Hi Luigi,
You have to check:
1. Size of pads (too long under the components body).
2. Size of opening in stencil
3. P&P accuracy
4. Solder paste temperature (should be room temperature)
5. Oxidation of component / PCB [ads / solder paste particles
Reuven


On Wed, Feb 22, 2012 at 8:33 AM, <[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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