A printed circuit board (PCB) represents the electrical connections between component packages and pads using copper tracks. Common PCB materials include copper conducting layers and insulating epoxy resin layers. Footprints define the pattern components occupy on the board. Initially, components are placed loosely in airwire form, resembling a messy ratsnest that is later disentangled through routing. Routing connects components by selecting shortest paths between pads while adhering to design guidelines regarding component placement, trace widths, and thermal reliefs.

1. Printed Circuit Board, PCB
A PCB is the representation of the electrical connections of
the pads (footprints) of component packages with solder pads
and signal tracks.
Footprint
Solder
Pad
Track

2. PCB Materials
Conducting layers are typically made of thin copper foil. Insulating layers
dielectric are typically laminated together with epoxy resin prepreg.
The board is typically coated with a solder mask that is green in color .
Type of prepreg: FR-2 (Phenolic cotton paper), FR-3 (Cotton paper and
epoxy), FR-4 (Woven glass and epoxy), FR-5 (Woven glass and
epoxy), FR-6 (Matte glass and polyester), G-10 (Woven glass and
epoxy), CEM-1 (Cotton paper and epoxy), CEM-2 (Cotton paper and
epoxy), CEM-3 (Woven glass and epoxy), CEM-4 (Woven glass and
epoxy), CEM-5 (Woven glass and polyester
FR-4 is most common material used today

3. Component Footprint
Define as:
• The pattern and space on a board taken up by a
component
• A graphic software representation of a component.
Categorized into:
1. Thru hole device
– Axial-leaded
– Radial-leaded
2. Surface Mount device

4. Radial-leaded Capacitor Layout footprint
Axial-leaded Resistor Layout footprint

5. Component Package
• A type of PCB component which contains a chip and acts
to make a convenient mechanism for protecting the chip
while on the shelf and after attachment to a PCB.
• With its leads soldered to a printed circuit board, a
package serves as the electrical conduction interface
between the chip and the board.

6. TO-92 TO-220
DIP8 DO-41

7. Airwire / Ratnest
• Airwires are to be seen only in the layout view (PCB).
• They represent the electrical connections (signals) between
pads and often intersect each other.
• They are disentangled later by the use routing. A
multitude of airwires are called ratsnest.
• “rat's nest” is used to describe a particularly messy or
disordered environment.

8. Multitude airwires
create disorderly
environment,
commonly
called as ratnest
Air-wire

9. Component Placement
Guideline
• place components close to each other while trying to keep the routing
process in mind.
– As you place components, turn and flip each one around to make the
guide lines as straight and as short as possible.
X
√

10. Guideline (cont.)
• place components on the circuit board just like they are drawn on
the schematic.
– That means that components that are directly connected to each other will
be placed closely together.
• It is usually easiest to place the big components first (like
microcontrollers, op amps, etc.) and then place all of the little
ones that connect to them (resistors, capacitors, etc.) around them.
Schematic Layout

11. Guideline (cont.)
• completely place all components on the circuit board before
beginning any routing.
• try to leave at least 30 mils between components, and 50 mils
between a component and the edge of the circuit board.
Mil?
a unit of distance equal to
0.001 inch: a "milli-inch,"
One mil is exactly
25.4 microns.
50 mils 30 mils
1 mil = 1/1000 inch

12. Routing
Routing is the process of selecting paths in a net to connect
components according to its connection.
Guideline
• In general, the shortest routes are the best.
– This is because long routes tend to generate more electrical
noise, or pick up more electrical noise.

13. Guideline (Grounding)
• Ground is the most important signal on all circuit boards
• In a well-grounded board, the ground signal is a solid reference
for every signal on the board
• ground net with very narrow traces, it will have higher resistance
than if you route it with wide traces
• At high frequencies, a wide trace is less inductive than a narrow
one.
– This means that the high frequency AC resistance of your ground is
lower.
• Conclusion: ground must be route using wide traces

14. Power Plane
• To achieve better grounding cooper pouring and power
plane for ground route is introduced.
• Benefit of abundance grounding:
– to provide built-in shielding from external interference
– help quiet any noisy routes
– reduce cross coupling on the board.
– provides a good safe path for static electricity, thus increasing
the reliability of your finished board

15. Ground Routing Ground Plane
a.k.a Copper
Pouring

16. Thermal Relief
• A thermal relief pad is a PCB pad connected to a copper pour
using a thermal connection.
• It looks like a normal pad with copper "spokes" connecting it to the
surrounding copper.
• A pad directly connected to the copper pour would be difficult to
solder, since the heat provided by the soldering iron will quickly
leak away from the pad and into the copper pour (due to high
thermal conductivity of copper).
• A thermal connection restricts the heat flow, making the pad easier
to solder.

17. Thermal Relief
Solder pad on cooper
pouring.
Relief heat transfer to
ease soldering