The footprint is the physical representation of the actual component on the PCB. A footprint is the arrangement of pads in SMD (in surface-mount technology) or through-holes (in through-hole technology) used to physically attach and electrically connect a component to a printed circuit board.
Manufacturers provide instructions on how to define the size of the component dimensions on the PCB in the form of footprint. Also, in the absence of footprint, IPC standards which is globally accepted can be followed. Typically for the creation of surface-mount footprints the IPC-7351B standard is used and for creating through-hole components the IPC-2221B standard is used. These standards provide calculations and directives that should be applied to the component’s mechanical dimensions while creating the footprint.
Below example is a representation of a pads(yellow) of a footprint and the component mounted on it.
The design process
- Padstack creation
- Component outline
- Placement outline
- Component Origin
- Keepout outline
For SMD pad creation, you typically use the rule pin length + 0.4mm for defining the length of the pad and you typically use the pin width + 0.2mm for defining the width of the pad. On the other hand, for through-hole components, you have to typically use the pin diameter + 0.3mm extra as drill diameter. It is important to also define the solder mask clearance for the pads to make sure the copper will remain exposed. This clearance is typically the same areas as the pad size. For SMD components you also have to define the solder paste mask. This will tell how much and where the solder paste is required. Here you will also use the same size as the pad size.
The assembly outline is drawn to match the maximum dimensions of component body and pin profile. The assembly outline should be drawn with 0.1mm width in the assembly top layer.
The silk screen provides information about the device’s orientation and reference designator. Great care should be taken that the silk screen does not cross exposed copper of the pads, but otherwise encircle the entire footprint. You must remain 0.2mm away from pads and the outline should be drawn with 0.1mm width.
This outline surrounds the entire body of the component. Height information for the component is also included in the Placement Outline. The outline should cover the component body or pads whichever is the largest and should add an additional 0.05mm.
The origin is used by the pick and place machine to locate the part on the PCB. It is recommended to make sure the component origin is located on a pin. This prevents placing components off the grid during component placement.
It is important to not place components too close to one another, this to make sure you can correctly place the parts on the PCB during assembly using automated production lines. To facilitate this the footprint design includes the keepout outline.
We at Flex-ES offer complete library development and maintenance solutions that are in line with the IPC Standards and we deliver components with customer specific standards if required. Working with ISO certified companies, ensures a process-oriented approach and a component library management system that delivers high quality components. The process includes a 4-eye review system. We have a component database of more than a million parts that are IPC 7351B compliant. Also we support all leading EDA tools and have a dedicated team to quickly turnaround the component requests”.