PCB Design Basics

The PCB (Printed Circuit Board) is the base of any electronic device. In general designing a PCB is just interconnecting the components by any conductive material (mostly copper) tracks, which can be easily soldered.

When designing a PCB one needs to consider so many parameters, neglecting any one of will affect the proper functioning of the circuit. We are here discussing about such parameters, so one can easily design a professional grade PCB.

Base Laminate

The PCB design processes starts right from the selection of the base laminates can be used.

There are two basic types of laminates available, Glass Epoxy and Paper phenolic. Glass epoxy is more expensive and has better electrical characteristics than Paper phenolic. So Paper phenolic is used in simple circuits.

There are 4 types of base laminates are available FR2, FR3, FR4 and FR5.

FR2

FR3

FR4

A brief comparison of PCB laminates

Property	FR2	FR3	FR4	FR5
Name	Paper Phenolic	Epoxy Paper	Epoxy woven Glass Fiber	True Epoxy woven Glass Fiber
Temperature withstand	105 degree C	120-130 degree C	140 degree C	210 degree C
Mechanical Strength	Ok	Good	Very Good	Very Good
Cost	Low	Medium	High	Very High
Dielectric Strength	750V/mil	MV/m	20 MV/m	MV/m
Application	Single Layer	2 Layer	8 Layer	High grade Multi layer PCB

Copper Foil cladding

Add caption

Second important thing while designing the PCB is the copper foil. There are two types, Type-A and Type-B. Type-A is 98% pure copper, while type-B is 99.9% pure copper. The thickness of the copper cladding is measured in microns or µ meter. For selecting proper clad thickness, trace resistance should be calculated first.

The trace resistance R is 

R = SL/WT

Where

S   is Specific resistance of Copper.

            L   is Length of trace.

            W is Width of trace.

            T  is Thickness of copper cladding.

 Resistance of the trace affects the voltage drop across the trace & heating of the trace. When the passing current through trace is considerably high. With low current circuits we should not bother about the trace resistance.

Capacitance formed by trace and laminates

The copper foils on either side of the laminate forming a capacitor of value C can be calculated by

C = (0.887)EA/D u farad

Where

            A is overlapping area in cm²

D is thickness of base laminate

E is dielectric constant of base laminate

Also, there is a capacitance formed by the two adjacent traces, which is very difficult to calculate. So the above calculation has to be considered for first approximation only.

Base Material	Dielectric Constant
FR2	4.8
FR3	4.8
FR4	5.4
FR5	5.4

Inductance of the traces

                             The inductance of the traces is varies inversely with the ratio of the thickness to width. In most application where low frequency circuits are used, trace inductance and capacitance is not a critical thing. But in the frequencies at VHF & UHF level the inductance and capacitance formed by traces should be analyzed in a proper way. 

              The inductance & capacitance of traces can also create delay in signal propagation. The delay can be increased with increasing thickness of the trace and decreasing thickness of the base laminate. So it while using thinner base laminate, traces should be thinner to compensate each other.

Width of the traces

                                                Width of the traces can be considered by a thumb rule that is 

“The width of the Power Supply trace should be double of the Signal trace, while the width of the Ground line should be double of the Power Supply line.”

It is recommended that the copper should not be etched from the unused areas on the PCB, instead it should be the part of the ground line.

It has two advantages:

1. This will reduce the voltage drop in the ground pattern & minimize the falls triggering of logic gates.
2. Reduces the etching time and cost due to less areas need to etched.

Placement of the traces

The placement of the traces is also an important parameter in PCB design. Few points should keep in mind while designing the PCB:

1. The Ground and Power Supply trace should be placed such that a capacitance coupling forms between them.
2. 
   
   
3. In a double sided PCB the Power Supply trace and Ground trace can be on either side of the PCB.                                                                                                                                                                    
4. The Signal trace should be exactly above the Ground trace.
   
5. Any delicate signal, low level signal or very high impedance signal trace, should be shield by ground trace. Ground traces should be placed both side of it. It will reduce the interference from adjacent signal traces.
   
6. In FET circuits a ground trace should be placed between Gate terminal and Source, Drain terminals.
   
7. Wherever Digital and Analogue both types of circuits are in a same board separate ground line should be placed, connect them a point which is nearer to power supply ground. 

Size of the PCB

                          The size of the PCB should also be properly selected. It should be as smaller as possible. A big also needs more place and packaging material for it. The manufacturing cost is also more. But big PCB designs are less complicated to do repair work. Smaller PCBs are more complicated layout, which takes more time in repairing and troubleshooting. While small PCB design having low manufacturing cost and needs less storage.

Some more PCB design considerations

1. The PCB layout design should be simplified twice or thrice so that it will be compact in size and number of jumpers can be reduced as possible.
2. Before start to design the circuit should be studied properly and divided into separate blocks for easy signal flow, try to reduce the signal traces, while increase in jumper could be tolerable.
3. Test points, Potentiometers, trimmers etc should be placed at easily accessible location, sometimes 
   could be access from outside of the cabinet.
4. Connectors, jacks, on-board fuses, switches should be on the perimeter of the board. Before go further in designing consider is there any component which needs Heat-sink? If so then make room for the same. Heat-sinks should be separated from heat sensitive components and well ventilated
5. For heavier components extra mounting support should be provided on the PCB.
6. Multi-layer PCB should have two dedicated layers, one for power supply and the other for ground.
7. Before passing the design for manufacturing, design validation process is very important for error free product.
8. So many CAD softwares are available in the market, always use a licensed product.
9. Most CAD packages are only for design purpose not for simulation of the design; hence designer should have adequate knowledge to produce a professional grade PCB. 

Here is a detailed check list for PCB Designing