There are a couple things to watch with this, esp. if solder-masked after  IR
fusing. Often saw problems with this process if UV cured liquid solder mask
used. If output of UV lamps not monitored, only a partial cure results in a
weak  mask*. Then when soldering takes place, the tin-lead under the mask melts
and  moves, tearing the thin weak mask. Then as the PWA cools, flux residues
suck  under the mask through the minute tears in the mask. Such entrapped
residues  leak out over time, possibly compromising the reliability of the PWAs.

Readers should be aware that the fusing fluids are essentially strong water
soluble fluxes which can have a significant negative impact on the SIR of the
completed assemblies. Such results have been reported at China Lake Soldering
 Seminar and Internepcon Japan. So now one could potentially have water
soluble  fluxes and assembly fluxes seeping out through the minute solder mask
tears.  This could often be detected by the ionic contamination levels not going
to zero  during the duration of the test, or by continuing to show high levels
if the  ionic testing is repeated.

*UV lamps start to degrade the minute they are turned on, in contrast to
incandescent lamps. One published example stated that three passes through the
UV solder mask curing cycle were needed to reach the desired level of solder
mask cure.

Bill  Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO  80526
Tel: 970.207.9586 Cell: 970.980.6373
email:  [log in to unmask]


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