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

[Dave N6NZ]

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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