Generally, a footprint could reveal a lot of information about something. In this term, footprints serve as the result of something that arrived and left a pattern. Footprint is often a term of concern for printed circuit board designers. Footprints in PCB design refers to a pattern of a component that will be soldered there.
It could be a surface mount capacitor or a through-hole connector. Every part of a printed circuit board that requires soldering must have a footprint. Sometimes referred to as landing patterns, footprints are necessary in PCB production. Therefore, it is important to design land patterns to exact standards.
Engineers require footprint for manufacturing process. Your component footprints need to adhere to some standards just like the PCB design. Creating component footprints is a valuable skill.
What is a PCB Footprint?
A PCB footprint is an arrangement of through holes or pads used to electrically connect and physically attach a component to a board. A footprint can be also known as a land pattern. The footprint on a printed circuit board must match leads arrangement on a component.
Component manufacturers usually produce several pin-compatible products. This enables the systems integrators to transform the specific component in use without changing the PCB footprint. Many component manufacturers design footprints for their components. Land patterns look like the physical dimensions of the component. This land pattern provides an idea about the placement of component and location of traces.
Also, designers use vias and traces to interconnect land patterns on the PCB. Regardless of the component type, footprint is important. Whether it is ball grid array, SMT, or THT, a land pattern is crucial for PCB assembly. PCB footprints reveal where you need to place components in a PCB layout. Therefore, they are part of the production files sent to a manufacturer once production begins.
Accuracy is important when designing a printed circuit board footprint. An incorrect footprint will make the production process a mess. Component designators are usually encoded in your component model and this will appear on the layout when you place a land pattern. A component footprint comprises several parts which include silkscreen, , and more.
What does a PCB Footprint Comprise?
A printed circuit board footprint or land pattern comprises the following elements:
Courtyard and outline
This is a keep out area or boundary in your land pattern. Thus, you can’t place other components within the PCB footprint courtyard.
Mechanicals
There are some components featuring mechanical elements. These elements may extend above a component and cause a collision with other components. An example is a heat sink that has no electrical connection to the printed circuit board. Also, you can define this in the mechanical layer in your circuit board layout.
Hole or pad locations
Your footprint will also include the location of a hole or pad. This reveals where to locate SMD pad or THT hole. Also, these locations are important for soldering. Therefore, you must design them accurately.
Reference designator
The reference designator is an alphanumeric code that offers a unique identifier of the component in a PCB layout and schematic.
3D model
Footprints often feature a corresponding schematics symbol. The 3D model allows you to view the boards in 3D. Also, it allows 3D clearance evaluation between components.
Pin-1 designator
This reveals the correct orientation to the assembler. Some components feature arbitrary orientation.
How to Create a PCB Footprint
Designers make use of design software to create PCB footprints. Most times, PCB footprints of some standard packages are in the PCB library of the software. The land pattern must match the actual physical size of the component. Otherwise, it won’t align well with the pads. Land patterns provide information as regards the distance between pads, reference designators, and more.
The different parts of a footprint include:
- Manufacturer part number
- Silkscreen
- Component centroid
- Connectors and special components
- 3D model step file
- Assembly details
- Mounting holes/SMT pads
- Component boundary
- Component orientation marking and Pin 1 marking
There are guidelines for creating a PCB footprint. Also, these guidelines include how to inspect, rework, and test components.
- Begin with exact information
Make sure you have the right specifications for your part. Also, ensure you adhere to relevant standards.
- Create footprint from the ground up
Here you need to begin with the pad information. Ensure you have everything you need. Then, create the outline of the component at the greatest material width. After this, include reference designators, silkscreen outlines, and other features.
- Ensure you include all necessary data
There can be problems during a PCB layout if the footprint is incomplete. For example, if CAD tools state that you need to consider the height of the part, don’t ignore it.
- Be careful of adding too much data
Don’t add unnecessary information to your footprint. If you include extra data, it will be replicated. Ensure you review your footprint for accuracy.
Basics of PCB Component Footprint
A PCB component footprint should comprise pieces of information. These include silkscreen outlines, reference designators and more. The information mentioned above is invaluable and should be in your component land pattern. Also, CAD tools are important as they help you create 2D footprints in various file formats. There are important points to keep in mind when designing PCB footprints:
Look out for SMD pad/ through hole sizes and locations
Land patterns need to comprise accurately size pads around the component’s edge. Also, mounting pads should be smaller than the land pads. Include the locations of the hole for through-hole components. The mounting holes or pads on these components must have correct sizes.
Include Pin-1 indicators and polarity
Polarized capacitors and diodes have cathode and anode. Therefore, you need to indicate it in the land pattern. For quad packages and Dip, you have to include some indicators of Pin-1 in the land pattern.
Keep datasheets handy
A complete datasheet will reveal a drawing f the component with necessary information like pad location and pad sizes. For instance, power MOSFETs may include a die-attached paddle or an integrated heat sink. It is important you include these details in the component footprint. Also, this is crucial for high speed and high voltage designs.
Understand package sizes
Your component footprint features the component package’s outline. This outline has to overlap the through-holes or pads on some particular SMD components. Sometimes, these dimensions are not revealed in a datasheet since the component is in a standardized package. Some examples are BGA, PLCC, and LQFP components.
PCB Footprint Checklist
Some designers have approved PCB libraries used for creating layouts. There are cases where a client sends a library with pre-made footprints for COTS parts or proprietary components. Therefore, it is crucial to check through a PCB footprint checklist to be certain your footprints match the components. This helps you to know if the land patterns are accurate.
Here are important things to check out for in component data before a PCB layout begins:
Dimensions
You have to check the pad pitch, pad sizes, and outline them with CAD tools. It is easy to detect an incorrect component if the dimensions don’t match. This will help you to know if the wrong package was in the footprint.
MPN
Some components are available in multiple packages. Also, there is always a particular code at the MPN to identify a particular package. Check if the package in the footprint matches the package of the manufacturer according to the specified part number.
Revision and lifecycle status
Most times, designers fail to check these points. Avoid end-of-life (EOL) components or NRND. Also, ensure the component footprint matches the manufacturer’s most recent revision.
Basic footprint match
You can use this to detect an incorrect footprint on a component. For instance, if some connectors and other components don’t have standardized packages, you can easily spot a mismatch.
Problems Associated with PCB Footprints
It is important to follow the PCB footprint guidelines to avoid some problems. These problems can only occur when there are no guidelines in place.
Small component outlines
You can’t assemble land patterns with small outlines using automated production lines. You may be able to manually assemble the parts depending on how severe the error is. However, that could take more expense and time. The manufacturer may reject the circuit board if the error is large.
Incorrect pad sizes
Very small pads can result in breakout issues for THT parts. Also, this can cause bad solder joints for surface mount design. Too large pads can occupy room for trace routing.
Incorrectly spaced pads
This is another problem associated with PCB footprints. Too close or too far through hole pads can result in component insertion problems during assembly. Also, SMT design pads too far or close from each other can result in the formation of insufficient solder.
Standards for Creating a Footprint
There are certain industry standards for PCB component footprint. Also, the quality of a footprint must meet these industry standards.
IPC-7352 (PCB Footprints)
There are certain requirements provided by the IPC-77351 for SMT designs and footprint standards. According to this standard, there must be maximum solder joint quality after placing components. In addition, the IPC-7351 standard offers instructions on how to design new land patterns and utilize existing ones. Some of the tips include:
- Ensure the spacing around the component footprint for rework and inspection is enough.
- Don’t ignore the recommendations of the manufacturer. Ensure you strictly adhere to the recommendations for better results.
- Test points for functional tests are necessary
- Use a common package type whenever it is possible. There are options to transform the existing package footprints to the necessary requirements. Also, you should create a land pattern from the beginning if the package type isn’t in the library.
ANSI Y32.2-1975 (schematic symbols)
This land pattern standard states that you must arrange schematic symbols by pin number. It doesn’t allow the use of a pin function to arrange symbols. There is PCB CAD software that enables you to decide if you want your schematic symbols to comply with this standard.
ISO 10303-21 (3D STEP models)
This land pattern standard focuses on STEP models utilized in 3D MCAD/ECAD software. It is a file format specification. It is easier to adhere to this industry standard if your ECAD software features a footprint generator.
Conclusion
We hope you now understand what a pcb footprint is. However, if there’s any section where you need some clarification, please contact us here.
