Enclosed are a few suggestions and/or concerns for "rat-bite" breakaway tabs:

     1)   Depending on the board thickness, the number of tabs along the
     periphery will vary.  Typical for 0.062" fabs, one tab every 2.5"-3" is
     sufficient, for 0.031" thick fabs, one tab every 2.0"-2.5", for 0.020"
     thick fabs, one tab for every 1.0-1.5".  Remember, the tabs are designed to
     support the weight of the board and components through SMT reflow and/or
     wave soldering while being relatively easy to break off after component
     assembly and soldering.

     2)  Typically, the "rat-bite" tab consists of a series of holes drilled
     side-by-side along a flat portion of the board outline in an unrouted web
     of PCB base material.  The idea is that the tab can be snapped off either
     manually or by flexing the tab with a pair of needle-nose pliers.  The
     weakest point of the tab will be along the diameter of the row of holes.
     To minimize the board cost in drilling, you might use the smallest via hole
     size already in use on the board, so the PCB fabricator will not have to
     change drills for the additional features.  A rule of thumb:  Pitch of the
     holes should be 2x the hole diameter (e.g. for 0.020" holes,
     center-to-center spacing should be 0.040").

     3)  To minimize tab residue (if the design will allow it, drill the
     breakaway holes with their diameter along the board outline.  Part of the
     drilled hole will be inside the outline but the tab residue outside of the
     outline will be negligable.

     4)  As far as location of tabs relative to component placement, tabs should
     be be as far away from small SMD components as practical, since the flexing
     of the board during manual singulation could fracture solder joints or the
     component body itself. With reference to component orientation, no SMD
     component within a 0.3" radius of the tab should be mounted perpendicular
     to the edge of the board.

These guidelines are intended to assist you in a design that fits your specific
application.  They are based on past experience in SMD assembly panelization for
high-volume assembly and not meant to be the only solution that works.

Hopefully, by avoiding these "potholes in the road of life" , your journey will
be smoother.

Bill Fabry
Plantronics, Inc.
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