Bill
Very slightly off-topic, but not really. Firstly, if you are using a
ceramic component, I suppose you are working for a hi-rel application.
This implies that it must be cleaned. Whatever cleaning process you use,
the fluids must not only get under the components, they must also be
easily replaced during rinsing and not simply captured and held in place
by capillary forces. This means that there must be sufficient space.
Secondly, if your component is 20 pin, it is presumably fairly small.
I have a rule of thumb that for components which allow free access of
cleaning fluids across two opposite sides with the distance between them
< 2,5 mm, the minimum spacing should be 0,1 mm, but add 0,1 mm for every
extra 2,5 mm or part thereof. This assumes that the components are all
oriented to permit maximum fluid flow. If all sides are leaded, such as
a QFP, or the orientation is such as to be normal to the fluid flow,
then this impedes fluid flow and the spacing should be doubled. This
applies to solvents of all types, including water and vapour phase.
For these reasons, I second the notion of converting your LCCC to
J-lead. Even small LCCCs tend to accumulate dangerous paste residues
around the inside of the solder joints, no matter how they are cleaned.
This can be easily seen by carefully removing the components by using a
fine-grain diamond saw (dry) along the four sides, so that the centre
drops out, leaving just the indents of the castellation: you will see
the flux residues, as sure as eggs is eggs.
Hope this helps.
Brian
Bill Robinson wrote:
>
> Greetings All,
>
> I would like to know the best height for spacing 20 pin LCCCs off the
> board, and the best means of achieving this height. My concern is that
> there be some compliance to allow CTE mismatch between the part and the
> board during themal cycling. Any recommendations?
>
> Thanks,
>
> Bill
>
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Brian Ellis
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