[sdiy] PTH vias

[Seb Francis]

Hi Vesa,

Thanks for your answer.

Vesa Lahteenmaki wrote:
>> Hi,
>>
>> Does anyone know whether it's a bad idea to cover PTH vias with 
>> solder resist?  By default my PCB software leaves a gap in the solder 
>> resist which means that you can't place silkscreen legend on top of a 
>> via.
>>
>> Looking at some commercial PCBs I see that quite often vias are not 
>> covered with solder resist.  Is there an electrical reason for this?
>>
>> Nowadays my PCBs are Immersion Silver finish (which is a 1 micron 
>> thick plating of silver on the copper) if that makes a difference?
>>
>> TIA,
>> Seb
>>
> Certainly not recommended ! Would cause several problems, search with 
> keyword "via tenting".
> Normal liquid soldermask would disapper into the hole anyway, so it 
> would not really cover the hole .

Ok, so it depends on the type of soldermask used.  Thanks to knowing the 
right name for this I found this document which is helpful about 
different soldermask types:
FINDING THE SOLDER MASK FOR YOUR NEEDS
http://www.precisioncircuits.com.au/cid/022.html

I would settle for just covering the pad part of the via with soldermask 
and leaving the hole uncovered - this would be much less disruptive for 
any silkscreen legend placed over a via.  But unfortunately my PCB 
software only allows to specify a positive guard gap around vias, or no 
soldermask hole at all.



> Sometimes some or all vias must be filled for in circuit testing ( 
> board held in place with vacuum ), but this is a special process ( via 
> plugging ) and done on one side only. Otherwise process chemicals 
> might be trapped in the hole and could cause failure later.
>

This is something that does concern me - what I've read on the internet 
about tenting describes only 3 variations:
- Soldermask covers top
- Soldermask covers bottom
- Vias 'plugged' from bottom
It is not ever described that the soldermask covers top&bottom.  Yet my 
PCB software only allows the same soldermask on top and bottom which as 
you say might trap chemicals in the hole.  I guess I could make a 
workaround and export the Gerber file for the top resist, then change 
the via settings, then export the Gerber file for the bottom resist.


Seb