Introduction to PCB: Printed Circuit Board

Printed Circuit Boards are primarily an insulating material used as base, into which conductive strips are printed.

The base material is generally fiberglass, and the conductive connections are generally copper and are made through an etching process

The main PCB board is called the motherboard, the smaller attachment PCB boards are called daughter boards or daughter cards

PCB board design defines the electrical pathways between components

It is derived from a schematic representation of the circuit

When it is derived, or imported from a schematic design, it translates the schematic symbols and libraries into physical components and connections.

A PCB design package allows the designer to define and design on multiple layers

Many of these are physical layers such as:
• Signal Layer
• Power plane Layer
• Mechanical Layer

And some are special layers such as:
• Solder & Paste Mask Layers
• Silkscreen or Top Overlay Layers
• Drill guides
• Keep-out Layer

And some CAD specific display-only layers:
• Ratsnest layer
• DRC Errors

Signal Layers are the tracks that represent where copper needs to be placed.

They are designed in the positive

There are no conventions for how thick signal traces can be. The width of your signals depend on
• Electrical nature of the trace (e.g. power traces are generally thicker than signal traces)
• Routing and space constraints
• Manufacturing constraings! Not all manufacturing houses/processes can handle fine traces
• For signals you can use 8 mil trace widths with 8 mil spacing

Thicker traces provide lesser resistance and inductance

Internal Layers are generally used for Power Planes

They are drawn in the negative, in other words tracks placed on this layer represent the void

A signal name such as VCC or GND can be assigned to an Internal Plane and the CAD tool
will automatically connect pads to the plane, greatly simplifying PCB routing

Internal planes can also be split into sub planes or split planes

Mechanical Layers are special in that they can be added or viewed in any layer

Used to provide drill and cut-out guides during the manufacturing process

Keepout layers generally follow the mechanical layer to indicate to the CAD tool an area should not be routed over. This is useful as an autorouting constraint.

Solder mask is usually the green coating on a PCB board which is designed to insulate and protect the underlying copper traces from enviornmental factors, and is also used to prevent bridging (shorting) traces during wave soldering

Solder mask usually covers everything on the PCB board except for pads and vias, though it is good practice to cover vias, especially if dealing with BGA components. This process is called tenting the vias

Solder mask is shown on the CAD tool as a negative image. I.e. where there is solder mask
“shown” is where there will be NO soldermask.

Paste Mask
• Paste mask is similar to solder mask, except that it
is used to create solder paste screens which can
then be used to solder SMDs in a hot re-flow
soldering process

• Multilayer is a simple way of adding an object such as mechanical drawing or a pad to ALL the PCB layers. Any object on this layer will automatically be added to all the PCB layers

Drill Layer – There are two drill layers
• The Drill Guide Layer is used for legacy equipment and is generally not in use today
• The Drill Drawing Layer provides coded plots of board hole locations

The silkscreen layer is also known as Overlay.
Top Overlay refers to the silkscreen on top of the board, and Bottom Overlay refers to silkscreen on bottom

This is the layer onto which the component designators are printed (R1, R2, …) so as to identify individual components during component placement of the board

They are also used during the PCB routing process to indicate the outlines of your
components. This helps you in placing (or not placing) components too close to one another, or too close to the edge of the board.

Make sure your silkscreen doesn’t run over any exposed copper (such as pads)!

Multi-layer PCB boards are the norm for complex designs, in particular those
which have size constraints

The simplest of multi-layer boards is a 4 layer board which provides two internal
power planes and two outer signal layers

More complex multi-layer boards can reach up to 24 layers!

Some PCB material limit the number of layers, such as flexible PCB boards, but the advantages of using flexboards can

By using dedicated power and ground planes, you drastically reduce the complexity of your routing, not to mention reducing inductance and impedance across the board

Prevent ground loops from hapening

Providing good grounding is critical to the functionality of a PCB board

On a multi layer board, one of the layers should be dedicated ground and all ground signals should have vias into this ground plane.

Avoid chaining grounds in order to prevent ground loops from occurring

Use bypass capacitors to smooth out power spikes by components that suddenly draw significant current

Bypass caps should be placed as close to the component’s power pins as possible

You should use 100 nf caps as the norm, with lower capacitance for higher frequencies, and higher capacitance for lower frequencies

Viewing all layers in a PCB document at once can quickly get out of hand

In order to aid the designer, CAD tools provide several options for managing and showing active layers

One technique is to view the PCB in a draft mode.
In this mode objects are drawn only with their outline

Another technique is to view them one layer at a time

Some CAD tools have a “see through” mode enabling you to view multiple layers at the same time

Finally you can enable/disable each layer

PCB Boards are primarily designed in imperial units (inches) as opposed to metric units (mm).

A thousands of an inch is called mil (not to be confused with mm), where:
• 100 mils = 0.1 inch = 2.54 mm

The reason for using imperial units in a PCB document is because most of the
components were manufactured according to imperial pin spacing. The practice continues even today!

Pads come in all sorts of shapes and sizes. They can be through-hole or SMD but they all follow the same general guidelines

Through-hole pads are used for leaded components like resistors and capacitors are round, with about a 60 mil inner diameter

Dual in-line Packages (DIP) components like ICs fair better with an oval shaped through-hole pad, with 60 mils in one dimension and 100 mils in another

Surface mount (SMD) components adhere to the chip specs and are generally rectangular

Pin 1 of any chip footprint should always be a different shape: rectangular for DIPs and oval for SMDs

Vias are special pads which connect electrical signals from one side of your board to another

In special circumstances, from one layer to another without crossing all layers (blind or
buried vias)

Blind and buried vias are to be avoided at all cost, they are difficult to debug and rework

Vias are made of conductive material which are called Plated Through-hole.

There is really no difference between Vias and Pads except that the CAD tools manage them
differently to allow more complex operations on vias.

Vias are generally much smaller than pads

Component placement is an extremely important function of the designer

Components should be placed according to their connections to other components, thermal
considerations, mechanical requirements, as well as signal integrity and routability

Components which have connections to each other should be placed in the same vicinity

For example, a processor should be placed very close to the RAM and Flash ICs on which it relies

Components should also be placed on a grid, usually a 100 mil grid, in order to provide for a symmetric flow of routing where tracks and components are lined up

CAD tools provide auto-router and board wizard functionality

In reality, PCB designers don’t use an auto-router

The technology behind an auto-router has a science of its own, drawing from disciplines such as artificial intelligence, heuristic algorithms, and ultimately attempting to solve the traveling salesman problem

The irony is, only a very experienced PCB designer can take advantage of an auto-router, and even then, it is only used for a fraction of the board

Internal layers should be designed at least 15” away from the edge of the PCB board. This is to ensure that the manufacturer of the board does not cut into the copper or short the planes. You can employ the keepout layer for this purpose.

Each manufacturing house has their own manufacturing guidelines for minimum space width, copper-to-edge distance, via and holesizes, annular ring, etc.

Many assembly houses require that you provide your PCB boards in a panelized format.
Panelizing a design means fitting multiple boards on a standard panel. Once the assembly is done, they are cut into individual PCB boards

Gerber, otherwise known as RS-274-D, is the industry standard photo plotting language

It is used by photo plotter equipment which use light to “draw” a line

It was developed in the 1980s, and has since been modified in many ways

It is an ASCII format file, which instructs the photo plotter to perform one of four basic functions:
• Move to location X,Y
• Select the correct aperture tool
• Open, close, or flash shutter
• End of line instruction (the ‘*’ character)

In order for a board manufacturer to build the PCB boards, they need a set of gerber files

The PCB CAD tool is capable of generating gerber files for every PCB layer

However, an aperture library and sometimes a drill file must be generated separately

As discussed, gerber files instruct the equipment where to go and what to do. The aperture list specifies which tool to use. Aperture files come with different resolutions, it is best to send the highest resolution aperture list possible to the manufacturer

Excellon drill files which may also be sent separately are used to tell the equipment what size holes to drill and where.

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