Another depends response.
Machine, flux, solder, PCB design, component and board finish all
contribute.
Preheat can be applied before the machine with an oven, preheat in the
machine, using the the solder to touch near the solder connection, running
slow enough to pre heat the next joints, using an oversize nozzle, having
the solder close, not touching the solder joint (not a very effiecient
method)
Soldering speeds can exceed 10mm/s under certain conditions.
Machine manufaturer may suggest 4 to 6 mm/s.
300C plus is not unusual on a robotic selective solder machine using SAC305.
A manual solder fountain at this temperature would eat up the PCB's as the
timing is tough to control.
Selective soldering requires examination of the voiding in the joint,
confirmation that the knee wall thickness is not destroyed by too long a
dwell, basically qualify the product and process. Changing fluxes, solders,
PCB finish process speeds, inerting and process temperatures can all cause
dramatic changes in results.
An EMS refused to use the specified flux and SAC305 because it would cost
too much. The result was a pile of scrap, missed delivery, dissatisfied
customer (me) who had not been informed of the changes and an EMS looking
at quadruple the capital cost if they used the less expensive material due
to the increased cycle times if they implemented the less expensive
materials. Changing from alcohol to water base fluxes takes much more
preheat and energy to remove the water instead of alcohol. SAC305 melting
point is 7 degrees lower than the substitute material. This process can
bite back if care is not taken with change.
Eroded pins, delamination, voids, insufficient hole fill, excess flux,
solder balls, bridges and skips can result from short term thinking change.
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