Re: gEDA-user: bending design rules for TSSOP20 -- need advice

[Dave N6NZ <n6nz@xxxxxxxx>]

That's interesting information, thanks.

I'm thinking I misintrepreted "mask swell" to mean .008 copper-to-mask, 
as opposed to mask diamater = pad diameter + 0.008, which would yield 
.004 mask-to-copper.  If so, then I end up with .008 of mask between 
pads.  In any case, I need to verify with them exactly what the rule means.

-dave


Steve Meier wrote:
> One shop I work with asks for soldermasks to be at least 5 mills wide.
> If you have two adjacent pads with solder mask between the pads that
> soldermask should be at least 5 mills wide. So I suspect for PCBexpress
> that they need the soldermask to be at least 8mills wide. The issues has
> to do with alignment tolerences. Soldermask put on with a stencil  has
> worse tolerences the soldermask applied with a photo imagable process.
>
> Steve Meier
>
> Dave N6NZ wrote:
> > Here is the rule in question from PCBexpress web site:
> >
> > Solder mask swell is at least .008 larger than copper surfaces to keep
> > mask off pads.
> >
> > Am I interpreting that correctly?  That I need .008 all around the
> > copper pad?  Or is a mask .008 wider than the pad sufficient?
> >
> > -dave
> >
> > Dave N6NZ wrote:
> > > Thought I just saw a thread on this topic, but I deleted the whole
> > > works and can't find it in the archives.
> > >
> > > I'm trying to reconcile a data sheet for a TSSOP-20, 0.65mm lead
> > > pitch package with PCBexpress's design rules.  The problem: 26mil
> > > l.p. and 10mil pad width leaves 16mil btw pads.  The rule: "8mil
> > > between mask and copper" leaves exactly 0 mils of mask between pins.
> > >
> > > So.... can I bend something here and get a reasonable board? 
> > > Something like: go to an 8mil pad width so that I get 2mil of mask in
> > > between. Pad is too skinny, but I guessing should reflow well...
> > > those of you with more experience than me at SMT need to clue me up. 
> > > Will 2mil of mask be too skinny to work well? Other option: live with
> > > no mask between pins and hope I don't bridge.
> > >
> > > Thoughts?
> > >
> > > -dave
> > >
> > >
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> >
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