Ralph Mowery wrote:
Does the flux in solder really get old ?

Just getting into the SMD soldering and the tubes of solder paste have
an expiration date on them of about a year and some want to be kept in
the refrigerator when not being used. Also bought some liquid flux and
it mentions an expiration date about a year or so later.

The problem with solder paste is surface oxidisation of the very small
particles of solder in it, and possibly saponification of the flux due to
its reaction with the oxidised solder. Keep it refrigerated in a ziplock
bag or hermetically sealed container with a silica gel sachet to control
humidity, at the lowest possible storage temperature as the rate of
degradation doubles with every 10 deg C increas in temperature, and allow
it to warm to room temperature an hour before use.

Fluxes are a more complex matter. Liquid Rosin fluxes are unlikely to
degrade over decades if securely stoppered. Liquid Synthetic fluxes may
be more problematic and if any component can further polymerise, the
expiry date may be justified. If kept cool and well stoppered, in a dark
place or opaque bottle, the chances are that it will still be good well
after the expiry date. Gel fluxes may be stable or may tend to seperate
or degrade. Again, cool and dark may help.

Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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