Hi Bob,
Please use "the rule of 36":    Take the weight of the parts in grams and
divide that number by the total area (sq. in.) of the pads of the device.
If the resultant is greater than 36, the part will probably fall off
during reflow.  You will have to do a few tweaks to either the amount of
solder deposited, or the size of the pads to dial this in.
Kerry

Kerry McMullen
Principal New Product Mfg. Engineer
LTX Corporation
825 University Avenue
Norwood, MA 02062-2643
(T) 781-467-5468
(C) 508-631-1832
(F) 781-461-0993



bob wettermann <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
09/12/2006 07:18 AM
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Subject
[TN] Surface Tension of Solder vs Part Mass in Reflow






Dear Technetters:

  I have a thought question for you SMT process engineers out there.

  We are attempting to have a part selectively drop off in a reflow oven
on a 2-sided l-f SMT assembly. In order for this to happen we will have to
make sure during the reflow cycle that the part in question falls off
while the others remain.

  The question is how does one calculate the force due to surface tension
holding a part to the PCB while the solder is in the  liquidus phase? If
we know this then we believe we have a method to attach the right amount
of weight to the device to make it fall off.
  \
  Thanks .

  BWET


Bob Wettermann

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