Good morning Charles,
>
> Does anybody have a specification for these particular topics :
>
> - Time allowed between water soluble paste printing and reflow ?
> - Time allowed between reflowed PCB assembled with waters soluble solder
> paste and the cleaning operation ?
I don't believe that any of the process level specfications (IPC at least)
discuss the time between any solder paste print and entry into reflow.
Considering the wide range of pastes available, it would completely depend
on the rheological properties of the paste. I doubt any specification could
be drafted that adequately defined such a parameter, especially for a
"one-size-fits-all" approach.
For the time between reflow and cleaning, I also doubt there is a limit in
present specs. MIL-STD-2000 quoted a time of 30 minutes, but that was based
on the old high-solids rosin fluxes, which quickly polymerized. If you did
not initiate cleaning in that time span, the rosin became very hard to
remove. Present day water soluble fluxes have a MUCH lower rosin percentage
and do not suffer from this problem. As a guideline, cleaning should be as
quick as manageable without thermal shock, i.e. don't run it across the wave
and drop into the cleaner without some cool down time.
How long could you go? It depends on the aggressiveness of the flux. If you
are using a low activity flux, then there is likely to be little corrosion
going on while you wait to clean. On the other hand, if you are using zinc
chloride, better clean immediately or your circuits get etched away.
>
> Is there a different specification between the different types or activity
> of organic fluxes? If it so, what is it?
If the flux is qualified to J-STD-004, one of the designators in the flux
symbol will tell you how active the flux is. The activity is classified as
L, M, and H, meaning Low, Medium, and High. It classification also has a
listing for halide content - a "0" means halide-free, and a "1" is
halide-bearing. So the most benign flux is L0, then L1, then M0, then M1,
etc. Organic fluxes are type OR and would have classifications of ORL0,
ORL1, etc. Most water solubles are of medium activity or higher. I have
never seen an ORL0 or ORL1, but I suppose they could exist.
> Is it a problem if we double
> reflow th PCB (double sided board) before the cleaning? Depending on the
> quantity of a giving lot, the time between the first printing and the
> cleaning operation may be long (see couple of hours, the cleaning machine
> is not in line with the SMT machines), that why I'm interested to know if
> there is a spec.
Many manufacturers have a two-reflow process (IR reflow then wave) without an
intermediate cleaning step. It can be done because solder pastes put down
far less flux than wave soldering. One thing to consider though is that flux
residues get progressively harder to remove with each exposure to reflow
temperatures.
>
> Also, does anybody have a specification for the baking operation to
prevent
> or eliminate moisture in plastic components (IC, QFP, BGA, etc...). Is
> there a relationship between the exposure time in humidity vs baking time
> or is there a fixed set of temperature and time parameters?
I agree with the other responder who listed J-STD-020.
>
> Thanks in advance, it would be very appreciated if someone could share any
> specification or recommendation.
You're welcome.
Doug Pauls
CSL
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