Printed Circuits Dielectric Base Materials
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A printed circuit is a conductive circuit pattern on one or both sides of an insulating substrate. Multilayer boards with tens of levels of circuitry are manufactured with conducting thru-holes to interconnect the circuitry levels. The conductive pattern can be formed by any of several techniques after which component lead holes are drilled or punched in the substrate and components are installed and soldered in place. Printed-circuit construction is ideal for assembly of circuits that employ miniature solid-state components. Its advantages over conventional chassis and point-to-point wiring include:
- Considerable space savings over conventional construction methods is usually a result.
- A complex circuit may be modularized by using
several small printed circuits instead of a single larger one. Modularization simplifies troubleshooting, circuit modification, and mechanical assembly in an enclosure.
- Soldering of component leads may be accomplished in an orderly sequence by hand or by dip or wave soldering.
- A more uniform product is produced because wiring errors are eliminated and because distributed capacitances are constant from one production unit to another.
- The printed-circuit method of construction lends itself to automatic assembly and testing.
- Using appropriate base metals, flexible cables or flexible circuits can be built.
- By using several layers of circuits (in proper registry) in a sandwich construction, with the conduct ors separated by insulating layers, relatively complex wiring can be provide.
Printed-Circuit Base Materials
Rigid printed-circuit base materials are available in thicknesses varying from ‘/64 to ½ inch. The important properties of the usual materials are given in Table 19. For special applications, other rigid or flexible materials are available as follows:
(A) Glass-cloth Teflon (polytetrafluoroethylene, PTFE) laminate.
(B) Kel-F (polymonochlorotrifluoroethylene) lamin ate
(C) Silicone rubber (flexible)
(D) Glass-mat-polyester-resin laminate.
(E) Teflon film.
The most widely used base material is NEMAX XXP paper-base phenolic.
Copper is used almost exclusively as the conductor material, although silver, brass, and aluminum also have been used. The common thicknesses of foil are 0.0014 inch (1 ozlft2) and 0.0028 inch (2 oz/ft2). The current-carrying capacity of a copper conductor may be determined from Fig. 20.
The most widely used production methods are:
(A) Etching process, wherein the desired circuit is printed on the metal-clad laminate by photographic, silk-screen, photo-offset, or other means, using an ink or lacquer resistant to the etching bath. The board is then placed in an etching bath that removes all of the unprotected metal (ferric chloride is a commonly used mordant for copper-clad laminates). After the etching is completed, the ink or lacquer is removed to leave the conducting pattern exposed.
(B) Plating process, wherein the designed circuit pattern is printed on the unclad base material using an electrically conductive ink, and, by electroplating, the conductor is built up to the desired thickness. This method lends itself to plating through punched holes in the board for making connections from one side to the other.
(C) Other processes, including metal spraying and die stamping.
Conductor protective finishes are required on the circuit pattern to improve shelf-storage life of the circuit boards and to facilitate soldering. Some of the most widely used finishes are:
(A) Hot-solder coating (done by dip-soldering in a solder bath) is a low-cost method and gives good results where coating thickness is not critical.
(B) Silver plating used as a soldering aid but is subject to tarnishing and has a limited shelf life.
(C) Hot-rolled or plated solder coat gives good solder ability and uniform coating thickness.
(D) Other finishes for special purposes are gold plate, for corrosion resistance and solder ability and electroplated rhodium over nickel, for wear resistance. Insulating coatings such as acrylic, polystyrene, epoxy, or silicone resin are sometimes applied to circuit boards to improve circuit performance under high humidity or to improve the anchorage of parts to the board. Conformal coatings are relatively thick and tend to smooth the irregular contour of the mounted items; they add less mass than encapsulation. A protective organic coating (unless excessively thick) will not improve the electrical properties of an insulating base material during long exposure to high humidity. On two-sided circuit boards, where the possibility of components shorting out the circuit patterns exists, a thin sheet of insulating material is sometimes laminated over the circuit before the parts are inserted.
Before a printed-circuit layout is made, the circuit must be bread boarded and tested under the anticipated final operating conditions. This procedure will permit operating deficiencies and quirks to be detected and corrected before the time-consuming process of producing the circuit board is begun. It is important to note that certain circuits may operate differently on a printed-circuit board than on a breadboard, and appropriate corrective steps may be necessary. For example, inductive coupling between foil patterns may cause unwanted oscillation in high-frequency or amplifier circuits.
All features (terminal areas, contacts, board boundaries, holes, etc.) should be arranged to be centered at the intersections of a 0.100-, 0.050-, or 0.025-inch rectangular grid, with preference in the order stated. Many components are available with leads spaced to match the standard grids. Devices with circular lead configurations and a few other multiplied devices are exceptions that require special attention and dimensioning. Following this grid-layout principle simplifies drafting and subsequent machine operations in board manufacture and assembly.
Drilled holes must be employed if the stated requirements for punched holes cannot be met, or if the material is not of a punching grade. Drilling is less detrimental to the laminate surrounding the hole; punching may cause crazing or separation of the laminate layers.
The diameter of punched holes in circuit boards should not be less than the thickness of the base material.
The distance between punched holes or between holes and the edge of the material should not be less than the material thickness.
Punched-hole tolerance should not be less than ±0.005 inch on the diameters.
Hole sizes should not exceed by more than 0.020 inch the diameter of the wire to be inserted in the hole. With smaller holes, hand insertion of the wire is difficult. Machine insertion requires the larger allowance. Clinching of the lead is desirable if the clearance is larger.
Tolerances with respect to the true-grid location for terminal area centers and for locating edges of boards or other locating features (datums) should not exceed on the board: 0.014 inch diameter for conductor widths and spacings above 0.031 inch; 0.010 inch diameter for conductor widths and spacing 0.010 to 0.031 inch, inclusive. Tolerances on other dimensions (except conductor widths and spacing) may be larger. Closer tolerances may be needed if machine insertion is required.
Terminal area diameters should be at least (A) 0.020 inch larger than the diameter of the flange or projection of the flange on eyelets or standoff terminals, or the diameter of a plated-through hole, and (B) 0.040 inch larger than the diameter of an unsupported hole. Since the terminal area should be unbroken around the finished hole, the diameter should be further increased over the above minimum to allow for the permitted hole-position tolerance. Conductor widths should be adequate for the current carried. See Fig. 20, For a given conductor-width and copper-thickness intersection, proceed vertically to the allowable temperature-rise line and then horizontally to the left to determine the permissible current. An additional 15% derating is recommended for board thicknesses of 1/32 inch or less, or for conductors thicker than 0.004 inch (3 oz). The normal ambient temperature surrounding the board plus the allowable temperature rise should not exceed the maximum safe operating temperature of the laminate. For ordinary work, copper conductor widths of 0.060 inch are convenient; with high-grade technique (extra cost), conductor widths as small as 0.0 10 inch can be readily produced.
Conductor spacing requirements are governed by the applied voltage, the maximum altitude, the conductor protective coating used, and the power-source size. The guide in Table 20 is suggested.
Preparation of Artwork
In preparing the master artwork for printed circuits, careful workmanship and accuracy are important. When circuits are reproduced by photographic means, much retouching time is saved if care is taken with the original artwork.
Artwork should be prepared on a dimensionally stable material. Tracing paper and bristol board are now outmoded, and specially treated (toothed) polyethylene terephthalate (Mylar, Cronar) base drafting films are used for most printed-circuit layouts. The layout pattern may be produced by one of the following methods:
1. Hand application of opaque, permanent black ink
2. Pressure-sensitive tape
3. Hand-cut stencil made from self-adhesive opaque film
4. Preformed self-adhesive layout patterns
Artwork should be prepared to a scale that is two to five times oversize. Photographic reduction to final negative size should be possible, however, in one step.
Avoid the use of sharp corners when laying out the circuit. See Fig. 21.
The centers of holes to be manually drilled or punched in the circuit board should be indicated by a circle of ‘/32-inch diameter (final size after reduction). See Fig. 22. This feature is not needed on each board if templates or numerically controlled machine tools are used for hole preparation; however, it is still a convenience for checking drawings, master artwork, and photographic negatives.
When drawing the second side of a printed-circuit board, corresponding centers should be taken directly from the back of the drawing of the first side.
In addition to the illustration of the circuit pattern, the trim line, registration marks, and two scale dimensions at right angles should be shown. Nomenclature, reference designations, operating instructions, and other information may also be added.
All components should be inserted on one side of the board if practicable. In the case of boards with the circuit on one side only, the parts should be inserted on the side opposite the circuit. This allows all connections to be soldered simultaneously by dip-soldering.
Dip-soldering consists of applying a flux, usually a rosin-alcohol mixture, to the circuit pattern and then placing the board in contact with molten solder. Slight agitation of the board will ensure good fillets around the wire leads. In good present technique, the circuit board with its components assembled (on one side only) has its conductor pattern passed through the crest of a “wave” of molten solder; all junctions are soldered as the board progresses through the wave. The flux, board temperature, solder temperature, and immersion time are interrelated and must be adjusted for best results. Long exposure to hot solder is detrimental to the insulating material and to the adhesive that joins the copper foil to the insulation. For hand dipping, a 5-second dip in a 60/40 tin-lead solder bath maintained at a temperature of 450 degrees Fahrenheit will give satisfactory results.
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After solder-dipping, the residual flux should be removed by a suitable solvent. Be sure the solvent is compatible with the materials used in the component parts mounted on the board; solvents frequently dissolve cements or plastics and marking inks, or cause severe stress cracking of plastics.
To secure the advantages of machine assembly:
(A) Components should be of similar size and shape, or separate inserting heads will be required for each different shape of item.
(B) Components of the same size and shape must be mounted using the same terminal lead spacing at all points.
(C) Different values of a part, or even different parts of similar shape and sizes (if axial-lead style) may be specially sequenced in a lead-taped package for insert ion by one programmed insertion head.
(D) A few oddly sized or shaped components may be economically inserted by hand after the machine insertion work is completed.
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