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Date: | Mon, 7 Oct 1996 16:01:54 -0400 |
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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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