Rudolph
It is not easy to answer all your questions. I would guess that the
bromide level on the bare boards may come from the FR system BUT, if the
boards are HASL, my guess it is more likely to come from the HASL flux.
It would depend also on how you extracted it. If it was done over
several hours at a high temperature, then yes, it could be leached out
from the boards.
It would seem that your reflow flux is probably hydrochloride activated.
This is relatively unusual for a specific "no-clean" flux. The fact that
the bromide level drops is possibly because there is still some free
hydrochloride which is driven off by the heat. This is more consistent
with the HASL flux, rather than the board FR components, which may
actually increase after reflow.
I have no explanation of the high bromide level at the end. This may be
an empirical error if only one test was made, or is it consistent over a
number? I can say it is most probably of external origin. What are you
adding to the board between wave soldering and the end? Are you cleaning
anything with a brominated solvent, such as CBM or nPB, even locally?
Quoting max Br and Cl levels on a "no-clean" board is, IMHO,
meaningless. However, I would suggest that, to meet the low Cl levels,
you must change your paste to a halide-free one.
Hope this helps
Brian
Rudolph wrote:
>
> Hi TechNet:
>
> Recently I have sent out number of no-clean assemblies at different
> stages to an outside lab for a contamination evaluation. ( not
> analysis) The numbers came back, and I was surprise to see how much
> more bromide and chloride had been collected during the assembly
> process. The result is shown below:
>
> At the bare PCB level, bromide level is 6.2 , while chloride is 1.2
> ( all units are in ug/in sq.)
> At the post reflow stage, bromide drops to 3.0, and the chloride goes
> up to 16.8
> After wave soldering, bromide stays at 3.5, and the chloride drops to
> 8.1
> At the final assembly level( no manual rework), bromide level soars to
> 30.8, and chloride is 9.6
>
> I have called my paste, flux and alloy bar suppliers and confirmed
> that non of these material contain any bromide and chloride in them.
>
> The original 6.2 ug/in sq bromide at the PCB level is most likely from
> the fire retardant material used on the FR-4. But can someone help me
> to explain where did the additional 25 ug/in sq of bromide come from
> after the wave soldering process? what about the chloride level? Do
> these numbers look normal to you?
>
> One of my customers has a cleanliness spec' calls for an assembled
> board with no more than 7.0 ug/sq in of bromide and 2.5 ug/sq in of
> chloride when they receive the board.
>
> Do you think this requirement is reasonable for an assembled board
> built with a no-clean process?
>
> Any kind of inputs and comments are greatly appreciated. Thank You.
>
> Rudolph Yu
>
>
>
>
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