How do PCB manufacturers know how to orient polarized components


Let's say I have diode or a polarized capacitor in my circuit.

How do PCB manufacturers know how to orient the part on the PCB?

When I create the part in Altium for example, I just make up which pin is pin 1 and which pin is pin 2. As long as the schematic matches the PCB, it won't throw an error at me.

In other words, is there something that relates the pins on the schematic and the pads on the PCB to the real-life leads of the component?

I have not seen on diode datasheets where it says "pin 1 should be the cathode, pin 2 should be the anode", unless it does say that and I am dumb.

Best Answer

In other words, is there something that relates the pins on the schematic and the pads on the PCB to the real-life leads of the component?

None, just like there is no standard of whether the anode and cathode in a diode symbol or footprint should be pin 1 or pin 2. It's all up to you to use the appropriate footprint with the schematic symbol you use.

The component in the tape can be any which way and you have to look at the datasheet or the markings on the component sitting physically in the tape reel to know.

You sort it out with 2-way human communication (i.e. notes to the assembler. A diagram is best and clear silkscreen markings are even better.) and the guy sets up the pick and place machine does it.

I've had instances where the guy ignored diagrams accompanying my notes and assumed the silksreen "dots" (not dots as much as little random linear ticks which were artifacts of the footprints in the library I was using) floating around different components on the board were indicators for pin 1. All the ICs came backwards. The IC footprints being used in that case did not have clear pin 1 markings on the silkscreen other than the fact it was always to the top-left when the was read upright RefDes which was why I included a diagram. The diodes in that case were not part of the diagram but not a problem either because I went through their footprints and manually sure all the footprints used had a white polarity line or the actual diode symbol for their silkscreen. After all, I was going to be the one debugging and reworking the board and I needed to know what the polarity was by looking at the board. I assume the guy setting up the pick and place just looked at how the diode sat in the reel since the SMD packaging also had a white polarity line. I doubt he went to the datasheet for every single diode and looked at the packaging information.

This is a much bigger problem for some components like SMD photodiodes which, due to requiring an unobstructed face, cannot have polarity markings on the component. The only way to know which is anode and cathode is to look at which pin is bigger and smaller, but to know which is which you need to go to the datasheet. In that case you really do have to go to the datasheet and look at the packaging information to see which way they come in the reel. It's a major pain. I would probably send a snapshot of every diode datasheet page containing that info as part of the notes.

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