Hi Alex -
     
     I haven't had time to work out the math but MPM has published this 
     formula:
     
                Weight of component (in grams)/ Total pad mating area in 
     square inches
     
     The grams/square inch must be less than or equal to 30 to keep things 
     from falling off the board. 
     
     I recommend you run this formula  and compare it to a surface tension 
     - force type comparison just to be on the safe side.
     
     
     PS - have you done any  Impedance spectroscopy testing lately?
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Surface Tension
Author:  [log in to unmask] at ccmgw1
Date:    10/5/96 12:05 PM


Can someone help me with a formula to approximate my design rules to 
solve the problem of falling chips on second side reflow. The chips on 
the second side of the board seem to be to heavy for the superficial 
tension that is supposed to hold them down while going through the reflow 
oven for the second time.  I want to set PCB  design rules depending on 
the weight of the chip, surface area of the chip's leads and pad sizes. I 
know that there are many more variables to consider but a simple rule of 
thumb formula will do.  Or if anybody knows about the exact science to 
deal with this problem I will appreciate the feedback. (Can't use Glue)
     
Thank you much.
P.S.  This is my first time in ThechNet and I thank you all for the 
forum.
     
Alex Basauri
SMT Engineering
Trimble Navigation LTD
The GPS Solution!!
Sunnyvale, CA
[log in to unmask]
(408) 736-4924
     
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