Manufacturing

PCB CAM

1. Input of the fabrication data.

1. Verification of the data; optionally DFM

1. Compensation for deviations in the manufacturing processes (e.g. scaling to compensate for distortions during lamination)

1. Panelization

1. Output of the digital tools (copper patterns, solder resist image, legend image, drill files, automated optical inspection data, electrical test files,...)

1. Silk screen printing uses etch-resistant inks to create the protective mask.

1. Photoengraving uses a photomask and developer to selectively remove a UV-sensitive photoresist coating and thus create a photoresist mask. Direct imaging techniques are sometimes used for high-resolution requirements. Experiments were made with thermal resist.

1. PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') 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 (if relevant) z axis.

1. Laser resist ablation Spray black paint onto copper clad laminate, place into CNC laser plotter. The laser raster-scans the PCB and ablates (vaporizes) the paint where no resist is wanted. (Note: laser copper ablation is rarely used and is considered experimental.)

• Silk screen printing – Used for PCBs with bigger features

• Photoengraving – Used when finer features are required

• Print onto transparent film and use as photo mask along with photo-sensitized boards (i.e., pre-sensitized boards), then etch. (Alternatively, use a film photoplotter)

• Laser resist ablation

• PCB milling

• 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.

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. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) 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.Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern. 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 (exposed to light through a mask and then developed which removes the unexposed film). 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.

Panelization

Copper patterning

Large volume

Small volume

Hobbyist

Subtractive, additive and semi-additive processes

Manufacturing starts from the PCB fabrication data generated by computer aided design, such as Gerber layer images, Gerber or Excellon drill files, IPC-D-356 netlist and component information. The Gerber or Excellon files in the fabrication data are never used directly on the manufacturing equipment but always read into the CAM (Computer Aided Manufacturing) software. CAM performs the following functions:

Panelization is a procedure whereby a number of PCBs are grouped for manufacturing onto a larger board - the panel. Usually a panel consists of a single design but sometimes multiple designs are mixed on a single panel. There are two types of panels: assembly panels - often called arrays - and bare board manufacturing panels. The assemblers often mount components on panels rather than single PCBs because this is efficient. The bare board manufactures always uses panels, not only for efficiency, but because of the requirements of the plating process. Thus a manufacturing panel can consist of a grouping of individual PCBs or of arrays, depending on what must be delivered.

The panel is eventually broken apart into individual PCBs; this is called depaneling. Separating the individual PCBs is frequently aided by drilling or routing perforations along the boundaries of the individual circuits, much like a sheet of postage stamps. Another method, which takes less space, is to cut V-shaped grooves across the full dimension of the panel. The individual PCBs can then be broken apart along this line of weakness. Today depaneling is often done by lasers which cut the board with no contact. Laser panelization reduces stress on the fragile circuits.

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. (Alternatively, a conductive ink can be ink-jetted on a blank (non-conductive) board. This technique is also used in the manufacture of hybrid circuits.)

The method chosen depends on the number of boards to be produced and the required resolution.

The (semi-)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.

Chemical etching is usually done with ammonium persulfate or ferric chloride. For PTH (plated-through holes), 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.Chemical etching

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 litre 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 exposed 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.