Please see technical note TN0018 https://my.st.com/resource/en/technical_note/cd00134799.pdfOn page 5 you can find basic rules for land pattern dimensions.
Basically it means, there won’t be any solder mask between the pads. It is ok for prototyping.
You should discuss it with your PCB manufacturer and assembly plant. If they are able to make narrow solder mask you can modify the solder mask expansion to have solder mask also between pads. For example I use following settings:
A = PCB land length = LGA solder pin length + 0.1 mmB = PCB land width = LGA solder pin width + 0.05 mmC = Solder mask opening length = PCB land length + 0.05 mmD = Solder mask opening width = PCB land width + 0.05 mm
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