All three questions were addressed in a book I wrote in the early 1980s.
One point I must emphasise: do you mean free chlorine used to disinfect
drinking water or do you mean chloride anions? Chemically and
electrically, there is a world of difference and you (and those who
answer you) must be precise in this terminology. Only chlorides will
show up in an ionic contamination test. One problem is the method used
in chlorinating drinking water; some small communities may use sodium
hypochlorite which liberates free chlorine enough to kill off
microorganisms but each bug will cause it to form a molecule or two of
sodium chloride. The latter may improve the sales of antihypertensives
in the local pharmacy!
I agree wholeheartedly with your point 3 and have promoted low pressure,
high volume cleaning as being preferable since my erstwhile company
introduced its first aqueous cleaner in 1975.
Re airknife "cleaning" (also discussed in my book, now out of print),
good airknifing from multiple angles (we used rotary airknives) at
velocities of the order of 50-100 m/s will remove at least ~90-95% of
the water and the residual contamination dissolved therein. Very often,
even with non-DI water, this is largely sufficient as the remaining
evaporative drying, even under components, leaves very little and often
fairly harmless (e.g., calcium carbonate) residues. Qualification
testing of your whole soldering/reflow/cleaning/drying processes is
nevertheless necessary.
Brian
On 01/02/2011 19:08, Forrester, Michael (H USA) wrote:
> I have three questions I hope someone can help me with:
>
> 1) We have a vendor that uses "city water" directly into their SMT
> PCA washer. I believe DI water is industry standard.
> Does anyone know if it is documented anywhere? Spec?
>
> 2) We had an issue where there was Chlorine contamination and gold
> dendrites found inside some standard gull-wing 6 pin SMT ICs.
> This issue seems to be batch related. The chip manufacturer
> claims that chlorine is never used in their process. Should the SMT
> process expect to handle the possibility that the wash solution
> may get into a part? The PCA vendor uses unfiltered "city water",
> with no cleaner, in the wash process and does a ROSE test on one
> board of each lot. Since the ROSE test passes, the PCA vendor
> is blaming the part manufacturer for having "leaky" parts, since
> the thinking is the wash is getting into the part and trapped.
> 3) One issue I have is the water pressure in the wash is 80-100 psi.
> I think this is too strong, and should be half of that? You want to
> flood the board with water, not hit the board and bounce off?
>
> Thank you in advance.
>
> Best Regards,
>
> Michael Forrester
> Sr. Product Engineer
>
>
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