Steve
If I understand your problem right, I would do the assembly a little
differently. Start by putting every component normally, except the
shield, clean, stick the shield in place with a conductive epoxy, making
sure you do not add contamination.
IMHO, you have as much chance of cleaning under the shield (and, if you
solder the shield in place, even after cleaning the rest, as a pig has
in hell not ending up as roast pork. You are going to have impossible
flux residues inside it and, with a "no-clean" flux, there is every
chance they will have sublimated everywhere.
Cleaning method. The problem with vapour phase is which solvent?
HCFC-141b is not permitted in the USA and some other countries. HCFC-225
is permitted in some countries but even the methanol azeotrope is not a
brilliant solvent for flux residues and is very expensive and somewhat
toxic. HFCs and HFEs (blended) are all expensive and very ineffective.
TCE/ethanol azeotrope may be OK but is very aggressive (may attack the
substrate or plastic components) and is somewhat toxic (very cheap,
though). This leaves nPB blends which I cannot and do not recommend,
although they are unregulated for the moment. It would appear that they
may be VERY toxic (one manufacturer puts it more or less on a par with
carbon tet and two others refuse to sell it as a solvent blend because
of toxicity) and seems to be straight on line for very strict
regulation. It is also ozone-depleting but not yet in the Montreal
Protocol but could become so. See
http://groups.yahoo.com/group/solvents_group
for more details.
Ultrasonics: before adopting ultrasonics in production, check the
reliability of cleaned assemblies. There are some components that don't
like them at all. Especially ceramic-cased semiconductors, quartz
crystals, some multilayer ceramic capacitors etc. The problem is that
they may not cause the component to fail in the immediate but to weaken
it so that it fails prematurely after a number of
days/weeks/months/years of service.
Baking: Thorough drying is obviously necessary before potting/coating.
With normal assemblies, baking is not mandatory, provided that you can
ensure all volatiles are eliminated. This may be done by baking, putting
in a vacuum, thorough drying or a long sojourn in a dry atmosphere, as
you wish. If the drying process is reasonably good, and you use vacuum
coating/potting/impregnation, I recommend you draw a vacuum for a few
minutes before the actual compound is introduced.
Hope this helps
Brian
Steve Abrahamson wrote:
>
> Howdy,
>
> I have got a major mess on my hands. We may soon be running a double sided
> SMT product that needs to be fully cleaned due to a conformal coating
> process.
> Both sides of the assembly have two 2" x 1" x .3" RF Shields that are
> soldered to the PCA. There are SMT components under the shield, and the
> shields have two .040" holes on top- the rest of the shield is sealed.
> Just in case we cannot clean 100% of the residue out of these shields, the
> customer requires a no-clean chemistry solder paste. (They claim no clean
> residue has caused problems with past due to issues with the battery
> pack...I am not much of an electrical guy, and cannot refute them).
> They are pushing us towards a vapor degrease cleaning method, since they
> had used the process sucessfully during the design phase with good luck.
>
> Question 1: If we did use a non aqueous solvent for cleaning, especially
> one that has evaporative characteristics (any HCFC replacement), could we
> avoid the "bake out" process?
>
> Question 2: Would a cold soak process with somthing like Bioact be a viable
> alternative- and be performed without a bake out?
>
> Question 3: Would ultrasonics be valuable (if we had alternating frequency
> such as with the Smartsonic system), or could more damage be done.
>
> Steve Abrahamson
> 208-898-2695
> [log in to unmask]
>
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