[sdiy] Soldermask for fine pitch TQFP packages

[Simon Brouwer]

IMO you don't need soldermask between the pads. I looked at a few PCB's 
we developed at work that have 0.5 mm pitch components on them and while 
a number of them do seem to have soldermask between the pads there was 
also one without.
For hand soldering I don't believe it makes any difference, and in 
automated production I assume it will be reflow soldered in which case 
it shouldn't either.
Running the risk of having a significant area of the pads not solderable 
is definitely worse.

Seb Francis schreef:
> 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
>
>
>
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>


-- 
Vriendelijke groet, 
Simon Brouwer. 

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