The following guidelines, the third in a series,
apply to making your SMT
components "assembly ready" .
The info below continues the outline we started
in our last 2 newsletter concerning the "best
practices" of board design for assembly.
SMT diodes and LEDs
Make sure to designate the cathode (-) end
with a bar in the silk screen. For LEDs "-"
and "+" symbols in the silk screen are very
helpful for assembly and inspection to make
sure they light up the first time.
Test Points
Test point pads should be a minimum of 0.030"
in diameter and at least 0.050"
center-to-center away from each other.
Make sure the test point pads are accessible
after the board is assembled.
To keep test fixtures and testing costs down,
place all test points on the same side of the
board, as far away from each other as
possible, and keep pad diameters as large a
possible.
Wires
With the exception of jumper wires do NOT
design wires for direct soldering to the
circuit board. Instead, use an insulation
displacement connector (IDC) or crimp on
style terminal to the end of the wire.
Try to eliminate all jumper wires.If jumper
wires are absolutely necessary , use standard
lead spacing and zero ohm resistors.
Fiducials
In general a minimum of two (four is ideal)
round fiducials per assembly side are
required. Make sure the fiducials are across
from each other, in opposite corners at
non-symmetrical points. If the board is
placed in an array, fiducials are required on
both the assembly and rails. It is also good
practice to use two different sizes of
fiducials should be used per side.
If space permits, local fiducials should be
added for fine pitch components (in addition
to the three general fiducials for the
board). Local fiducials reduce the
possibility for misplacement, particularly on
larger more densely packed boards.
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