The
following guidelines, the last in a series, apply to making your
prototype board build "assembly ready" .
The info below
continues the outline we started previously concerning the "best
practices" of board design for assembly.
Silk
screen Use a silk screen outline on the board to indicate the
preferred location of things like serial number labels and other
required stamps or labels. It is good practice to make sure the silk
screen of polarized devices are designed so that it is not covered
after of components are placed. A screen line width of 0.010" is
preferred for component outlines. Text sizes should be greater than
0.050" in order to be readable to the inspector of the assembly.
Minimum spacing of 0.010" is recommended between the silk screen and
pads
Pads A good rule of thumb for the pad diameter of a
through-hole component is twice its finished hole
diameter.
Vias Do not place exposed vias underneath 0805 or
smaller chip components. A good rule of thumb for via pad diameter
design is that it should be 0.020" larger than the diameter of the
finished hole for that via. It is also good practice to make sure
that the cross-sectional area of a via is equal to or greater than
the cross-sectional area of the trace connected to it.
A minimum
soldermask web of 0.003" is preferred between SMT pads and
vias.
Board
Outline/Edges It is recommended that a minimum 0.010" line width
should be used to draw the board outline. When you are creating
Gerber files for manufacturing, the board outline should be included
on every layer in order to insure that it provides for common
features for alignment. Remember to keep copper traces a minimum of
0.010" away from routed edges and 0.020" away from V-scored edges.
In addition discrete SMT components should be a minimum of 0.030"
away from board edges that are V-scored or have breakaway tabs. It
is our recommendation that components should be kept at least 0.030"
away from the board edges to prevent their cracking during
depanelization. "V-scoring" is preferable to
breakaway-tabs. | 
