With respect to solder protect ant on PWB land patterns, what is "white tin" ?
 

• Is it only tin and alloy or combination of metal and something else?
• How is it applied to the land pattern?

With respect to etch back on 1 oz copper. We would like to design boards with lands, for 20 mil  pitch QFPs,  that measure between 10 and 11 mils wide. Some of our board houses ask us to bump our fab drawings to 12 mil while others asked us not to. Any opinions out there?