What are they?
Figure 1
- A printed circuit is a conductive pattern glued to one or both sides of an insulating material.
- Holes are punched or drilled through the conductor and board to allow the interconnection of electronic parts.
- In the case of a double sided board, the holes are plated to provide a connection between the conductors on both sides of the board.
- This method provides considerable space savings over hand wiring and allows for automated insertion and soldering of parts.
- A more uniform product is produced because wiring errors are eliminated.
- The insulating material thickness may vary from 0.015” to 0.500”.
- The most widely used base material is NEMA-XXXP paper base phenolic.
- Copper is the most common conductive material glued to the base.
- The common thicknesses of the copper foil are 0.0014” (1 oz./sq. ft.) and 0.0028” (2 oz./sq. ft.).
- For single sided boards, the copper is laminated to the board and then screened and etched away.
- Double sided boards use a plating process and conductive ink to achieve the desired layout.
DESIGN RULES
After the breadboard has been tested, there are some designs rules used to layout the printed circuit board. A few of these basic rules are listed here:
- Diameter of punched holes should not be less than 2/3 the board thickness.
- Distance between punched holes or between holes and board edge should not be less than the board thickness.
- Holes should not exceed more than 0.020” of the diameter of the wire to be inserted in the hole (machine insertion may require more, but leads should be “clinched”).
- Conductor widths should be large enough to carry current peaks. A width of one tenth of an inch (1 oz./sq. ft. copper) will increase in temperature 10°C at a DC current of 5A.
- Conductor spacing must be capable of withstanding applied voltages. If a voltage difference of 500 volts exists between two copper runs, they must be separated by at leads 0.03” to prevent breakdown.
- Avoid the use of sharp corners when laying out copper (see Figure 1). Sharp corners produce high electric fields that can lower breakdown. Sharp corners will also make it easier for copper to peel from the board.
- Heavy parts must be mounted to prevent board damage if the unit is dropped.
- The printed circuit board must be fastened to prevent leads from touching the case or any other object mounted near the board.
- Mounting hardware must be designed to prevent board stress (warping or excessive torque).
THE TOP LEGEND
- The component side of a printed circuit board should always have a drawing showing the placement of the parts and their schematic marking (R1, R2, etc.).
- This drawing is called the Top Legend.
- When a board needs to be repaired, the schematic becomes the “road map” and the top legend becomes the “address” on the part.
- Figure 2 shows the correlation between the Schematic and the Top Legend.
Figure 2
SOLDER
The Electronic Glue
Different parts have been discussed. A printed circuit board to interconnect these parts has been discussed. Now it’s time to talk about the “Electronic Glue” called Solder.
- Soldering wire is composed of Tin and Lead with a rosin or acid core.
- Acid core solder should never be used on electronic boards since the acid will damage the components.
- Acid core solder is mainly used to attach metals (copper water pipes for example).
- When tin and lead are mixed, the melting point of the mixture is lower than the melting point of either tin or lead.
- The point at which the melting point is the lowest is when the mixture equals 63% tin and 37% lead.
- This is called the eutectic ( u tek’tik) point of the mixture.
- An alloy of 50% tin, 32% lead, and 18% cadmium is the lowest melting point solder, but the cost is greater than the more commonly used 60/40 type.
- The most common flux placed at the center of this hollow wire alloy is Rosin based.
- Removing the flux from the board requires a chemical that can dissolve rosin.
- In recent years many water soluble fluxes have been developed.
- These fluxes can be removed by washing the boards in water.
- After the parts are placed in the holes on the printed circuit board, their leads should be trimmed and bent.
- A good mechanical connection will improve the soldering capability of the parts by forcing the part and copper on the board to rise to the same temperature.
- Positioning the soldering iron correctly and using the right amount of heat are crucial to a good solder job.
OTHER MECHANICAL PARTS
Figure 3
- There are many other mechanical parts used by manufacturers of electronic equipment.
- Most of them fall into the category of switching or connecting circuits.
- In Figure 3, five of the six parts shown are used to switch or connect signals to the printed circuit board.
- Only the spacer falls into a different category, called mounting.
- The switch is used to redirect current or voltage from one circuit to another.
- The wire nut is used to hold two twisted wires together and insulate them (prevent them from being bare and exposed) at the same time.
- The PC board male and female connectors are used to attach wires from controls or other circuits to the printed circuit board.
- The RCA Phono Jack is used to bring a signal (for example from a phonograph needle) to the printed circuit board.
- The spacer holds the printed circuit board away from the case to prevent leads from shorting to the case.
MOUNTING HARDWARE
Figure 4
- There are many different methods for mounting printed circuit boards.
- The simplest method is using machine screws and spacers.
- Figure 4 shows some of the common screw heads used by electronic manufacturers.
- The oval head screw in Figure 4 has a tapered end that will cut into the metal and make a thread for the body of the screw.
- The self-threading screw eliminates the need for a nut and lockwasher but can produce metal fragments that must be removed to prevent shorts from occurring.
AUTHORS
1.Bunty B. Bommera
2.Dakshata U. Kamble