Alright let us do a quick re-cap of what we have covered so far.
We have given an overview of some basic materials and the initial process for developing imaging for inner layers. We have covered the lamination and drill process.
Now we are coming down the home stretch.
Now that the panels are drilled we need to do the outer layer image and copper plating process.
That begins with dry-film resist being coated on the outer layer panels. This is a similar process that was done when making the images for the inner layers of the board.
The film will cover the entire surface including the drilled holes.
The film is then exposed on a light table using the image of the outer layers. That is then developed off of the copper. Where the film is exposed to light the film hardens creating an image of the circuit pattern. Any holes that become exposed will be plated through with copper.
The next step then is called Copper “Pattern” Plate. This is an electroplating process that plates copper onto the exposed metal surfaces. Typically it will be plated to a thickness of around 1mil (0.001”). This can vary and in some cases there can be much more copper plated if the design calls for that.
The already exposed copper has a thin layer of tin plating applied to it. This protects the copper for the traces and in the holes for the next steps in the process.
The panels need to be cleaned prior to the soldermask being applied. It must be free of any oxidation and contamination prior to the mask being applied. This is typically some type of pumice scrub.
LPI Solder Mask is then applied. This is a photo-sensitive epoxy based ink. It covers the panel completely and is dried but not completely cured.
Then just like during the image process for the boards the panels are exposed to light and the soldermask pattern. The panels run through a developer and what is not needed is removed.
Most of your board finishes are applied to the remaining exposed copper at this point. In some cases finishes are applied prior to mask but that is not typical.
In the next article we will talk about some of the possible finishes that could be applied to a pcb.
Silkscreen is also applied at this point and baked along with the solder mask and is ready for the final steps.