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.
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