What is the PCB Footprint? | 2021 Newest

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Thur, Mar 04, 2021

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What step should I study first if I want to get my PCB?

What are the PCB footprint and PCB library? How to design the PCB footprint? This blog may help you.

Part 1: What is PCB Footprint

pcb footprint1 .jpg

The PCB footprint is a pattern for the soldering of an electronic component. The pattern is the fundamental part of the design of a printed circuit board. However, the electronic components are of different types and nature. These may be surface mount capacitors or a through-hole connector.

Part 2:What is the PCB Library?

The collection of one or more footprints of the printed circuit board for electrical components represents a PCB library.

A PCB library refers to the arrangement of the physical pad for the electronic component.

However, the other mechanical properties of the electronic components are also represented by the PCB library.

A PCB library can also represent the physical shape of the electronic component in 3D form. It also contains the information of the 3D model in the form of STEP files that can be added to the footprint.

Part 3:What is the Netlist in PCB?

There are many connections between the different components on a printed circuit board. All these connections come under the netlist in the PCB.

Therefore, the netlist contains the electrical connections in the textual format. The generation of the netlist in the printed circuit board comes from the production data.

The role of the netlist is very important in the connection integrity of the printed circuit board. We can locate the missing connections or incorrect connections with the help of the netlist. This happens with the help of a test named as E-test.

pcb footprint 2.jpg

Part 4: PCB Footprint Design Guidelines

The following guidelines are important for the design of the PCB footprint.

Ensure symmetric layout

If the layout of the design of the component is symmetric, it will prevent you from the difficulty of tombstoning. The process of the detachment of any side of the component is tombstoning. This process occurs due to the unbalanced force on the component. Retain opposite pads of the same size

Retain opposite pads of the same size

It is important to keep in mind that the opposite pads of the component have the same size and shape. This will help you prevent the tombstoning and shifting of the sides of the component.

Ensure adequate solder mask dam

The application of the solder mask dam usually occurs between the footprint pads and other elements of the electronic component. Therefore, you should ensure that this solder mask dam is adequate between these parts. This process helps to prevent solder bridging. The solder bridging occurs when the joints are present too close to each other.

Make sufficient clearance between pads and board edge

You should also take care of the creation of sufficient clearance between the board edge and footprint pads. This process helps you to do the process of depanelization. The process of dividing the panel into smaller parts is depanelization. During this process, the mounting and assembly of the electrical component also occur.

So, you should also include the following FDA guidelines for the successful and proper placement of the components.

Include pin 1 indicators

The pin 1 indicators have the important role of checking the correct connections of the pins. They can locate if any wrong or inappropriate pin connection is present. They ensure the correct attachment of the pins to the pads.

Use polarity indicators

For the indication of the polarized components, the use of the polarity indicators is very essential. This also helps in the correct functioning of the electrical components. The reason is that the reverse polarity of the components can cause catastrophic effects on the operation of the PCB.

For example, the diode acts in the conduction mode in normal cases. If it has the reverse polarity in the circuit operation, it will function as an open circuit.

Include component outlines

The component outlines are helpful in the creation of the correct placement of the electrical components. Moreover, it is also important to make the pad connection areas the same.

If you follow these guidelines during the process, it will ensure the correct placement of the electronic components on the printed circuit board. You should not mix the functions of the other components with them during this process.

Part 5: Types of the Footprint in PCB

The footprint describes both the electrical connection of the component and other information: silkscreen printed on the board, data for the assembly drawing, etc. 

Footprints can be created manually or using the wizard. PM created with the help of the wizard can be modified manually. There are two types of footprints in the system:

  • common seats
  • the footprint for one component

Generic footprints describe typical packages, so they are reusable and stored within the library. 

Footprints for complex / unique components (eg BGAs) can be created directly "inside" one component - these are footprints for one component. This approach allows you to simplify the library because the likelihood of reusing complex footprints is extremely low. In any case, the necessary data can be used by copying.

The seats can be divided into two classes:

Mechanical seats

Electrical footprints are intended to describe the installation of electronic components. 

Electric seats

Mechanical seats are used to describe the placement of additional elements on the board, such as heat sinks. 

In other words, mechanical footprints are not designed for routing. They cannot include electrical connections.

Any footprint contains various objects, the composition of which is determined by the type of footprint. Objects are placed on "footprint layers" that allow you to interpret the actual board layers.

Seat composition

Footprint descriptions can contain various objects. Their possible set is determined by the type of footprint.

A mechanical footprint can include the following objects:

  • Component body boundaries
  • Mounting holes
  • Places of glue application
  • Graphic marking
  • Assembly drawing information
  • Regions (design rule changes)

An electrical seat can include the following items:

  • Contact pads
  • Hull boundaries
  • Mounting holes
  • Tracks (between pads)
  • Vias
  • Fiducial points
  • Places of glue application
  • Graphic marking
  • The value of the attribute (characteristics) of the component
  • Assembly drawing information
  • Regions(design rule changes)

Creating a footprint

The system has two types of contact pads:

  • Planar contact pads (SCP) - located on the outer sides of the board.  The control panel does not require data on the internal layers of the board.
  • Through-hole pads (SKP) are pads with a through-hole.  For UPC, you need to enter data about the inner layers of the board.

In addition to the indicated types of contact pads, using this editor, mounting holes and reference points are created (foil pads freed from the protective mask, to which the conductor is not connected).

pcb footprint3.jpg

Further, in a specialized editor, footprints are formed from contact pads and other elements. It should be noted once again that both silk-screen printing and information for the assembly drawing (marked in yellow) are set on the footprint.

pcb footprint 4.jpg

The seats can have various modifications designed for different mounting densities. Like: high, medium, and low (in accordance with the IPC-7351A standard). 

In the Delta Design system, these three modifications can be implemented within one footprint. In order to create a footprint with different modifications, it is necessary to create the appropriate pads.

The pads will differ for different mounting densities. When switching between different mounting densities, the pads will change accordingly. However, the coordinates of the pads will be preserved. Silkscreen printing and assembly drawing data for each mounting density are configured separately. 

By placing such a footprint on the board, you can easily change the mounting density, rather than using another component in the project. Besides, this mechanism allows you to simultaneously change the density even for all components on the board. 

For example, if you were unable to route the board, you can simply increase the packing density and try again using the freed-up space. To facilitate the designer's work, footprints can be created using the footprint wizard. The wizard creates footprints for typical packages such as BGA, SOIC, BQFP, and many others.

pcb footprint 5.jpg

A footprint created using the wizard can be modified in the footprint editor. Typical elements (contact pads) can be replaced with non-standard ones; this makes it easy to create even complex non-standard footprints.


Conclusion

I hope this blog can help you with the PCB footprint design. If you’re done with your design, work with a reliable PCB manufacturer to have your board produced. PCBONLINEis a one-stop manufacturing and assembly service provider in the PCB field. Besides, if you’re not that sure about your board design, you can ask us for free tech support and Gerber checking. And if you trust us, you can also let us manufacture the board for you. What's more, if you are interested in this article, there is the second one which write the related topic of PCB footprints. please click:  https://www.pcbonline.com/blog/how-do-you-make-and-change-a-footprint-in-pcb.html