What is on th PCB

Let’s start with a PCB based on FPGA extension training without soldered components.

The main elements that make up a PCB can be seen in the diagram.

Component

Component refers to a component that is soldered to a PCB.

PCB carries various components. There are many different types of components and their packaging on the PCB varies greatly.

Even the same type of component is available in many different packages. For example, a 14-pin op-amp LM324 is available in DIP14, SO-14, SSOP14, and so on.

Devices can be simply categorized into two broad groups based on package type.

(1) Through hole devices:

Holes need to be punched in the circuit board to hold the pins in place. The pins of the component span the Top layer or Front layer and Bottom layer or Back layer.

(2) Surface mount devices:

Components appear on only one layer, either on the top layer or on the bottom layer.

But a device is only on one of the layers.

As the system of the product runs faster and faster, the integration of the device is getting higher and higher.

Surface mount devices (SMDs) became mainstream.

Because it is small, cheap, and has good high-frequency characteristics.

And circuit boards are easier to process and produce, and the density of the boards can be higher.

Additionally, you should be careful to distinguish between two English terms related to surface mount devices:

(1) SMT, is the abbreviation for Surface Mount Technology.

(2) SMD, is the abbreviation of Surface Mount Device.

Pads

The pin of the device is secured to the board by a pad and is electrically connected to other pads. The pads are an important part of the device package.

Depending on the type of packaging of the device. It has through hole pads that run through the top and bottom layers.

As well as surface mount pads that are only on the top or bottom layer.

Some devices have both pads and they come in a variety of shapes and sizes.

When building component packages with a CAD tool, set them up correctly according to the definitions in the datasheet for that device.

When using the package that comes with the tool or downloading a package from the Internet, be sure to double-check it. Make sure that the size of the pads meets the specifications in the device manual.

Layer

The PCB is layered, with each layer having a different function and definition. Components are generally in the Top layer or Bottom layer of the PCB.

In addition to the “physical” layers that connect the signals, there are also layers that are used for processing, installation, and information indication.

We generally refer to single-panel, double-panel, 4-layer boards, etc. as layers with electrical connections (signals, power, ground, etc.).

However, the layers in CAD tools are much broader.

When designing PCBs with CAD tools, it is important to clearly understand the meaning and role of these layers. And to set up and use them correctly.

The layers of the circuit board is almost invisible to the naked eye. If you want to learn, please refer to "How to identify the number of layers of a circuit board with the naked eye" (just click the link).

Track

Track is the copper wire used on the PCB to make electrical connections between devices and devices. The Wire on the schematic is the ideal connection.

Because there is no impedance, current limitation, and interference with each other.

However, the actual wires on the board have a certain impedance (depending on the length and width of the wires, the board, the vias, etc.).

There are also practical problems such as mutual electromagnetic interference between the wires.

After all, space on the board is limited.

The length and width of the wires, as well as the distance between wires, are set according to the physical space of the board and the number of signals to be connected.

Depending on the nature of the signal and the constraints of the alignment space on the board, the width varies.

For example:

(1) Alignments carrying large currents should be as short and thick as possible.

(2) Differential lines carrying signals in pairs should be as equal in length as possible.

(3) The transmission impedance of high-speed digital signal lines should match the output impedance of the transmitting device and the input impedance of the receiving device as much as possible.

PCB board alignment

Via

If the circuit cannot realize all signal alignments on one level. Then the signal lines have to be connected across the layers by means of Via.

The form of the via and the size of the aperture depend on the characteristics of the signal and the requirements of the processing plant.

The Via can be compared to an underground tunnel in life.

Depending on the setup of the connecting signal layer, there are three main types of vias

Three types of perforations

(1) Through hole.Connected to the upper and lower levels, It is visible above and below.

(2) Buried via. Inside the board, connecting two layers inside the board that are not visible on the surface.

(3) Blind via. Only one side is visible, the other is not. Blind hole connects a signal from a surface layer to one of the internal signal layers.

The shape and size of the vias are not set randomly. This depends on the characteristics of the connected signals and the process requirements of the PCB processor.

Silkscreen

The Silk Screen is used to mark the outline, orientation, numbering, and remarks of components on the PCB. This is for easy identification.

The name of the silk screen varies from one CAD software to another.

For example, the top layer of the silkscreen in Altium Designer is called Top overlay,. And the bottom layer is called Bottom overlay.

Screen Printing on PCB

Solder mask

In order to prevent short-circuiting of signal lines that should not be connected for any reason, the PCB should be protected with a solder mask.

The PCB should be protected with a solder mask layer. This is a layer of oil that is used to insulate the top and bottom layers where there are no pads.

The solder mask prevents the solder from short-circuiting two wires with different signals.

Its presence also prevents short circuits between wires and solder joints during reflow soldering, wave soldering and manual soldering.

It also prevents oxidization of the copper layer and improves the reliability of the PCB.

Different colors of boards are actually different solder-mask colors selected

Different colors of solder resist are used to get different colors of boards.

When the same board is processed, different colors of solder resist are selected. And different colors of boards are obtained.

Many companies or teams purposely use a certain color for a certain series of boards to show off their style. For example, Spark Fun’s boards are basically all red.

Even the sides of the same board may use different colors of solder-mask. For example, the Arduino Uno board has a green color on the front and part of the back is white.

When choosing a different solder-mask color when processing a fast board, the price and delivery time may vary. In general, green is the most versatile and fastest.

Alignment hole

On many boards there are usually some round holes with an inner diameter of 2.5~3mm near the perimeter. They are called Mounting Hole.

Some of these holes are used to connect the circuit boards that hold the physical laminate in place.

For prototypes under development, holes are punched in the four corners even if no other boards need to be connected.

This is to facilitate debugging. Without interfering with other electrical connections, the four holes can be connected to “ground” on the board.

A more reliable way is to connect an oscilloscope probe and a grounding clamp to the “ground” on the board during debugging.

This eliminates the need to solder a “ground” test point. Of course, the “ground point” of the high-speed circuit must be close to the signal under test.

In addition

When commissioning, installing plastic or metal screws on the four positioning holes can provide support.

Placed on the experimental bench needs to keep a certain distance from the bench surface.

Avoid wires and components on the table top to cause a short circuit of the signal on the board.

Positioning holes for connection to the motherboard (two on the left side)