PCB (Printed Circuit Board)
What is a Printed Circuit Board or PCB ?
A printed circuit board or PCB, is a plate or board where different elements are placed to form an electrical circuit and it contains the electrical interconnections between them.
In its simplest form, a PCB is a plastic board reinforced with glass. It uses conductive tracks, pads and other similar features etched from copper sheets and laminated onto a non-conductive substrate.
These copper lines (known as traces) allow electrical charge to flow through the PCB, providing power to the different components that are situated systematically on the board.
Capacitors, resistors and other components are soldered onto the PCB where needed.
Fig.1 : A 2 layer PCB
Layers of a PCB
The simplest PCBs are single sided boards i.e., it contains copper tracks and interconnects only on one of its surfaces. These types of boards are known as 1 layer printed circuit board or 1 layer PCB.
The most common type of PCB’s contain 2 layers, that means , it has interconnects on both surfaces of the board.
However, depending on the physical complexity of the design ( PCB layout ), the boards can be manufactured of 8 or more layers.These new layers have their own copper trace formations.
An assembly process is required to mount the electrical components on the printed circuit boards(PCBs).
This process can be either done by hand or through specialized machinery.
This process requires the use of solder to place the components on the board.
For avoiding or to prevent the solder to accidentally short-circuit two tracks from different nets, pcb manufacturers apply a finish or varnish called soldermask on both surfaces of the board.
The most common color of soldermask used in printed circuit boards is green, followed by red and blue.
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In EDA software (Electronic design automation), generally exist a rule associated to the expansion of the soldermask. This rule specifies the distance that exists between the pad’s borders and the soldermask’s border.
This concept is illustrated in figure 2(a).
Silkscreen or Overlay
Silk-screening is the process of printing information on the soldermask.
This process is normally used on the component side to identify components, test points, PCB and PCBA part numbers, warning symbols, company logos and manufacturer marks.
The silkscreen can be printed on both surfaces of the board.However, the work required to produce two silk-screens is simply twice as much as one and hence the price is high.
Silkscreening requires specially formulated inks. Standard colors are Black, white and yellow. Non-standard colors can be requested but will impact your cost and your lead-time.
Figure. 2 shows a region of a circuit, where all the printings are made in white.
As discussed above, the PCBs can be made of several layers.
While designing a PCB using EDA software, all the layers need to be specified.
Since a PCB has both conductive and non-conductive layers, hence we are going to use the term layer without the suffix “CAD” only when referring to conductive layers. If we use the term “CAD Layer” then we will refer to all kinds of layer, that is both conductive and nonconductive.
The CAD Layer stackup is as following:
Figure.3 shows 3 different stackups.
Fig.3: (a) 2 layers stackups , (b) 4 layers stackups, (c) 6 layers stackups
The orange color highlights the layers in each stackup.
The stackup height, or board thickness can vary depending on the application, however the most used is 1.6 [mm] or 62 [mils].
In some countries [thou] is used as a synonym for [mils]. ( 1 [mil] = 0.001 [inch] = 0.0254 [mm] )
There is a great variety of electronic component packages in market now.
It is common to find several types of packages for one device. For example you can find the same integrated circuit in QFP’s and LCC’s packages.
Basically there exist 3 big families of electronic packages such as :
- SMD/SMT (surface mount device/surface mount technology)
- BGA (Ball grid array)
All the components that have pins and are intended to be mounted through a plated hole in the PCB, belong to this group.
This kind of component is soldered to the opposite side of the board from which the component was inserted.
Generally these components are mounted on one surface of the board only.
All the components that are soldered in the same side of the board from which the component was placed, belong to this group.
The advantage of this type of package is that it can be mounted on both sides of the PCB.
Also, these components are smaller than the thru-hole type, which allows the design of smaller and denser printed circuit boards.
BGA(Ball grid array):
These types of components are frequently used for high density pin integrated circuits.
Since the pins are made of solder balls that have to be melted for making the electrical contact with the pads, therefore it is required to have specialized machinery to solder them to the printed circuit boards .
BGA components are ideal for high frequency integrated circuits due to the very small parasitic inductances present in the joint between the pad and the balls.
These type of components are very common in computer hardware like motherboards and video accelerator cards.
It is a small surface of copper in a PCB that allows soldering the component to the board.
There are 2 types of pads. Such as : thru-hole and SMD (surface mount).
Thru-hole pads are used for introducing the pins of the components, so that they can be soldered from the opposite side from which the component was inserted. These types of pads are very similar to a thru-hole via.
The SMD pads are intended for surface mount devices, or in other words, for soldering the component on the same surface where it was placed.
As shown in Figure. 4 , this PCB has 4 components.
The components IC1 and R1 have 8 and 2 SMD pads respectively, however the components Q1 and PW have 3 thru-hole pads.
Fig.4: SMD and Thru-hole pads
It is a conductive path which is used to connect 2 points in the PCB.
For example, for connecting 2 pads or for connecting a pad and a via, or between vias.
The width of the tracks depends on the currents that flow through them.
Fig.5 shows the tracks on the PCB that is connecting IC1 and IC2.
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