The quality of the surface finish on top of the copper traces
Printed circuit boards (PCB) are used basically on every electronics product. On top of the PCB, copper traces are used to connect components together.
Attaching components on top of pure copper is difficult, because copper oxidises very quickly. Therefore, additional surface finish is normally needed. There are several options for surface finish. Typical metallic ones are for example solder, tin, silver, and gold over nickel (ENIG). As an example, the structure of ENIG is shown in the picture on the right.
Especially, for corrosion resistance the quality of the surface is critical.
In this week’s picture, a very bad quality ENIG surface is shown with lots of pinholes (the black dots). The pinholes allow impurities and corrosive elements to reach copper and can cause considerable corrosion problems. In this case the pinholes were seen in the edge areas of ENIG and it caused very severe corrosion in salt spray testing. In this case the quality was bad and, therefore, the corrosion was expected. However, even a much smaller number of pinholes can cause problems, so it is good to make sure that the surface finish is of good quality especially for harsh environments.
As we in Trelic do lots of adhesive attachments, we commonly use ENIG surface finishes. Adhesives behave differently than solders. When ENIG is used in solder attachments, solder forms intermetallic layers (IML) with the surface finish. Adhesives, on the other hand, do not consume ENIG during the attachment process, because no IML is formed. Therefore, in case of adhesive attachments, some ENIG always stays on the PCB after processing and is vulnerable for instance to corrosion.