When to apply ENIG.
ENIG (Electroless Nickel Immersion Gold) is applied to the bare copper of a printed circuit board during the fabrication process. When in the fabrication process should be a concern to the contract manufacturer or OEM responsible for assembling the components onto the bare board. If a fabrication drawing states that the printed circuit board shall have SMOBC (Solder Mask Over Bare Copper) then ENIG must be applied after the application of solder mask. There are some exceptions to the rule.
- The fabrication drawing or OEM fabrication specification clearly states that ENIG shall be applied to all outer layer copper surfaces prior to the application of solder mask. I have had one customer specify this as a requirement. The desire was to have an extra layer of plated nickel covering all copper surfaces under the solder mask. The assembled boards operated in a corrosive environment. Over time the environment attacked the copper through the solder mask and conformal coat. This customer felt that it was worth the extra cost to plate everything for the extra layer of metal.
- The manufacturer of the bare board doesn’t know what they are doing. This is sometimes the case with printed circuit board manufacturers that do not process ENIG on site. In my previous post I list several manufacturing pit falls that impact the ENIG process. These companies may not appreciate the level of surface perfection required for ENIG since they don’t run the process on site. They may manufacturer a good board with HASL (Hot Air Solder Leveling) but have issues with ENIG. HASL is aggresive enough to burn away or remove many forms of contamination that may be on the copper to be plated. Hence their standard process may result in mystery non-conformances that come and go at the outside service or on the assembly line. In order to avoid problems with an outside service that applies the ENIG for them. Bare board manufacturers that process ENIG on site understand that the preceding process steps have a huge impact on the ENIG process. If they haven’t figure that out then you may want to change vendors.
The drawback to applying ENIG prior to the application of solder mask is a lower bond strength of the mask to the metal surface. The mask bond strength of a SMOBC board is greater than mask over gold. Prior to the application of solder mask the boards are cleaned and scrubbed. The cleaning component removes copper oxidation. This is typically done with a low concentration of sulfuric acid. At this point the copper is very smooth with next to no topograph. Solder mask shall not adhere very well unless the copper is roughened up. This is typically done with a pumice scrubber immediately after cleaning. This is where the problems occur with ENIG applied prior to solder mask. The immersion gold is only 2 to 5 micro-inches thick. The scrubbing operation needs to be minimized to prevent the gold from being scrubbed away. The minimized operation results in a lower bond strength between the mask and gold when compared to the mask and copper. Some assemblers may find that the solder mask flakes off of ENIG plated areas under the solder mask. When you apply solder mask over ENIG it happens. If you specify solder mask to be applied after ENIG then you have to live with it. If you specify SMOBC and you get ENIG under the mask then you need to talk to your vendor.