Grahm:
- The U.S. military standards (DOD-STD-2000 & Mil-Std-2000) had required that
chip resistors with externally deposited elements be mounted with the
resistive element facing away from the pwb.  The major concerns at that time
were: 1) leaving the resistive element "open" and available to precision
(laser) trimming after mounting and 2) assuring that the resistive element was
"open" to the action of whatever cleaning system was used (avoiding having
flux or other process residue trapped between the resistive element and the
pwb surface if the chip was mounted face-down).  Heat dissipation was a
concern too, but was considered less important than the above (if a chip needs
max. convective cooling mount it on it's side [billboard] to expose the
maximum surface to the environment - DISCLAIMER; other concerns apply to this
type mounting.]
- There was some discussion of this subject (face up/down) in the last meeting
where Revision C of ANSI/IPC-J-STD-001 is being thrashed out.  I believe the
consensus, after all was said and done, is that there is not sufficient data
to justify continuing to require that the resistive element be mounted facing
away from the pwb surface (someone with a better memory may correct me on
that).  Anyway, I believe the new, revised, value engineered, issue of J-
STD-001 will not require that the resistive element face away from the pwb.
- Hope this helps some.
Regards, Jim Moffitt, Moffitt Enterprises Consulting.

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