Re: gEDA-user: solder masks, etc.

[Dan McMahill <dan@mcmahill.net>]

On Tue, May 11, 2004 at 12:42:17PM -0700, David Koski wrote:
> Hello,
> 
> In the following file:
> 
> http://kosmosisland.com/island/david/test_1.pcb
> 
> I have a SOT-428_Transistor (Q1) footprint that I made. With a solder mask as
> shown in the file, I cannot get the large pad closer than about 170 mils from
> the edge of the board. It is as close as it can get in the above referenced
> board. If I make the solder mask parameter small, the pad can be placed next to
> the edge of the board. What am I missing?

I think PCB is preventing the soldermask relief from extending beyond the
edge of the board.

> Also, I have only done prototypes without solder masks so far. What is the
> "right" amount of mask? I noticed that even library footprints vary as can be
> seen in the file referenced above.

There isn't really a "right" amount.  A conservative rule would be to have a 5 mil
soldermask relief around the pads, so for example a 20 x 20 mil square pad would
have a (5 + 20 + 5) x (5 + 20 + 5) square soldermask opening.  Another rule
would be to ensure at least 5 mils of soldermask (not the relief, but where the
soldermask is) between pads.  Now, clearly on a fine pitch part with a 20 mil
pin pitch, you can't have 15 of those 20 mils taken up with soldermask opening
and soldermask.  In those cases, I've used a 5 mil clearance and removed the
(now less than 5 mils) of any remaining soldermask between pads.

Hope this helps a little.

-Dan

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