Dwight,
            As a general rule I cover all vias except on prototypes when acess to the 
via is useful. With very fine pitched and high
density stuff this gets pretty esential. Exposed vias can lead to bridging from one 
via to another or from a via to a pad because of
scrap solder paste. If you have BGAs on the board, then NOT tenting the vias, at least 
immediately under these devices is asking for
trouble.

There will be a difference in the electrical chacteristics of a via covered in solder 
resist, in particular it will alter its
chacteristic impedance slightly. However, taken as a part of an entire routed track 
this effect is, I believe, negligable. Even with
controlled impedance signals, the via itself has a greater impact than the solder 
resist over the via. Bear in mind that even a good
PCB manufacture tends to quote +/-10% tolerance on for most digital signal CI traces.

The vias filling with solder does imprve their current handling capability slightly, 
but I understand the effect is fairly small.

Some manufactures don't like tented vias because for lager vias the bit of solder mask 
that would cover the hole breaks off and they
get annoying bits of slightly sticky solder mask dust floating about. My feeling about 
this is: If I want the vias covered, and you
don't want to build em that way then I'll take em to someone else.


Hope this is of some help.

Regards,

Rob Malos,
Cyborg Design





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