[sdiy] Soldermask for fine pitch TQFP packages

[John Luciani]

On Wed, Nov 26, 2008 at 7:35 PM, Seb Francis <seb at burnit.co.uk> wrote:
> John Luciani wrote:
>>
>> One more spec to watch is the minimum width required for the soldermask.
>> If the gap results in a soldermask strip that is less than the minimum
>> width I increase the gap to the smallest size required to remove the
>> strip.
>>
>
> This is a bit tricky as my PCB package only allows the guard gap to be
> specified by pad, or globally, but not by package.  And unfortunately the
> default for all the pads is 10mil ...
>
> So I either just set a global 4mil, or I would have to go through and
> manually change all the pads to 4mil, except any pads that are used by
> packages with pad gap between 8mil and 8mil + min soldermask width.  I'm not
> even sure if this is possible since pads can be shared by multiple packages.

Can't you script this? I create all of my footprints using scripts. The mask is
set pad by pad by the script.

>
> What's the worst that could happen if I inadvertently specified a soldermask
> strip that was less than the minimum?  Would it just not stick properly and
> risk peeling off?  Or can something more problematic happen?

I doubt it would end up being an issue, especially for hand assembly.
The thin strip may peel off but it wasn't needed anyway. I doubt
it would peel off any soldermask that you did want.

I try to avoid possible problems when I design the footprints.
I don't make the mask strips too thin and I don't run silkscreen over
copper. All of this is down using scripts. I do not create any
footprints manually.

(* jcl *)

-- 
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