Because PCB's are a custom, manufactured part, the fabrication process begins with customer supplied data files, drawings, and specifications. Once a purchase order and data files are received, fabrication proceeds through a series of operations as follows (see Manufacturing Flowchart); Pre-production Engineering, Computer-Aided Manufacturing (CAM)/photoplot, inner layer processing, drilling, through-hole sensitizing, outer layer imaging, plating/etching, soldermask coating, hot air solder leveling (HASL), silkscreen, final rout, electrical test, and shipping.
Pre-Production Engineering: Once data files and PO are received, Production Planning reviews all data, drawings, and specifications for completeness. If complete, Planning then develops the production process flow, determines raw material requirements, and sends data package to CAM.
CAM proceeds to run the data through design analysis, panelizes parts for production, develops drill, rout, and AOI programs, sends data to photoplotter to create film, and netlist file to test.
Inner Layer Processing: Dry Film operators receive Production traveler with material requirements from Planning. Material is issued and prepared it for imaging. Dry Film is applied, photo image is exposed and developed. Panels are then forwarded to the Plating department for etching of copper features, stripping of Dry Film, and sent on to AOI for inspection and tooling punch. Following AOI acceptance and tooling punch, panels are moved to Lamination where they have Oxide applied and placed into lamination fixtures for lamination.
Drilling: Panels come to drilling from Lamination. Drill file installed and First Article (FA) drilled to check program accuracy and registration of holes to internal layers. Once verified, remainder of panels will be drilled.
Through-hole sensitize and plate: After drilling, panels are alternate layers of dielectric and copper. In order to connect all layers together and create a conductive path through the holes, a seed coat of copper is deposited. Prior to deposition of copper, the holes are prepared and any epoxy smear removed through a series of chemical baths ending with copper deposition in the Electroless Copper bath.
Outer Layer Processing: Following electroless copper deposition, the panels are ready for the outer layer image to be applied. Dry Film once again coats the panels and has the outer layer circuitry imaged on and developed. Panels are then moved to the Electroplating area where copper is electroplated on the surface and through the holes creating a continuous path. Tin/Lead is then electroplated to act as an etch resist during the etching process. After etch, dry film is stripped and the panels are either reflowed or have the Tin/lead removed. It is at this point that any additional surface coatings like nickel and gold are applied.
Pre- Soldermask Inspection: All panels are inspected either with AOI and/or visually for any functional or visual defects. Utilizing customer drawings and specifications, all physical characteristics are verified including registration, hole sizes, thickness, etc.
Soldermask application: Liquid Photo-imageable (LPI) Solder mask is applied using an automated screen printer. Panels are then baked as a tack cure, the image applied, developed and final baked. Solder Reflowed panels are moved directly into Silkscreen for the Component Identification markings; Soldermask over bare copper (SMOBC) panels are sent to Hot Air Solder Leveling (HASL) and then back to Silkscreen.
Final Rout: At this point, the panels are complete with the exception of cutting individual boards from the manufactured panels. One panel is routed as an FA to verify program, then all panels routed.
Electrical Test: Using an electronic netlist, all boards are tested using a “bed of nails” test machine or a “Flying Probe” test machine. Flying Probe is very useful for small increments such as prototypes; whereas for production, the bed of nails or fixture test is more appropriate. Both methods are equally reliable, just difference in time and expense.
Final Inspection and Shipping: PCB’s are now complete and functionally sound, they must now be inspected for cosmetic type defects. Once accepted, all are moved into shipping for transport to customer.
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