The printed circuit board undergoes a series of plating procedures in order to enhance its physical appearance and electrical conductivity. Plating is an essential procedure in PCB fabrication.
This article contains the final steps performed by a standard PCB fabrication company. It employs the “SES” process, coats the soldermask, applies the solder finishes, and silkscreens the labels.
1.Strip the Photoresist
After the panel has undergone photoengraving and a series of plating procedures, the photoresist shall be removed to prepare for another etching process. The dry film resist is stripped, thereby exposing the copper foil. The tin-plated surface is not affected during the strip. This is the first step in the commonly phrased “SES” or “strip-etch-strip” process.
2.Etch the Unwanted Copper from the Outer PCB
Any surface not covered with tin plating will be removed. This is the second step to the “SES”. After the etching process, PCB fabrication is electrically completed. The only tasks to do are to finish the outer layer’s appearance, conform to the customer’s mechanical specifications, and do a final inspection.
3.Strip the Tin Lead
This step is all about preparing and cleaning the PCB, in order to apply soldermask. All the tin is chemically removed, thereby exposing the bare copper metal and the resin panel. This is the last step in “SES”.
4.Paint over using SolderMask
Another photoresist lamination process is done. The objective, however, is to protect the lead holes.
The soldermask is usually a green protective layer. It is an acrylic epoxy paint that is used to protect the copper lines. SMOBC, or solder mask over bare copper, is the process of applying this green layer to clean and coat the board.
Before applying the soldermask, the copper board must be clean and free from oxidation and contamination. The surface must also be clean to improve adhesion to the mask.
A soldermask is just another photosensitive coat. The panel should be coated completely and then left to dry. If the coating can already be touched, another photoengraving procedure is done. The dried output, however, must not be dried long enough to be considered cured.
The lead holes are to be stripped of the coating, because these are the areas that will be soldered. The mask will also protect the panels from possible electrical shorts during assembly. It can protect the board from contamination, electrostatic discharge and handling damages, as well.
The soldermask can then be cured thru oven baking. Some PCB fabrication companies, however, use infrared heating sources. Some companies also nickel- or gold-plate their board’s connectors immediately after the soldermask’s final cure.
5.Create Solder Finishes
This process is usually done after PCB assembly. It usually contains procedures that are necessary to solder components, by using machines and technology-advanced techniques. It can also be done in order to ship the board products, as is, without any components attached.
There are a number of available options in creating a solder finish. It can be lead-free, immersion tin, immersion silver, Electroless Nickel Immersion Gold (ENIG), Organic Solderable Preservative (OSP) or hard gold.
It can also be a rare metal finish like palladium.
6.Use Silkscreen to Create Identification
This is the finishing touch. This procedure is done to provide the customer some component identification. Labels such as R1, C1, or U1, are displayed onto the PCB, to identify the location of the components. This is necessary to provide documentation to make troubleshooting easier. The panels are again baked after silkscreen painting. This is done to cure the ink.
This article focusses on the final touches of PCB fabrication. It involves the “SES”, and various curing and finishing procedures. At this point, you can either have your working electronics circuit, or just the completed multi-layered PCB. There are other procedures that come after silkscreen printing like cutting, final testing and shipping. The fabrication tasks, however, ends here. You can have a perfect container for your electronics circuits, if you can find a perfect partner who will do the fabrication for you.