A2d. solder masks - Solder resists (masks) are placed on the circuit board to ensure that only exposed areas of the board are coated with solder during wave, drag, or dip soldering. They prevent solder bridges from forming across dielectric areas of the board between traces, pads, lands, and holes. The masks are made from epoxy or other thermosetting plastic materials that remain on the board and provide protection to it, and increased insulation between circuit paths and components. Screening and photographic methods are used to apply the mask material. With the screening method, the thermosetting material is screened on and cured with ultraviolet light or heat. Photomasks provide a more accurate but more expensive approach and are used for finer-pitched boards. The wet or dry photographic film is applied to the board and processed with a light exposure, development of the film, and removal of the unwanted portion. Wet film is applied by spraying, dipping, curtain coating or roller coating. Dry film is laminated to the board with vacuum equipment.  

Temporary solder masks may be applied by similar methods to portions of the board to shield them during wave soldering. Dummy plugs, tape, or precut shapes are sometimes temporarily assembled to the board for the same purpose.  

A third type of coating for unassembled boards is a temporary solderable protective film coating. This type of coating protects circuit and pad areas from contamination with dust or dirt and from tarnish during storage before soldering. The coating typically is removed automatically by the heat of soldering or the activity or solvent action of the soldering flux.  

A2e. separating boards (depanelling) - Individual circuit boards are separated from a panel of several boards by CNC (computernumerically- controlled) routing machines. Routing cutters of 1/8 in (3 mm) diameter are commonly used. Beveling or chamfering, to put a tapered edge on contact fingers, is an accompanying operation to routing. It is performed by an angle-ground or tilted CNC routing tool. Slots and grooves are sometimes machined into the board with the same equipment. An alternative board separation method, less common, is blanking but it entails the expense of making a blanking die for each board design.  

A2f. silkscreen identification - various identifying and instructional information is printed on the circuit board as one of the final bareboard operations. Conventional screen printing techniques are used with epoxy ink followed by drying or curing.  

A3. multilayer boards - are made by laminating double-sided boards together with internal layers of board material. If two double-sided boards are combined with an internal dielectric board layer, a four-layered board will result. This construction is called cap sheet lamination . Another method for a four-layer board, the foil lamination construction, uses on double-sided board in the middle, covered on top and bottom with a dielectric board layer, and then, on each of them, an external layer of copper foil. Fig. 13A3 shows both arrangements. Boards with 16 or more circuit layers can be created if enough layers of lamination and enough boards are combined. The internal wiring traces are completed before lamination. Also before lamination, sheets of prepreg (uncured reinforced plastic dielectric material), and copper foil, if used, are sheared or purchased to the panel size needed and are cleaned. They and the boards may be drilled for tooling holes to maintain alignment during the balance of the process. The pitch width is normally narrow for multilayer boards since they are used in more sophisticated equipment with more complex and more concentrated circuitry.  

The complete procedure for cap sheet board construction is as follows: 1) In several steps, the copper-foil board surfaces are treated with resist, and non-circuit areas of copper are etched away, leaving copper circuit traces, as is done with regular single- or double-sided boards. 2) surface oxidation - These circuit traces are subjected to heated oxidizing chemicals to provide a black copperoxide surface which improves adhesion of the laminate. 3) The boards are rinsed, and then baked to remove absorbed moisture. 4) lamination - The boards are laid in a carefully aligned stack with layers of prepreg , uncured reinforced plastic lamination sheets. (Epoxy/glass sheets are most common.) Temporary “caul” plates, often with alignment pins, hold the stack in alignment. 5) curing - The stack is placed in a press that has heated platens and the stack is kept under pressure until the lamination plastic has cured. This may be done with the stack, and possibly the press, in a vacuum, to reduce the amount of pressure needed and reduce slippage. 6) cooling - The stack is cooled under pressure in another press. 7) stress relieving - The stack may be baked in an oven for several hours at about 325 ° F (160 ° C) to reduce internal stresses and avoid warpage. 8) Caul plates are removed and any plastic flash is removed. Edges are trimmed, if necessary. 9) Drilling for vias can now take place. Drilled holes are deburred. 10) Drilled via holes are copper plated by the electroless method. 11) Solder masking and electroplating of solder for external surfaces, as with double-sided boards, follows.  

The procedure for boards with the foil lamination construction is similar. Internal double-sided boards are processed to produce copper circuit traces having black oxide surfaces, with the operation sequence outlined above. Boards are rinsed, baked, and stacked with layers of prepreg between the conductive surfaces and the layers of cleaned copper foil on the top and bottom of the stack. The stack is cured, cooled, and stress relieved as outlined above. Tooling holes are added if not already in place and via holes are drilled, deburred and electroless copper plated. The copper foil surfaces of the multilayer board are converted to wiring patterns with the methods outlined above, including plating. Solder masking is applied and solder plating, as with double-layer boards, follows.  

Multilayer boards are also made using the additive approach.  

Fig. 13A3 Two ways to construct a four-layer, printed circuit board. With both methods, sheets of prepreg , partially cured sheets of epoxy resin reinforced with glass cloth, are used. View a) shows cap sheet lamination, using two double-sided boards (with circuit paths delineated) and one sheet of prepreg. View b) shows one double-sided board with circuit paths, two prepreg sheets and two sheets of copper foil. In both examples, the sheets are bonded together with pressure and sufficient heat to fully cure the epoxy. Multilayer boards with fewer or more layers are similarly constructed.

A4. making flexible printed circuit boards - These boards use heavy flexible film as a base instead of a rigid, glass-reinforced board. Originally used simply for carrying multiple leads between components that have some motion between them, these boards now contain complex circuits including those that are double sided and, sometimes, multiple layered. Three materials are prominent in construction of flexible boards. All are thermosetting plastics with high temperature resistance: polyimide (Kapton ® ) plastic film is used in the most critical applications and has the highest temperature resistance and highest cost; liquid crystal polymer (LCP) film has similar characteristics at a somewhat lower cost; and polyester film is used in less critical applications where low costs are more important. Electronic devices are surface mounted on these films and are connected primarily with reflowed solder, though some boards use conductive epoxy instead. Traces on the boards are of copper. Copper is provided by foil that is bonded to the film base, and traces are produced in the copper by the subtractive process. (See A1b.) Traces may be coated with an organic coating to preserve solderability, by tin electrolytically, nickel by electroless plating, or silver by immersion-dispersion. Boards are often given a protective elastomer coating ( conformal coating . See M.) after assembly is complete. This coating is cured by ultra-violet energy that passes through the flexible base film and cures the coating on both sides.