On 14/01/2012 09:33, Alan wrote:
In message , geoff
wrote

Surface mount devices are often glued to the pcb

Not in my experience. Solder is the only thing holding the component on
the board on 99.99% of commercial gear.

Its slightly less common than it was, since solder paste is now almost
universally used, and that tends to hold components in place until
soldering. On the earlier surface mount stuff where it was flow soldered
(usually on the "chip wave" rather than in the main bath) they had to be
glued fairly well else they would simply fall off the board once it was
inverted and fed through the wave.

There may be board coatings on boards for the military market which make
rework almost impossible for those with out the right tools.

Conformal coatings don't always make it much harder to remove and
replace stuff (although some are a right pig!), but it can be a pain
getting the coating back in place properly.

The only time I've seen glue used is when a physically very large
component has been fitted and the manufacturer has used a "blob" of glue
to bond to component case to the board to prevent differential physical
movement - to help prevent vibration failures.

Wogging as GEC used to call it...

(real vibration situations would usually call for stuff being tied down,
or clamped in place. Often with holes designed into the boards to make
that possible - or sometimes by fixing to the heat ladder when there is one)

--
Cheers,

John.

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