Bev C./Alcon:  I don't read the words on 5-13 & 5-14 of A-610C to "require"
the use of spacers.  If you "elect" to use spacers, then they need to meet
the attributes listed on those pages.  Standoff/Spacers for components are
used for three primary reasons.  The primary reasons are:  to elevate the
component enough so that the cleaning system can extract process residuals;
to elevate the component enough to that visual inspection can confirm/deny
wetting on the primary (component) side of the pwa; to provide a resilient
mounting surface that will function to mechanically support the component
during shock/vibration throughout a wide temperature range.  Note that
resiliency can be achieved either by selection of material or by design of
non-resilient material (like the springs in your car).  When no vibration or
shock are anticipated, but cleaning and inspection are still considerations
you may want to consider frangible spacers (sugar wafers or soda straws that
survive the mass soldering operation and are later dissolved by your cleaning
system).
- Additional reasons for using spacers may be, as one respondent pointed out,
is to  keep the first 0.050 inch of lead below the coating meniscus out of
the solder connection area because most component manufacturers will not
assure/warrant the first 0.050 inch to be solderable.  Plunging the coating
meniscus down to the surface of 0.062 in. thick pwb is risky business as you
may end up with only 0.010 inch of wettable lead in the pth.  A spacer can
also provide more than adequate electrical isolation of the (sometimes
conductive) component body from any surface circuit traces.  Conductive
cooling of components is a minor considerations and obviously dependent on
the spacer material.  Remember that spacers typically have three or four
points (bumps or legs) that contact the pwb surface and little conductive
cooling is achievable through that small contact area.  The "bumps" or "legs"
of spacers need to be at least 0.010 inch tall, anything more will not
improve cleaning (but may assist visual inspection).  The higher the
component sits above the board surface the worse the response to
vibration/shock will be.  Hope the above helps some.  Regards, Jim Moffitt,
Technical Director, Electronics Training Advantage, Indianapolis

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