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Geotech
USA
22 Posts |
 Posted - 23 Apr 2025 : 20:32:05
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Let's say I have a single opamp like a LM741. Pins 1, 5, and 8 are unused; my schematic symbol does not include them but the PCB footprint obviously does. What happens is that often (but not always) the PCB side thinks they need to bee connected to something and adds flylines to some other node, usually a ground or supply. It's not consistent; sometimes it's just pin 5 that it wants to hook up, sometimes pin 8, sometimes 2 of the unused pins. Don't recall all 3 unused pins getting flylined.
I know the pin assignments include the ability to force a pin to be unused but this is only available if the pin is included on a schematic symbol. How do you do this for the extra PCB pins? |
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Peter Johnson
United Kingdom
510 Posts |
Posted - 24 Apr 2025 : 10:24:11
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Personally, I've never seen this happen so I'd like to turn this question on its head. The nets that are being connected to unused pins, what's their function? Clearly from your description they're already used in the design. Have you tried creating a netlist from the schematic and checking the LM741 in that list? Is this limited to one design? If not, is it just the LM741? Clearly something's amiss but you'll need to give more details for any chance of working out what.
Have you checked the pin assignments? It's possible for a single schematic pin to be mapped to multiple pcb pins. If this has been done accidentally, it would cause unused pins to be connected, especially if the separator were a comma rather than a plus sign. |
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Geotech
USA
22 Posts |
Posted - 28 Apr 2025 : 17:30:31
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| I've sent an example circuit. |
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Unused pins want to be connected