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4 Terminal PCB Footprint

MikeSD

I have a component (MOSFET) that is a 3 terminal device, in TO-92.  In a surface mount, it's 4 pins (3 pins and a tab).

 

I'm not sure the best way to deal with this.  On the datasheet, each connection is D, G, S and D(tab).  But on the footprint it's numbered (1, 2, 3, 4).

I have two options.

1) Make the footprint 4 pins (each differently numbered, 1, 2, 3 and 4).  But this requires adding a 4th pin on my component in the schematic capture, which looks strange. Most transistors are only 3 pin devices.

2) Make a footprint that has 3 pins with the center pin large to include the tab area.  But this removes routing options under the componet.

I initially tried 4 pins with numbers 1, 2, 3 and 2 for the tab but got an error becuse two pins had the same number.

 

What is the best or most appropriate way of handling this?

  • I tried the last 2 suggestions.

     

    1) I created a mechanical pin, and the schematic symbol as a 3 pin device.  Everything went ok except in PCB, I couldn't figure out how to attach a net.  I still need to connect the net to the same point as the drain.

    2) I also created a 4 pin schematic symbol and a PCB footprint with 4 pins.  Hid the schematic pin (4).  In PCB the net wasn't connected to anything and like above, I couldn't figure out how to assign the net.  Perhaps that can be done in schematic capture but I couldn't see a way to do it.  If I use "line", that extra line is visible and doesn't look right.

     

     

  • Whoops,

    I didn't know you needed to connect the tab to a net when I suggested using a mechanical pin.  For a pin to connect to a net normally, it needs to be a connect pin in the physical symbol.  Also, the pin needs to be assigned to the net in the schematic or the logical symbol.  Unfortunately, I also am not familiar with how to do that in Capture.  Of course, you always have non-elegant solutions like putting in a static shape over the mechanical pin and assigning a net to the shape, then waiving the DRC; but then if you have to move the symbol or change the net name, it's more work.  Hopefully, you can figure out how to assign the net in Capture.

    Good luck.

  • You could also add a shape or line to a dumb layer, and add that layer to your gerber outputs.   The only problem with that though is there is no DRC for it, and your netlist will not match the connectivity.     I think your best option would be to add the 4th pin to the schematic as pin 4 and be done with it.

  •  I'm using 4-pin symbol. But 2-nd and 4-th placed in same position. Then turn off the display of pin numbers and names.

    The only drawback - in connection to this pin(s) arise junction point.

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