WLCSP footprint inconsistency

[Stefan]

Hi!

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 smaller than the reference design

1. What is the correct pad-size and soldermask exansion for Non-solder mask defined land pattern?
2. What is the correct pad-size and soldermask exansion for Solder mask defined land pattern?
3. Also the Paste stencil opening is missing from the specifications. Rererence design has 0mm expansion. Is this correct?

[Stefan Birnir Sverrisson]

Hi Stefan

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.