I’ve noticed that a lot of CAD library footprints for two-pin polarized parts have pin one pointed up as zero degree rotation. According to IPC, pin 1 pointed to the left is zero degree rotation.
Why is this such a common error? I can’t be certain, but I have a pretty good idea.
Surface mount parts, as everyone knows, generally come in reels of tape. It stands to reason, that the parts would be placed into the tape at a standard zero-degree rotation. They generally do. Before putting a perplexed look on your face, take a look at the image below.
When looking at the tape, it’s a pretty natural thing to pull it out and hold it horizontally – with pin 1 up – perpendicular to our angle of vision. Makes sense. It’s not a stretch to look at this strip of tape and end up assuming that pin one is up at zero rotation.
However – the machines are the ones being spoken to. Not humans. The machines get the parts in line with their line of vision. That puts pin one on the left.
For more to the part rotation story, tune your browser dial to here. Or just scroll down a little bit. It’s right below.
The long and winding reel leads to your PC board. Not your door.