Printed Circuits

LECTURE 4

PRINTED CIRCUITS

· A printed circuit has conductors that are thin strips of metal, usually copper, bonded to a baseboard (refer as Printed Circuit Board or PCB).

· The base is made of laminations or layers of special paper or glass mat joined by phenolic or epoxy resins.

· The electronic components are mounted on the other side of the base.

· They are connected to the conductors by soldering.

· Such a circuit assembly is more compact than 1 that is hand-wired.

· It can also be mass-produced efficiently to be used in all kinds of electrical and electronic products.

· In its original form, at least 1 entire surface of a PCB is covered with copper foil that is bonded to it.

· This forms a copper-clad laminate.

· For some PCBs, both sides of the baseboard are copper-clad.

· 4 steps must be done to made PCB:

1. Layout Diagram.

2. Transfer of the Pattern.

3. Etching, Cleaning, and Drilling.

4. Soldering.

· LD: Is a pattern of the conductors that are to be on the board.

· LD: Shows which surfaces of the copper are to be protected from an etching solution, which is used to remove the unwanted copper foil from the board.

· LD: Is made with adhesive printed-circuit tape or drawn with ink.

· LD: A good 1st step is to arrange the components of the circuit on a sheet of graph paper according to the schematic diagram.

· LD: If this arrangement is suitable for the PCB, then the layout is roughed / sketched in on paper with a pencil or pen before being put into finished / detailed form.

· TOP: The simplest way to transfer a layout to the copper-clad laminate is to lay out the diagram directly on the surface of the laminate with tape.

· TOP: The pattern is usually transferred to the surface of the laminate by screen-printing or by a photographic process.

· TOP: Screen-printing is a process by which a resist is put on the copper surface through a stencil of the layout diagram, where the unwanted copper (part not protected by the resist) is removed from the laminate with an etching solution.

· TOP: A layout diagram can also be transferred to a PCB by using copper-clad material coated with a photosensitive resist compound as mentioned in 3 steps below:

1. A picture of the layout diagram is taken.

2. The negative is carefully inspected to ensure that all dark areas are opaque / dense / solid.

3. The negative is placed on the copper-clad laminate and exposed to strong light where the copper surfaces then resist the effects of the etching / engraving / drawing solution.

· ECD: After the layout diagram has been transferred to the copper surface, the copper-clad laminate is put in an acid or alkaline etching solution.

· ECD: This solution eats thru the unprotected copper, leaving only the copper under the layout.

· ECD: Next, the tape or resist is simply removed from the base.

· ECD: The remaining conductors are then cleaned.

· ECD: As needed, holes are drilled or punched thru the conductors and the base laminate for component wires.

· ECD: The baseboard (PCB) is now ready for the components to be mounted.

· ECD: The components can be inserted / mounted either by manually or thru auto-insertion process, such as Surface Mounted Technology (SMT).

· S: When all components have been mounted on the PCB, the assembly is ready for soldering.

· S: Is commonly done in a 1-step process known as wave soldering.

· S: Waves of melted solder are washed into each of the joints that connect components leads to the conductors.

· S: This kind of soldering takes only a short time.

· S: It is, therefore, well suited for use in mass production.

· S: Be careful, enough heat should be applied to the joint to let the solder flow onto all surfaces being joined.

· S: Too much heat may cause the conductor to separate from the baseboard.

· S: For most PCB soldering, a light-duty soldering iron (25 or 40 W) will provide enough heat.

· S: Using too much solder may cause it to flow to another conductor or terminal.

· A 60/40 solder (60% tin + 40% lead) is normally acceptable for PCB work.

· S: After soldering, the assembly is ready for final inspection and testing (using jig tester).

· A single unit or board of a PCB is often called a module or a circuit pack.

· A module may contain the components of a specific circuit, such as a rectifier or an amplifier, or of a circuits’ combination.

· In many electronic devices, modules are designed to be plugged into a circuit.

· Modules of this kind often have plated terminals that are inserted into clip sockets.

· If a module proves to be defective, it can then be repaired or replaced without disturbing any other parts of the circuit system.