Hi Luigi,
I  agree about ghosts. I have thought so too many a times. In this case,
the ghost can be bustered by having a look in Wassink, I quote:

The cause of solder balls can be
(i) Incorrec distribution of flux, resulting from maladjusment of fluxer or
transporting jigs, or from board material that id badly wetted by flux.
(ii)oxidation of solder at the position where the board loses contact with
the solder bath or of wave (also a fluxing faultl).
(iii)contamination of the solder.
(iiii) incorrect pre-drying of the solder paste
(v)  use of poor quality of the solder paste.

Another way to find the ghost is to investigate the process window. Tt
consists of so many parameters, that I prefer to send the troubleshooting
map to you offline instead.

Then we can continue the discussion.

Before I close, I can tell you that we have seen similar issues and I
recall that we saw such swarms of microballs only in the VP line. When the
subject is passing the firsr zone too fast, volatiles are still within the
paste. When in the lower part, the instant heat makes the slurry boil and
before the solder has liquidified, a number of solder particles escaped and
were spread around, more or less. The size variation mirrors the paste's
nature namely solder particles from nn um  to xx um.

The map will come in minutes, got to find it first..

Inge

-



On 22 February 2012 07:33, <[log in to unmask]> wrote:

> Hello everybody.
>
> Any  idea about random presence of microballs (diameter 0.1 to 0.5 mm)
> beneath some chips (R and C 0805, 1206,1210, 1812) ?
> We use a SnPb process with a vapor phase oven.
> We use same types of PCB, same process on same SMT line in a clean room
> from years and the balls are randomly present from some months now.
> They could be present in one lot of PCB and not the following; beneath a
> component on one PCB and not on the same component on other PCB; we have
> balls one day and not the day after; the balls disappear for weeks and
> re-appear after; ...
> I have checked different lots of solder paste, origin of PCB's, all
> parameters of printing, P&P and the cleaning (co-solvent station). The
> thermal profiles are same.
> As it's hi-rel production I cannot change easely products and process.
> I suspect a process parameter but which ?
> I ask myself if there is not a ghost in the line .. :-)
>
> Best regards.
>
> CANTAGALLO Luigi
> Technology Engineer
>
>
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