I know Keysight says 5 in-lbs for 3.5 mm and 8 in-lbs for SMA. But I was looking through numerous websites to see if there really is a standard. And it appears there is not (that I have found). Some sites give different torque for SMA depending on the connector material. Some sites, rather than 8 in-lbs, state 7 to 10 in-lbs. Is there an official spec on a truly official document? Or is this yet another non-standardized standard value?
I suppose since so much of the equipment we use where SMA and 3.5 mm torque matters is Keysight (HP, Agilent) equipment, that is to some degree, a standard.
I grabbed this from Keysight about a year ago for a connector care class
Here is one that I find a bit more useful as far as informational on connectors. Its a great teaching and reference tool.
http://www.npl.co.uk/upload/pdf/prec-coax-connect_guide_v3.pdf (http://www.npl.co.uk/upload/pdf/prec-coax-connect_guide_v3.pdf)
MIL-STD-348 covers SMA dimensional tolerances. You will find SMA on pages 70 - 73 here:
http://www.santron.com/uploads/resources/MILSTD348REVB.pdf
I found nothing there regarding torque.
3.5mm is covered by IEEE 287 which is available for a fee. I don't own that standard.
Because of the manufacturing variations that will exist between manufacturers I would be very surprised if any standard dictated the torque requirement. I think you'd be dictating everything including materials and thread finish if you did.