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Footprints for Small Polarized Capacitors

olebon

Sorry about an elementary question, I am still in process of transition from PADS where such questions never arise.

I have a problem with adding polarized caps.  For example, I need to use a 0805 Tantalum 10uF. If I follow tutorials and and just drop in a 'C' from "discrete.lib" and then try to assign an existing footprint from "C:\Cadence\SPB_17.4\share\pcb\pcb_lib\symbols" all I can find to fit is a non-polarized "smc0805.dra". Trying to avoid  designing a new footprint I downloaded a cap from Ultralibrarian, but their footprint "F98-S_AVX"does not show any polarity either.

I am not really a PCB designer, but an embedded engineer and design not very professional prototype boards once per 6 months to be cleaned later. To fix the issue on my prototypes I simply add '+" to the silkscreen where it has to be in the actual footprint.

So my question is, what is the proper way to add '+' to 0805 or 0603 packages of polarized capacitors? Thanks!

  • in reply to olebon

    You've got a mismatch somewhere.   Also, you might have a path issue.

    One of the things about Allegro is that it caches the symbol (footprint) so having a netlist with one definition and a symbol with another gets tricky.  So here's a quick test:

    In OrCAD, double-check to see that the pin NUMBER is 1,2  in the schematic.  Then in Allegro/OrCAD PCB open up the symbol for the cap and check the PIN number of each pin.  Then once both match, save the symbol as "SMCT3216_B" .  Go back to OrCAD and change the respective symbol PCB_FOOTPRINT property to "SMCT3216_B" as well.  Then re-netlist the design and bring in back into the PCB editor.  See if that corrects the issue.  

  • in reply to redwire

    Thanks, this helped. I opened the 'smct3216.dra' and found that they use 'p' and 'N' not as pin names, but pin NUMBERS. After I used these confusing numbers in my copies of capacitor models the problem is gone. 

    As a C programmer I can guess that they use uint8_t to store pin number and when the user enters 'P' the software reads it as 80 simply using ASCII value of the character. IMHO this is a bug. 

  • in reply to olebon

    Glad you got it worked out.  It's those pesky details that bite us all.  Another issue to put in your files is pin 1,2 swapped on a polarized capacitor.  JEDEC has a definition of which pin should be negative and which one positive but often those get reversed and "poof" the polarized cap blows up because of mixups in the footprint vs the schematic.  Sigh.

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