Doug
Sunny, about 30°C, beautiful spring-like weather with the birds making
an infernal row and the hills a blaze of colour before everything gets
seared next month. Just added a bit to the garden irrigation system to
finish the last piece of automatic watering before it gets hot.
Slight variations of opinion for Rudolph, but roughly similar. :-) The
rejoinder was sent off long before I saw yours!
Best regards
Brother Ellis
Douglas Pauls wrote:
>
> 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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