Pad and Via
size: One should maintain the largest practical annular
ring to insure a good mechanical, as well as electrical and
thermal connection. There is a wide variety of pad sizes and
shapes available to accomplish this. Octagonal pads are far
superior to round.
Eddy's rule: the
pad or via size should be 50% greater in diameter than the
diameter of the hole.
Eddy's Inverse:
the hole should be 2/3 the diameter of the pad.
Plating
through and tolerances:
Hole sizes should be specified as
finished hole sizes. This compensates for the loss of
diameter in plating through the hole; the hole will be around
.002" smaller than drilled. This figure represents an
average that can vary by as much as 30%. Larger holes will
take more plating than small ones as a rule and holes near the
outside of a panel will also take more copper. On the other
side of the equation, the plating thickness will be greater than
the .002" decrease in hole diameter. Due to etch-back and internal
stress, you may have as much as .002" copper thickness. This
thickness on both sides of the hole adds up to .004" total copper,
yet the hole is only .002" smaller.
Obviously, one needs
to be careful in considering hole sizes, as there are a great many
variables including accuracy of hole location and tolerances
within the component to be placed.
A good practice is to
allow .005" extra hole diameter to accommodate all of these
variables. This also allows de-soldering of defective parts
during repair.
