Hi Yuan -
I use the same formula - the total pad/ball mating area (after collapse)
divided into the weight (in grams) of the package. I have used a not too
exceed number of 30 g/ in sq. but have gone up to 44 g/in sq.   So the weight
per joint will vary with the number of balls, the size of the balls and the
pitch.
Hope this helps.
Regards,
Phil Zarrow

ITM Consulting
Durham, NH  USA
www.ITM-SMT.com
T: (603) 868-1754
F: (603) 868-3623
EM:[log in to unmask]

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