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. | 
