1. Printed Circuit Board
Printed Circuit board is essential for building the circuit. The PCB is used to arrange the components and connect them with electrical contacts. Generally to prepare a PCB requires lot of effort like designing the PCB layout, fabricating and testing the PCB. Commercial type PCB design is a complicated process involving the drawing using PCB design software like ORCAD, EAGLE, making mirror sketch, etching, tinning, drilling etc. On the other hand a simple PCB can be made easily. This procedure will help you to make a homemade PCB.
Making a homemade PCB
Required Material for PCB:
- Copper clad board – It is available in different sizes.
- Ferric chloride solution – For etching ( Removing the copper from unwanted area
- Hand drill with bits of requited size.
- OHP marker pen, sketch paper, carbon paper etc.
Step by Step PCB Design Process:
- Cut the Copper clad board using a Hacksaw blade to get the required size.
- Clean the Copper clad board using soap solution to remove dirt and grease.
- Draw the diagram on the sketch paper using the OHP pen as per the circuit diagram and mark the points to be drilled as dots.
- On the opposite side of the sketch paper, you will get the impression of the diagram in the reverse pattern. This is the Mirror Sketch used as the PCB tracks.
- Place the Carbon paper over the Copper coated side of the clad board. Place the Mirror sketch over it. Fold the sides of the papers and fix it with cello tape.
- Using a ball point pen, draw the mirror sketch applying some pressure.
- Remove the papers. You will get the carbon sketch of the mirror sketch on the Copper clad board.
- Using the OHP pen, draw the carbon markings present on the copper clad board. The drilling points should be marked as dots. The ink will dry easily and the sketch will appear as lines on the copper clad board.
- Now start etching. It is the process of removing unused copper from the board using chemical method. To achieve this, a mask must be placed on the copper which is to be used. This part of the masked copper acts as the conductor for the flow of electric current. Dissolve 50 gms Ferric chloride powder in 100 ml Luke warm water. (Ferric chloride solution is also available). Place the copper clad board in a plastic tray and pour the Etching solution over it. Frequently shake the board to dissolve the copper easily. If it is done in sunlight, the process will be fast.
- After removing all the copper, wash the PCB in tap water and dry it. Copper tracks will be under the ink. Remove the ink with Petrol or Thinner.
- Drill the soldering points using the hand drill. Drill bit size should be
- IC holes – 1 mm
- Resistor, Capacitor, Transistor – 1.25mm
- Diodes – 1.5 mm
- IC Base – 3mm
- LED – 5mm
- After drilling, coat the PCB using varnish to prevent oxidation.
Make a simple tester on a piece of Plywood to test the components quickly before making a circuit. It can be easily built using drawing pins, LEDs and resistors. The tester board can be used to check, Diodes, LED, IR LED, Photodiode, LDR, Thermister, Zener diode, Transistor, Capacitor and also to check the continuity of fuses and cables. It is portable and battery operated. It is highly useful for the project builders and reduces the job of multimeter testing.
Take a small plywood piece and using drawing pins make points of contact as shown in the photo. The connections between the contacts can be made using thin wire or steel wire.
Connect 9 volt battery and start to test the components.
1. Points X and Y are used to test and to determine the value of Zener (It is difficult to read the value printed on the Zener diode). Place the Zener with correct polarity between the points X and Y. Make sure that it is in firm contact with the points X and Y. You can use cello tape to fix the Zener. Then using a digital multimeter, measure the voltage between points A and B. It will be the value of the Zener. Note that, since 9 volt battery is used; only zeners below 9 volts can be tested.
2. Points C and D are used to test different kinds of diodes like Rectifier diode, Signal diode, LED, Infrared LED, Photo diode etc. LDR and Thermisters can also be tested. Place the component between C and D with correct polarity .Green LED will light. Reverse the polarity of the component (except LDR and Thermister) Green LED should not light. Then the component is good. If the green LED lights when changing the polarity, the component is open.
3. Points C, B and E are used to test the NPN transistor. Place the Transistor over the contacts so as make the collector, base and emitter in direct contact with the points C, B and E. Red LED will light feebly. Press S1. Brightness of LED increases. This indicates that the transistor is good. If it is leaky, even without pressing S1, LED will be bright.
4. Points F and G can be used for continuity test. Fuses, cables etc can be tested here for continuity. The continuity of transformer windings, relays, switches etc can be easily tested.The same points can also be used to test capacitors. Place the + ve of the capacitor to point F and negative to point G. Yellow LED will turn on fully first and then fades. This is due to the charging of the capacitor. If it is so, the capacitor is good. The time taken to dim the LED depends on the value of the capacitor. Higher value capacitor will take a few seconds. If the capacitor is damaged, the LED will either turn on fully or will not turn on.
2. Chip on Board
Chip on Board is a semiconductor assembly technology where the microchip is directly mounted on the board and electrically connected using wires. Different forms of Chip On Board or COB are now used to make Circuit boards instead of the conventional assembly using a number of components. These chips make the circuit board compact reducing both the space and cost. Main applications include toys and portable devices.
2 types of COB:
- Chip and wire Technology: The microchip is bonded to the board and connected by wire bonding.
- Flip Chip Technology: The microchip is bonded with solder bumps at the points of intersection and is soldered inversely on the board. It is done using conductive glue onto the organic pcb. It was developed by IBM in 1961.
The COB essentially consists of an unpackaged semiconductor die attached directly on to the surface of a flexible PCB and wire bonded to form the electrical connections. On to the Chip an epoxy resin or silicon coating is applied to encapsulate the chip. This design provides high packaging density, enhanced thermal characteristics etc. The COB assembly uses the C-MAC Micro technology which offers fully automated assembly of the chip. During the assembly process, a wafer of bare die is cut and placed onto an LTCC or thick ceramic or flexible PCB and then wire wounded to give the electrical connections. The die is then protected using the Glob top or Cavity fill encapsulation techniques.
Manufacturing a Chip on board involves 3 major steps:
1. Die attaching or die mounting: It involves applying glue to the substrate and then mounts the chip or the die over this adhesive material. This adhesive may be applied using techniques like dispensing, stencil printing or pin transfer. After attaching the adhesive is exposed to heat or UV light to attain strong mechanical, thermal and electrical properties.
2. Wire Bonding: It involves connecting the wires between the die and the board. It also involves chip to chip wire bonding.
3. Encapsulation: Encapsulation of die and bond wires are done by spreading a liquid encapsulating material over the die. Silicone is often used as encapsulant.
Advantages of Chip on Board
- There is no need for component mounting which reduces the substrate weight and the assembly weight.
- It reduces the thermal resistance and number of interconnects between the die and the substrate.
- It helps achieve miniaturization which can prove cost effective.
- It is highly reliable due to lower number of solder joints.
- It is easy to market.
- It is adaptable to high frequencies.
A Simple Working Application of COB
A Simple Melody circuit of Single Music COB used in door bell is shown below. The chip is too small with electrical contacts. The chip is a ROM with prerecorded music. The chip works off 3 volts and the output can be amplified using a single transistor amplifier.