[sdiy] Soldermask for fine pitch TQFP packages

[ASSI]

On Wednesday 26 November 2008, Seb Francis wrote:
> Batch PCB tell me their soldermask tolerance is +/-4mil.  So it
> seems like I should use a guard gap of 4mil around the pads when
> specifying the soldermask.  In the case of this TQFP package this
> would mean no soldermask at all.

You don't need soldermask where there is no copper, so this would be 
OK.

> And in general for larger pads, is it useful to specify a guard gap
> at all, or just let up to 4mil of soldermask cover the edge of the
> pads?

I like to have the pads in the clear, but it's not strictly necessary.

> I'm primarily looking at hand soldering, but am also interested how
> I should specify the same thing if I ever got to the point of
> automated assembly.

For hand soldering (unless you have an hot air soldering "iron") I 
would perhaps chose a slightly narrower pad footprint than normal to 
mitigate the tendency for bridging a little bit.


Achim.
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