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

[Steve Meier <smeier@xxxxxxxxxxxxxxxxxxx>]

Dave,

PCBExpress requirement of a clearence around the pads of at least 8
mills meets the IPC specifications for silk screened soldermask.

For the geometry of the TSSOP-20 with a lead pitch of 26 mills and lead
widths of 10 Mills your soldermask width between pads would be zero. As
you stated.

If you are going to use a shop that only does a silk screened soldermask
then I suggest you use a gang clearence around the group of pads and
risk the solder shorting the pads.

However, switching to a photo imageable soldermask process reduces the
solder mask clearence down to 3 to 5 mills. This would leave a
soldermask bridge between pads of at least 6 Mills. I, don't see
pcbexpress suporting this process though. You probably should contact
them. I have had problems in the past with gang soldermask clearences.
It puts a lot more preassure on the assembly process.

The reference document I am using is IPC-SM-782A "Surface Mount Design
and Land Pattern Standard"

Steve Meier 


On Sat, 2006-10-21 at 21:42 -0700, 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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