Printed circuit board manufacturing


Printed circuit board manufacturing is the process of manufacturing bare printed circuit boards and populating them with electronic components. It includes all the processes to produce the full assembly of a board into a functional circuit board.
In board manufacturing, multiple PCBs are grouped on a single panel for efficient processing. After assembly, they are separated. Various techniques, such as silk screening and photoengraving, replicate the desired copper patterns on the PCB layers. Multi-layer boards are created by laminating different layers under heat and pressure. Holes for vias are also drilled.
The final assembly involves placing components onto the PCB and soldering them in place. This process can include through-hole technology or surface-mount technology .

Design

Manufacturing starts from the fabrication data generated by computer aided design, and component information. The fabrication data is read into the CAM software. CAM performs the following functions:
  1. Input of the fabrication data
  2. Verification of the data
  3. Compensation for deviations in the manufacturing processes
  4. Panelization
  5. Output of the digital tools
Initially PCBs were designed manually by creating a photomask on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. The finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards.
Modern PCBs are designed with dedicated layout software, generally in the following steps:
  1. Schematic capture through an electronic design automation tool.
  2. Card dimensions and template are decided based on required circuitry and enclosure of the PCB.
  3. The positions of the components and heat sinks are determined.
  4. Layer stack of the PCB is decided, with one to tens of layers depending on complexity. Ground and power planes are decided. A power plane is the counterpart to a ground plane and behaves as an AC signal ground while providing DC power to the circuits mounted on the PCB. Signal interconnections are traced on signal planes. Signal planes can be on the outer as well as inner layers. For optimal EMI performance high frequency signals are routed in internal layers between power or ground planes.
  5. Line impedance is determined using dielectric layer thickness, routing copper thickness and trace-width. Trace separation is also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals.
  6. Components are placed. Thermal considerations and geometry are taken into account. Vias and lands are marked.
  7. Signal traces are routed. Electronic design automation tools usually create clearances and connections in power and ground planes automatically.
  8. Fabrication data consists of a set of Gerber format files, a drill file, and a pick-and-place file.

    Panelization

Several small printed circuit boards can be grouped together for processing as a panel. A panel consisting of a design duplicated n-times is also called an n-panel, whereas a multi-panel combines several different designs onto a single panel. The outer tooling strip often includes tooling holes, a set of panel fiducials, a test coupon, and may include hatched copper pour or similar patterns for even copper distribution over the whole panel in order to avoid bending. The assemblers often mount components on panels rather than single PCBs because this is efficient. Panelization may also be necessary for boards with components placed near an edge of the board because otherwise the board could not be mounted during assembly. Most assembly shops require a free area of at least 10 mm around the board.

Depaneling

The panel is eventually broken into individual PCBs along perforations or grooves in the panel through milling or cutting. For milled panels a common distance between the individual boards is 2–3 mm. Today depaneling is often done by lasers which cut the board with no contact. Laser depaneling reduces stress on the fragile circuits, improving the yield of defect-free units.

Copper patterning

The first step is to replicate the pattern in the fabricator's CAM system on a protective mask on the copper foil PCB layers. Subsequent etching removes the unwanted copper unprotected by the mask.
  1. Silk screen printing uses etch-resistant inks to create the protective mask.
  2. Photoengraving uses a photomask and developer to selectively remove a UV-sensitive photoresist coating and thus create a photoresist mask that will protect the copper below it. Direct imaging techniques are sometimes used for high-resolution requirements. Experiments have been made with thermal resist. A laser may be used instead of a photomask. This is known as maskless lithography or direct imaging.
  3. PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and z axis.
  4. Laser resist ablation involves spraying black paint onto copper clad laminate, then placing the board into a CNC laser plotter. The laser raster-scans the PCB and ablates the paint where no resist is wanted.
  5. Laser etching, in which the copper may be removed directly by a CNC laser. Like PCB milling above, this is used mainly for prototyping.
  6. EDM etching uses an electrical discharge to remove a metal from a substrate submerged into a dielectric fluid.
The method chosen depends on the number of boards to be produced and the required resolution:
  • Large volume:
  • * Silk screen printing – Used for PCBs with bigger features
  • * Photoengraving – Used when finer features are required
  • Small volume:
  • * Print onto transparent film and use as photo mask along with photo-sensitized boards, then etch.
  • * Laser resist ablation
  • * PCB milling
  • * Laser etching
  • Hobbyist:
  • * Laser-printed resist: Laser-print onto toner transfer paper, heat-transfer with an iron or modified laminator onto bare laminate, soak in water bath, touch up with a marker, then etch.
  • * Vinyl film and resist, non-washable marker, some other methods. Labor-intensive, only suitable for single boards.

    Etching

The process by which copper traces are applied to the surface is known as etching after the subtractive method of the process, though there are also additive and semi-additive methods.
Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern. The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates. As more copper is consumed from the boards, the etchant becomes saturated and less effective; different etchants have different capacities for copper, with some as high as 150 grams of copper per liter of solution. In commercial use, etchants can be regenerated to restore their activity, and the dissolved copper recovered and sold. Small-scale etching requires attention to disposal of used etchant, which is corrosive and toxic due to its metal content. The etchant removes copper on all surfaces not protected by the resist. "Undercut" occurs when etchant attacks the thin edge of copper under the resist; this can reduce conductor widths and cause open circuits. Careful control of etch time is required to prevent undercut. Where metallic plating is used as a resist, it can "overhang" which can cause short circuits between adjacent traces when closely spaced. Overhang can be removed by wire-brushing the board after etching.
In additive methods the pattern is electroplated onto a bare substrate using a complex process. The advantage of the additive method is that less material is needed and less waste is produced. In the full additive process the bare laminate is covered with a photosensitive film which is imaged. The exposed areas are sensitized in a chemical bath, usually containing palladium and similar to that used for through hole plating which makes the exposed area capable of bonding metal ions. The laminate is then plated with copper in the sensitized areas. When the mask is stripped, the PCB is finished.
Semi-additive is the most common process: The unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed bare original copper laminate from the board, isolating the individual traces. Some single-sided boards which have plated-through holes are made in this way. General Electric made consumer radio sets in the late 1960s using additive boards. The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes to produce conductive vias in the circuit board.
Industrial etching is usually done with ammonium persulfate or ferric chloride. For PTH, additional steps of electroless deposition are done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.