When your PCB data goes to fabrication the soldermask layers are examined to determine if apertures have been modified (are not 1:1).
If your soldermask layers are not 1:1 then the fabricator will most likely NOT use your soldermask layers and they will try to use the solder paste layer to determine the actual pad sizes.
Fabricators shrink or expand the soldermask to optimize your design for best process yields.
Soldermask webs are removed if the soldermask features between pads of fine-pitch parts are less that 3 mils wide.
Gang mask is a term used to describe removing the soldermask between closed spaced pins.
Here is and example of a gang masked (SOIC, SOP) with a pin to pin pitch of 0.65mm.
Laser Direct Imaging (LDI) has greatly improved soldermask registration and reduced the soldermask swell requirements, which allow for wider soldermask webs around fine pitch parts.
I'm on a mission this month to determine what is currently the best practice for soldermask.
Should we provide 1:1 data to the fabricators ?
Research:
According to Tom H. who is the father and Champion of the IPC-7351 Land patterns:
Designers have two choices.
"Use a stock library with 1:1 scale masks and create fabrication and assembly notes for solder mask and paste mask adjustments".
"Create a custom library for a PCB layout and tell the manufacturer's not to touch (adjust) the Gerber data."
Source:
http://www.pcblibraries.com/forum/advice-for-solder-mask-paste-mask-layers_topic484.html
PCB Libraries has created a CAD tool neutral CAD Library Solution for Land Patterns.
http://www.pcblibraries.com/
By default this tool creates pads and soldermask 1:1.
