A diode is one of the most widely-used semiconductors. Diodes keep currents flowing properly. You can think of diodes as one-way streets. They send current in one direction, known as the “forward direction.” If the current tries to come back in the reverse direction, they block the current.
When it comes to surface mount technology (SMT) for printed circuit board assembly, components are placed with the help of “pick-and-place” machines. These robotic devices place surface-mount elements onto a printed circuit board.
When these pick-and-place machines are programmed properly, the assembly process can be easy and error-free. One of the trickiest parts of programming the machines is figuring out the orientation of diodes. The modern diode symbol is an arrow. The orientation of the arrow indicates the direction that the current should flow. However, surface mount diodes don’t always use this arrow.
Light-emitting diodes can be tricky to mark because the markings can obstruct the light. Designs might include a notch on the side of the cathode or markings on pin 1. Others might have markings on the bottom of the LEDs that have nothing to do with the cathode. This might make perfect sense to the person making the plans, but the programmer may not understand their plans.
You might think that the rotation angles included in file formats like IPC-356D rule out any guesswork on the subject, but this data cannot always be counted on. It is based on assumptions that may not apply to all PCB designs. CAD is a great program, but it’s not 100 percent accurate and may give faulty information about diode polarity.
The mighty arrow
As you can see, there are many different ways for communicating polarity, but they can lead to confusion. The solution is to use a standard symbol, and that is a silkscreen-layer arrow. When you draw this arrow or diode symbol to indicate polarity, you can help the programmer know exactly which way you want the current to flow. You may have other markings on the packaging, cathode, or pin 1, but make sure to include that clearly defined arrow to eliminate any confusion about diode polarity.