Solder mask (negative layer) for QFN package

[liletian]

Hi Guys
It looks like normally people put a solder mask (negative layer) at the center of the QFN package. In other words, the QFN package in the PCB board is lower(thinner) than other place. I am thinking it is because in this way, you can easily put the chip on the right place?
Is it regular to have a solder mask (negative layer) for QFN package. For exaple, for QFN 5x5 package, you will put a 3.5x3.5 tstop(solder mask) at the center of the QFN package place. The only reason I can think is it probably easy to fit.
Can anyone comment on this?
Thanks a lot

 

[rupeshchannawar]

for soldering QFN Padon PCB, you have to give solder mask on the Pad...

 

[marce]

Have a look for IPC-7093 Design and Assembly Process Implementation for Bottom Terminated Components. Even if you have to actualy purchase it from the IPC it is worth it, it covers all aspects of dealing with QFN and other BTC's.