My PCB design software comes with some libraries that include some common components, like chip resistors and capacitors. I have noticed, however, that the footprint land pattern for the 0805 resistor is not identical to the 0805 capacitor.
I then did some googling, and discovered multiple IPC standards that don't seem to quite agree with each other.
Is there a reason an 0805 resistor would have a different land pattern than an 0805 capacitor? Is there a "best standard"? IPC-7351, IPC-SM-782, or what?
Answer
The current standard is IPC-7351B, which replaced IPC-7351A, IPC-7351 and IPC-SM-782 (in that order). Mentor graphics have a free PCB footprint viewer for Windows for all of the standard parts using this standard. Each part also includes a "courtyard" layer which defines how much space needs to be left around the component for manufacturing; useful when you are designing high density boards.
The standard itself consists of three versions but for most boards it is recommended that the "N" suffix (Nominal) footprints are used. Note that most parts do differ slightly from manufacturer recommended patterns but in my experience (and the experiences of my board loader these footprints do tend to meet manufacturing requirements very well.
As for you query about the 0805 resistor vs 0805 capacitor, I suspect that the footprints have been designed to best attach the parts; while they are fairly similar in the horizontal plane, capacitors do tend to be slightly higher so maybe the differences in the footprints take this into account.
No comments:
Post a Comment