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SMD pads come in basically two shapes: rectangular and rounded rectangle. Does either offer advantages? For instance for etching, or HF signal integrity?

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For the most part, no. It doesn't matter. Or, to put it a different way: For about 95% of the designs out there, the shape of the pad does not matter. There are some things to consider, however, before just assuming that your pads are correct:

  • BGA's need round pads of a very specific size.
  • The pad has to be big enough to hold enough solder paste to do the job. There needs to be enough solder to create a proper fillet for reliable soldering. The size of the stencil, along with the size of the pad, will determine how much solder is available for that pin.
  • Some RF guy is going to say that the shape matters. I am not an RF guy. But I do design digital circuits with 2.5+ GHz signals and the pads don't matter for me.
  • Sometimes the shape of the pad is used to identify Pin 1. Square pad for pin 1, rounded pad for the other pins.
  • One might argue that pads with rounded corners are less likely to de-laminate. I have no idea if this is true or not. But if it is true, it can only make a difference if there is a lot of rework being done on the PCB. For normal automated assembly of PCB's, this doesn't seem to matter.
  • If the part manufacturer recommends a specific pad size/shape, then you better follow what they say.
  • Square pads will get slightly rounded off in the etching process. For medium to large pads you'll never notice. For small pads you might care. I almost never care.
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