[sdiy] Soldermask for fine pitch TQFP packages

[Seb Francis]

Seems like I have a lot of questions for this list at the moment!

I've been trying to work out the best way to specify the PCB for fine 
pitch TQFP packages.  In particular I'm not sure about the guard gap for 
the soldermask.

The package in question is a 12x12mm 80-lead TQFP.  This has 0.5mm pitch 
leads and a recommended land pattern of 0.3mm (11.8mil) wide pads with 
0.2mm (7.9mil) gaps between pads.

I'm looking at using Batch PCB for the prototype PCBs and since their 
min spacing is 8mil, I will use 0.29mm (11.4mil) wide pads with 0.21mm 
(8.3mil) gaps.

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.

Is it advisable to have no soldermask around these fine pitch pads?  Or 
is it better to have no guard gap and risk that up to 4mil of the pads 
could be covered with soldermask?

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'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.

Anyone with experience in this?

Seb