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SA2007
India
3 Posts |
 Posted - 07 Mar 2007 : 04:12:18
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I am looking for PCB footprints for surface-mount capacitors, in sizes A, B and D.
I can't find any corresponding library entries for these. ?
As these are fairly standard, I would guess that they're there somewhere?
ETA:
I looked at Kemet's data sheet "Surface Mount - Mounting Pad Dimensions and Considerations" for Tantalum Capacitors: I guess that the library PCB symbols C0402 - C7257 correspond to the various alphabetic case sizes? In which case I am confused to see "C7243" - should this be C7343, aka size D?
I spotted this interesting note on the datasheet: quote:
The larger case sizes are more difficult and it is not recommended that these be waved soldered. Pad designs are presented for the adventurous
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Edited by - SA2007 on 07 Mar 2007 14:03:42 |
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Peter Johnson
United Kingdom
490 Posts |
Posted - 08 Mar 2007 : 05:02:28
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The surface mount devices included as standard were based on the first issue of the IPC-SM 782 ANSI standard for surface mount footprints. They were created in 1991, so they are now rather old. They're still usable, but the pad sizes are rather generous by today's standards, as production equipment has improved. I suspect that the case nomenclature predates the current system, which is why it's confusing.
Most of them were dimensioned to be suitable for wave soldering as well as reflow. Wave soldering, being inherently more crude, does require larger pads. We do sell a supplementary library based on the latest release of the above standard, where the footprints are smaller as they're intended just for reflow, but if you only have a limited number of footprints, you could design your own. The pcb footprint wizard is now clever enough to work out optimum or near optimum pad sizes based on the component dimensions.
Your comment about wave soldering the larger capacitors is understandable. Above a certain component height, the body of the component creates a shadow on the trailing edge of the wave, so dry joints are a very real risk. It's not so bad if the component is across the flow, as the first part of each pad will be soldered, but if it's in line, the second pad is definitely at risk.
There's another issue with gull wing leads as well. As the solder wave breaks away, it often doesn't do it cleanly for the last pair of pads, leaving a bridge. Because of this it's good practice to add an extra pair of pads, so that a SO14 device would use a SO16 footprint, so the two trailing dummy pads can be bridged to the last pair of 'real' pads without electrical error. In the jargon, these are called 'robbers'.
Peter Johnson
Technical Support
Number One Systems |
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SA2007
India
3 Posts |
Posted - 08 Mar 2007 : 22:08:20
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| Can you confirm whether the "C7243" should be "C7343"? |
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Peter Johnson
United Kingdom
490 Posts |
Posted - 09 Mar 2007 : 06:36:00
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We've done some research on this. It appears that the recommended footprint for 7243 has evolved over time, so this one is fairly different from the current recommendation. However, by comparing it with the current recommendation for footprint 7343, it seems that it's quite similar, but a little larger. That implies that the C7243 footprint could be used as a flow solder version of the 7343 footprint, but I would urge you to check this yourself, as we can't take responsibility if it goes wrong.
Peter Johnson
Technical Support
Number One Systems |
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PCB footprints for tantalum caps