Bill,
This is good information. 
Thank you. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Wednesday, September 21, 2005 11:40 AM
To: [log in to unmask]
Subject: Re: [TN] Fused tin lead plating versus HASL

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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