Alex,
     
     I don't know for sure but I would think that theoretically, the amount 
     of weight that could be supported by molten solder "bumps" on the 
     bottom side of a pcb should be: 
     
     [suface tension of solder( approx 400 micronewtons/mm) x the perimeter 
     of a "pad"(mm) x #pads] - weight of the solder on the pads(rho*g*Vol  
     in micronewtons)
     
     Don't forget that the surface tension of the solder must also support 
     the weight of the solder itself along with the component you are 
     trying to support. Also, if the component lead perimeter immersed in 
     the solder on the pad is less than the actual pad perimeter, then I 
     would think that there would be less surface tension available to the 
     component for support so we would have to reduce the available surface 
     tension proportionally. Maybe the final formula would be:
     
     {[gamma solder*pad perim*#pads]-(rho*g*V)]*(lead perim*#leads)/(pad 
     perim*#pads)}
     
     I'll be curious to see what you come up with. Good luck.
     
     Regards,
     
      Mark Kwoka
      Harris Semiconductor
     
     
______________________________ Reply Separator _________________________________
Subject: Surface Tension
Author:  [log in to unmask] at smtp
Date:    10/5/96 9:30 AM
     
     
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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