The electroplating processes that electrically plates copper onto the exposed metal surfaces is next. The copper will be plated up to a thickness of approximately 1 mil (0.001”), depending on the required final finish for the panel.
The copper plating step is followed by plating tin onto all the exposed copper surfaces. The tin will be used as an etch resist to maintain the copper traces, hole pads and walls during the outer layer etch process.
The developed dry film resist is now removed from the panel. The tin plating is not affected. Any holes that were covered with resist are now open and will be non-plated. This is the first step in the common
phrase “strip-etch-strip”or “SES”process
Copper is now removed from all parts of the panel that are not covered by tin. The tin resists the chemicals used to etch away the copper. Only the pads and traces from the artwork
are left behind on the panel surface. The “E”of SES
Then the tin is chemically removed leaving behind a bare copper and laminate panel. The surface pads, traces and plated through holes are the exposed copper. This is the
last step in strip-etch-strip
Clean and Prep for Solder Mask
The exposed copper surface pads, traces and plated through holes must be clean and free of oxidation prior to applying solder mask. During the cleaning process the surface is scrubbed with pumice to improve adhesion of the mask as well as to remove any surface contamination
LPI Solder Mask Application
A photo-sensitive epoxy based ink is applied, completely coating the panel. It is then dried to the touch but not final cured. Using a method identical to image, the panels are exposed to a light source through a film tool. Then the panel is developed exposing the copper pads and hole defined by the artwork.
Solder Mask Cure
Solder mask is normally cured by baking in an oven; however, some fabricators are using infrared heat sources. The primary purpose of the mask is to restrict
the areas that will be covered with solder. It also protects panels from contamination, handling damage and possible electrical shorting during assembly and installation.
At Advanced Circuits, whenever possible, legend or nomenclature would be screened on the panels before further processing. Also, at Advanced the nickel and gold plating for edge connectors occurs immediately after final cure of the solder mask and legend
There are a number of processing options that can occur depending upon the desired final finish. Currently we can provide our customers with a SnPb or lead free solder finish, hard gold, Electro less Nickel Immersion Gold (ENIG), immersion tin or immersion silver. Other finishes include Organic Solderable Preservative (OSP), soft or bondable gold and a number of other “exotic”finishes like palladium. “Normal”processing would continue with the application of solder.
Legend, Silkscreen, Nomenclature, Component Designator
Ink is silkscreened onto one or both sides of the panel depending on the requirements of the customer. The printing usually dictates component placement, part number or name, date code, logo or other specified information. Panels are then baked to cure the ink
Hot Air Solder Leveling (HASL)
The panels are coated with flux –a viscous compound that promotes even coating of the copper. Then the panels are dipped completely into a bath of molten solder. The
solder covers all exposed metal surfaces. As the panel is removed from the solder, high pressure hot air is directed at both sides of the panel. The “blast”of air
removes excess solder from the holes and smoothes the surface of the pads.
Rout, Fabrication, Score, Bevel
After HASL the boards are cut to size on a CNC machine or router. Most panels have the individual parts routed out into single pieces or arrays of varying sizes. Boards or arrays can
also be scored so that they can be easily broken apart after assembly.
Then the boards are generally checked for cleanliness, sharp edges, burrs and other fabrication requirements. Chamfers, slots, countersinks and bevels are added
during the rout & fabrication processes.
Bare Board Electrical Test
Boards are tested for opens and shorts in the circuitry, in one of the last steps of production. Test programs can be loaded directly onto various types of test machines or used to create
specific fixtures and test programs.
Boards are visually inspected to assure they meet our customers’requirements, industry specifications and
AS standards, as well as having the physical dimensions and hole sizes verified.
Packaging and Ship
Circuit boards meeting the acceptability standards are counted, shrink wrapped and readied for shipment along with all the required certificates, samples, cross
sections, etc. All of these are packaged for shipment using products made from renewable resource