I would like for your help on the subject of GRIDS.
Two paragraphs in the new draft of IPC-2614 seem outdated, and I would
propose removing or revising them, but I thought I should check with you
guys first

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6.4.7
Board design may use multiple grid systems to accommodate the component
termination and conductor routing patterns. Area array packages with
different pitches (grids) are handled by using compatible grid variations
that allow the conductor routing escapes from the inner land and hole
locations to the fan out patterns. A particular board area should use a grid
that accommodates most of the component and the via holes needed to connect
the circuit. Since a grid system is used to locate most holes, each hole
location need not be individually dimensioned. Holes located off grid should
be individually dimensioned on the profile (board outline) drawing or on the
drill pattern, whichever is more convenient.
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6.5 Grid Systems
Grid systems are used to locate components, plated-through and non-plated
holes, conductor patterns, surface mount land patterns, and other features
of the printed board and its assembly. When printed board features fall off
grid, they shall be individually dimensioned and toleranced on the
Fabrication Data Set. These dimensions may appear either on the profile
(board outline) view or on the drill pattern whichever is more convenient.
The centroid of components should be placed on grid with the pin one
orientation defined as required in IPC-7231 and IPC-7351. The component
pitch characteristics define the termination land pattern feature locations.
Grid systems shall be located with respect to a minimum of two datum points.
The grid increment shall be specified on the Fabrication Data Set (or may be
embedded in the CAD system data base). The selection of the grid increment
is based on the component terminal location for through-hole components, or
component center point for surface mount components. Typical grid increments
are multiples of 0.13 mm [0.005 in] for through-hole components, and 0.05 mm
[0.002 in] for surface mount components.
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Some CAD systems must employ an internal grid as part of the overall
database structure, but this is transparent to the user and is different
than the grid systems proposed above.

Do any of you ever dimension anything simply because it is off-grid?
Would an "off grid" hole or component ever present a problem to anyone?
Would specifying the grid increment in the "fabrication data set" be of any
use to anyone?

thanks,
Jack

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