I realize this should be simple but I'm following the gEDA how to create footprints document and trying to understand how the numbers derived for specific footprints. For example: the pad definitions in the 0805.fp is as follows:
Pad[-3543 -393
-3543 393
5118 2000 5718 "1" "1" "square"]
Pad[3543 -393
3543 393
5118 2000 5718 "2" "2" "square"]
Excuse the crude sketch.
According to the gEDA how to create footprints document the dimensions for the 0805 are (these values are taken from the IPC-SM-782A Standard):
C X Y Z G Grid
74.8 59.1 51.2 126.0 23.6 157.5 x 315.0
So from the document the pad sizes as follows:
|<-------- C -------->|
-------------------------------------------
| |
| ----------- ----------- | ----------
| | | | | |
| | | | | | | |
| | | | | | | | X
| | | | | | | |
| |_______| |_______| | _______
| |<- Y -->|<-- G -->| |
|_____________________________|
|<---------------- Z ------------------>|
Calculated dimensions:
C = 3543 * 2 = 7086 = 70.86 mils
X = 5118 = 51.18 mils
Y = 393 * 2 + thickness = 5904 = 59.04 mils
G = C - thicknes = 7086 - 5118 = 1968 = 19.68 mils
Z = C + thickness + clearance = 7086 + 5118 + 2000 = 14204 = 142.04 mils
If I calculated these values correctly, why are they off from the values in the IPC-SM-782A standard?
If I didn't calculate them correctly, where did I go wrong?
Is this a standard clearance?