Alex,
     Our factory uses the JPL (joint perimeter length) to determine if a
part will stay on during second side reflow.  The surface tension force
equals the JPL times the STC. 
     Fst = JPL * STC
      where Fst = Liquid solder surface tension in milligrams,             
            JPL = Joint Perimeter Length in centimeters,
            STC = Surface Tension Constant in Dynes per centimeter             
                  (note: one dyne will hold approx. .5 milligrams of wt.)
     On our line the STC of the solder we are using is 376 Dyne per cm of
JPL.  As an example, if we measure one cm around the perimeter of a pad
(length*2 + width*2) we can expect that pad to have 376 dynes of STC and
be able to hold (376*.5) = 188 milligrams of weight. So, if your part is
less than 188 milligrams it will stay on during second side reflow.
     Hope this helps.
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   /  Mark R. Peltier          [log in to unmask]  /
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 On 5 Oct 1996, Alex Basauri wrote: 

> 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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