When a “printed circuit board” is designed and sent out for manufacturing, the manufacturer requires that a “footprint”, essentially a map, be created for the board. This shows where the parts are to go on the PCB. On this copper pattern occasionally the whole outline of where specific components or parts are actually drawn in. Other times a virtual outline is made and this is able to be seen on a computer screen and on the assembly schematics but not on the actual board itself.
Polarity Marks:
When designing the PCB’s polarity indicators have to be designated. There are certain guidelines for different types of components:
Batteries And Capacitors:
Most have a plus (+) sign, indicating positive polarity on the silk screen transfer, closest to the positive pad. The indicator stripe for the positive polarity that is on the capacitor is to be placed closest to the positive sign.
LED’s And Diodes:
LED’s (light-emitting diode) and other types of diodes, polarity is indicated by a ‘diode arrow symbol’. This is a triangle plus a bar. The bar matches the cathode end of the indicator strip that is on the diode itself.
IC’s Or Integrated Circuits:
A ‘dot’ or ’1′ is the polarity mark you find for an “IC”. There is some preference to have additional marks identified, so each pin will have corresponding numbers in the silkscreen.
Additional information in regards to polarity marks:
All polarity marks must be visible after mounting the part so it can be seen after inspection. (It’s useless to put the polarity mark where no one can see it). It’s okay to put the polarity mark underneath a tubular-packaged diode, since it is possible to look “around” the mounted diode and see the polarity mark.
Obstacles
When talking about PCB’s, references to an obstacle refer to a graphical characteristic in the footprint of the card. They are places where for instance a component is going to sit. If you were using an auto router software to design your PCB’s card you would enter in the obstacles for reference as areas for the program to stay away from when mapping out a design.
Text
An example of the type of text printed on a circuit board is the reference designator. These are displayed on the silkscreen layer and usually the mechanical layer. Text is also used for serial numbers or design revision numbering.
Origins
When each footprint is placed onto the circuit board for layout, you can place it on the board by using its origin. The origin could be the pin 1 center or the body origin depending on how you define it in the footprint. This is critical for precisely placing components or sensor arrays that must be aligned to an optics system.
Always remember, you need to plan for spacing, components, your reference designator and origin. This quick overview of PCB footprints is meant to offer enough information to help you get an initial understanding of what you need to consider when planning out a printed circuit board
