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The Manufacturing Process for Printed Circuit Boards
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Although each printed circuit board fabricator slightly modifies the manufacturing process based
on the company's area of specialty, most manufacturers follow industry standard processes
and use consistent materials. The manufacturing process can be broken down into three phases:
Pre-manufacturing, Manufacturing,
and Post-manufacturing. The following white paper details
the typical process for taking a printed circuit board design and making it a reality.
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Pre-Manufacturing:
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The first step for anyone looking to order a printed circuit board is to select the manufacturer that specializes in the specific type of circuit board needed. For example, customers may be looking for large quantity, mass production boards, they may need a single prototype, or they require a small lot of highly customized boards.
Once a manufacturer is selected, it is important to have the printed circuit board design reviewed by certified PCB CAM engineer. This step is critical to make sure all files contain the necessary data to build the pcb. Checking the design prior to placing an order reduces the chance for holds, avoids embarrassing surprises, and eliminates the potential for delays. Some pcb manufactures offer this service for free through their websites.
Once the design is finalized, customers place an order with the manufacturer by sending purchase order instructions, Gerber files, drill files, and fabrication drawings.
(See glossary for a
definition of terms) Once the order is received, all of the files are sent for Engineering/CAM review.
Engineering/CAM review consists of an exhaustive checklist, which once complete, is kept with each part number's tooling file. Once the review checklist passes, the job is tooled using state-of-the-art computer software and hardware. Tooling consists of laser-plotted artwork, drill file, CNC rout file, job traveler, and an engineering file. All tooling is stepped and repeated for optimum utilization of an 18" x 24" panel complete with plating enhancements, test tooling, and registration holes.
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Manufacturing:
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For a multi-layer printed circuit board, the first process is to select the proper thickness laminate, i.e. “0.008 for a 6 layer pcb and copper foil – usually half ounce. Each inner circuit is transferred to the copper panel using photographic dry film material called resist. The resist is hot-roll laminated onto the copper panel. The film tooling is exposed onto the panel using a 5-kilo Watt light source. The light exposed panels are then processed through a developer to remove the resist from the unwanted copper. Next the exposed copper is etched off the panel leaving the resist-covered pads and traces behind. Finally the resist is stripped from the panel leaving the exposed bare copper circuits behind.
Total time is about 3 hours.
The inner layers are then stacked and pinned alternately with thin sheets of epoxy glass pre-preg, that separates the copper layers. The outer layers are made with a foil of copper. The stack is pinned between two heavy metal plates creating a "book." This book is put in a heated hydraulic press for about 2 hours at 350 degrees F and a pressure of 180 pounds per square inch.
Once pressed, these panels look just like double-sided laminate and are ready for drilling. For double-sided panels, drilling is the first process. The panels are pinned to the table of a CNC drill and the drill program is loaded. The proper size drill bits are automatically used and changed during the process. At the completion of the process the panels are inspected and any burrs are removed from the panels surfaces.
Electroless copper is the next step in the process. A thin layer of copper, about 50 millionths thick, will cover the entire panel and inside the drilled holes. There is a series of chemicals that condition, clean, and activate the surface inside the holes before the panels are suspended in the copper bath.
The primary images of the top & bottom layers are applied using the same dry film plating resist and developing process as the inner layers. Once they’re developed the panels are ready for the copper plating procedure. The panels are cleaned and activated chemically, then connected to a rack that is placed inside a large volume tank of copper solution. At 17amps/square foot of copper, the panels are electro-plated with a goal of achieving the addition of about one mil of copper in the holes and on the surface. Tin is then plated over the copper. During the entire time the dry film resist remains on the panels to prevent plating where there are no circuits.
After plating, the dry film plating resist is stripped off the panel leaving the unwanted copper exposed. This copper is chemically etched off the panel leaving only the tin covered copper circuitry behind. Finally the tin is stripped from the panel leaving bare copper circuits.
The bare copper panels are scrubbed with a pumice slurry to increase adhesion on the surface of the copper. Then solder mask ink is applied directly over the bare copper. Liquid Photo Imageable (LPI) solder mask is screened onto the panel and it is tack dried in an oven. The panels are exposed to a light source similar to the imaging process. The panel is then developed to remove any unexposed mask from the pads and holes. Finally, the panel is baked to cure the remaining mask to its permanent state.
The legend or component designator is then screened onto the panel using a screen stencil, which is photographically made from the legend film tool. It’s just like screen-printing on a t-shirt. Once completed the panels are cured in an oven to complete the screening process.
Next, the panels are cleaned, coated with flux and dipped into a tank of molten solder. The Hot Air Solder Leveler (HASL) coats the exposed copper pads and holes with molten solder. Using hot air forced through “air knives” at high pressure it also smoothes the surface and clears the holes of excessive solder as the panels rise out of the solder pot.
Last, but not least, in the manufacturing process is the CNC routing process. The panels are pinned to a backup material beneath a series of router heads. The CNC program is loaded into memory, and router bits are loaded into each head. Normally, a 0.093" size bit is used to cut out the individual printed circuit boards. After routing any special fabrication processes like beveling gold fingers are done before the pcbs are released to final inspection and test.
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Post Manufacturing:
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Electrical testing of each printed circuit board may take place on either a universal grid or a flying-probe test machine. The choice of machines depends on technology and quantity of boards to be tested. The grid machine is best for large quantities of boards while the flying-probe style tester works well for lower quantities and boards with complex geometries.
Once tested, a final inspection takes place before packing and shipping.
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