Rule of thumb for BGA padstacks?

[randychase]

Tom, what is the general rule of thumb for soldermask voids?

This is for a .8mm BGA which would use .4mm pads.

[Tom]

Randy,

I'm attaching an Intel specification regarding BGA soldermasks. In this area there are no "Rule of Thumb" general rules. A discussion needs to be held with the people who are doing the assembly process. We make all of our padstacks with a 1:1 Paste Mask and the assembly shop needs to know this in the assembly notes. This way they can adjust the BGA Paste Mask data to create the stencil. The Solder Mask data (over size or under size info) that is gathered from the assembly shop should be added to the fabrication notes so that the PCB manufacturer can properly adjust the BGA Solder Mask to meet the Assembly Shop needs.

Please see - http://pcbstandards.com/forums/showthread.php?s=&threadid=251 for an update regarding this subject. The picture in the attached ZIP file from Intel is NOT what IPC is telling the industry.