Rudolph,

You have an interesting case.  Did the outside lab you used interpret the
results for you?  If so, what did they say?  I saw many such cases when I
was at CSL.  I suspect that the reason that you are seeing the residue
levels jump around so much is due to your bare boards.  If you can trust
your flux and paste vendors, and you are truly using halide free materials,
then the only source of these residues is prior to entry into your assembly
operation.

In the scenario that you give, it is not likely that you are using the same
test sample for extraction at the different points of the assembly process.
You are probably using different assemblies on a sample basis.  Therefore,
you may have differing bare board cleanliness levels.  If you are dealing
with multiple vendors, I can almost guarantee that you will get different
residue levels on incoming boards.  I presume that all of the boards are
coming in tin-lead coated.  HASL fluxes and fusing fluids generally come in
one of three flavors:  high in chloride (e.g. HCl), high in bromide (HBr),
or high in both.  And I can just hear Brother Ellis warming up the keyboard
to straighten me out on this score.  Anyway, if the post-HASL or
post-fusing cleaning process is not adequate to remove the HASL flux, or
the laminate/solder mask absorbs the flux or fusing fluid, then you get
bare boards with high levels of chloride and bromide.  It is not unusual
for a high volume fabricator, such as Photocircuits or Sanmina, to have a
chloride-based HASL line running in parallel to a bromide-based HASL line,
both of which may be producing your bare boards.  Therefore, in one pack of
bare boards, you can have a mix of high chloride and high bromide.  An even
wider mix in residues if you have multiple vendors.  I once worked with a
vendor with 20 different board sources.  Imagine the mix of residues there.

It is normal for bromide levels to increase with each exposure to reflow
temperatures.  Such exposure tends to make the laminate and solder mask
more porous, so more flame retardant bromide can be extracted in the ion
chromatography analyses.  I recognize your customers cleanliness spec as
CSL's recommended guidelines for a no-clean assembly operation.  A value of
7 for bromide suggests to me that you are using either CEM laminate or
polyimide, which have lower amounts of added bromide flame retardant.

The bottom line is that the residue levels are probably not jumping around
due to your assembly processes, but are due to the varying condition of the
boards going in to your process.

You ask the question of whether your customer's cleanliness spec is
reasonable for a no-clean assembly process.  Since I had a large part in
developing that recommended cleanliness level, OF COURSE it is reasonable.
Brilliantly written.  Impeccible data.  Sage advice.  As Graham Naisbitt
might say, "All that rot".

If you have other questions, post 'em.  I'll give it another shot.

Doug Pauls
Rockwell Collins

I can just hear the incoming rejoiner coming from Cyprus.  How is the
weather there today Brian?

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