products.

The picture represents a third application often used with salt corrosion testing: the reliability study of PCB materials. In this case we compared PCBs without any additional protective layers, as the aim was to compare the corrosion resistance of surface finishes used on top of copper. ENIG, organic solderability preservative (OSP) and immersion silver (ImAg) surface finishes were used.

The results showed that there are significant differences between the surface finishes. The ImAg PCBs had excellent corrosion resistance. The OSP PCBs had decent performance, although clearly poorer than that of the ImAg PCB. ENIG PCBs had severe corrosion and they failed very quickly. The results showed that if after reflow soldering PCB has plain areas with ENIG coating, it is very important to consider how these areas are protected from corrosion. Simplest way would be to avoid them.

If you have any questions or needs regarding salt corrosion or reliability testing in general, please contact us! We are happy to help.