"Jim Williams" <[log in to unmask]> wrote

>     Some designers make an effort to maintain a web of soldermask between 
>     the solder pads on SMT devices, where pad spacing permits. When the 
>     spacing of fine pitch devices preclude the web, it is omitted. I have 
>     heard that the webs are used to reduce solder bridging. 
>     
>     Thinking through this issue, I find webs being used where pad spacing 
>     is large enough so as to make solder bridging unlikely, while webs are 
>     omitted when the pad spacing is close enough to make solder bridging 
>     most likely. 
>     
>     I am interested in any factual data that supports the reason for, or 
>     the value of, soldermask webs.
>     
>     Thank you in advance for your response.
>     
        Several others have written that with a capable process including
proper stencil design, proper pad design, and a properly controlled printing
process, fine pitch can be performed without solder dams between fine pitch
pads.  I don't disagree, but one benefit may be enhnced cleaning,
paticularly after any rework.  

        Soldermask tends to be smoother than the fiberglass surface, thus
allowing easier removal of residues.  I am currently working such an issue
on a connector design where we are still proofing our manufacturing process.
One assembly has solder dams, and its sister card does not.  the card
without solder dams is presenting much higher incidences of residues between
lands.

Good Luck, Steve Mikell, Process Engineer, Soldering & Cleaning
SCI Systems, Government Division, Plant 13
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