Outer Layer Image and Soldermask
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.
Once the copper is plated up the dry film resist that is left needs to be removed as well as the copper that is underneath it.
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 resist is then stripped from the boards. The boards will then go through the SES or strip-etch-strip process.
The copper that is now exposed and not covered by tin will be etched away. Only the traces and the pads around the holes and other copper patterns will remain.
Then the remaining Tin that is covering the traces and holes is chemically removed leaving behind the copper and the exposed laminate of the pcb.
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.
The solder mask is typically then baked in an oven so that it can cure and harden and adhere to the board.
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.