https://devzone.nordicsemi.com/f/nordic-q-a/11290/wlcsp-footprint-inconsistencyHi! If you look at the reference design for nrf51x22_ceaa the pad size is 0.18mm with a 0.076 mm solder mask expansion BUT in application note nAN-38 (page 6) it is recommended to use NSMD with the formula 0.9*155um = 0.14mm witch is much smalleren-USTelligent Community 13Tue, 19 Jan 2016 15:00:18 GMThttps://devzone.nordicsemi.com/thread/42462?ContentTypeID=1Tue, 19 Jan 2016 15:00:18 GMT137ad170-7792-4731-bb38-c0d22fbe4515:76fec55d-1352-4ab4-89e7-9d2c0e94ce76Stefan Birnir Sverrisson<p>Hi Stefan</p> <p>You are correct, there is a little inconsistency between the documents you mention. In fact, the PCB pad optimal diameter should be 90% or 100% of the UBM diameter. The requirement is not very strict and pad diameter between 0.14 and 0.20mm is perfectly fine, but larger than 0.20mm can be risky. So the reference design should be correct for pad size and solder mask expansion for NSMD land pattern. If you are considering SMD land pattern, the pad not covered by solder mask should also be 0.14mm to 0.20mm.</p><div style="clear:both;"></div>